7/10/2016

Pump Sizing

This article will focus on calculating the process data required to size a pump:
- suction pressure
- discharge pressure
- NPSHa
- differential pressure and head
- shaft power
- design conditions

The function of a pump is move fluid (liquid, slurry) from Point A to Point B. The pump must deliver enough pressure (or head) to ensure the fluid can overcome frictional losses to achieve the desired arrival pressure at Point B.


In this example, we'll consider pumping hydrocarbon fluid from a scrubber to a separator. 

1) Suction pressure, Ps
Ps = Pa - ⌂Pf,s + ⌂Ph,s
where Pa = operating pressure at Point A, ⌂Pf,s = frictional losses in suction line, 
⌂Ph,s = static gain between Point A and pump suction nozzle

2) Discharge pressure, Pd
Pd = Pb + ⌂Pf,d + ⌂Ph,d
where Pb = operating pressure at Point B, ⌂Pf,d = frictional losses in discharge line, 
⌂Ph,d = static gain between Point B and pump discharge nozzle

3) NPSHa
Available Net Positive Suction Head is an important variable, always ensure that the required NPSH (NPSHr, determined by vendor) is less than NPSHa, else we risk cavitation to the pump. 

Cavitation is the damage to the pump due to presence of bubbles; hence vapor pressure, Pvap is an important variable too because that's the pressure when the first bubble forms. 

NPSHa = Ps - Pvap + ⌂Ph,s

4) Differential pressure and head, ⌂P and H
⌂P = Pd - Ps
H = ⌂P/ρg
where g = gravitational constant (9.81m/s)

5) Shaft power
There's a difference between hydraulic (absorbed) and shaft power: 

i) hydraulic: the energy transferred to the fluid to increase velocity and head
P = Q.⌂P/36

ii) shaft: power supplied by the motor, considering pump efficiency, ╖
P = Q.⌂P/36.╖

6) Design pressure
Typically a design pressure should already be assigned based on the max operating pressure and margin, so we should check that the selected design pressure is adequate for this pump.

Shut-off pressure is the maximum pressure at minimal flow (ie pump running dry). This value is determined by the vendor but as a good estimate, we can use the equation below:

Design P = 1.2 x Pshut-off

References:
1. API RP14E

*Feel free to comment/share your knowledge :)

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