Chap09 Centrifugal Pumps & Piping

Chapter 9. Centrifugal Pumps & PipingProprietary: - for the exclusive use of Amoco Production and other wholly owned subsidiaries of Amoco Corporation.

9.1

Centrifugal pumps are ideal for the low pressure, high flow rate requirements ofhydrocyclones and mixing systems. Unlike constant-volume piston pumps, centrif-ugal pumps provide constant head. Consequently, the pump and associated pip-ing system must be correctly sized and designed to deliver the required flow rateand desired head. This section briefly describes how centrifugal pumps work andprovides guidelines for the design, installation and operation of centrifugal pumpsand piping systems.

Principle of Operation

The centrifugal pump consists of a rotating impeller mounted inside a casing(Figure 9.1). Fluid enters the casing at the center (the eye of the impeller). As theimpeller spins, the fluid is accelerated to the circumference by the curved impellervanes. The accelerated fluid exits the impeller and enters the pump casing wherethis kinetic energy is converted into pressure energy. Although the pump can oper-ate against a closed discharge valve, it is not recommended. When there is noflow, all the pump power is dissipated into the fluid. This will cause the pump andmotor to quickly overheat.

A drive shaft connected to the impeller transmits power from the driver. A stuffingbox or seal is normally used to prevent leakage. The most common driver for cen-trifugal pumps is the a.c., fixed-speed, induction motor. Variable-speed motors areavailable, but rarely required for drilling rig applications. The motor is joined to thepump shaft by a flexible coupling. Drivers are usually three-phase motors. Therotation of the pump should be checked when it is installed to make sure that it isrotating in the proper direction.

Centrifugal pumps are usually constructed of a cast-steel housing with cast-ironinternal parts. Long-life packages offer hard-facing on the high-wear areas of thepump. Wear-resistant tungsten carbide seals are also available. Both are highlyrecommended.

The pump performance curves in Appendix E, Pump Performance Curves, illus-trate that the head generated by centrifugal pumps decreases very little as theflow rate is increased. Conversely, the flow rate through hydrocyclones is notaffected much by head. Note, however, that hydrocyclones are designed to oper-ate at a certain amount of head. Less or more may be detrimental to their perfor-mance. Therefore, the pump should be sized to provide the correct head at theflow rate dictated by the hydrocyclones.

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9.2

Sizing Centrifugal Pumps

1. Determine the total flow rate needed. For a hydrocyclone manifold, the flowrate is calculated by:

2. Determine the total head required. For most hydrocyclones, the required inlethead is 75 ft. The total head required from the pump is:

where:

Figure 9.1 Typical Centrifugal Pump. Kinetic energy is converted into pressure energy by the rotating impeller vanes to provide consistent head.

Q (gpm) # of Cones Flow Capacity Cone=

Ht 75 ft Lift Height (ft) Friction Losses (ft)+ +=

Centrifugal Pumps & Piping

Lift Height is the height between the hydrocyclone manifold and the mud sur-face (not the pump suction).Friction Losses are the equivalent loss of head through lines, elbows andtees. For most installations, this is generally between 2 and 5 ft. If long lineProprietary: - for the exclusive use of Amoco Production and other wholly owned subsidiaries of Amoco Corporation.

9.3

lengths and/or numerous elbows and tees are present, use a worksheet asshown in Table 9.1 to calculate the actual friction losses. An example calcula-tion is provided.

3. Using the pump performance curve for your pump (Appendix E, Pump Perfor-mance Curves), find the intersection of the total flow rate required (Step 1)and the total head required (Step 2). Choose the impeller size which corre-sponds to this point. If the intersection point falls between impeller sizes,choose the next larger impeller size.

4. Determine the required horsepower to drive the pump. Using the pump perfor-mance diagram for your pump, find the intersection point for the impeller sizedetermined in Step 3 and the total flow rate (Step 1). Read the correspond-ing horsepower required at this point. Interpolate between the horsepowercurves when necessary. This is the horsepower required to pump water. Forany mud weight, the required brake horsepower (BHP) is calculated by:

Centrifugal Pump Sizing Example

Problem:

Determine the pump requirements, given the following desilter arrangement:

12-50 gpm conesRequired head - 75 ftMaximum Mud Density - 10 ppgPiping System as shown in Figure 9.2

Solution:

1. Calculate Total Flow Rate

BHPmudmud ppg( ) BHPcurve

8.34--------------------------------------------------------------=

Q 12 cones 50 gpmcone-------------- 600 gpm= =



9.4

2. Calculate the pump discharge head:

A. Using a worksheet such as Table 9.1, list the length and size of each pipe,and the number and size of each fitting.

B. From Table 9.2, find the friction loss coefficients (C) for each item listed.C. Calculate the friction loss for each item.

D. Sum the friction losses to arrive at the total friction losses.

E. The total required head is the sum of the required hydrocyclone head, thefeet of lift and the friction losses.

3. From the Pump Performance Curves (Appendix E, Pump PerformanceCurves), select a pump which will provide the required head and flow rate. For this example, a Harrisburg Series 250 6 x 5 x 14 pump operating at1150 rpm with a 14 in. impeller will provide 95 ft of head at 600 gpm.

4. Determine the Horsepower required.

At 95 ft of head and 600 gpm, this pump will require 25 HP to pump water.Correcting for 10 ppg mud:

Figure 9.2 Centrifugal Pump Sizing Example.


BHPmudmud ppg( ) BHPcurve

8.34--------------------------------------------------------------=

10( ) (25)8.34

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9.5

Table 9.1 Detailed Worksheet for Pump Sizing

Equipment Information

Required Flow Rate Q (gpm) = 600Required Hydrocyclone Head HH (ft) = 75Feet of Lift HL (ft) = 6

Tabulation of Friction Losses

N is the length of pipe (ft) or number of Fittings

N Pipe Length or Fitting Type Size (in.)= Friction Loss (ft)

1 Extended Entrance 6 = .02

10 Suction Line 6 = .24

15 Discharge Line 5 = .87

2 Short Elbows 5 = .93

Total Friction Loss, HF (ft) = 2.06Total Required Head, HT (ft)

HH + HL + HT = HT

75 + 6 + 2 = 83 ft

30 HP=

N C QR2

1 106-----------------------------

1( ) 0.664( ) 600( )2

1 106-------------------------------------------

10( ) 0.0664( ) 600( )2

1 106-------------------------------------------------

15( ) 0.1612( ) 600( )2

1 106-------------------------------------------------

2( ) 1.29( ) 600( )2

1 106----------------------------------------

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9.6

Table 9.2 Friction Loss Coefficients for Pipe Fittings2

Nominal Pipe Size

Actual Inside

Diameter (Inches)

Straight Pipe1

e e Y

Welding Ell 90Standard Weight

Short Long

For 45 EllsUse 65% of Values

Given Below

20

-

-

-

-

8 --

-

-

-

-

562911

37227

3.841.29.66

2.19.81.40

.203

.061

.038

.125

.039

.023

RD

= 1 RD

= 1.5Std Wt

Gate Valve FULLY OPEN

Long Ell Threaded

45Ell Threaded

Cone Entrance

Standard Tee

Threaded

Square Entrance

90 Ell Threaded

Reduced Tee

Threaded

Extended Entrance

Square Ell

Standard Tee

Threaded

Butterfly Valve FULLY OPEN

Swing Check Valve FULLY OPEN

GlobValvFULLOPEN

1/23/4

.622

.82410,0482,300

4,019920

8,0342,300

11,0533,220

17,0824,830

33,15896,600

-

-

50,24016,100

190,9152,90

11-1/41-1/2

1.0491.3801.610

62814767

37710361

879264148

1,130338182

1,632514276

3,3261,028546

-

-

-

5,0201,616877

18,195,7293,035

22-1/2

3

2.0672.4673.068

18.77.32.3

18.77.34.6

56229.1

74.729.411.4

9344

18.3

1878834

20581

18.3

28014053

1,083507196

456

4.0265.0476.065

.5485

.1612

.0664

1.10.48.20

2.741.13.53

3.841.29.66

5.492.101.00

10.974.031.99

4.941.13.73

17.66.453.19

62.022.911.3

81012

7.98110.02012.000

.0156

.0047

.0019

.062

.024

.011

.172

.061

.030

.203

.080

.038

.312

.118

.057

.624

.235

.114

.203

.075

.042

.515

.197

.095

3.491.32.64

1. Based on pipe friction factor, f = .047. For other function factors, C = [31,088 f/d5] * L where L has units of feet and d has units of inches.2. Adapted from IADC Mud Equipment Manual, Handbook 4, Centrifugal Pumps and Piping Systems.


Estimating Impeller Size

In many instances, we are dealing with existing equipment and need to determinethe pump impeller size to estimate output capacity. The impeller size of a centrifu-Proprietary: - for the exclusive use of Amoco Production and other wholly owned subsidiaries of Amoco Corporation.

9.7

gal pump can be estimated by the following procedure:

1. The fluid density, pump rpm, a valve on the pump discharge and an accuratepressure gauge between the pump and valve are required.

2. With the pump running, close the discharge valve and read the pressure.Note: Limit time to less than 3 minutes.

3. Convert pressure read in Step 2 to head (feet).

4. Plot the head from Step 3 on the pump performance curve for 0 gpm. Esti-mate the effective impeller size.

Pipe Sizing

As was evident in the centrifugal pump sizing example, the pipe diameter and thedesign of the piping system will affect the size of the pump and the horsepowerrequirements. Suction and discharge lines should be as short as practical andsized to flow at velocities in the range of 5 to 10 ft/s. Low velocities will allow solidsto drop out in the lines. High velocities erode elbows and cause distribution prob-lems in the hydrocyclone manifold. Inadequate suction line size can cause cavita-tion in the pump. Also, the suction line should have no elbows, tees or reducerswithin 3 pipe diameters of the pump suction flange.

Pipe velocity can be calculated using the following equation:

where:

Q = flow rate, in gal/mindi = inside diameter of the pipe, in inches

As a quick reference, the maximum and minimum recommended flow rates forcommon pipe diameters are listed in Table 9.3.

v ft s( ) Q2.48( ) di

2

---------------------------=


Pr

Table 9.3 Recommended Flow Rates for Pipe

Nominal Pipe Diameter

Recommended Flow Rates, gpmoprietary: - for the exclusive use of Amoco Production Company and other wholly owned subsidiaries of Amoco Corporation.

9.8

Suction Head Requirements (NPSH) The suction line of the pump must be submerged to prevent vortexes in the suc-tion tank or air locking of the pump. Centrifugal pumps require a net positive suc-tion head (NPSH) to prevent cavitation and subsequent damage to the pump. TheNPSH required is a function of the pump design and the flow rate. NPSH curvesare included on the Centrifugal Pump Performance figures to find the minimumNPSH needed. The amount of NPSH available must then be determined.

As a shortcut, the minimum submergence for 6 in. and 8 in. suction lines as afunction of flow rate is provided in Figure 9.3. These curves may be used for mostapplications where the suction line is short and straight.

If the intersection of your submergence depth and flow rate fall near the line, adetailed determination of suction head should be made using the following equa-tion:

Schedule 40 Minimum@ 4 ft/s

Maximum@ 10 ft/s

3/4 7 161 12 26

1-1/4 20 471-1/2 26 63

2 45 1052-1/2 60 150

3 95 2303-1/2 130 310

4 160 4005 260 6256 360 9008 650 155010 1000 255012 1400 3500

NSPH ft( ) Patm0.052 m---------------------------= dsubmergence

Pvapor0.052 m---------------------------

Vs2

2g------ Hfs+

Centrifugal Pumps & PipingProprietary: - for the exclusive use of Amoco Production and other wholly owned subsidiaries of Amoco Corporation.

9.9

where:

Patm = uncorrected barometric pressure, psi (Figure 9.4)dsubmergence = height from pump suction to fluid level, ftPvapor = vapor pressure of liquid, psi (Figure 9.5)pm = mud density, lb/galvs = velocity of suction line fluid, ft/sg = gravitational constant = 32 ft/s2Hfs = friction head losses in the suction line, ft

NPSH Example

From the previous example, the required NPSH from the Pump PerformanceCurves for our flowrate and impeller size is approximately 4.5 ft. Head loss in thesuction line were calculated in the worksheet example to be 0.26 ft.

If the rig elevation is 1000 ft and the mud circulating temperature is 100oF., theavailable NPSH is determined as follows:

Figure 9.3 Minimum Suction Line Submergence. Points below or right of the lines should be avoided.


Pr

1. Patm = 14.2 psia (from Figure 9.4)

2. dsubmergence = 6 ft

3. Pvapor = 0.95 psia (from Figure 9.5)oprietary: - for the exclusive use of Amoco Production Company and other wholly owned subsidiaries of Amoco Corporation.

9.10

4.

5. The available NPSH is:

Since only 4.5 ft is required, there is sufficient NPSH available.

Suction Line Entrance

A properly-designed entrance will minimize friction loss, reduce air entrainmentand will reduce the amount of dead volume before suction is lost. Various designsare compared in Figure 9.6.

Installation and Operating Guidelines

1. Eliminate manifolding wherever possible.

2. Keep air out of the mud by degassing, having adequate suction line submer-gence and installing baffles to break vortices.

3. Do not restrict flow on the suction side of centrifugal pumps.

4. Install a pressure or head gauge between the pump and the first valve.

5. Do not completely close off discharge for more than 3 minutes.

6. Suction and discharge lines should be as short and straight as practical.

7. Size lines to achieve velocities of 5 - 10 feet per second.

8. Install pumps to run with flooded suctions. Check NPSH.

9. Check direction of rotation.

v Q2.48( ) di

2

---------------------------

4002.48 6.0562-------------------------------- 4.38 ft/s= = =

NPSH ft( ) 14.20.052 10-------------------------= 6 0.95

0.05 10----------------------

4.38( )22 32.4------------------- 0.64+ 30.54 ft=


10. To reduce start up load on the electric motor, start the pump with the dis-charge valve partially open, then open fully once the pump is up to speed.This will also reduce shock loading on the downstream equipment.Proprietary: - for the exclusive use of Amoco Production and other wholly owned subsidiaries of Amoco Corporation.

9.11

Figure 9.4 Elevation vs. Barometric Pressure.



9.12

Figure 9.5 Vapor Pressure as a Function of Fluid Temperature.Fluid Temperature, F

Centrifugal Pumps & PipingProprietary: - for the exclusive use of Amoco Production and other wholly owned subsidiaries of Amoco Corporation.

9.13

Figure 9.6 Pump Suction Pipe Entrances. The recommended designs reduce friction loss and air entrainment.


Pr

Summary

A centrifugal pump provides constant head, which is ideal for the low pres-sure, high flow rate requirements of hydrocyclones and mixing systems. Cen-oprietary: - for the exclusive use of Amoco Production Company and other wholly owned subsidiaries of Amoco Corporation.

9.14

trifugal pumps are constructed of a cast-steel housing with cast-iron internalparts. Hardfacing on the high-wear areas and tungsten carbide seals are rec-ommended.

Centrifugal pumps must be sized to provide the required head. Charts of headversus flow rate for the most common centrifugal pumps supplied in AppendixE, Pump Performance Curves. A procedure to correctly size centrifugalpumps is outlined in this section.

Suction and discharge piping should be short as possible to reduce frictionlosses. The piping should be sized to flow at velocities in the range of 5 to10 ft/s to prevent solids settling or erosion problems. Tables and charts aresupplied to estimate the friction losses in pipe and fittings.

The suction line of the pump must be submerged to prevent vortexes in thesuction and subsequent air locking of the pump. Guidelines are presented fordetermination of minimum submergence depth. Designs for suction lineentrances are also illustrated.

Chapter 9. Centrifugal Pumps & PipingPrinciple of OperationSizing Centrifugal PumpsCentrifugal Pump Sizing ExampleEstimating Impeller Size

Pipe SizingSuction Head Requirements (NPSH)NPSH ExampleSuction Line Entrance

Installation and Operating GuidelinesSummary

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Chap09 Centrifugal Pumps & Piping