Centrifugal Pump Centrifugal Pump Course Course

Dec-01 to Dec.03, 2003, Jubail Dec-01 to Dec.03, 2003, Jubail Intercontinental Hotel, Al-Jubail, Intercontinental Hotel, Al-Jubail,

K.S.A K.S.A under the auspices ofunder the auspices of

The Arabian Petrochemical The Arabian Petrochemical Company,Company,

PETROKEMYA (SABIC Affiliate)PETROKEMYA (SABIC Affiliate)Prepared & Presented By:Prepared & Presented By:Syed Ali Hussain,Syed Ali Hussain,Maintenance Reliability Group,Maintenance Reliability Group,

ContentsContents

Why we require pump?Why we require pump? General Types of pumpsGeneral Types of pumps

Dynamic, Positive displacement types.Dynamic, Positive displacement types. What is Centrifugal?What is Centrifugal?

Working principle of Centrifugal pumpWorking principle of Centrifugal pump General Components of centrifugal pumpGeneral Components of centrifugal pump Stationary components Casings, seals, nozzles. Bearing Stationary components Casings, seals, nozzles. Bearing

housings etc.housings etc. Rotating componentsRotating components ImpellerImpeller Specific Speed and Impeller profileSpecific Speed and Impeller profile Shaft and shaft sleeveShaft and shaft sleeve CouplingCoupling

Why we require pump?Why we require pump?

Since ages man think of transferring the Since ages man think of transferring the liquid from the lower surface level to Higher liquid from the lower surface level to Higher surface level. The need to transfer water from surface level. The need to transfer water from an area of lower level to area of Higher level an area of lower level to area of Higher level give birth to develop some mechanical device give birth to develop some mechanical device to fulfill the requirement. Pump was one of to fulfill the requirement. Pump was one of the outcome of the thought processes of the the outcome of the thought processes of the old scientists and technologists. Present day old scientists and technologists. Present day pumps are used to transfer liquid from a pumps are used to transfer liquid from a lower level to higher level by consuming lower level to higher level by consuming some energy provided by the driver. some energy provided by the driver.

General Types of PumpsGeneral Types of Pumps

Generally there are two types of Generally there are two types of pumps.pumps.

Positive Displacement.Positive Displacement. KineticKinetic

Positive DisplacementPositive Displacement::The fluid is forced to move because it The fluid is forced to move because it is displaced with some piston, vane, is displaced with some piston, vane, screw or roller. Positive Displacement screw or roller. Positive Displacement pumps act to force water into a systempumps act to force water into a system

regardless of the hindrance which may oppose regardless of the hindrance which may oppose the transferthe transferDynamic:Dynamic: The pump moves with some speed and adds The pump moves with some speed and adds energy to the liquid thus increasing the energy to the liquid thus increasing the pressure of the liquid and consequently the pressure of the liquid and consequently the fluid is transferred. fluid is transferred.

We are discussing centrifugal pump which lies We are discussing centrifugal pump which lies under this heading. The centrifugal pump is under this heading. The centrifugal pump is sub class of Dynamic pumpsub class of Dynamic pump

What is Centrifugal ?What is Centrifugal ?

Centrifugal is a term pertaining to direction of Centrifugal is a term pertaining to direction of the force. This is a force which moves or tends the force. This is a force which moves or tends

to move away from the center of a circle. to move away from the center of a circle. Centrifugal force is the force of spinning.Centrifugal force is the force of spinning.

In the case of centrifugal pumps the liquid is In the case of centrifugal pumps the liquid is entered at the eye of an impeller and is passes entered at the eye of an impeller and is passes

through the periphery of the impeller. This through the periphery of the impeller. This increases both the pressure and velocity of theincreases both the pressure and velocity of the

liquid being pumped. When the liquid leaves liquid being pumped. When the liquid leaves the outer periphery of impeller, there is the outer periphery of impeller, there is

continuous increasing area at the volute which continuous increasing area at the volute which converts theconverts the

the velocity of the fluid into pressure the velocity of the fluid into pressure energy and thus at the discharge nozzle of energy and thus at the discharge nozzle of the pump, we get high pressure.the pump, we get high pressure.

There are energy conversions in Centrifugal There are energy conversions in Centrifugal pump operation. The mechanical energy pump operation. The mechanical energy from the driver (Electric motor, turbine, from the driver (Electric motor, turbine, engine) is converted into Velocity or Kinetic engine) is converted into Velocity or Kinetic energy; the velocity is converted into energy; the velocity is converted into pressure (potentialpressure (potential

Energy). The pump is also known as Energy). The pump is also known as Velocity machine.Velocity machine.

General Components of General Components of

Centrifugal PumpsCentrifugal Pumps A centrifugal pump has two main components:A centrifugal pump has two main components:

A rotating component comprised of an impeller A rotating component comprised of an impeller and a shaftand a shaft

A stationary component comprised of a casing, A stationary component comprised of a casing, casing cover, and bearings.casing cover, and bearings.The general components, both stationary and The general components, both stationary and rotary, are depicted in Figure. The main rotary, are depicted in Figure. The main components are discussed in brief below. The components are discussed in brief below. The next figure shows these parts on a photograph of next figure shows these parts on a photograph of a pump in the field.a pump in the field.

Stationary ComponentsStationary Components   CasingCasing

Casings are generally of two types: volute and Casings are generally of two types: volute and circular. The impellers are fitted inside the circular. The impellers are fitted inside the casings.casings.

1. 1. Volute casingsVolute casings build a higher head; build a higher head; circular circular

casingscasings are used for low head and are used for low head and high high capacity.capacity.

A volute is a curved funnel increasing in area to the A volute is a curved funnel increasing in area to the discharge port as shown in Figure. As the area of the discharge port as shown in Figure. As the area of the cross-section increases, the volute reduces the speed cross-section increases, the volute reduces the speed of the liquid and increases the pressure of the liquid.of the liquid and increases the pressure of the liquid.

One of the main purposes of a volute casing is to One of the main purposes of a volute casing is to help balance the hydraulic pressure on the shaft of help balance the hydraulic pressure on the shaft of the pump. However, this occurs best at the the pump. However, this occurs best at the manufacturer's recommended capacity. Running manufacturer's recommended capacity. Running volute-style pumps at a lower capacity than the volute-style pumps at a lower capacity than the manufacturer recommends can put lateral stress manufacturer recommends can put lateral stress on the shaft of the pump, increasing wear-and-tear on the shaft of the pump, increasing wear-and-tear on the seals and bearings, and on the shaft itself. on the seals and bearings, and on the shaft itself. Double-volute casings are used when the radial Double-volute casings are used when the radial thrusts become significant at reduced capacities.thrusts become significant at reduced capacities.

2. 2. Circular casingCircular casing have stationary diffusion vanes have stationary diffusion vanes surrounding the impeller periphery that convert surrounding the impeller periphery that convert velocity energy to pressure energy. Conventionally, velocity energy to pressure energy. Conventionally, the diffusers are applied to multi-stage pumps.the diffusers are applied to multi-stage pumps.

The casings can be designed either as solid casings or The casings can be designed either as solid casings or split casings. split casings. Solid casingSolid casing implies a design in which implies a design in which the entire casing including the discharge nozzle is all the entire casing including the discharge nozzle is all contained in one casting or fabricated piece. A contained in one casting or fabricated piece. A split split casingcasing implies two or more parts are fastened implies two or more parts are fastened together. When the casing parts are divided by together. When the casing parts are divided by horizontal plane, the casing is described as horizontally horizontal plane, the casing is described as horizontally split or axially split casing. When the split is in a split or axially split casing. When the split is in a vertical plane perpendicular to the rotation axis, the vertical plane perpendicular to the rotation axis, the casing is described as vertically split or radially split casing is described as vertically split or radially split casing. casing. Casing Wear rings act as the seal between the Casing Wear rings act as the seal between the casing and the impeller.casing and the impeller.

Suction and Discharge NozzleSuction and Discharge Nozzle

The suction and discharge nozzles are part of the casings The suction and discharge nozzles are part of the casings itself. They commonly have the following configurations.itself. They commonly have the following configurations.

1. 1. End suction/Top dischargeEnd suction/Top discharge - The suction nozzle - The suction nozzle is located at the end of, and concentric to, the shaft is located at the end of, and concentric to, the shaft while the discharge nozzle is located at the top of while the discharge nozzle is located at the top of the case perpendicular to the shaft. This pump is the case perpendicular to the shaft. This pump is always of an overhung type and typically has lower always of an overhung type and typically has lower NPSHr because the liquid feeds directly into the NPSHr because the liquid feeds directly into the impeller eye. impeller eye.

2. 2. Top suction Top discharge nozzleTop suction Top discharge nozzle -The suction -The suction and discharge nozzles are located at the top of the and discharge nozzles are located at the top of the case perpendicular to the shaft. This pump can case perpendicular to the shaft. This pump can either be an overhung type or between-bearing type either be an overhung type or between-bearing type but is always a radially split case pump.but is always a radially split case pump.

3. 3. Side suction / Side discharge nozzlesSide suction / Side discharge nozzles - The - The suction and discharge nozzles are located at the sides suction and discharge nozzles are located at the sides of the case perpendicular to the shaft. This pump can of the case perpendicular to the shaft. This pump can have either an axially or radially split case type.have either an axially or radially split case type.

Seal Chamber and Stuffing BoxSeal Chamber and Stuffing Box Seal chamber and Stuffing box both refer to a Seal chamber and Stuffing box both refer to a chamber, either integral with or separate from the chamber, either integral with or separate from the pump case housing that forms the region between the pump case housing that forms the region between the shaft and casing where sealing media are installed. shaft and casing where sealing media are installed. When the sealing is achieved by means of a When the sealing is achieved by means of a mechanical seal, the chamber is commonly referred to mechanical seal, the chamber is commonly referred to as a Seal Chamber. When the sealing is achieved by as a Seal Chamber. When the sealing is achieved by means of packing, the chamber is referred to as a means of packing, the chamber is referred to as a Stuffing Box. Both the seal chamber and the stuffing Stuffing Box. Both the seal chamber and the stuffing box have the primary function of protecting the pump box have the primary function of protecting the pump against leakage at the point where the shaft passes against leakage at the point where the shaft passes out through the pump pressure casing. out through the pump pressure casing.

When the pressure at the bottom of the chamber is When the pressure at the bottom of the chamber is below atmospheric, it prevents air leakage into the below atmospheric, it prevents air leakage into the pump. When the pressure is above atmospheric, the pump. When the pressure is above atmospheric, the chambers prevent liquid leakage out of the pump. chambers prevent liquid leakage out of the pump. The seal chambers and stuffing boxes are also The seal chambers and stuffing boxes are also provided with cooling or heating arrangement for provided with cooling or heating arrangement for proper temperature control. Figure below depicts an proper temperature control. Figure below depicts an externally mounted seal chamber and its parts. externally mounted seal chamber and its parts.

Gland:Gland: The gland is a very important part of the The gland is a very important part of the seal chamber or the stuffing box. It gives the seal chamber or the stuffing box. It gives the packing or the mechanical seal the desired fit on packing or the mechanical seal the desired fit on the shaft sleeve. It can be easily adjusted in axial the shaft sleeve. It can be easily adjusted in axial direction. The gland comprises of the seal flush, direction. The gland comprises of the seal flush, quench, cooling, drain, and vent connection ports quench, cooling, drain, and vent connection ports as per the standard codes like API 682.as per the standard codes like API 682.

Throat BushingThroat Bushing:: The bottom or inside end of the The bottom or inside end of the chamber is provided with a stationary device called chamber is provided with a stationary device called throat bushing that forms a restrictive close throat bushing that forms a restrictive close clearance around the sleeve (or shaft) between the clearance around the sleeve (or shaft) between the seal and the impeller.seal and the impeller.

Throttle bushingThrottle bushing refers torefers to a device that forms a a device that forms a restrictive close clearance around the sleeve (or restrictive close clearance around the sleeve (or shaft) at the outboard end of a mechanical seal shaft) at the outboard end of a mechanical seal gland.gland.

Internal circulating deviceInternal circulating device refers to device refers to device located in the seal chamber to circulate seal located in the seal chamber to circulate seal chamber fluid through a cooler or barrier/buffer chamber fluid through a cooler or barrier/buffer fluid reservoir. Usually it is referred to as a fluid reservoir. Usually it is referred to as a pumping ring.pumping ring.

Mechanical Seal:Mechanical Seal: For sealing of the pump.For sealing of the pump.

Bearing housingBearing housing

The bearing housing encloses the bearings mounted on The bearing housing encloses the bearings mounted on the shaft. The bearings keep the shaft or rotor in the shaft. The bearings keep the shaft or rotor in correct alignment with the stationary parts under the correct alignment with the stationary parts under the action of radial and transverse loads. The bearing action of radial and transverse loads. The bearing house also includes an oil reservoir for lubrication, house also includes an oil reservoir for lubrication, constant level oiler, jacket for cooling by circulating constant level oiler, jacket for cooling by circulating cooling water.cooling water.

Rotating ComponentsRotating Components ImpellerImpeller

The impeller is the main rotating part that provides The impeller is the main rotating part that provides the centrifugal acceleration to the fluid. They are the centrifugal acceleration to the fluid. They are often classified in many ways.often classified in many ways.

Based on major direction of flow in reference to the Based on major direction of flow in reference to the axis of rotationaxis of rotation

Radial flowRadial flow Axial flowAxial flow Mixed flowMixed flow

Based on suction typeBased on suction type Single-suction: Liquid inlet on one side.Single-suction: Liquid inlet on one side. Double-suction: Liquid inlet to the impeller Double-suction: Liquid inlet to the impeller

symmetrically from both sides.symmetrically from both sides.

Based on mechanical construction Based on mechanical construction (See Figure(See Figure)) Closed: Shrouds or sidewall enclosing the Closed: Shrouds or sidewall enclosing the

vanes.vanes. Open: No shrouds or wall to enclose the Open: No shrouds or wall to enclose the

vanes.vanes.

Semi-open or vortex typeSemi-open or vortex type. .

Specific SpeedSpecific Speed

““Specific Speed” is expressed as:Specific Speed” is expressed as:

NS= NQNS= NQ1/21/2/H/H3/43/4

Where N: rpm, Q: capacity (gpm), H: head (ft)Where N: rpm, Q: capacity (gpm), H: head (ft)

Specific speed may be further defined as the Specific speed may be further defined as the revolutions per minute at which geometrically similar revolutions per minute at which geometrically similar impellers would run if they were of such size as to impellers would run if they were of such size as to discharge one gallon per minute against a one-foot discharge one gallon per minute against a one-foot head. The physical meaning of specific speed has no head. The physical meaning of specific speed has no particular value, being a dimensionless number, particular value, being a dimensionless number, largely used as “type” number. It is a constant for largely used as “type” number. It is a constant for all similar pumps and does not change with the all similar pumps and does not change with the speed of the samespeed of the same pump. The specific speed pump. The specific speed determines the general shape or class of impeller. determines the general shape or class of impeller.

As the specific speed increases, the ratio of the As the specific speed increases, the ratio of the impeller outlet diameter, D2, to the inlet or eye impeller outlet diameter, D2, to the inlet or eye diameter, D1, decreases. This ration becomes 1.0 diameter, D1, decreases. This ration becomes 1.0 for a true axial flow impeller.for a true axial flow impeller.Radial flow impellers develop head principally Radial flow impellers develop head principally through centrifugal force. Pumps of higher specific through centrifugal force. Pumps of higher specific speed develop head partly by centrifugal force and speed develop head partly by centrifugal force and partly by axial force. A higher specific speed partly by axial force. A higher specific speed indicates a pump design with head generation more indicates a pump design with head generation more by axial forces and less by centrifugal forces. An by axial forces and less by centrifugal forces. An axial flow or propeller pump generates its head axial flow or propeller pump generates its head exclusively through axial forces. Radial impellers exclusively through axial forces. Radial impellers are generally low-flow, high-head design, where are generally low-flow, high-head design, where axial flow impellers are high-flow, low-head designs.axial flow impellers are high-flow, low-head designs.Increased speeds without proper suction conditions Increased speeds without proper suction conditions often cause serious problems from vibration, noise, often cause serious problems from vibration, noise, and pitting. and pitting.

Closed impellers require wear rings and these wear Closed impellers require wear rings and these wear rings present another maintenance problem. Open rings present another maintenance problem. Open and semi-open impellers are less likely to clog, but and semi-open impellers are less likely to clog, but need manual adjustment to the volute or back-plate need manual adjustment to the volute or back-plate to get the proper impeller setting and prevent to get the proper impeller setting and prevent internal re-circulation. Vortex pump impellers are internal re-circulation. Vortex pump impellers are great for solids and "stringy" materials but they are great for solids and "stringy" materials but they are up to 50% less efficient than conventional designs. up to 50% less efficient than conventional designs. The number of impellers determines the number of The number of impellers determines the number of stages of the pump. A single stage pump has one stages of the pump. A single stage pump has one impeller only and is best for low head service. A impeller only and is best for low head service. A two-stage pump has two impellers in series for two-stage pump has two impellers in series for medium head service. A multi-stage pump has three medium head service. A multi-stage pump has three or more impellers in series for high head service.or more impellers in series for high head service. Wear ringsWear rings: Wear ring provides an easily and : Wear ring provides an easily and

economically renewable leakage joint between economically renewable leakage joint between the impeller and the casing. clearance becomes the impeller and the casing. clearance becomes too large the pump efficiency will be lowered too large the pump efficiency will be lowered causing heat andcausing heat and

vibration problems. Most manufacturers require that vibration problems. Most manufacturers require that you disassemble the pump to check the wear ring you disassemble the pump to check the wear ring clearance and replace the rings when this clearance and replace the rings when this clearance doubles.clearance doubles.

Shaft Shaft The basic purpose of a centrifugal pump shaft is to The basic purpose of a centrifugal pump shaft is to transmit the torques encountered when starting transmit the torques encountered when starting and during operation while supporting the impeller and during operation while supporting the impeller and other rotating parts. It must do this job with a and other rotating parts. It must do this job with a deflection less than the minimum clearance deflection less than the minimum clearance between the rotating and stationary parts.between the rotating and stationary parts.

Shaft SleeveShaft Sleeve Pump shafts are usually protected Pump shafts are usually protected from erosion, corrosion, and wear at the seal from erosion, corrosion, and wear at the seal chambers, leakage joints, internal bearings, and in chambers, leakage joints, internal bearings, and in the waterways by renewable sleeves. Unless the waterways by renewable sleeves. Unless otherwise specified, a shaft sleeve of wear, otherwise specified, a shaft sleeve of wear, corrosion, corrosion,

and erosion-resistant material shall be provided to and erosion-resistant material shall be provided to

protect the shaftprotect the shaft . . The sleeve shall be sealed at The sleeve shall be sealed at one end. The shaft sleeve assembly shall extend one end. The shaft sleeve assembly shall extend beyond the outer face of the seal gland plate. beyond the outer face of the seal gland plate. (Leakage between the shaft and the sleeve should (Leakage between the shaft and the sleeve should not be confused with leakage through the not be confused with leakage through the mechanical seal). mechanical seal).

Coupling:Coupling: Couplings can compensate for axial growth Couplings can compensate for axial growth of the shaft and transmit torque to the impeller. Shaft of the shaft and transmit torque to the impeller. Shaft couplings can be broadly classified into two groups: rigid couplings can be broadly classified into two groups: rigid and flexible. Rigid couplings are used in applications and flexible. Rigid couplings are used in applications where there is absolutely no possibility or room for any where there is absolutely no possibility or room for any misalignment. Flexible shaft couplings are more prone misalignment. Flexible shaft couplings are more prone to selection, installation and maintenance errors. to selection, installation and maintenance errors. Flexible shaft couplings can be divided into two basic Flexible shaft couplings can be divided into two basic groups: elastomeric and non-elastomericgroups: elastomeric and non-elastomeric

Elastomeric couplings use either rubber or polymer Elastomeric couplings use either rubber or polymer elements to achieve flexibility. These elements can elements to achieve flexibility. These elements can either be in shear or in compression. Tire and rubber either be in shear or in compression. Tire and rubber sleeve designs are elastomer in shear couplings; jaw and sleeve designs are elastomer in shear couplings; jaw and pin and bushing designs are elastomer in compression pin and bushing designs are elastomer in compression couplings. couplings.

Non-elastomeric couplings use metallic elements Non-elastomeric couplings use metallic elements to obtain flexibility. These can be one of two to obtain flexibility. These can be one of two types: lubricated or non-lubricated. Lubricated types: lubricated or non-lubricated. Lubricated designs accommodate misalignment by the designs accommodate misalignment by the sliding action of their components, hence the sliding action of their components, hence the need for lubrication. The non-lubricated designs need for lubrication. The non-lubricated designs accommodate misalignment through flexing. accommodate misalignment through flexing. Gear, grid and chain couplings are examples of Gear, grid and chain couplings are examples of non-elastomeric, lubricated couplings. Disc and non-elastomeric, lubricated couplings. Disc and diaphragm couplings are non-elastomeric and diaphragm couplings are non-elastomeric and non-lubricated. non-lubricated.

   Auxiliary ComponentsAuxiliary ComponentsAuxiliary components generally include the following Auxiliary components generally include the following

piping systems for the following services: piping systems for the following services:

Seal flushing , cooling , quenching systemsSeal flushing , cooling , quenching systems Seal drains and ventsSeal drains and vents Bearing lubrication , cooling systemsBearing lubrication , cooling systems Seal chamber or stuffing box cooling, heating systemsSeal chamber or stuffing box cooling, heating systems Pump pedestal cooling systems Pump pedestal cooling systems   Auxiliary piping systems include tubing, piping, Auxiliary piping systems include tubing, piping,

isolating valves, control valves, relief valves, isolating valves, control valves, relief valves, temperature gauges and thermocouples, pressure temperature gauges and thermocouples, pressure gauges, sight flow indicators, orifices, seal flush coolers, gauges, sight flow indicators, orifices, seal flush coolers, dual seal barrier/buffer fluid reservoirs, and all related dual seal barrier/buffer fluid reservoirs, and all related vents and drains.vents and drains.

All auxiliary components shall comply with the All auxiliary components shall comply with the requirements as per standard codes like API 610 requirements as per standard codes like API 610 (refinery services), API 682 (shaft sealing systems) etc.(refinery services), API 682 (shaft sealing systems) etc.

ContentsContents Pressure and HeadPressure and Head Absolute pressureAbsolute pressure Specific GravitySpecific Gravity Head and pressure conversion formulaHead and pressure conversion formula Suction Head, Discharge Head, Total headSuction Head, Discharge Head, Total head Vapor pressure and NPSHVapor pressure and NPSH NPSH, NPSHA, NPSHRNPSH, NPSHA, NPSHR NPSH problems and cavitationNPSH problems and cavitation NPSH Calculation.NPSH Calculation. Cavitation characteristicsCavitation characteristics

Pressure and HeadPressure and Head

PressurePressure is force acting on a unit of area. It is is force acting on a unit of area. It is expressed in Pounds/square inch or Newton/ expressed in Pounds/square inch or Newton/ square meter. square meter.

HeadHead is the height of the liquid. is the height of the liquid. Atmospheric Pressure

Liquid Pressure

10 psig

The gauge is showing a pressure of 10 psig. This is The gauge is showing a pressure of 10 psig. This is gauge pressure and is not including the gauge pressure and is not including the Atmospheric pressure which is 14.7 psi.Atmospheric pressure which is 14.7 psi.

Absolute pressureAbsolute pressure::

Absolute pressure=Gauge pressure + Atmospheric Absolute pressure=Gauge pressure + Atmospheric pressurepressure

Absolute pressure of the tank shown in the previous Absolute pressure of the tank shown in the previous slide is 10+14.7=24.7 psia.slide is 10+14.7=24.7 psia.

Absolute pressure will always be greater than Gauge Absolute pressure will always be greater than Gauge pressure.pressure.

A 10 feet of water exerts a pressure of 4.33 A 10 feet of water exerts a pressure of 4.33 psig. A 100 feet of water column exerts a psig. A 100 feet of water column exerts a pressure of 43.3 psig. Dividing 4.33/10 and pressure of 43.3 psig. Dividing 4.33/10 and 43.3/100, we see that for each 1 foot of 43.3/100, we see that for each 1 foot of water 0.433 psig pressure is exerted.water 0.433 psig pressure is exerted.

4.33 psig

43.3 psig

10 feet

100 feet

Specific GravitySpecific Gravity is the weight of the substance is the weight of the substance divided by the weight of the same volume of divided by the weight of the same volume of water. The specific gravity of water is 1. A liquid water. The specific gravity of water is 1. A liquid having a specific gravity of less than 1 will weigh having a specific gravity of less than 1 will weigh less than water for an equal amount. If the weight less than water for an equal amount. If the weight is less then the force exerted will also be less and is less then the force exerted will also be less and hence for an equivalent height the head of the hence for an equivalent height the head of the liquid having a Sp. Gravity of less than 1 will be liquid having a Sp. Gravity of less than 1 will be lower than that of water. lower than that of water.

The formula for converting Head into pressure is The formula for converting Head into pressure is

Head=Presssure/sp.gravity.0.433Head=Presssure/sp.gravity.0.433

Suppose the gauge is showing a pressure of 30 psig Suppose the gauge is showing a pressure of 30 psig for a liquid with Sp. Gravity of 0.7. The head will for a liquid with Sp. Gravity of 0.7. The head will be 30/0.7.0.433=98.9 feet. be 30/0.7.0.433=98.9 feet.

Suction HeadSuction Head is the sum of the pressure changed to head, is the sum of the pressure changed to head, plus the velocity changed to head, at the inlet of the plus the velocity changed to head, at the inlet of the pump.pump.

Discharge HeadDischarge Head is the pressure at the discharge changed is the pressure at the discharge changed to head. to head.

Total HeadTotal Head is the discharge head minus Suction head. It is is the discharge head minus Suction head. It is the actual distance pump pushed the liquid at the the actual distance pump pushed the liquid at the expense of energy. expense of energy.

Reference for height measurement is the pump centerline.Reference for height measurement is the pump centerline.

Total head can be estimated by measuring the heights at Total head can be estimated by measuring the heights at the suction and discharge tanks and subtracting these the suction and discharge tanks and subtracting these heads or by reading the pressure gauges at the suction heads or by reading the pressure gauges at the suction and discharge and converting them to head measurement and discharge and converting them to head measurement and subtracting them.and subtracting them.

For a suction lift system Suction lift is added to the For a suction lift system Suction lift is added to the discharge head to calculate the Total Head.discharge head to calculate the Total Head.

Suction Head

Discharge Head

Total Discharge Head

Suction Lift

Discharge HeadTotal Head

Suction Lift System

Vapor Pressure and Net Positive Vapor Pressure and Net Positive Suction Head (NPSH)Suction Head (NPSH)

Vapor PressureVapor Pressure is the Pressure at which the is the Pressure at which the liquid will transform into vapor at a given liquid will transform into vapor at a given temperature. Vapor pressure is a function of temperature. Vapor pressure is a function of temperature. temperature. Understanding the phenomenon, when the Understanding the phenomenon, when the Absolute suction pressure is not high enough. Absolute suction pressure is not high enough. Liquids vaporize or evaporate at the pump suction. Liquids vaporize or evaporate at the pump suction. In a liquid the vapors exert a pressure before they In a liquid the vapors exert a pressure before they escape. Vapor pressure is the pressure of the escape. Vapor pressure is the pressure of the vapor that is trapped in the liquid. Vapor pressure vapor that is trapped in the liquid. Vapor pressure causes the liquid to vaporize or evaporate. causes the liquid to vaporize or evaporate. Heating a liquid increases the its vapor pressure. Heating a liquid increases the its vapor pressure. To keep the liquid at the pump from vaporizing, To keep the liquid at the pump from vaporizing, the absolute suction pressure must be higher than the absolute suction pressure must be higher than the Vapor pressure at the given temperature.the Vapor pressure at the given temperature.

Net Positive Suction Head (NPSH)Net Positive Suction Head (NPSH) is a common is a common term used in Centrifugal pumps. It can only be term used in Centrifugal pumps. It can only be understand by understanding the terms NPSH understand by understanding the terms NPSH available and NPSH required. available and NPSH required.

NPSHNPSH (available) (available) is the energy level expressed in is the energy level expressed in terms of liquid height at the suction flange of the terms of liquid height at the suction flange of the centrifugal pump, with the liquid vapor pressure as its centrifugal pump, with the liquid vapor pressure as its datum. Ordatum. Or

It is the difference between Total suction head and It is the difference between Total suction head and vapor pressure of the liquid in feet of liquid, at the vapor pressure of the liquid in feet of liquid, at the suction flange.suction flange.

It can be attained by subtracting all the losses like It can be attained by subtracting all the losses like entrance losses, losses through fittings, elbows, entrance losses, losses through fittings, elbows, strainers etc. and the vapor pressure from the strainers etc. and the vapor pressure from the Absolute Suction head.Absolute Suction head.

NPSH available is specified by the user of the pump. NPSH available is specified by the user of the pump.

NPSHNPSH (required) (required) is the minimum head required needed at is the minimum head required needed at the suction of a pump to enter the impeller without the suction of a pump to enter the impeller without vaporizing. Orvaporizing. OrThe reduction in total head as the liquid enters the The reduction in total head as the liquid enters the Pump. NPSH required is determined by the Pump Pump. NPSH required is determined by the Pump manufacturer. manufacturer. It is worth to know that why NPSH available is related to It is worth to know that why NPSH available is related to Vapor pressure. The impeller imparts energy to the liquid Vapor pressure. The impeller imparts energy to the liquid only in liquid form. From the model on the next slide shows only in liquid form. From the model on the next slide shows that there will be some loss of energy when the liquid will that there will be some loss of energy when the liquid will travel from the suction flange to the impeller eye, where it travel from the suction flange to the impeller eye, where it meets the impeller vanes. Therefore head (pressure) at the meets the impeller vanes. Therefore head (pressure) at the vane tip is less than the head (pressure) available at the vane tip is less than the head (pressure) available at the suction flange. As soon as the static head in the area suction flange. As soon as the static head in the area between suction flange and the vane tip reaches vapor between suction flange and the vane tip reaches vapor pressures, the liquid starts vaporizing. NPSH A is therefore pressures, the liquid starts vaporizing. NPSH A is therefore related to Vapor pressure at its pumping temperature; related to Vapor pressure at its pumping temperature; indicates how much energy is available for the passage indicates how much energy is available for the passage from suction flange to leading edge of the impeller vanes. from suction flange to leading edge of the impeller vanes.

A B D

E

Entrance lossEntrance loss FrictionFriction TurbulenTurbulence ce friction, friction, Entrance Entrance loss at loss at vane tipvane tip

Increasing Increasing pressure due to pressure due to impellerimpeller

C

INCREASING PRESSURE

A B C D E

Relative Pressures in the Entrance section of a pump

Point of Lowest pressure where Vaporization starts

As long as the actual energy loss, NPSH R As long as the actual energy loss, NPSH R remains less than NPSH A the total head will remains less than NPSH A the total head will exceeds the vapor pressure , which guarantees exceeds the vapor pressure , which guarantees that the pumpage will remain liquid during its that the pumpage will remain liquid during its travel. travel.

NPSH R varies as the square of the proportionate NPSH R varies as the square of the proportionate speed increase. In general standard centrifugal speed increase. In general standard centrifugal pump will not perform well if gas entrainment pump will not perform well if gas entrainment exceeds 1%.exceeds 1%.

The mandatory margin between NPSH A and The mandatory margin between NPSH A and NPSH R is usually 1 foot.NPSH R is usually 1 foot.

NPSH Problems and CavitationNPSH Problems and CavitationIf NPSH A is not greater than NPSH R, there will be a If NPSH A is not greater than NPSH R, there will be a marked reduction in Head or Capacity or even a marked reduction in Head or Capacity or even a complete failure to operate. Excessive vibrations may complete failure to operate. Excessive vibrations may take place when there is vapor passing through the take place when there is vapor passing through the impeller. Cavitation will take place which results in impeller. Cavitation will take place which results in Pitting and erosion of the impeller.Pitting and erosion of the impeller.

CavitationCavitation is a phenomenon in which bubbles will is a phenomenon in which bubbles will form if, during liquid flow its static head becomes form if, during liquid flow its static head becomes lower than its vapor pressure at the prevailing lower than its vapor pressure at the prevailing temperature. As soon as energy input causes the temperature. As soon as energy input causes the pressure to become greater than the vapor pressure pressure to become greater than the vapor pressure the bubbles will suddenly collapse or implode. As the the bubbles will suddenly collapse or implode. As the vapor collapse, the adjacent walls are subjected to a vapor collapse, the adjacent walls are subjected to a tremendous shock from the in rush of liquid into the tremendous shock from the in rush of liquid into the cavity left by the bubble. This shock actually flakes off cavity left by the bubble. This shock actually flakes off small bits of metal and the parts small bits of metal and the parts

Take on the appearance of having been badly Take on the appearance of having been badly eroded. This erosion shows up not at the point of eroded. This erosion shows up not at the point of lowest pressure where the bubble is formed, but lowest pressure where the bubble is formed, but further down stream where the bubble collapses. further down stream where the bubble collapses. The most sensitive areas usually are the low The most sensitive areas usually are the low pressure sides of the impeller vanes near the inlet pressure sides of the impeller vanes near the inlet edge and the front shroud where the curvature is edge and the front shroud where the curvature is greatest. greatest.

A standard method to observe and judge A standard method to observe and judge cavitation on the test stand or in the field is to cavitation on the test stand or in the field is to hold the speed constant and reduce the suction hold the speed constant and reduce the suction pressure at various capacities. A 3% drop in pressure at various capacities. A 3% drop in discharge pressure is considered evidence of discharge pressure is considered evidence of cavitation. It is plotted in the Head capacity curve cavitation. It is plotted in the Head capacity curve and it is determined as the NPSH R of the pump. and it is determined as the NPSH R of the pump. More explanation is given in coming slides. More explanation is given in coming slides.

NPSH CalculationNPSH Calculation

-Z

+Z

Pt

Pt

Ps

Ps

-Zps

+Zps

Pump centerline

The formula includes the following abbreviations and The formula includes the following abbreviations and signs which are as follow.signs which are as follow.

Pa= Absolute pressure in atmosphere surrounding Pa= Absolute pressure in atmosphere surrounding gage.gage.

Ps= Gage pressure at the suction may be negative or Ps= Gage pressure at the suction may be negative or positive according to the arrangement.positive according to the arrangement.

Pt= Absolute pressure on free surface of liquid in the Pt= Absolute pressure on free surface of liquid in the closed tank connected to the pump suction.closed tank connected to the pump suction.

Pvp= Vapor pressure of the liquid being pumped.Pvp= Vapor pressure of the liquid being pumped.

hf= Lost head due to friction.hf= Lost head due to friction.

V = Average velocityV = Average velocity

Z Z = Vertical distances may be positive or negative = Vertical distances may be positive or negative depending upon the arrangement.depending upon the arrangement.

W = Specific weight of liquid at pumping temperature.W = Specific weight of liquid at pumping temperature.

The formula is as follows.The formula is as follows.NPSH available = (Pa-Pvp) / w +Ps/w + Zps =VNPSH available = (Pa-Pvp) / w +Ps/w + Zps =V22 /2g /2gOrOrNPSH available = (Pt-Pvp) /w +Z –hfNPSH available = (Pt-Pvp) /w +Z –hf

Cavitation characteristicsCavitation characteristics of a centrifugal pump is to of a centrifugal pump is to cause a break down in the normal Head Capacity cause a break down in the normal Head Capacity curves. This is done by holding the speed and suction curves. This is done by holding the speed and suction pressure constant and varying the capacity, or by pressure constant and varying the capacity, or by holding the speed constant and reducing the suction holding the speed constant and reducing the suction pressure at various capacities. Either of these methods pressure at various capacities. Either of these methods will produce a breakdown in the Head characteristics as will produce a breakdown in the Head characteristics as shown. The Hydraulic Institute has permitted a drop in shown. The Hydraulic Institute has permitted a drop in head of 3% to be accepted as evidence that cavitation head of 3% to be accepted as evidence that cavitation is present under test conditions. It is recommended to is present under test conditions. It is recommended to operate the pump above the break away sigma if noise operate the pump above the break away sigma if noise and vibrations are to be avoided.and vibrations are to be avoided.

Normal Q-H curveNormal Q-H curve

% Head Normal

Break Away

% Capacity Normal

ContentsContents Pump performance Characteristic CurvesPump performance Characteristic Curves Head, Capacity, BHP, Efficiency, B.E.P Head, Capacity, BHP, Efficiency, B.E.P Affinity LawsAffinity Laws

PUMP PERFORMANCE; PUMP PERFORMANCE; CHARACTERISTIC CURVESCHARACTERISTIC CURVES

The performance characteristics of a centrifugal The performance characteristics of a centrifugal pump are clearly defined by its performance pump are clearly defined by its performance curves. They are also known as Head Capacity curves. They are also known as Head Capacity curve or simply Q-H curves. Prior to discuss the curve or simply Q-H curves. Prior to discuss the characteristic curves, following terms are to be characteristic curves, following terms are to be familiarized in order to get a clear idea.familiarized in order to get a clear idea.Capacity Capacity is the volume of liquid per unit time is the volume of liquid per unit time delivered by the pump. It is denoted by Q and the delivered by the pump. It is denoted by Q and the units are gallons/minute or gpm, munits are gallons/minute or gpm, m33 /sec. /sec.Head Head is sum of the gauge pressure head and the is sum of the gauge pressure head and the velocity head at the discharge flange minus the velocity head at the discharge flange minus the sum of corresponding heads at the suction flange sum of corresponding heads at the suction flange equals the energy in foot-pounds added per equals the energy in foot-pounds added per pound of liquid pumped and is called Total head pound of liquid pumped and is called Total head developed by the pump. It is expressed in feet or developed by the pump. It is expressed in feet or meters and is usually denoted by H.meters and is usually denoted by H.

BHPBHP is the Horse power required by the pump for any is the Horse power required by the pump for any flow rate of the liquid with a specific gravity of 1.0. It flow rate of the liquid with a specific gravity of 1.0. It is denoted as is denoted as

BHP= QH(Sp.gravity)/3960. In SI Power units are Watt BHP= QH(Sp.gravity)/3960. In SI Power units are Watt and is given by P=9797QH(Sp.gravity) when Q is in and is given by P=9797QH(Sp.gravity) when Q is in mm33 /sec and Head is in meters /sec and Head is in meters

NPSH is the Net Positive suction head required at NPSH is the Net Positive suction head required at some particular flow rate. The explanation is given in some particular flow rate. The explanation is given in previous slides. The unit is feet or meters.previous slides. The unit is feet or meters.

Efficiency is equal to the rate at which imparts energy Efficiency is equal to the rate at which imparts energy to the liquid, divided by the rate at which the pump to the liquid, divided by the rate at which the pump requires energy. It is denoted in percentage. The requires energy. It is denoted in percentage. The formula is as follow.formula is as follow.

EfficiencyEfficiency= QH. Specific Gravity/3960.hp X 100= QH. Specific Gravity/3960.hp X 100

From the performance curves some general principles From the performance curves some general principles are set which help us in making the basic concept ofare set which help us in making the basic concept of

Centrifugal pump performance.Centrifugal pump performance.

When the Total Head decreases, the pump capacity When the Total Head decreases, the pump capacity increases except at very low capacity.increases except at very low capacity.

As the level of suction head is dropped Total Head will As the level of suction head is dropped Total Head will increase resulting in decreasing the capacity.increase resulting in decreasing the capacity.

Power requirement increases with increasing the flow Power requirement increases with increasing the flow rate. rate.

NPSH requirement increases with the flow rate.NPSH requirement increases with the flow rate.

The efficiency is relatively low at high and low flow rates.The efficiency is relatively low at high and low flow rates.

Best Efficiency Point (B.E.P)Best Efficiency Point (B.E.P) is the point on the curve is the point on the curve at which the pump operation is most efficient and at which the pump operation is most efficient and therefore most economical.therefore most economical.

By pinching the Discharge valve of the pump, the By pinching the Discharge valve of the pump, the capacity decreases and so the NPSH requirement and capacity decreases and so the NPSH requirement and correspondingly the Head increases. correspondingly the Head increases.

Capacity Q

Total Head in

Efficiency

Total Head NPSH

Efficiency

Horse Power

Horse Power

NPSH

Shut off head

B.E.P

The head produced by the pump is limited to that The head produced by the pump is limited to that which is developed at shut off (zero capacity). Even if which is developed at shut off (zero capacity). Even if there is blockage in the discharge line the pressure there is blockage in the discharge line the pressure will not exceed the shut off value. Correspondingly the will not exceed the shut off value. Correspondingly the pump driver will not be overloaded as the horsepower pump driver will not be overloaded as the horsepower required will be decreasing as the flow is throttled required will be decreasing as the flow is throttled back. back.

AFFINITY LAWSAFFINITY LAWS

Any machine, which imparts velocity and Any machine, which imparts velocity and converts velocity to pressure, can be converts velocity to pressure, can be categorized by a set of relationships, which categorized by a set of relationships, which apply to any dynamic conditions. These apply to any dynamic conditions. These relationships are referred to as the “Affinity relationships are referred to as the “Affinity Laws”. They can be described as similarity Laws”. They can be described as similarity processes, which follow the following rules:processes, which follow the following rules:

Capacity varies as the rotating speed – that is, Capacity varies as the rotating speed – that is,

the peripheral velocity of the impeller. the peripheral velocity of the impeller. Head varies as the square of the rotating speed Head varies as the square of the rotating speed BHP varies as the cube of the rotating speed. BHP varies as the cube of the rotating speed.

The affinity laws apply to centrifugal gas The affinity laws apply to centrifugal gas compressors as well as to centrifugal pumps, compressors as well as to centrifugal pumps, but are most distinctlybut are most distinctly

useful for estimating pumps performance at useful for estimating pumps performance at different rotating speeds, or impeller diameters different rotating speeds, or impeller diameters starting with pumps with known characteristics.starting with pumps with known characteristics.The basic variations can be analyzed by these The basic variations can be analyzed by these relationships: relationships:

1 - By changing speed and maintaining constant 1 - By changing speed and maintaining constant impeller diameter, pump efficiency will remain the impeller diameter, pump efficiency will remain the same, but head, capacity, and BHP will vary same, but head, capacity, and BHP will vary according to the Laws. according to the Laws.

2 - By changing impeller diameter, but maintaining 2 - By changing impeller diameter, but maintaining constant speed the pump efficiency for a diffuser constant speed the pump efficiency for a diffuser pumppumpwill not be affected if the impeller diameter is not will not be affected if the impeller diameter is not changed by more than five percent. Note that the changed by more than five percent. Note that the change in efficiency will occur if the impeller size is change in efficiency will occur if the impeller size is reduced sufficiently to affect the clearances between reduced sufficiently to affect the clearances between the casing and the periphery of the impeller. the casing and the periphery of the impeller.

However, the head, capacity and BHP will vary as follows:However, the head, capacity and BHP will vary as follows:

Parameter Parameter Variation 1 Variation 1 Variation 2Variation 2

1. With impeller 1. With impeller diameter, D, held diameter, D, held

constantconstant

2. With speed, N, 2. With speed, N,

held constantheld constant

CapacityCapacity A. Q1/Q2 = N1/N2.A. Q1/Q2 = N1/N2. A. QI/Q2 = D1/D2A. QI/Q2 = D1/D2

HeadHead B. H1/H2= B. H1/H2= (N1/N2)2(N1/N2)2

B. H1/H2 =(DI/D2)2 B. H1/H2 =(DI/D2)2

Where Where Q = Capacity, gpm Q = Capacity, gpm

H = Total Head, Feet H = Total Head, Feet

BHP = Brake Horsepower BHP = Brake Horsepower

N = Pump Speed, rpm N = Pump Speed, rpm

Parameter Parameter Variation 1 Variation 1 Variation 2Variation 2Brake HorsepowerBrake Horsepower C.BHP1/BHP2=(N1/C.BHP1/BHP2=(N1/

N2)N2)33 C.BHP1/BHP2=(D1/C.BHP1/BHP2=(D1/D2)D2)33

These relationships may be manipulated in any These relationships may be manipulated in any mathematically valid way. The common mathematically valid way. The common denominator being a speed change, the denominator being a speed change, the relationships may be complied as follows: relationships may be complied as follows:

Q2Q2 = (N2/N1).Q1 = (N2/N1).Q1 (4)(4) H2 = (N2/N1)2 H1H2 = (N2/N1)2 H1 (5) (5)

P2 = (N2/N1)3 P1P2 = (N2/N1)3 P1 (6(6) )

ContentsContents

Pump operationPump operation Discharge recirculationDischarge recirculation Minimum flow controlMinimum flow control Effect of viscosity on pump performanceEffect of viscosity on pump performance Parallel and Series operationParallel and Series operation Capacity regulationCapacity regulation

PUMP OPERATION.PUMP OPERATION.

We will discuss different Terms which are We will discuss different Terms which are frequently used in Today's Industry as far as frequently used in Today's Industry as far as Centrifugal pump jargon is concerned.Centrifugal pump jargon is concerned.

Internal Recirculation.Internal Recirculation. Effect of Pump performance due to Viscosity. Effect of Pump performance due to Viscosity. Minimum FlowMinimum Flow Capacity RegulationCapacity Regulation Parallel and Series Operation.Parallel and Series Operation.

Internal RecirculationInternal Recirculation

There is a small flow from Impeller Discharge to There is a small flow from Impeller Discharge to SuctionSuction

through the wearing rings. This takes place at all the through the wearing rings. This takes place at all the capacities but does not usually contribute in raising capacities but does not usually contribute in raising the temperature of the liquid being pumped unless the temperature of the liquid being pumped unless operation is near shut off.operation is near shut off.

When the capacity is reduced by throttling a When the capacity is reduced by throttling a secondary flow reversal at the suction or discharge secondary flow reversal at the suction or discharge tip of the impeller vanes occur. tip of the impeller vanes occur. Suction Suction recirculationrecirculation is the reversal of flow at the impeller is the reversal of flow at the impeller eye and travel upstream with a rotational velocity eye and travel upstream with a rotational velocity approaching the peripheral velocity of the diameter. approaching the peripheral velocity of the diameter. A rotating annulus is formed and hence this annulus A rotating annulus is formed and hence this annulus moves upstream from the impeller inlet and through moves upstream from the impeller inlet and through the core of the annulus an axial flow is produced. the core of the annulus an axial flow is produced. Shearing between annulus and the axial flow Shearing between annulus and the axial flow through the core produces vortices which form and through the core produces vortices which form and collapse, producing noise and cavitation at the collapse, producing noise and cavitation at the suction side of the pump. suction side of the pump.

Guide vanes may show cavitation damage from Guide vanes may show cavitation damage from the impingement of the back flow from the the impingement of the back flow from the impeller eye in Suction recirculation.impeller eye in Suction recirculation. Discharge RecirculationDischarge Recirculation is the reversal of flow is the reversal of flow at the Discharge tip of the impeller vanes. The at the Discharge tip of the impeller vanes. The high shear rate between the inward and outward high shear rate between the inward and outward relative velocities produces vortices that cavitate relative velocities produces vortices that cavitate and usually attack the pressure side of the vanes.and usually attack the pressure side of the vanes.

The tongue or diffuser vanes may show cavitation The tongue or diffuser vanes may show cavitation damage on the impeller side from operation in damage on the impeller side from operation in discharge recirculation.discharge recirculation.

Suction Recirculation

Discharge Recirculation

Viscosity and effect on pump designViscosity and effect on pump design and and operation operation

There are some significant differences in pump There are some significant differences in pump behavior, and system response, when pumping fluid behavior, and system response, when pumping fluid other than water. One of the major factors is other than water. One of the major factors is viscosity. This is a characteristic of all fluids, viscosity. This is a characteristic of all fluids, including water, different for each fluid. It is of including water, different for each fluid. It is of course obvious that fluids differ from solids; it is not course obvious that fluids differ from solids; it is not so obvious that fluids can approach solids so that a so obvious that fluids can approach solids so that a distinction may be blurred. distinction may be blurred.

Effect on Pump InstallationEffect on Pump Installation The viscosity of the liquid is a very important factor in The viscosity of the liquid is a very important factor in

the selection of a pump. It is the determining factor the selection of a pump. It is the determining factor in frictional head, motor size required and speed in frictional head, motor size required and speed reduction necessary. Frequently, for high viscosity reduction necessary. Frequently, for high viscosity liquids, it is more economical to use a large pump liquids, it is more economical to use a large pump operating at a reduced speed since the original operating at a reduced speed since the original higher total installation cost is more that offset by higher total installation cost is more that offset by reduced maintenance and subsequent longer life of reduced maintenance and subsequent longer life of the unit. the unit.

Effect of viscosity on performanceEffect of viscosity on performance

Liquids of high viscosity offer more resistance to Liquids of high viscosity offer more resistance to flow than water. This increased friction occurs both flow than water. This increased friction occurs both in the piping and in the pump itself. More energy is in the piping and in the pump itself. More energy is required to force the liquid through the flow required to force the liquid through the flow passages in the pump casing and through the pump passages in the pump casing and through the pump impeller. The disc friction of the impeller; that is, impeller. The disc friction of the impeller; that is, the resistance of the liquid to the rotation of the the resistance of the liquid to the rotation of the impeller, is greater when pumping a high viscosity impeller, is greater when pumping a high viscosity liquid. These friction losses within the pumpliquid. These friction losses within the pump

result in the pump generating a lower head and result in the pump generating a lower head and reduce the volume of liquid handled by the pump. reduce the volume of liquid handled by the pump. These losses also result in a lowering of the pump These losses also result in a lowering of the pump efficiency and an increase in the brake horsepower efficiency and an increase in the brake horsepower requirements. requirements.

The reduction in head, capacity, and efficiency and The reduction in head, capacity, and efficiency and the increase in horsepower requirements of a pump the increase in horsepower requirements of a pump handling a viscous liquid depend to a large extent on handling a viscous liquid depend to a large extent on the type pump, the design of the passages inside the type pump, the design of the passages inside the pump, the design of the impeller, the size of the the pump, the design of the impeller, the size of the pump, the capacity of the pump and the pump pump, the capacity of the pump and the pump speed. speed. The following equation are used for determining the The following equation are used for determining the viscous performance when the water performance of viscous performance when the water performance of the pump is known: the pump is known: Qvis = CQ X Qw Qvis = CQ X Qw Hvis = CH X Hw Hvis = CH X Hw E vis = CE X Ew E vis = CE X Ew bhp vis = QviS X Hvis X sp. gr. / 3960 X E vis bhp vis = QviS X Hvis X sp. gr. / 3960 X E vis CQ, CH and CE are determined from graphs, which CQ, CH and CE are determined from graphs, which are based water performancesare based water performances

The following equations are used for approximating The following equations are used for approximating the water performance when the desired viscous the water performance when the desired viscous capacity and head are given and the values of CQ capacity and head are given and the values of CQ and CH must be estimated form graph using QviS and CH must be estimated form graph using QviS and Hvis, as: and Hvis, as:

Qs (approx.) = Qvis / CQQs (approx.) = Qvis / CQHw (approx.) = Hvis/ CH Hw (approx.) = Hvis/ CH

MINIMUM FLOW CONTROLMINIMUM FLOW CONTROL Although a constant – speed centrifugal pump will Although a constant – speed centrifugal pump will operate over a wide range of capacities, it will operate over a wide range of capacities, it will encounter difficulty at low flow. In general, low-flow encounter difficulty at low flow. In general, low-flow problems are worse for large high – energy pumps, for problems are worse for large high – energy pumps, for pumps handling hot or abrasives – laden liquids, for pumps handling hot or abrasives – laden liquids, for pumps designed for high efficiency at best efficiency pumps designed for high efficiency at best efficiency point (BEP), and for pumps for low net positive suction point (BEP), and for pumps for low net positive suction head (NPSH). head (NPSH). Source of trouble at reduced flow Source of trouble at reduced flow The sources of the pump’s distress are fourfold – The sources of the pump’s distress are fourfold – thermal, hydraulic, mechanical, and abrasive wear. thermal, hydraulic, mechanical, and abrasive wear. Thermal Thermal Inescapable energy conversion loss in the pump warns Inescapable energy conversion loss in the pump warns the liquid. the liquid. Hydraulic Hydraulic When flow decreases far enough, the impeller When flow decreases far enough, the impeller encounters suction or discharge recirculation, or encounters suction or discharge recirculation, or perhaps both. Flashing, perhaps both. Flashing,

cavitations, and shock occur, often with vibration cavitations, and shock occur, often with vibration and serious damage. and serious damage. Mechanical Mechanical Both constant and fluctuating loads in the radial Both constant and fluctuating loads in the radial

and axial directions increase as pump capacity and axial directions increase as pump capacity falls. Bearing damage, shaft and impeller falls. Bearing damage, shaft and impeller breakage, and rubbing wear on casing, impeller, breakage, and rubbing wear on casing, impeller, and wear rings can occur. and wear rings can occur.

Axial-flow and mixed-flow pumps with high specific Axial-flow and mixed-flow pumps with high specific speed give comparatively higher head and take speed give comparatively higher head and take comparatively more power at low flow. A bypass comparatively more power at low flow. A bypass system may be necessary not only to reduce system may be necessary not only to reduce stress but also to prevent motor overload. stress but also to prevent motor overload. Abrasive wear Abrasive wear Liquids containing a large amount of abrasive Liquids containing a large amount of abrasive particles, such as sand or ash, must flow particles, such as sand or ash, must flow continuously through the pump. If flow decreases, continuously through the pump. If flow decreases, the particles can circulate insidethe particles can circulate inside

the pump passage and quickly erode the impeller, casing, the pump passage and quickly erode the impeller, casing,

and even wear rings and shaft.and even wear rings and shaft. FACTORS IN DESIGN OF A MINIMUM FLOW FACTORS IN DESIGN OF A MINIMUM FLOW CONTROL SYSTEM CONTROL SYSTEM

Pump size Pump size

Higher capacity, brake power, specific speed, and Higher capacity, brake power, specific speed, and suction specific speed all tend to make a suction specific speed all tend to make a minimum flow control system difficult to design minimum flow control system difficult to design and costly to build and operate. and costly to build and operate.

Discharge Pressure Discharge Pressure

High discharge pressures mean high head loss in High discharge pressures mean high head loss in bypass lines. Liquids that can flash and cavitate bypass lines. Liquids that can flash and cavitate demand special precautions in valving, orifices, demand special precautions in valving, orifices, and piping. and piping.

Available heat sink Available heat sink By pass flow must go far enough upstream to prevent By pass flow must go far enough upstream to prevent progressive temperature buildup or flow disturbance progressive temperature buildup or flow disturbance in the pump suction. This may mean a simple in the pump suction. This may mean a simple discharge back to an un insulated inlet line or discharge back to an un insulated inlet line or discharge to a receiving tank or cooler with enough discharge to a receiving tank or cooler with enough area and enough inflow of cool liquid to handle the area and enough inflow of cool liquid to handle the thermal load. Bypass flow can discharge into a de-thermal load. Bypass flow can discharge into a de-aerator storage tank, a condenser, a flash tank, or a aerator storage tank, a condenser, a flash tank, or a cooling pond. Altitude, distance, and pressure inside cooling pond. Altitude, distance, and pressure inside the receiving tank are also factors, as is the fact that the receiving tank are also factors, as is the fact that the interior must be available for inspection and for the interior must be available for inspection and for repair of spray or distribution pipes and orifices. repair of spray or distribution pipes and orifices. Continuous bypass system Continuous bypass system Figure illustrates a simple system, with a bypass line Figure illustrates a simple system, with a bypass line branching off the pump discharge upstream of the branching off the pump discharge upstream of the main line check valve and containing a fixed orifice main line check valve and containing a fixed orifice dimensioned by analysis to provide minimum required dimensioned by analysis to provide minimum required pump flow. pump flow.

PUMPSHUT OFF VALVE

CHECK VALVE

SHUT OFF VALVE

BY PASS LINE

ORIFICE

SHUT OFF VALVE

RECEIVING TANK

CONTINUOUS BY PASS OPERATION

The bypass receiving tank must be at a lower The bypass receiving tank must be at a lower pressure than pump discharge and may be the pump pressure than pump discharge and may be the pump suction source or some other vessel than will return suction source or some other vessel than will return the liquid to the pump (if it is not to be wasted). the liquid to the pump (if it is not to be wasted). Locating the bypass branch off before the discharge Locating the bypass branch off before the discharge check valve as shown keeps backflow from process or check valve as shown keeps backflow from process or flow from a parallel operating pump from going back flow from a parallel operating pump from going back to the receiving tank or back through the pump to the receiving tank or back through the pump during a pump shutdown. during a pump shutdown. The choice of size for the bypass pipe depends on The choice of size for the bypass pipe depends on flow, equivalent line length, and how close the liquid flow, equivalent line length, and how close the liquid temperature is to the boiling point. If the pressure temperature is to the boiling point. If the pressure breakdown after the orifice results in flashing flow, breakdown after the orifice results in flashing flow, the orifice should discharge into the larger receiving the orifice should discharge into the larger receiving tank. In that case, eliminate the shutoff valve tank. In that case, eliminate the shutoff valve downstream of the bypass. downstream of the bypass.

On – off bypass of minimum flow On – off bypass of minimum flow In figure a bypass line runs to a below – surface In figure a bypass line runs to a below – surface spray pipe in an overhead pressurized receiver. For spray pipe in an overhead pressurized receiver. For low bypass flows and low heads of cool water, the low bypass flows and low heads of cool water, the on-off valve maybe single-stage. Control is simple, on-off valve maybe single-stage. Control is simple, with a dead band of 2 to 5 % of design – point with a dead band of 2 to 5 % of design – point delivery. The meter, downstream of the bypass delivery. The meter, downstream of the bypass branch off, serves system control functions. Bypass branch off, serves system control functions. Bypass piping is full of water, eliminating the danger of a piping is full of water, eliminating the danger of a sudden impact of a water slug on the orifice. sudden impact of a water slug on the orifice. Should the bypass piping discharge above the water Should the bypass piping discharge above the water level in the receiving tank, a check valve in this line level in the receiving tank, a check valve in this line is advisable. Without a check valve, the water in the is advisable. Without a check valve, the water in the bypass piping will drain back through an idle pump to bypass piping will drain back through an idle pump to the level in the receiving tank, if this is the suction the level in the receiving tank, if this is the suction source. When the pump is started, water slugging source. When the pump is started, water slugging could occur in the empty portion of the pipe and could occur in the empty portion of the pipe and fittings. fittings.

PUMPSHUT OFF VALVE

CHECK VALVE

SHUT OFF VALVE

ORIFICE

SHUT OFF VALVE

RECEIVING TANK

ON OFF BY PASS OPERATION

CONTROL

ON OFF CONTROL VALVE

SPRAY PIPE

METERING ORIFICE

Automatic Bypass Control System Automatic Bypass Control System A single element that combines the pump check A single element that combines the pump check valves, pipe tee, pressure – reducing orifice, flow valves, pipe tee, pressure – reducing orifice, flow meter, and control valve can be an on-off or meter, and control valve can be an on-off or modulating bypass control, as in figure lift check acts modulating bypass control, as in figure lift check acts as a flow meter. This opens and closes a pilot valve, as a flow meter. This opens and closes a pilot valve, which operates, and integral – control pressure – which operates, and integral – control pressure – reducing valve. The element is self-powered, requiring reducing valve. The element is self-powered, requiring no external source of electricity or air, and is designed no external source of electricity or air, and is designed to be fail-safe. The highest-pressure drops can be to be fail-safe. The highest-pressure drops can be handled with commercially available systems. They handled with commercially available systems. They system design must suit the pump characteristics. system design must suit the pump characteristics. As in other bypass system, protection is necessary to As in other bypass system, protection is necessary to prevent flashing in the piping down steam of the prevent flashing in the piping down steam of the control element. A fixed orifice for on-off control or a control element. A fixed orifice for on-off control or a variable orifice for modulating control is needed at the variable orifice for modulating control is needed at the end of the bypass line and at the receiving tank wall. end of the bypass line and at the receiving tank wall. No anti flash orifice is required if the automatic control No anti flash orifice is required if the automatic control is at the end of the bypass line.is at the end of the bypass line.

PUMPSHUT OFF VALVE

SHUT OFF VALVE

SHUT OFF VALVE

RECEIVING TANK

AUTOMATIC BY PASS OPERATION

AUTOMATIC BYPASS ASSEMBLY

CAPACITY REGULATIONCAPACITY REGULATION

CAPACITY REGULATIONCAPACITY REGULATION Capacity variation ordinarily is accomplished by a Capacity variation ordinarily is accomplished by a change in pump head, speed, or both simultaneously. change in pump head, speed, or both simultaneously. The capacity and power input of pumps with specific The capacity and power input of pumps with specific speeds up to about 4000 increase with deceasing speeds up to about 4000 increase with deceasing head, so that the drivers of such pumps may be head, so that the drivers of such pumps may be overloaded if the head falls below a safe minimum overloaded if the head falls below a safe minimum value. Increasing the head of high – specific speed value. Increasing the head of high – specific speed pumps decreases the capacity but increases the pumps decreases the capacity but increases the power input. The drivers of these pumps should either power input. The drivers of these pumps should either be able to meet possible load increases or be be able to meet possible load increases or be equipped with suitable over load protection. Capacity equipped with suitable over load protection. Capacity regulation by the various methods given below may regulation by the various methods given below may be manual or automatic. be manual or automatic.

Discharge Throttling: Discharge Throttling:

This is the cheapest and most common method of This is the cheapest and most common method of capacity modulation for low and medium-specific speed capacity modulation for low and medium-specific speed pumps. Usually its use is restricted to such pumps. pumps. Usually its use is restricted to such pumps. Partial closure of any type of valve in the discharge line Partial closure of any type of valve in the discharge line will increase the system head so that the will increase the system head so that the

system-head curve will intersect the head capacity system-head curve will intersect the head capacity curve at a smaller capacity. Discharge throttling curve at a smaller capacity. Discharge throttling moves the operating point to one of lower efficiency, moves the operating point to one of lower efficiency, and power is lost at the throttle valve. This may be and power is lost at the throttle valve. This may be important in large installations, where more costly important in large installations, where more costly methods of modulation may be economically methods of modulation may be economically attractive. Throttling to the point of shutoff may cause attractive. Throttling to the point of shutoff may cause excessive heating of the liquid in the pump. This may excessive heating of the liquid in the pump. This may require a bypass to maintain therequire a bypass to maintain the

necessary minimum flow or use of different necessary minimum flow or use of different method of modulation. This is particularly method of modulation. This is particularly important with pumps handling hot water or important with pumps handling hot water or volatile liquids, as previously mentioned. volatile liquids, as previously mentioned. Suction Throttling Suction Throttling If sufficient NPSH is available, some power can be If sufficient NPSH is available, some power can be saved by throttling in the suction line. Jet engine saved by throttling in the suction line. Jet engine fuel pumps frequently are suction throttled fuel pumps frequently are suction throttled because discharge throttling may cause because discharge throttling may cause overheating and vaporization of the liquid. At overheating and vaporization of the liquid. At very low capacity, the impellers of these pumps very low capacity, the impellers of these pumps are only partly filled with liquid, so that the power are only partly filled with liquid, so that the power input and temperature rise are about one-third the input and temperature rise are about one-third the values for impellers running full with discharge values for impellers running full with discharge throttling. The capacity of condensate pumps throttling. The capacity of condensate pumps frequently is submergence –controlled, which is frequently is submergence –controlled, which is equivalent toequivalent to

suction throttling. Special design reduces cavitations suction throttling. Special design reduces cavitations damage of these pumps to negligible amount. damage of these pumps to negligible amount. Bypass Regulation Bypass Regulation All or part of the pump capacity may be diverted from All or part of the pump capacity may be diverted from the discharge line to the pump suction or other the discharge line to the pump suction or other suitable point through a bypass line. The bypass may suitable point through a bypass line. The bypass may contain one or more metering orifices and suitable contain one or more metering orifices and suitable control valves. Metered bypasses are commonly used control valves. Metered bypasses are commonly used with boiler-feed pumps for reduced capacity with boiler-feed pumps for reduced capacity operation, mainly to prevent overheating. There is a operation, mainly to prevent overheating. There is a considerable power saving if excess capacity of considerable power saving if excess capacity of propeller pumps is bypassed instead of using propeller pumps is bypassed instead of using discharge throttling. discharge throttling. Speed Regulation Speed Regulation This can be used to minimize power requirements and This can be used to minimize power requirements and eliminate overheating during capacity modulation. eliminate overheating during capacity modulation.

Steam turbines and internal combustion engines are Steam turbines and internal combustion engines are readily adaptable to speed regulation at small extra readily adaptable to speed regulation at small extra cost. A wide variety of variable cost. A wide variety of variable speed mechanical, speed mechanical, magnetic, and hydraulic drives are available, as well as magnetic, and hydraulic drives are available, as well as both ac and dc variable-speed motors. Usually variable both ac and dc variable-speed motors. Usually variable – speed motors are so expensive that they can be – speed motors are so expensive that they can be justified only by an economic study of particular case. justified only by an economic study of particular case.

Regulation of Adjustable vanes Regulation of Adjustable vanes

Adjustable guide vanes ahead of the impeller have Adjustable guide vanes ahead of the impeller have been investigated and found effective with a pump of been investigated and found effective with a pump of specific speed Ns = 5700. The vanes produced a specific speed Ns = 5700. The vanes produced a positive pre whirl which reduced the head, capacity, positive pre whirl which reduced the head, capacity, and efficiency. Relatively little regulation was obtained and efficiency. Relatively little regulation was obtained from the vanes with pumps having ns = 3920 and from the vanes with pumps having ns = 3920 and 1060.1060.

Adjustable outlet diffusion vanes have been used with Adjustable outlet diffusion vanes have been used with good success on several large European storage good success on several large European storage pumps for hydroelectric developments. Propeller pumps for hydroelectric developments. Propeller pumps with adjustable – pitch blades have been pumps with adjustable – pitch blades have been investigated with good success. Wide capacity investigated with good success. Wide capacity variation was obtained at constant head and with variation was obtained at constant head and with relatively little loss in efficiency. These methods are relatively little loss in efficiency. These methods are so complicated and expensive that they probably will so complicated and expensive that they probably will have very limited applicable in practice. have very limited applicable in practice. Air AdmissionAir Admission Admitting air into the pump suction has been Admitting air into the pump suction has been demonstrated as a means of capacity regulations, with demonstrated as a means of capacity regulations, with some saving in power over discharge throttling. some saving in power over discharge throttling. Usually air in the pumped liquid is undesirable, and Usually air in the pumped liquid is undesirable, and there is always the danger that too much air will causethere is always the danger that too much air will cause

the pump to lose its prime. The method has rarely been the pump to lose its prime. The method has rarely been used in practice but might be applicable to isolated used in practice but might be applicable to isolated causes.causes.

Parallel OperationParallel Operation:: Parallel operation of two or more pumps is a common Parallel operation of two or more pumps is a common

method of meeting variable – capacity requirements. By method of meeting variable – capacity requirements. By starting only those pumps needed to meet the demand, starting only those pumps needed to meet the demand, operation near maximum efficiency can usually be operation near maximum efficiency can usually be obtained. The head – capacity characteristics of the obtained. The head – capacity characteristics of the pumps need not be identical, but pumps with unstable pumps need not be identical, but pumps with unstable characteristics may give trouble unless operation only on characteristics may give trouble unless operation only on the steep portion of the characteristic can be assured. the steep portion of the characteristic can be assured. Care should be taken to see that no one pump, when Care should be taken to see that no one pump, when combined with pumps of different characteristics, is forced combined with pumps of different characteristics, is forced to operate at flows less than the minimumto operate at flows less than the minimum required required preventing recirculationpreventing recirculation.. Multiple pumps in a facility Multiple pumps in a facility provide spares for emergency service for the down time provide spares for emergency service for the down time needed for maintenance. Parallel pump operation is needed for maintenance. Parallel pump operation is beneficial where the system resistance curve is flat.beneficial where the system resistance curve is flat.

Series OperationSeries Operation for centrifugal pumps is carried for centrifugal pumps is carried out to increase the head. It enforces an identical out to increase the head. It enforces an identical flow through each pump where the discharge flow through each pump where the discharge head is sum of the heads developed by each head is sum of the heads developed by each pump. pump.

As a general rule Series operation of pumps is As a general rule Series operation of pumps is beneficial where the system resistance curve is beneficial where the system resistance curve is steep.steep.

See the curves on the next slide. See the curves on the next slide.

H

Q

PARALLEL OPERATION

PARALLEL

SERIES OPERATION

SYSTEM RESISTANCE CURVES

SERIES

Centrifugal Pump Troubleshooting Centrifugal Pump Troubleshooting GuideGuide

Pump not reaching Design Flow ratePump not reaching Design Flow rate

AA. Probable Cause:. Probable Cause:

Insufficient NPSH (Noise may or may not present)Insufficient NPSH (Noise may or may not present)

Recommended Remedy:Recommended Remedy:

Recalculate NPSH available. It must be greater than Recalculate NPSH available. It must be greater than NPSH required by pump at desired flow. If not:NPSH required by pump at desired flow. If not:

Re-design suction piping, holding number of elbows Re-design suction piping, holding number of elbows and number of planes to a minimum to avoid and number of planes to a minimum to avoid adverse fluid rotation as it approaches the impeller.adverse fluid rotation as it approaches the impeller.

B. Probable Cause:B. Probable Cause:

System head greater than anticipated.System head greater than anticipated.

Recommended Remedy:Recommended Remedy:

Reduce system head by increasing pipe size Reduce system head by increasing pipe size and/or reducing number of fittings. Increase and/or reducing number of fittings. Increase impeller diameter. impeller diameter.

C. Probable Cause:C. Probable Cause:

Plugged impeller, suction line, or casing: product Plugged impeller, suction line, or casing: product fibrous or contains large size solids.fibrous or contains large size solids.

Recommended Remedy:Recommended Remedy:

For fibrous material:For fibrous material:

1. Reduce length of fiber when possible,1. Reduce length of fiber when possible,

Consider oversized pump.Consider oversized pump.

D. Probable cause:D. Probable cause:

Entrained air-leak from atmosphere on suction side.Entrained air-leak from atmosphere on suction side.

Recommended Remedy:Recommended Remedy:

1. Check suction side gaskets and threads for 1. Check suction side gaskets and threads for tightness.tightness.

2. Properly repack stuffing box.2. Properly repack stuffing box.

3. Install Vortex breaker3. Install Vortex breaker

E. Probable Cause:E. Probable Cause:

Entrained gas from process.Entrained gas from process.

Recommended Remedy:Recommended Remedy:

Process generated gases may require a large pump.Process generated gases may require a large pump.

F. Probable Cause:F. Probable Cause:

Speed too lowSpeed too low

Recommended Remedy:Recommended Remedy:

Check motor speed against design speedCheck motor speed against design speed

G. Probable cause:G. Probable cause:

Direction of rotation wrong.Direction of rotation wrong.

Recommended Remedy:Recommended Remedy:

Reverse any two of three leads on a three phase Reverse any two of three leads on a three phase motor.motor.

H. Probable cause.H. Probable cause.

Impeller too small.Impeller too small.

Recommended Remedy:Recommended Remedy:

Replace with proper diameter impeller.Replace with proper diameter impeller.

I. Probable cause: I. Probable cause:

Impeller clearance too largeImpeller clearance too large

Recommended Remedy:Recommended Remedy:

Reset impellerReset impeller

Pump not reaching design Head (TDH)Pump not reaching design Head (TDH)

A. Probable Cause:A. Probable Cause:

Check all items under “Check all the items Check all items under “Check all the items referred in Pumps not reaching design flow referred in Pumps not reaching design flow rate.”rate.”

Recommended Remedy:Recommended Remedy:

Refer to remedies listed under Problem No.1Refer to remedies listed under Problem No.1

No-Discharge or Flow- Pump RunningNo-Discharge or Flow- Pump Running

A. Probable Cause:A. Probable Cause:

Not properly primedNot properly primed

Recommended Remedy:Recommended Remedy:

Repeat priming operation, recheck instructions.Repeat priming operation, recheck instructions.

B. Probable Cause:B. Probable Cause:

Suction lift too high.Suction lift too high.

Recommended Remedy:Recommended Remedy:

1. Rearrange piping.1. Rearrange piping.

2. Increase Suction head if possible.2. Increase Suction head if possible.

3. Determine if larger impeller would be better.3. Determine if larger impeller would be better.

4. Select new pump to handle higher suction lift.4. Select new pump to handle higher suction lift.

C.Probable CauseC.Probable Cause

Impeller, Suction line, or casing is plugged.Impeller, Suction line, or casing is plugged.

..

Recommended Remedy:Recommended Remedy:

For fibrous materials:For fibrous materials:

1. Reduce length of fiber when possible.1. Reduce length of fiber when possible.

2. Consider oversized pump2. Consider oversized pump

D. Probable cause:D. Probable cause:

Direction of rotation wrongDirection of rotation wrong

Recommended Remedy:Recommended Remedy:

Reverse any two of three leads on a three phase Reverse any two of three leads on a three phase motormotor

E. Probable cause:E. Probable cause:

Entrained air-leak from atmosphere on suction side.Entrained air-leak from atmosphere on suction side.

Recommended remedy:Recommended remedy:

Check 1-DCheck 1-D

Recommended Remedy:Recommended Remedy:

For fibrous materials:For fibrous materials:

1. Reduce length of fiber when possible.1. Reduce length of fiber when possible.

2. Consider oversized pump2. Consider oversized pump

D. Probable cause:D. Probable cause:

Direction of rotation wrongDirection of rotation wrong

Recommended Remedy:Recommended Remedy:

Reverse any two of three leads on a three phase Reverse any two of three leads on a three phase motormotor

E. Probable cause:E. Probable cause:

Entrained air-leak from atmosphere on suction side.Entrained air-leak from atmosphere on suction side.

Recommended remedy:Recommended remedy:

Check 1-DCheck 1-D

Pump operates for short period, then looses Pump operates for short period, then looses prime.prime.

A. Probable Cause:A. Probable Cause:

Insufficient NPSH.Insufficient NPSH.

Recommended Remedy:Recommended Remedy:

Recalculate NPSH available. It must be greater than Recalculate NPSH available. It must be greater than NPSH required by pump at desired flow. If not:NPSH required by pump at desired flow. If not:

Re-design suction piping, holding number of elbows Re-design suction piping, holding number of elbows and number of planes to a minimum to avoid and number of planes to a minimum to avoid adverse fluid rotation as it approaches the impeller.adverse fluid rotation as it approaches the impeller.

B. Probable cause:B. Probable cause:

Entrained air-leak from atmosphere on suction side.Entrained air-leak from atmosphere on suction side.

Recommended remedy:Recommended remedy:

Check 1-DCheck 1-D

Excessive Noise-Wet endExcessive Noise-Wet end

A.A. Probable cause:Probable cause:

Cavitation-Insufficient NPSH availableCavitation-Insufficient NPSH available

Recommended remedy:Recommended remedy:

Refer to remedy under 1-A.Refer to remedy under 1-A.

B. Probable Cause:B. Probable Cause:

Abnormal fluid rotation due to complex suction piping.Abnormal fluid rotation due to complex suction piping.

Recommended Remedy:Recommended Remedy:

Re-design suction piping, holding number of elbows and Re-design suction piping, holding number of elbows and number of planes to a minimum to avoid adverse number of planes to a minimum to avoid adverse fluid rotation as it approaches the impeller.fluid rotation as it approaches the impeller.

C. Impeller rubbingC. Impeller rubbing

Recommended Remedy:Recommended Remedy:

1.1. Check and reset Impeller ClearanceCheck and reset Impeller Clearance

2.2. Check out board bearing assembly for axial end Check out board bearing assembly for axial end play.play.

Excessive Noise –Power endExcessive Noise –Power end

A.A. Probable causeProbable cause

Overloaded bearing which is indicated by flaking or Overloaded bearing which is indicated by flaking or spalling of the bearing raceways.spalling of the bearing raceways.

Recommended Remedy:Recommended Remedy:

Check to be sure that actual operating conditions Check to be sure that actual operating conditions do not exceed the maximum allowable suction do not exceed the maximum allowable suction pressure and the maximum allowable specific pressure and the maximum allowable specific gravity.gravity.

B. Probable cause:B. Probable cause:

Bearing contamination appearing on the raceways Bearing contamination appearing on the raceways as a scoring, pitting, scratching, or rusting caused as a scoring, pitting, scratching, or rusting caused by adverse environment and entrance of abrasive by adverse environment and entrance of abrasive waste materials from atmosphere.waste materials from atmosphere.

Recommended Remedy:Recommended Remedy:

1.1. Work with clean tools in clean surroundings.Work with clean tools in clean surroundings.

2.2. Remove all dirt from housing before exposing Remove all dirt from housing before exposing bearings.bearings.

3.3. Handle with clean dry handsHandle with clean dry hands

4.4. Use clean solvents and flushing oil.Use clean solvents and flushing oil.

C. C. Probable cause:Probable cause:

Brinelling of bearing identified by indentations on Brinelling of bearing identified by indentations on the ball races, caused by improper handling during the ball races, caused by improper handling during fitting.fitting.

Recommended Remedy:Recommended Remedy:Be sure when mounting a bearing to apply the Be sure when mounting a bearing to apply the mounting pressure slowly and evenly. Press the mounting pressure slowly and evenly. Press the inner ring while fitting the bearing on the shaft and inner ring while fitting the bearing on the shaft and press against outer ring when fitting in housing.press against outer ring when fitting in housing.

D. D. Probable cause:Probable cause:False brinelling of bearing identified again by False brinelling of bearing identified again by either axial or circumferential indentations usually either axial or circumferential indentations usually caused by vibration of the balls between the races caused by vibration of the balls between the races in a stationary bearing.in a stationary bearing.Recommended Remedy:Recommended Remedy:

1.1. Correct the source of vibration.Correct the source of vibration.2.2. Where bearings are oil lubricated and employed in Where bearings are oil lubricated and employed in

units that may be out of service for extended units that may be out of service for extended periods, the shaft should be turned over periods, the shaft should be turned over periodically to re lubricate all bearing surfaces at periodically to re lubricate all bearing surfaces at intervals of one to three months.intervals of one to three months.

E.E. Probable Cause:Probable Cause:

Thrust overload on bearing identified by flaking ball Thrust overload on bearing identified by flaking ball path on one side of the outer race or in the case of path on one side of the outer race or in the case of maximum capacity bearings, may appear as a spalling maximum capacity bearings, may appear as a spalling of the races in the vicinity of the loading slot. These of the races in the vicinity of the loading slot. These thrust failures are caused by improper mounting of the thrust failures are caused by improper mounting of the bearing or excessive thrust loads.bearing or excessive thrust loads.

Recommended Remedy:Recommended Remedy:

1. 1. Follow correct mounting procedures for bearings.Follow correct mounting procedures for bearings.

2. 2. Check pump literature to be sure that actual conditions Check pump literature to be sure that actual conditions do not exceed the maximum allowable Sp. Gravity or do not exceed the maximum allowable Sp. Gravity or suction pressure limitations. suction pressure limitations.

F. Probable cause:F. Probable cause:

Misalignment identified by the fracture of ball retainer or a Misalignment identified by the fracture of ball retainer or a widewide

ball path on the inner race and a narrower cocked ball path on the inner race and a narrower cocked ball path on the outer race. Misalignment is caused ball path on the outer race. Misalignment is caused by poor mounting practices or improperly machined by poor mounting practices or improperly machined shafts. shafts.

Recommended remedy:Recommended remedy:

Check Housing bore dimensionally to be sure that Check Housing bore dimensionally to be sure that both bores are true with each other. Check shaft both bores are true with each other. Check shaft shoulders are squared with shaft center line. Run out shoulders are squared with shaft center line. Run out of the shaft is within limits.of the shaft is within limits.

G. Probable cause:G. Probable cause:

Bearing damaged by electrical arcing identified as Bearing damaged by electrical arcing identified as electro-etching of both inner and outer ring as pitting electro-etching of both inner and outer ring as pitting or cratering. Electrical arcing is caused by a static or cratering. Electrical arcing is caused by a static electric charge emanating from belt drives, electrical electric charge emanating from belt drives, electrical leakage or short circuiting.leakage or short circuiting.

Recommended Remedy:Recommended Remedy:

1.1. Where current shunting through the bearing cannot Where current shunting through the bearing cannot be corrected, a shunt in the form of a slip ring be corrected, a shunt in the form of a slip ring assembly should be inorporated. assembly should be inorporated.

2.2. Check all wiring, insulation and rotor windings to be Check all wiring, insulation and rotor windings to be sure that they are sound and all connections are sure that they are sound and all connections are properly made.properly made.

3.3. Where pumps are belt driven, consider the Where pumps are belt driven, consider the elimination of static charges by proper grounding elimination of static charges by proper grounding or consider belt material that is less generative.or consider belt material that is less generative.

H. H. Probable Cause:Probable Cause:

Bearing damage due to improper lubrication, Bearing damage due to improper lubrication, identified by one or more of the following.identified by one or more of the following.

1.1. Abnormal bearing temperature rise.Abnormal bearing temperature rise.

2.2. A stiff cracked grease appearance.A stiff cracked grease appearance.

3.3. A brown or bluish discoloration of the bearing A brown or bluish discoloration of the bearing race.race.

4.4. Failure of the ball retainer.Failure of the ball retainer.

Recommended Remedy:Recommended Remedy:

1.1. Be sure that the lubricant is clean.Be sure that the lubricant is clean.

2.2. Be sure that proper amount of lubricant is used. Be sure that proper amount of lubricant is used.

3.3. Be sure proper grade of lubricant is used.Be sure proper grade of lubricant is used.

About the presenterAbout the presenter

Syed Ali Hussain is presently working as Pumps and Syed Ali Hussain is presently working as Pumps and Blowers Engineer in Maintenance Reliability Group. Blowers Engineer in Maintenance Reliability Group. He handles Reliability improvements and trouble He handles Reliability improvements and trouble shootings, condition monitoring in Pumps, blowers shootings, condition monitoring in Pumps, blowers and their drivers in Petrokemya. He is having eight and their drivers in Petrokemya. He is having eight years experience in Rotating equipment in Process years experience in Rotating equipment in Process industry. He was previously working with Pak Arab industry. He was previously working with Pak Arab Fertilizers Pakistan as rotating equipment engineer. Fertilizers Pakistan as rotating equipment engineer. He is a graduate Engineer from UET Lahore He is a graduate Engineer from UET Lahore Pakistan. He is Certified Vibration analyst I and life Pakistan. He is Certified Vibration analyst I and life member of Pakistan Engineering Council, Pakistan. member of Pakistan Engineering Council, Pakistan.