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SchlumbergerDowell

SOLIDS CONTROL HANDBOOKCentrifugal Pumps and Piping

Section 900

January 1998

Page 1 of 14

CONFIDENTIAL

Centrifugal Pumps and Piping1 Introduction.........................................................................................................................2

2 Principle of Operation.........................................................................................................2

3 Sizing Centrifugal Pumps...................................................................................................33.1 Centrifugal Pump Sizing Example..................................................................................4

3.2 Estimating Impeller Size ................................................................................................8

4 Pipe Sizing...........................................................................................................................84.1 Suction Head Requirements (NPSH).............................................................................9

4.1.1 NPSH Example ..................................................................................................12

4.1.2 Suction Line Entrance ........................................................................................13

5 Installation and Operating Guidelines.............................................................................13

6 Summary............................................................................................................................14

FIGURESFig. 1. Typical centrifugal pump. .............................................................................................3Fig. 2. Centrifugal pump sizing example. ................................................................................4Fig. 3. Minimum suction line submergence. ..........................................................................10Fig. 4. Elevation vs. barometric pressure. .............................................................................11Fig. 5. Vapor pressure as a function of fluid temperature. .....................................................12Fig. 6. Pump suction pipe entrances. ....................................................................................13

TABLESTable 1 Detailed Worksheet for Pump Sizing..........................................................................6Table 2 Friction Loss Coefficients for Pipe Fittings..................................................................7Table 3 Recommended Flow Rates for Pipe ...........................................................................9

Section 900

January 1998

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SOLIDS CONTROL HANDBOOKCentrifugal Pumps and Piping Schlumberger

Dowell

CONFIDENTIAL

1 Introduction

Centrifugal pumps are ideal for the low pressure, high flow rate requirementsof hydrocyclones and mixing systems. Unlike constant-volume piston pumps,centrifugal pumps provide constant head. Consequently, the pump andassociated piping system must be correctly sized and designed to deliver therequired flow rate and desired head. This section briefly describes howcentrifugal pumps work and provides guidelines for the design, installationand operation of centrifugal pumps and piping systems.

2 Principle of OperationThe centrifugal pump consists of a rotating impeller mounted inside a casing(Fig. 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 curvedimpeller vanes. The accelerated fluid exits the impeller and enters the pumpcasing where this kinetic energy is converted into pressure energy. Althoughthe pump can operate against a closed discharge valve, it is notrecommended. When there is no flow, all the pump power is dissipated intothe fluid. This will cause the pump and motor to quickly overheat.

A drive shaft connected to the impeller transmits power from the driver. Astuffing box or seal is normally used to prevent leakage. The most commondriver for centrifugal pumps is the a.c., fixed-speed, induction motor.Variable-speed motors are available, but rarely required for drilling rigapplications. The motor is joined to the pump shaft by a flexible coupling.Drivers are usually three-phase motors. The rotation of the pump should bechecked when it is installed to make sure that it is rotating in the properdirection.

Centrifugal pumps are usually constructed of a cast-steel housing with cast-iron internal parts. Long-life packages offer hard-facing on the high-wearareas of the pump. Wear-resistant tungsten carbide seals are also available.Both are highly recommended.

The pump performance curves in Appendix E, Pump Performance Curves,illustrate that the head generated by centrifugal pumps decreases very littleas the flow rate is increased. Conversely, the flow rate throughhydrocyclones is not affected much by head. Note, however, thathydrocyclones are designed to operate at a certain amount of head. Less ormore may be detrimental to their performance. Therefore, the pump shouldbe sized to provide the correct head at the flow rate dictated by thehydrocyclones.

SchlumbergerDowell


Section 900

January 1998

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CONFIDENTIAL

Fig. 1. Typical centrifugal pump.Note: Kinetic energy is converted into pressure energy by the rotating impeller

vanes to provide consistent head.

3 Sizing Centrifugal Pumps1. Determine the total flow rate needed. For a hydrocyclone manifold, the

flow rate is calculated by:

Q (gpm) = # of Cones x Flow Capacity/Cone2. Determine the total head required. For most hydrocyclones, the

required inlet head is 75 ft. The total head required from the pump is:

Ht = 75 ft + Lift Height (ft) + friction Losses (ft) where:

Lift Height is the height between the hydrocyclone manifold and themud surface (not the pump suction).

Friction Losses are the equivalent loss of head through lines, elbowsand tees. For most installations, this is generally between 2 and 5 ft. Iflong line lengths and/or numerous elbows and tees are present, use a

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January 1998

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CONFIDENTIAL

worksheet as shown in Table 1 to calculate the actual friction losses. Anexample calculation is provided.

3. Using the pump performance curve for your pump (Appendix E, PumpPerformance Curves), find the intersection of the total flow rate required(Step 1) and the total head required (Step 2). Choose the impeller sizewhich corresponds to this point. If the intersection point falls betweenimpeller sizes, choose the next larger impeller size.

4. Determine the required horsepower to drive the pump. Using the pumpperformance diagram for your pump, find the intersection point for theimpeller size determined in Step 3 and the total flow rate (Step 1).Read the corresponding horsepower required at this point. Interpolatebetween the horsepower curves when necessary. This is thehorsepower required to pump water. For any mud weight, the requiredbrake horsepower (BHP) is calculated by:

( )BHPmud mud ppg = x BHPcurve 8 34.3.1 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 Fig. 2

Fig. 2. Centrifugal pump sizing example.

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CONFIDENTIAL

Solution:1. Calculate Total Flow Rate

Q = 12 cones x 50 gpmcone

= 600 gpm

2. Calculate the pump discharge head:

A. Using a worksheet such as Table 1, list the length and size of eachpipe, and the number and size of each fitting.

B. From Table 2, find the friction loss coefficients (C) for each itemlisted.

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 hydrocyclonehead, the feet 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 flowrate.

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 pumpwater. Correcting for 10 ppg mud:

( )BHPmud mud ppg = x BHPcurve 8 34.( ) ( )

=

10 258 34.

= 30 HP

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Table 1 Detailed Worksheet for Pump Sizing

Equipment InformationRequired 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.) N

C QR2

1061

= FrictionLoss (ft)

1 Extended Entrance 6 ( ) ( ) ( )1 21

0.664 600

106

= .02

10 Suction Line 6 ( ) ( ) ( )10 21

0.0664 600

106

= .24

15 Discharge Line 5 ( ) ( ) ( )15 21

0.1612 600

106

= .87

2 Short Elbows 5 ( ) ( ) ( )2 21

1.29 600

106

= .93

Total Friction Loss, HF (ft) = 2.06Total Required Head, HT (ft)HH + HL + HT = HT75 + 6 + 2 = 83 ft

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Centrifugal Pumps and Piping

Section 900

January 1998

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CONFIDENTIAL

Table 2 Friction Loss Coefficients for Pipe Fittings 2

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3.2 Estimating Impeller SizeIn many instances, we are dealing with existing equipment and need todetermine the pump impeller size to estimate output capacity. The impellersize of a centrifugal pump can be estimated by the following procedure:

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

2. With the pump running, close the discharge valve and read thepressure. 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.

Estimate the effective impeller size.

4 Pipe SizingAs was evident in the centrifugal pump sizing example, the pipe diameterand the design of the piping system will affect the size of the pump and thehorsepower requirements. Suction and discharge lines should be as short aspractical and sized to flow at velocities in the range of 5 to 10 ft/s. Lowvelocities will allow solids to drop out in the lines. High velocities erodeelbows and cause distribution problems in the hydrocyclone manifold.Inadequate suction line size can cause cavitation in the pump. Also, thesuction line should have no elbows, tees or reducers within 3 pipe diametersof the pump suction flange.

Pipe velocity can be calculated using the following equation:

( ) ( )v ft s/ = Q

2.48 di2

where:

Q = flow rate, in gal/min

di = inside diameter of the pipe, in inches

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Section 900

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CONFIDENTIAL

As a quick reference, the maximum and minimum recommended flow ratesfor common pipe diameters are listed in Table 3.

Table 3 Recommended Flow Rates for Pipe

Nominal PipeDiameter

Schedule 40

Recommended Flow Rates,gpm

Minimum@ 4 ft/s

Maximum@ 10 ft/s

3/4 7 16

1 12 26

1-1/4 20 47

1-1/2 26 63

2 45 105

2-1/2 60 150

3 95 230

3-1/2 130 310

4 160 400

5 260 625

6 360 900

8 650 1550

10 1000 2550

12 1400 3500

4.1 Suction Head Requirements (NPSH)The suction line of the pump must be submerged to prevent vortexes in thesuction tank or air locking of the pump. Centrifugal pumps require a netpositive suction head (NPSH) to prevent cavitation and subsequent damageto the pump. The NPSH required is a function of the pump design and theflow rate. NPSH curves are included on the Centrifugal Pump Performancefigures to find the minimum NPSH needed. The amount of NPSH availablemust then be determined.

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As a shortcut, the minimum submergence for 6 in. and 8 in. suction lines asa function of flow rate is provided in Fig. 3. These curves may be used formost applications where the suction line is short and straight.

Fig. 3. Minimum suction line submergence.Note: Points below or right of the lines should be avoided.

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Section 900

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If the intersection of your submergence depth and flow rate fall near the line,a detailed determination of suction head should be made using the followingequation:

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

0.052 m -

Vs2

2g - Hfs

where:

Patm = uncorrected barometric pressure, psi (Fig. 4)dsubmergence = height from pump suction to fluid level, ft

Pvapor = vapor pressure of liquid, psi (Fig. 5)pm = mud density, lb/gal

vs = velocity of suction line fluid, ft/s

g = gravitational constant = 32 ft/s2

Hfs = friction head losses in the suction line, ft

Fig. 4. Elevation vs. barometric pressure.

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Fig. 5. Vapor pressure as a function of fluid temperature.

4.1.1 NPSH ExampleFrom the previous example, the required NPSH from the Pump PerformanceCurves for our flowrate and impeller size is approximately 4.5 ft. Head loss inthe suction 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 100F.,the available NPSH is determined as follows:

1. Patm = 14.2 psia (from Fig. 4)2. dsubmergence = 6 ft

3. Pvapor = 0.95 psia (from Fig. 5)

4. ( )v di=

Q2.48

ft / s

=

=

2400

2 48 6 05624 38

. .

.

5. The available NPSH is:

( ) ( )NPSH ft ft=

+

=

14 20 052 10

6 0 950 05 10

4 38 2

2 32 40 64 30 54.

.

.

.

.

.

. .

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

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Section 900

January 1998

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CONFIDENTIAL

4.1.2 Suction Line EntranceA properly-designed entrance will minimize friction loss, reduce airentrainment and will reduce the amount of dead volume before suction islost. Various designs are compared in Fig. 6.

Fig. 6. Pump suction pipe entrances.Note: The recommended designs reduce friction loss and air entrainment.

5 Installation and Operating Guidelines1. Eliminate manifolding wherever possible.

2. Keep air out of the mud by degassing, having adequate suction linesubmergence 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.

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9. Check direction of rotation.

10. To reduce start up load on the electric motor, start the pump with thedischarge valve partially open, then open fully once the pump is up tospeed. This will also reduce shock loading on the downstreamequipment.

6 Summary

A centrifugal pump provides constant head, which is ideal for the lowpressure, high flow rate requirements of hydrocyclones and mixingsystems. Centrifugal pumps are constructed of a cast-steel housingwith cast-iron internal parts. Hardfacing on the high-wear areas andtungsten carbide seals are recommended.

Centrifugal pumps must be sized to provide the required head. Chartsof head versus flow rate for the most common centrifugal pumpssupplied in Appendix E, Pump Performance Curves. A procedure tocorrectly size centrifugal pumps is outlined in this section.

Suction and discharge piping should be short as possible to reducefriction losses. The piping should be sized to flow at velocities in therange of 5 to 10 ft/s to prevent solids settling or erosion problems.Tables and charts are supplied to estimate the friction losses in pipeand fittings.

The suction line of the pump must be submerged to prevent vortexes inthe suction and subsequent air locking of the pump. Guidelines arepresented for determination of minimum submergence depth. Designsfor suction line entrances are also illustrated.

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