Process Engg.design Guide _pumps

Issued Book N Chapter N Rev. Page

GUIDE DOCUMENT GE 312 1.1.2. VIII 1 2/22

PROCESS ENGINEERING DESIGN GUIDE S.S. 1.2 : Process Equipment PART 1 SECTION 1 CHAPTER VIII Pumps PROCESS MANUAL (DATA BOOK)

GE 1 - ANG - rev. 0

CONTENTS

1. PUMP SELECTION

1.1. Pump selection 1.2. Flow control

2. CENTRIFUGAL PUMPS

2.1. Incidence of main operating parameters

2.1.1. Modification of impeller's speed (n, rpm) 2.1.2. Modification of impeller's diameter 2.1.3. NPSH

2.2. Main features of centrifugal pumps

2.2.1. Pumps operating in parallel 2.2.2. Rating point 2.2.3. Minimum flow 2.2.4. Materials of construction

2.3. Selection of centrifugal pumps 2.4. Power estimation

2.4.1. Shaft power 2.4.2. Pump efficiency 2.4.3. Viscosity correction factor 2.4.4. Electrical motor efficiency 2.4.5. Criteria for motor selection 2.4.6. Power at reduced capacity 2.4.7. Pump suction specific speed Nss

3. OTHER TYPES OF PUMPS

4. IMPACT OF DISSOLVED GASES ON THE NPSH




GE 1 - ANG - rev. 0 - EG-312-1128.doc

1. PUMP SELECTION

1.1. Pump selection diagram

Refer to the selection chart enclosed overleaf.

The borderlines of the various areas are conservative. They may be exceeded after Mechanical Department approval or based on Vendors data.

For reciprocating and rotary pumps, also refer to the tables given further in this chapter.

As a general rule, centrifugal pumps should always be preferred whenever they can be used, since they tend to be cheaper and more reliable.

They should generally not be considered in the following cases :

High viscosity (> 400 cSt) High differential head at low flowrate.

1.2. Flow control

Whatever the type of pump is, the flow can always be controlled by adjusting the speed ; at a fixed speed, flow can be often adjusted through control valve, throttling valve, etc.




GE 1 - ANG - rev. 0 - EG-312-1128.doc

PUMP SELECTION DIAGRAM




GE 1 - ANG - rev. 0 - EG-312-1128.doc

2. CENTRIFUGAL PUMPS

2.1. Incidence of main operating parameters

2.1.1. Modification of impellers speed (n, rpm)

For a given geometry :

The flowrate is directly proportional to n The head H is directly proportional to n2 The power P is directly proportional to n3 (at fixed efficiency) The efficiency is not affected significantly by a speed modification, excepted for

very low speeds where it slightly decreases.

2.1.2. Modification of impeller's diameter

At a given speed

The Impeller shape offers a constant cross sectional area (exit velocity triangles remain similar) :

- The flowrate is directly proportional to D - The head H is directly proportional to D2 - The power P is directly proportional to D3 (at fixed efficiency).

The cross sectional area through the impeller increases with the diameter :

- The flowrate is directly proportional to D2 - The head H is directly proportional to D2 - The power P is directly proportional to D4 (at fixed efficiency) - The efficiency slightly decreases when the impeller's diameter is reduced.

2.1.3. NPSH

It is directly proportional to n2, but does not vary with the diameter of the impeller (provided that the diameter reduction is less than 20%). The use of inducers allows to decrease the NPSH required.

n : Impeller speed (revolution per minute, rpm).




GE 1 - ANG - rev. 0 - EG-312-1128.doc

2.2. Main features of centrifugal pumps

2.2.1. Pumps operating in parallel

The performance curves should be drooping as much as possible to provide a good operating stability. This can be achieved if the angle between impeller blades is less than 18, which entails a slight reduction in pump efficiency.

2.2.2. Rating point

The pump rating point shall be located as close as possible from the maximum efficiency, but should not go beyond in terms of flowrate.

2.2.3. Minimum flow

A minimum flow shall always be maintained in a centrifugal pump. This may require a manual or automatic recirculation by-pass (restriction orifice, valve or "schroeder" check valve).

The minimum flow is specified by the pump supplier. It is usually 20 to 30% of the nominal flow, excepted for high-speed pumps, for which performance curves are bell-shaped and the minimum flow can be up to 50% of normal flow.

2.2.4. Materials of construction

Refer to the table enclosed overleaf.

2.3. Selection of centrifugal pumps

The main characteristics of various types of centrifugal pumps are summarized in the following table :

Centrifugal pumps

Flowrate

(m3/h)

Viscosity

(cSt)

Differential pressure

(bar)

Current speed (rpm.)

Efficiency

(%)

NPSH Required

(m)

Flow control

Sensitivity to solid impurities

Single-stage pump 2000 < 500 15 1500 10 to 80 1 to 4 Valve Low

Single-stage pump 1500 < 500 35 3000 10 to 80 1 to 5 Valve Low

Single-stage Sundyne pump

90 < 500 180 20000 max. 20 to 60 1 to 5 Valve Moderate

Double-stage pump 250 < 500 35 3000 10 to 75 2 to 5 Valve Moderate

Multi-stage pump 1500 < 500 140 3000 10 to 80 3 to 8 Valve Moderate




GE 1 - ANG - rev. 0 - EG-312-1128.doc

MATERIAL SELECTION TABLE FOR CENTRIFUGAL PUMPS

G-2 API Standard 610 (8th Edition August 1995)




GE 1 - ANG - rev. 0 - EG-312-1128.doc


H-2 API Standard 610 (8th Edition August 1995)




GE 1 - ANG - rev. 0 - EG-312-1128.doc


H-3 API Standard 610 (8th Edition August 1995)




GE 1 - ANG - rev. 0 - EG-312-1128.doc

2.4. Power estimation

2.4.1. Shaft power

)efficiencys'pump(x36)bar(Pxh/3m(Q

)kW(P

=

2.4.2. Pump efficiency

It can be estimated based on the chart enclosed further on.

2.4.3. Viscosity correction factor

It can be estimated based on the chart enclosed further on.

The impact on pump efficiency can be significant above 5 cSt, especially in the case of low flow and low pump head.

2.4.4. Electrical motor efficiency

It can be estimated based on the table enclosed further on.

The motor efficiencies are given for a 75% load (from maximum power), which is usually the case.

For motors exceeding 15 kW, the efficiency increases by 0.5% at full load and decreases by 2% at half load.

2.4.5. Criteria for motor selection

A standard nominal power shall be selected for the motor. The nominal power will be the standard nominal power immediately above the absorbed power at design conditions.

API 610 selection criteria shall be followed, i.e. the ratio of motor nominal power and pump shaft power at rating point shall not be less than :

1.25 if the shaft power at rating point is < 22 kW 1.15 if the shaft power at rating point is from 22 to 55 kW 1.1 if the shaft power at rating point is > 55 kW.




GE 1 - ANG - rev. 0 - EG-312-1128.doc

When the pump is specified for open-valve start-up, the absorbed power at maximum flow (end of the curve flow / head) shall be considered to select the motor, excepted for high specific velocity pumps for which the maximum absorbed power corresponds to minimum flow.

Practically, to calculate the power at maximum flow (end of the curve flow / head), the Mechanical Department will consider 125% of the flow at B.E.P. (Best Efficiency Point).

The standard nominal powers of electric motors are given alongside with motor efficiency values in the table enclosed further on.

Generally, low voltage motors (380 V) are used so long as the nominal power does not exceed 132 kW and high voltage motors (5500 V) for higher values. However, the limit between low and high voltage tends to increase, and a limit of 160 or even 200 kW may be considered. This is usually defined in a Project general specification.

Examples of calculations :

The shaft power required for a pump at design conditions is equal to 7 kW. From the table given further on, we obtain :

Motor efficiency : 85% for a 7.5 kW motor Motor efficiency : 86% for a 11 kW motor.

For the 7.5 kW motor, the absorbed power is 7 / 0.85 = 8.2 kW. This motor is not acceptable because the absorbed power (8.2) exceeds the nominal power (7.5).

For the 11 kW motor, the absorbed power is 7 / 0.86 = 8.1 kW. This motor is acceptable because the absorbed power (8.1) is lower than the nominal power (11).

Checking of the API 610 criteria

For the 11 kW motor, the ratio (motor nominal power / pump shaft power) equals 11 / 7 i.e. 1.57 > 1.25. The criteria is met.




GE 1 - ANG - rev. 0 - EG-312-1128.doc

Example n 2

The shaft power required for a pump at design conditions is equal to 12.2 kW. From the table given further on, we obtain :

Motor efficiency : 87 % for a 15 kW motor Motor efficiency : 88 % for a 18.5 kW motor.

For the 15 kW motor, the absorbed power is 12.2 / 0.87 = 14 kW. This motor is acceptable because the absorbed power (14) is lower than the nominal power (15). For the 18.5 kW motor, the absorbed power is 12.2 / 0.88 = 13.9 kW. This motor is acceptable because the absorbed power (13.9) is lower than the nominal power (18.5).

Checking of the API 610 criteria

For the 15 kW motor, the ratio (motor nominal power / pump shaft power) equals 5 / 12.2 = 1.22 < 1.25. The criteria is not met.

For the 18.5 kW motor, the ratio (motor nominal power / pump shaft power) equals 18.5 / 12.2 = 1.5 > 1.25. The criteria is met.

2.4.6. Power at reduced capacity

The power at reduced capacity will be estimated by the following method :

w Calculation of the power P at design flow Y w For a reduced flow x, the power P' will be :

+=

Y2

YxP'P

2.4.7. Pump suction specific speed nss

Nss = N x Q0.5 / (NPSH req

3/ 4)

where : N : Rotating speed of the pump rpm. Q : Flow by suction intake m3/h NPSHreq : NPSH required for the pump m

This criteria is used by the Mechanical Department for pump selection when a maximum value is imposed by the Client ; If not, it is only used as a guideline.




GE 1 - ANG - rev. 0 - EG-312-1128.doc

The Nss is calculated for the flow giving the best efficiency with the impeller maximum diameter. This means that for a same service, two pump suppliers will usually provide different Nss depending on the position of the operating point.

In some particular cases, an increase of the NPSH required (consequently of the NPSH available) can be desirable to get an acceptable Nss value. But this case is uncommon because few suppliers can propose two NPSH for a same pump model (different wheels for a same pump casing).

The Process Department does not have to verify this criteria ; It has to be done by the Mechanical Department.




GE 1 - ANG - rev. 0 - EG-312-1128.doc




GE 1 - ANG - rev. 0 - EG-312-1128.doc

Extract of Standard of the Hydraulic Institute, New York, USA 1955




GE 1 - ANG - rev. 0 - EG-312-1128.doc

EFFICIENCY OF ELECTRICAL MOTORS AT 75% LOAD

380 V 5500 V

Nominal P Efficiency Nominal P Efficiency kW % kW %

0.75 70 160 91 1.1 73 200 92 1.5 74 290 93 2.2 77 400 93.5 3 79 500 94 4 80 750 94.5 7.5 85 1 000 95 11 86 1 500 95.5 15 87 2 000 96 18.5 88 3 500 96.5 22 89 5 000 97 30 90 > 5000 97 37 90.5 45 91 55 91.5 75 92 90 92.5 110 93 132 93.5




GE 1 - ANG - rev. 0 - EG-312-1128.doc

3. OTHER TYPES OF PUMPS

The main features of reciprocating and rotary pumps are summarized on the tables enclosed further on.

Unless otherwise specified, the maximum flows indicated are limit values that may not be compatible with the maximum differential heads. For border cases, the Mechanical Department shall be consulted.

NPSH of reciprocating pumps

For reciprocating pumps, the pumped fluid is pulsed in the suction line. To produce this acceleration, a certain amount of energy is required. This energy, which adds to the frictional losses, is called "acceleration height". The NPSH available for a reciprocating pump can subsequently be calculated by the following formula :

NPSHa = (HA HB) + ( ) ( )[ ] 5.02AB2AB5VA HP10xxgTP(

+

Equation 1 (Note 1)

HA and HB : Height in m PA : Suction pressure in bar a TV : Vapour pressure of the pumped liquid in bar a g : Gravity factor in m / s2

r : Density in kg / m3

D PAB : Pressure drop due to friction in m HAB : Acceleration head




GE 1 - ANG - rev. 0 - EG-312-1128.doc

For centrifugal pumps, HAB is nil and DPAB does not depend on the pump. The NPSHa is independent of the selected pump.

For a reciprocating pump, HAB and DPAB depend on the pump, the NPSH available is dependent on the selected pump :

D PAB : Pressure drop due to friction, is calculated for the maximum liquid velocity HAB : Acceleration head, depends on the pumping rate.

During the preparation of the process specification, the characteristics of the pump are unknown and the NPSHa (from equation 1) cannot be calculated. The NPSH given in the process specification is the NPSHa for a non-pulsed flow, i.e. :

The NPSH available calculated is the one calculated as for a centrifugal pump, (HAB nil and D PAB calculated for an average velocity which corresponds to the average flow indicated in the specification).

Only after pump selection, provided that the piping layout at pump suction is defined, the NPSH available for the selected pump can be given. Subsequently, the following note is written on the process specification :

"The Mechanical Department must check with the supplier that the NPSH available corresponding to the pump is compatible with its NPSH required."

D PAB calculation :

The pressure drop due to friction is calculated as for a centrifugal pump, but the maximum liquid velocity, corresponding to the maximum instantaneous flow of the liquid in the suction pipe, has to be considered. Depending on the type of pump selected, the maximum instantaneous flow can be calculated by multiplying the average design flow by the following factor :

Single-acting pump

Double-acting pump

Simplex 3 2 Duplex 2 1.5 Triplex 2 1.3 Quadruplex 1.5 1.3 Others 1.3 1.3

Note - If a pulsation dampener is installed on the suction line, the factor to consider is 1.2 whatever the type of pump is.




GE 1 - ANG - rev. 0 - EG-312-1128.doc

HAB Calculation :

2AB D

LxNxQxKx712.0H = Equation 2 (Note 2)

Where :

HAB : In m Q : Average design flow in l/min N : Number of rpm D : Pipe diameter in mm L : Pipe length in m K : Factor depending on the type of the pump

Single-acting pump

Double-acting pump

Simplex 1.00 0.50 Duplex 0.50 0.29 Triplex 0.166 0.166 Quadruplex 0.125 0.10 Others 0.10 0.10

In case the suction line comprises several piping diameters, HAB must be calculated for each section, and the results added.

The equation 2 above derives from the following assumptions :

The motion generated by the pump is assumed to be a harmonical one, The fluid is assumed to be incompressible.

The first assumption is never realised. Usually, the motion generated by the connecting rod-crank assembly exceeds the maximum values of the harmonic curve. This excess depends on the ratio r = (length of the connecting rod / length of the stroke), which commonly ranges from 1.5 to 2.

The maximum acceleration head must be increased by 25 to 35%.

The second assumption is checked when the suction pressure is close to the atmospheric pressure, which is usually the case for reciprocating pumps.




GE 1 - ANG - rev. 0 - EG-312-1128.doc

Remark : When a pulsation dampener is installed on the suction line, the acceleration head between the pulsation dampener and the pump must be calculated from equation 2. The acceleration head between the drum and the pulsation dampener will be taken as 10% of the calculated value from equation 2. The two heads have to be added to obtain the total acceleration head.

Notes

1. There is a phase shift between the pressure drop due to friction D PAB and the pressure drop caused by acceleration HAB (acceleration is maximum when the velocity is minimum). This phenomenon is taken into account by the term [(PAB)2 + (HAB)2]05 in equation 1.

2. Some sources indicate equation 2 as being unrealistic when the suction line is very long.

4. IMPACT OF DISSOLVED GASES ON THE NPSH

The occurrence of dissolved gas in the liquids dramatically reduces the NPSH really available at the pump suction.

It is then necessary to take some margins into account, deriving from past experience. These margins shall be considered in the PDS and shall be clearly indicated to prevent any misunderstanding with the Mechanical Department.

The following recommendations can be found in the literature :

BFW pumps : divide by 1.25 Other pumps : divide by 2 or consider liquid at bubble point.




GE 1 - ANG - rev. 0 - EG-312-1128.doc

MAIN FEATURES OF RECIPROCATING PUMPS

Type of reciprocating pump

Flow

m3/h

Viscosity

cSt

Differential

pressure () bar

Usual

velocity rpm

Global

efficiency %

NPSH

required m

Remarks

Direct acting steam pump

100 (simplex) 250 (duplex)

1800 20 to 350 usually 700 and more possible

0-70 45-80 3-4 Speed adjustment by steam throttling. Max. temperature: 400C

Controlled piston pump (or plunger)

300 (multiplex) 1800 350 to 750 usually 2500 possible

20-450 55-85 4-5 Flow control : 0-100% Max. temp. : about 500C

Diaphragm pump with hydraulic control

20 by head 1000 350 usually 3000 possible (metal

diaphragm)

150-200 80 4-5 Max. temperature : 500C min. Temperature : -70C Very good tightness. Flow control : 0-100%. Typical use : loaded liquids. High viscosities.

Diaphragm pump with pneumatic control

50 1800 7 0-40

2 to 4 Max. temperature : 100C Applicable to loaded liquids (up to 90%)

Dosing pump : Maxi range Usual range

0-10 0-3

250000 500 3000

Possible

50-200 20 4-5 Extreme temperature : - 5000C max. - 2000C min. Flow control : 0-100% Flow accuracy : +/- 0.5%

() the maximum values indicated for flow and differential head may not be simultaneous.




GE 1 - ANG - rev. 0 - EG-312-1128.doc

MAIN FEATURES OF ROTARY PUMPS

Type of rotary pump

Flow (1)

m3/h

Viscosity

cSt

Differential pressure (1)

bar

Usual velocity

rpm

Global efficiency

%

NPSH required

m

Remarks

Gear 200 10 x 106 250 0-3000 50-90 (2) 3-4 Max. temperature : 3500C

Screw : - 2 screws - 3 screws

1000-2000 1000

1 x 106 2 x 106

200 200

2000 up to 15000

50-90 50-90 (2)

3-4 3-4

Max. temperature : 3000C

Vanes 400 1 x 106 25 0-1500 Max. temperature : 2600C

Hydraulic pumps : 200 bar, 20 m3/h, 2000 rpm.

Pistons up to 50 (3) up to 700 2000-3000 (3) These pumps are used in hydraulic systems for which the fluids have a 500/1000 cSt viscosity.

Deformable rotor 20 20000 2-4 200-1800 10 to 30 Max. temperature : 800C

Off centre Screw 200 1 to 200000 50 0-1500 50 1-5 Max. temp. : 80-900C Can handle erosive particles.

Lobes 200 1 to 200000 5-20 < 700 30-40 1-5 Max. temperature : 1200C

Deformable stator 20 25000 5-15 0-200 30-50 Max. temperature : 950C Can handle erosive particles.

Mouvex 120 3-5 250-1400 45-50 Max. temp. : 250-3000C

(1) The maximum values given may not be simultaneous. (2) These efficiencies commonly exceed 70% when the operating conditions (differential pressure + flow + viscosity) are in the

optimal range for the pump selected.

Process Engg.design Guide _pumps

Documents

Transcript of Process Engg.design Guide _pumps