CHE10209 Couplings Seals Bearings

Note: The source of the technical material in this volume is the ProfessionalEngineering Development Program (PEDP) of Engineering Services.

Warning: The material contained in this document was developed for SaudiAramco and is intended for the exclusive use of Saudi Aramco’semployees. Any material contained in this document which is notalready in the public domain may not be copied, reproduced, sold, given,or disclosed to third parties, or otherwise used in whole, or in part,without the written permission of the Vice President, EngineeringServices, Saudi Aramco.

Chapter : Process For additional information on this subject, contactFile Reference: CHE10209 R. A. Husseni on 874-2792

Engineering EncyclopediaSaudi Aramco DeskTop Standards

Miscellaneous Mechanical Components

Engineering Encyclopedia Process


Saudi Aramco DeskTop Standards

CONTENTS PAGES

INFORMATION

COUPLINGS 1

Flexible Disc Coupling 3

Flexible Diaphragm Coupling 5

Lubricated Gear Coupling 6

Coupling Life 7

GEARS 8

Speed Increasing Gears 8

Speed-Decreasing Gears 8

Gear Design 9

Lubrication of Gears 9

Power Consumption in Gears 9

VARIABLE SPEED COUPLINGS 10

BEARINGS 11

Journal Bearings 11

Thrust Bearings 13

Ball Bearings 14

BEARING LUBRICATION 15

Oil Ring Lubrication for Small Power Loads 15

Lubrication by Forced Circulation 16

PUMP SEALS 17

Packing 17

Mechanical Seals 19

Tandem Mechanical Seals 21

Double Mechanical Seals 22

Barrier Fluid System 23

COMPRESSOR SEALS 24

Labyrinth Seals 24

Oil Seals 25

VIBRATION MONITORING TECHNIQUES 26

VIBRATION PROBE TYPES 28

Non-Contacting Eddy Current Probe 28

Velocity or Seismic Sensors 29



Saudi Aramco DeskTop Standards

Thrust Bearing Monitoring 30

GLOSSARY 31



Saudi Aramco DeskTop Standards 1

COUPLINGS

The function of a coupling is to transmit power from one component of a machinery train to another and allowa certain amount of lateral and axial misalignment between the connected equipment. Some examples are:

• A motor to a pump.• A motor to a gear.• A gear to a compressor casing.• One compressor casing to another compressor casing.

Figure 1 shows some typical coupling locations.

Couplings must be designed to absorb movements due to slight misalignment between the shafts. Beforestartup, mechanics align machinery components; however, some small misalignment always remains. The twotypes of misalignment are parallel and angular. They are shown in Figure 2.

Coupling

Motor Gear CompressorCasing 1

CompressorCasing 2

FIGURE 1. LOCATIONS OF COUPLINGS




COUPLINGS (CONT’D)

Parallel Misalignment

Angular Misalignment

FIGURE 2. FUNCTIONS OF COUPLINGS -- (PARALLEL ANDANGULAR MISALIGNMENT)

Couplings perform one other function that is shown in Figure 3. They permit axial movement of the two shaftsrelative to each other, which is called free end float. The source of this movement is a change in shaft axialposition resulting from a change in temperature during operation.

FIGURE 3. FUNCTIONS OF COUPLINGS -- (FREE-END FLOAT)




Flexible Disc Coupling

Figure 4 shows the most common type of coupling, the flexible disc. This type of coupling is used on mostcentrifugal pumps and on some compressors. It transmits power from one shaft to the other through sets ofthin, flexible metallic discs. Flexing of the discs permits the movements necessary during each rotation. Theflexible discs also permit a limited amount of free end float, as shown in Figure 5.

An advantage of the flexible disc type coupling is that it does not require lubrication.

FIGURE 4. COUPLING TYPES - FLEXIBLE DISC, NONLUBRICATED




Flexible Disc Coupling (Cont’d)

FreeEndFloat

FIGURE 5. COUPLING TYPES - FREE END FLOAT




Flexible Diaphragm Coupling

Another type of nonlubricated coupling is the flexible diaphragm. This coupling is commonly used on largeloads with high speeds such as centrifugal compressors. Power is transmitted from one shaft to the otherthrough relatively thin flexible diaphragms that permit movement. Figure 6 illustrates the flexible diaphragmcoupling.

FIGURE 6. COUPLING TYPES - FLEXIBLE DIAPHRAGM - NONLUBRICATED




Lubricated Gear Coupling

An older type of coupling is the gear coupling shown in Figure 7. In this coupling, power is transmittedthrough gear teeth. All of the gears are concentric and they rotate at the same speed. However, the elementsare free to slide axially within each other, and this provides the required flexibility. A disadvantage of this gearis that it must be lubricated. Because poor lubrication can result in frequent coupling failures, this type ofcoupling is not commonly used today.

With Permission from Zurn Industries (Amerigear)

FIGURE 7. COUPLING TYPES - GEARS, LUBRICATED




Coupling Life

A coupling that is properly designed and installed should not experience failures during normal operation ofmachinery trains. Factors that can decrease coupling life are:

• A high torque stress on the coupling, greater than the design value.• Large misalignments between the two shafts.• Operation at relatively high speeds.• Corrosion or mechanical scratches on thin discs and diaphragms.• Dirt or sludge in a lubricated coupling.

A coupling can often tolerate any one of these contributing factors, but two or more in combination increasethe chances for a coupling failure.




GEARS

Gears are sometimes needed to change the speed of rotation from one component to the next. Some examplesare:

Speed Increasing Gears

Motors typically run at a maximum speed of 3600 rpm. Centrifugal compressors operate at 5000 to 13,000rpm. A speed-increasing gear is used to run a compressor at the higher speed.

Speed-Decreasing Gears

Steam turbine drivers run at high speeds, 5000 to 10,000 rpm. If the driven equipment is a pump or an electricgenerator, the speed required is typically 3600 rpm. A speed-decreasing gear is therefore required.

Gas turbines are also high speed machines running at 5000 to 10,000 rpm. Speed-decreasing gears are requiredfor pumps and electric generators driven by gas turbines.

The second stage of a centrifugal compressor may run at a higher speed than the first stage.

Reciprocating compressors are low speed machines, typically running at 500 to 1000 rpm. A speed-decreasinggear is necessary with most drivers, including 2-pole and 4-pole electric motors.

Gears are usually not needed for the following applications because the driver and the driven equipment canoperate at the same speed.

• An electric motor connected to a centrifugal pump.

• A steam turbine connected to a centrifugal compressor.

• A gas turbine connected to a centrifugal compressor.

• A slow-speed synchronous electric motor connected to a reciprocating compressor.




Gear Design

The three basic types of gears--spur, helical, and double helical--are shown in Figure 8.

Spur Helical Double Helical

FIGURE 8. TYPES OF GEARS

The spur gear is the simplest type of gear. The teeth are parallel to the axis of the shaft. This type of gear isnot used for large, high-speed loads.

In the helical gear, the teeth are not parallel to the shaft. Instead, they follow the shape of a helix, or spiral, onthe surface of the gear. This results in gradual loading of each tooth as the gear rotates. Vibration and noiselevels are much lower for helical gears than for spur gears.

A double helical gear has two sets of teeth included at opposite angles. This arrangement neutralizes the largethrust, or axial load, on the shaft, which is a disadvantage of the single helical gear. Double helical gears arecommon for large, high-speed loads.

Lubrication of Gears

Gears are lubricated by a spray of oil onto the teeth at the point of contact. The oil is circulated by a pump.The circulation system may be for the gear alone, or it may be combined with a system that lubricates all themajor bearings in the train.

Power Consumption in Gears

Gears are not 100% efficient in transmitting power. A power loss of 2 to 3% is normal.




VARIABLE SPEED COUPLINGS

Variable speed couplings permit the speed ratio to be changed during operation. The different types includehydraulic couplings, magnetic eddy current couplings, and v-belt couplings. These couplings are notcommonly used in Aramco. In fact, their use is declining in industry in general. It is now more common touse variable speed electric motors. Therefore, these couplings are not covered in detail in this course.




BEARINGS

Two types of bearings are used in rotating equipment. The first is the journal bearing, which supports theshaft and absorbs all radial forces. Radial forces are perpendicular to the shaft. The second type is the thrustbearing, which absorbs axial forces on the shaft. Axial forces are parallel to the shaft.

Journal Bearings

Journal bearings are usually sleeve bearings or tilting pad bearings. Typical journal bearing types are shownin Figure 9. The shaft rotates within a simple sleeve. A film of oil is maintained between the bearing and therotating shaft. The shaft does not touch the bearing but is supported by the dynamic forces of the oil. Ifcontact occurs, the sliding friction will damage the bearings in a short time.

FIGURE 9A. JOURNAL OR SLEEVE BEARINGS

FIGURE 9B. JOURNAL OR SLEEVE BEARINGS - TILT PAD




The rotation of the shaft causes the oil in the annular space to rotate. Since the annular space is smaller at thebottom of the shaft, the oil is forced into a narrow wedge at this point. This causes the oil pressure to be higherat the bottom of the shaft. The higher pressure supports the weight of the shaft and prevents contact betweenthe shaft and the bearing.

This type of bearing is the one most commonly used on large machinery such as centrifugal compressors,steam turbines, and gas turbines.

At high speeds, the simple sleeve bearing may cause unacceptable vibration, because the single oil wedge mayrotate, or orbit, around the shaft. Specially designed bearings with lobes or tilting pads (See Figure 9B)produce multiple oil wedges and greater stability at high speeds.




Thrust Bearings

Hydraulic, aerodynamic, and magnetic forces can cause axial forces on a shaft. These forces are absorbed bybearings called thrust bearings. Each compressor casing, turbine, and electric motor has at least one thrustbearing. A rotating collar on the shaft presses against stationary pads mounted to the casing. A rotating oilfilm between the collar and the pads supports the load without metal-to-metal contact in the same manner as ajournal bearing. See Figure 10.

Shoe PadsMounted toCasing

Soft Metal Surfaces

Thrust

CollarOil Out

Oil In

Shaft

Pivots or Levelling Plates

FIGURE 10. THRUST BEARINGS




Ball Bearings

Another type of bearing is the ball bearing or rolling contact bearing. Figure 11 is a diagram of a ball bearing.Ball bearings are commonly used in pumps. They absorb both the radial and the axial thrust forces on theshaft. Metal-to-metal contact does occur, but the contact is at small points that are constantly moving. Also,rolling friction occurs, not sliding friction. Therefore, only small amounts of heat are generated by the friction.

Outer Sleeve

Balls

Inner Sleeve

Ball Guide or Cage

FIGURE 11. BALL BEARING OR ROLLING CONTACT BEARING




BEARING LUBRICATION

Oil Ring Lubrication for Small Power Loads

Small pieces of rotating equipment such as pumps usually have oil-ring lubrication. See Figure 12. An oilreservoir is located below the shaft. An eccentric ring suspended on the shaft dips into the oil level. As theshaft rotates, this ring also rotates but at a slower speed. As the ring rotates through the oil, it picks up oil andcarries it up to the shaft. The oil then flows sideways into the bearing. A level indicator on the side of theequipment shows the level of oil in the reservoir. Operators check the level periodically and refill with oilwhen necessary.

Figure 12 also shows a constant level oiler, which keeps the level in the bearing housing constant until theglass bottle is empty.

FIGURE 12. BEARING LUBRICATION

Note that the glass bottle is not vented and does not show the oil level in the bearing housing. Level is checkedin the sight glass.




Lubrication by Forced Circulation

Large pieces of equipment such as compressors, turbines, and large electric motors have forced lubricationsystems. Oil is circulated through the bearings and through a cooler and filter to remove the heat generatedwithin the bearings and dirt. The major components are as follows:

• A reservoir to hold a supply of oil.

• Circulating pumps. Normally two pumps are provided, one driven by an electric motor, onedriven by a small steam turbine. Two pumps insure that circulation of the lubricating oil is neverlost, even during an electric power failure. Loss of circulation would cause significant damage tothe bearings.

• Piping carries the lubricating oil to and from each bearing. Usually small indicator glasses arelocated in these lines so that an operator can visually check that the oil is flowing.

• A cooler to remove heat generated by friction. The oil cooler may be a water cooler or an aircooler.

• A dual filter to remove solid particles greater than 10 microns from the oil.

• Control valves to control oil flow and pressure.




PUMP SEALS

Seals are required between the rotating shaft and the stationary casing of a pump to prevent the leakage ofliquid. Seals are mounted in the section of the casing called the stuffing box.

Packing

The original type of seal on pumps was packing. Packing consists of rings of flexible material in the annularspace between the shaft and the casing. See Figure 13. The rings are compressed by a device called the glandfollower. Compression of the packing rings causes a tight fit between the packing and the shaft and betweenthe packing and the casing. Lubrication of the surface between the packing and the shaft may be required. Ifso, a lubricant is injected into the space called the lantern ring. This fluid can also act as a barrier fluid if it ismaintained at a pressure higher than the pressure inside the pump.




The disadvantage of packing as a seal is that tightening is required during operation. A small but finite amountof leakage occurs with most packing services. Therefore, packing is not used very often today. Themechanical seal has become the standard sealing device.

FIGURE 13. PACKING TYPE - PUMP SEAL




Mechanical Seals

Most pumps today are equipped with mechanical seals rather than packing.Mechanical seals do not require tightening during operation. Leakage rates are extremely small. Figure 14 is adiagram of a mechanical seal. The principal parts of a mechanical seal are two seal rings. One ring is mountedon the shaft and rotates with the shaft; the other ring is stationary and is attached to the casing. Where the tworings touch each other, the surfaces are extremely flat and smooth. Therefore, contact between the two rings isso perfect that liquid cannot pass between them. A spring pushes the one ring against the other to maintaincontact. O-ring seals prevent flow of liquid through any other path.

FIGURE 14. MECHANICAL SEAL




If the pumping service is hot, or if the fluid viscosity is high, heat must be removed from the rotating seal. Aflushing liquid is circulated through the stuffing box to remove the heat. The flushing liquid also provideslubrication between the two seal surfaces. The flushing liquid can be a liquid circulated from the discharge ofthe pump if this liquid is clean. At other times, the flushing liquid may be a separate fluid. The flushing liquidalso prevents the accumulation of solids in the stuffing box.

Figure 15 shows several methods for providing flushing liquid to a pump seal.

FIGURE 15. SOME TYPICAL SEAL FLUSHING ARRANGEMENTS




Tandem Mechanical Seals

For most pumping services, a single mechanical seal is adequate. However, if the pump fluid is hazardous ortoxic, more than one seal may be required to make sure leakage does not occur. Two seals may be arranged intandem or as double seals. If one seal fails and leaks, the other seal serves as a backup and prevents leakage toatmosphere.

The tandem arrangement is shown on Figure 16. In this case, the inner seal contains the pressure. The pressurein the buffer zone between the two seals will be near atmospheric pressure. If the inner seal fails and leakageoccurs, liquid will accumulate in the buffer zone but will be contained by the outer seal. A level instrument ora pressure instrument in the buffer zone will sound an alarm to notify operators that leakage of the inner seal isoccurring.

If the outer seal fails first, only barrier fluid leaks to the atmosphere, not pumped fluid. Again an alarmindicates the malfunction. The barrier fluid is nonhazardous.

FIGURE 16. TANDEM MECHANICAL SEALS




Double Mechanical Seals

Double mechanical sealing is an alternative arrangement for two seals on a single shaft. See Figure 17. In thiscase, the pressure between the seals is higher than either the fluid pressure inside the pump or atmosphericpressure. Thus, if any leakage occurs, it is barrier fluid which leaks. Barrier fluid can leak into the product orout to the atmosphere. However, the product and the atmosphere are isolated from each other at all times. Thisarrangement is used for materials which are very hazardous, toxic, or corrosive. With this arrangement, thecorrosive liquid does not contact the sealing surfaces.

FIGURE 17. DOUBLE MECHANICAL SEALS




Barrier Fluid System

Figure 18 shows a typical barrier fluid system. A reservoir is provided at an elevation approximately 6 ftabove the pump seal. This reservoir is filled with a barrier fluid. The fluid circulates by thermal convection orby a pumping ring located on the rotating seal component to the seal and back to the container. A nitrogenconnection maintains pressure on the container. If both seals are functioning normally, the level in thereservoir and the pressure will remain constant and the respective instruments will function normally. If one ofthe seals leaks, an instrument will sound an alarm, notifying the operator of leakage.

FIGURE 18. TYPICAL BARRIER FLUID SYSTEM




COMPRESSOR SEALS

There are two major types of seals on compressor casings. Labyrinth seals minimize internal leakage, forexample, backflow from one impeller to the preceding impeller. Oil seals are located at the ends of the casingto prevent leakage of gas to the atmosphere.

Labyrinth Seals

In a compressor, leakage between the rotating impeller and stationary casing must be minimized in order tomaintain efficiency. Backflow of gas is minimized by means of labyrinth seals. See Figure 19. A labyrinthseal contains multiple teeth located very close to the rotating shaft. This results in a breakdown of pressureacross the seal with very little flow. Labyrinth seals are usually made of a soft metal so that they will notdamage the shaft, but as a result, they are very easily damaged by excessive vibration of the shaft.

FIGURE 19. LABYRINTH SEAL

Labyrinth seals can also be used as external seals on air compressors and gas turbines. Some leakage of theinternal gas to the atmosphere can be tolerated in these places.




Oil Seals

Most centrifugal compressors have oil seals at each end of the casing. See Figure 20. A bushing type oil sealshown has two rings mounted on the casing. These seal rings fit very closely to the shaft but with a finiteclearance. Seal oil is pumped into the space between the two seals and circulates through the annular spacesbetween the shaft and the seal rings. The pressure of the seal oil is controlled to a level 5 to 10 psi higher thanthe gas pressure on the other side of the ring. Seal oil pressure is also higher than atmospheric pressure. Thismeans that oil will flow outward through the seals, but gas and atmospheric air cannot flow through the seal.Seal oil is circulated by a pump.

Seal oil from the outer seal ring can be returned directly to the reservoir and the pump.

Oil that passes through the inner seal contains dissolved gas and must be treated differently. If the gas is clean,the oil passes through a low-pressure vessel to vent the gas. The oil is then returned to the system. If the gas isdirty or corrosive, the oil from the inner seal is usually discarded. The small pressure difference between theseal oil and the gas minimizes the leakage to the inner seal. Typical rates are 5 to 10 gallons of oil leakage perday.




VIBRATION MONITORING TECHNIQUES

Compressors, large turbines, and electric motors are expensive pieces of equipment. They are also usuallyessential to the operation of a process. One way to prevent mechanical failures is to continuously monitorequipment vibration levels. Very often, a gradual increase in vibration level precedes a significant failure. Ifincreased vibration can be detected, the machine can be shut down and repaired before the damage becomessignificant.

Hand-held devices are used for routine monitoring of vibration in pumps and small motors. Large equipment isfitted with permanently mounted vibration probes. When the vibration reaches a specified level, thesemonitors sound an alarm. They may also automatically shut down the machine.




FIGURE 20. OIL SEALS - CENTRIFUGAL COMPRESSORS




VIBRATION PROBE TYPES

Non-Contacting Eddy Current Probe

This type of probe is usually used for compressors, motors, and turbines. See Figure 21. A probe is installedin the bearing housing very near to the rotating shaft. A d.c. voltage signal is fed to the probe. Any metalmass near the tip of the probe will cause a loss of signal strength as the signal produces eddy currents in themetal. The losses are inversely proportional to the width of the gap between the probe and the shaft. Anelectronic instrument connected to the probe translates the signals into amplitude of vibration and frequency ofvibration. These devices can measure up to 5000 Hz.

The eddy current probe measures the distance between the shaft and the probe. Therefore, it can be used todetect radial vibrations and position of the shaft. Two radial probes are installed, at right angles, in each majorjournal bearing.

FIGURE 21. VIBRATION PROBE TYPES - EDDY CURRENT




Velocity or Seismic Sensors

The velocity sensor is usually used in equipment with ball bearings. A moving coil is attached to the bearinghousing. See Figure 22. A stationary magnet surrounds the coil. The movement of the coil generates a signalthat is monitored by an external electronics package. This type of monitor can measure much higherfrequencies than the eddy current monitor so it is used on gear housings. The vibration in gears can be muchhigher in frequency than in other types of equipment, because vibrations are generated by the teeth.

FIGURE 22. VIBRATION DETECTOR - VELOCITY (SEISMIC) SENSOR




Thrust Bearing Monitoring

Thrust bearing failure due to excessive wear is an important cause of machinery shutdowns. Monitors areinstalled to give early warning of a problem.

Two variables are normally monitored on a thrust bearing, the temperature of the bearing and the position ofthe shaft.

Temperature sensors are installed in the shoes of the thrust bearing. A high temperature at this positionindicates excessive load on the bearing,insufficient lubrication, or both.

The shaft position indicator is an eddy current sensor similar to the axial vibration monitor. The instrumentmeasures the distance between the tip of the probe and the thrust collar. As the thrust pads gradually weardown, the collar moves toward the probe.

A disadvantage of velocity sensors is large size. Recently, smaller sensors called accelerometers have beenapplied to measure vibrations other than shaft vibration. They operate by the piezoelectric principle and canmeasure high frequencies.




GLOSSARY

Accelerometer A device used to measure high frequency vibration of a gear housing orbearing housing.

Alignment The relative orientation of two shafts with respect to each other. For goodalignment, the shafts should be parallel and concentric.

Ball Bearing A bearing that has rotating spheres between two sleeves.

Barrier Fluid An intermediate fluid that prevents contact between a process fluid and theatmosphere.

Buffer Zone An area that contains a barrier fluid.

Coupling A device that connects the shafts of two machines.

Double Helical Gear A gear with two rows of teeth that are inclined to the shaft at oppositeangles.

Double Mechanical Seals Two seals installed back-to-back, with a high pressure barrier fluidbetween them.

Eddy Current Probe A type of vibration monitor that detects distance between the probe and ashaft.

Flexible Diaphragm A type of coupling that uses flexible diaphragms to transmit power, usedfor large equipment such as gas compressors.

Flexible Disc The most common type of coupling, usually used for pumps.

Flushing A liquid injected into a stuffing box to cool, lubricate, and clean a seal.

Forced Lubrication A system of bearing lubrication where oil is pumped into the spacebetween the bearings and the shafts.

Free-End Float Axial movement of the shaft.

Gear A device that transmits power and changes speed of rotation. Output speedis in a fixed ratio to input speed.




Gear Coupling A coupling that transmits power by means of gear-type elements. Allelements are concentric and rotate at the same speed, but axial motion ispossible.

Helical Gear A power transmission gear with teeth that are not parallel to the shaft.Each tooth follows the path of a helix on the surface of the gear.

Journal Bearing A bearing that absorbs the radial forces on a shaft.

Labyrinth Seal A seal made of a series of sharp-edged rings. The rings are very close to ashaft, without any sealing liquid.

Mechanical Seal A seal that prevents leakage by means of two rings whose faces fit togethervery closely. One ring is stationary, the other rotates.

Oil Ring A device used to lubricate bearings without forced lubrication. As itrotates, it dips into an oil reservoir and carries oil up to a shaft.

O-Ring A ring of flexible material used to seal an annular space in mechanicalseals where there is no relative rotation between the two surfaces to besealed.

Packing A method of sealing a rotating shaft, using rings of flexible materialaround the shaft.

Piezoelectric Producing an electric voltage by stressing a crystalline material, such asquartz.

Seal A device that prevents or minimizes leakage of fluid between a stationarycomponent such as a casing and a rotating component such as a shaft orimpeller.

Sleeve Bearing A cylinder-shaped bearing that fits around a shaft like a sleeve.

Spur Gear A gear with teeth that are parallel to the axis of the shaft.

Stuffing Box The part of a pump body that contains the mechanical seal or the packing.

Tandem Mechanical Seal Two mechanical seals, in series, on the same shaft.




Train A series of machinery components connected together; for example, driver,gear, and compressor.

Thrust Bearing A bearing that absorbs the axial force, or thrust, on a shaft.

Variable Speed Coupling A coupling that can continuously change the ratio of output speed to inputspeed.

Velocity Sensor A vibration probe that has a moving coil attached to a machine part. Thecoil moves inside a fixed magnet to generate a signal.

Documents

CHE10209 Couplings Seals Bearings