How To Size A Pump

Posted on November 11, 2009

To size a pump, you must define:

  • The flow rate of liquid the pump is required to deliver
  • The total differential head the pump must generate to deliver the required flow rate

This is the case for all types of pumps: centrifugal or positive displacement.

Other key considerations for pump sizing are the net positive suction head available (NPSHa) and the power required to drive the pump.

Pump System Diagram

 Pump System Diagram

Flow Rate

Usually, the flow rate of liquid a pump needs to deliver is determined by the process in which the pump is installed.  This ultimately is defined by the mass and energy balance of the process.

For instance the required flow rate of a pump feeding oil into a refinery distillation column will be determined by how much product the column is required to produce.  Another example is the flow rate of a cooling water pump circulating water through a heat exchanger is defined by the amount of heat transfer required.

Total Differential Head

The total differential head a pump must generate is determined by the flow rate of liquid being pumped and the system through which the liquid flows.

Essentially, the total differential head is made up of 2 components.  The first is the static head across the pump and the second is the frictional head loss through the suction and discharge piping systems.

Total differential head = static head difference + frictional head losses

Static Head Difference

The static head difference across the pump is the difference in head between the discharge static head and the suction static head.

Static head difference = discharge static head – suction static head

Discharge Static Head

The discharge static head is sum of the gas pressure at the surface of the liquid in the discharge vessel (expressed as head rather than pressure) and the difference in elevation between the outlet of the discharge pipe, and the centre line of the pump.

Discharge static head = Discharge vessel gas pressure head + elevation of discharge pipe outlet – elevation of pump centre line

The discharge pipe outlet may be above the surface of the liquid in the discharge vessel or it may be submerged as shown in these 3 diagrams.

Pump Discharge Above Liquid Surface

Pump Discharge Above Liquid Surface

Submerged Pump Discharge Pipe

Submerged Pump Discharge Pipe

Discharge Pipe Enters The Bottom Of The Vessel

Discharge Pipe Enters The Bottom Of The Vessel

Suction Static Head

The suction static head is sum of the gas pressure at the surface of the liquid in the suction vessel (expressed as head rather than pressure) and the difference in elevation between the surface of the liquid in the suction vessel and the centre line of the pump.

Suction static head = Suction vessel gas pressure head + elevation of suction vessel liquid surface – elevation of pump centre line

Note: gas pressure can be converted to head using:
Gas head = gas pressure ÷ (liquid density x acceleration due to gravity)

Pump Suction

Pump Suction

Frictional Head Losses

The total frictional head losses in a system are comprised of the frictional losses in the suction piping system and the frictional losses in the discharge piping system.

Frictional head losses = frictional losses in suction piping system + frictional losses in discharge piping system

The frictional losses in the suction and discharge piping systems are the sum of the frictional losses due to the liquid flowing through the pipes, fittings and equipment.  The frictional head losses are usually calculated from the Darcy-Weisbach equation using friction factors and fittings factors to calculate the pressure loss in pipes and fittings.

Darcy-Weisbach equation:

Darcy-Weisbach Equation

In order to calculate the frictional head losses you therefore need to know the lengths and diameters of the piping in the system and the number and type of fittings such as bends, valves and other equipment.

Net Positive Suction Head Available

The net positive suction head available (NPSHa) is the difference between the absolute pressure at the pump suction and the vapour pressure of the pumped liquid at the pumping temperature.

It is important because for the pump to operate properly, the pressure at the pump suction must exceed the vapour pressure for the pumped fluid to remain liquid in the pump.  If the vapour pressure exceeds the pressure at the pump suction, vapour bubbles will form in the liquid.  This is known as cavitation and leads to a loss of pump efficiency and can result in significant pump damage.

To ensure that the pump operates correctly the net positive suction head available (NPSHa) must exceed the net positive suction head required (NPSHr) for that particular pump.  The NPSHr is given by the pump manufacturer and is often shown on the pump curve.

Net positive suction head available = absolute pressure head at the pump suction – liquid vapour pressure head

Pump Power

Pumps are usually driven by electric motors, diesel engines or steam turbines.  Determining the power required is essential to sizing the pump driver.

Pump power = flow rate x total differential head x liquid density x acceleration due to gravity ÷ pump efficiency

 How To Size A Pump Example

Let’s look at an example to demonstrate how to size a pump.

30000 kg/hr of water needs to be pumped from one vessel to another through the system shown in the diagram below.  The water is at 20C, has a density of 998 kg/m3 , a vapour pressure of 0.023 bara and a viscosity of 1cP.  We’ll assume that the pump efficiency is 70%.

How To Size A Pump Example

Calculation

The calculation is presented below:

Pump Calculation

Results

Pump flow rate = 30 m3/hr

Pump total differential head = 134.8 m

Net positive suction head available = 22.13 m

Pump power = 15.7 kW

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8 Responses to “How To Size A Pump”

  1. Jason Blank
    Dec 18, 2012
    Reply

    What a neat program!


  2. Rohan.A
    Apr 19, 2013
    Reply

    Very effective calculator. How is it posible to obtain a calculator,

    Kind regards,

    Rohan


  3. Mike Gaynes
    Jul 09, 2014
    Reply

    Your write-up is thorough, a good review for me and very helpful. Thank you very much, Mike


  4. Nidhi
    Feb 11, 2015
    Reply

    Hey,
    Thanks for this article. It is great.
    The calculator you showed here, where can I get it to solve my own pump problem.
    Also if possible please can you give me the algorithm of the above calculation.

    Thank You very much.


  5. Foibe Uahengo
    Sep 29, 2015
    Reply

    Dear Sir/Madam

    Thank you for making this subject clear and to the point.

    I would like to ask for your permission to use your work for my classes if possible please.

    Thank you.

    Kind Regards
    Foibe Uahengo
    Transport Phenomena Lecturer


  6. Tom
    Mar 30, 2016
    Reply

    Super tool!!!!


  7. Paolo
    Jul 06, 2016
    Reply

    Hello,
    lovely calculator you’ve made. Is there a possibility to get it, this would help me a lot during the selection of pumps I have to make.
    Thank you very much in advance.
    Paolo



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