How To Size A Pump Blackmonk Engineering.pdf

7/27/2019 How To Size A Pump Blackmonk Engineering.pdf

http://slidepdf.com/reader/full/how-to-size-a-pump-blackmonk-engineeringpdf 1/9

How To Size A Pump

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 alltypes of pumps: centrifugalor positive displacement.

Other key cons iderations for pump sizing are the net positive suction head available (NPSHa) and the

power required to dr ive the pump.

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 proces s.

For instance the required flow rate of a pump feeding oil into a r efinery distillation column willbe

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.

TotalDifferentialHead

Search Keywords

Login Status

Sign up to ou r emails

Process Calculators

Search

You are not currently logged in.

Username:

Password:

gfedc Remember Me

Login

» Register

» Lost your Password?

Sign up for free emailupdates and subscriber

only information.

Name:

Email:

Sign Up

We hate spam and willnot shar e your details

with any third parties. You can unsubscribe at

any time.

Posted on November 11, 2009

BLOG CALCULATORS SERVICES ABOUT US CONTACT US LOGIN



rans er requ re .

TotalDifferentialHead

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 differentialhead is made up of 2 components. The first is the st atic head across the

pump and the second is the frictional head loss thr ough the suction and discharge piping syst ems.

Total differential head = static head differenc e + frictional head losses

Static Head Difference

The static head difference acr oss the pump is the difference in head between the dischar ge static head

and the suction static head.

Static head difference = discharge static head – suction static head

Discharge Static Head

The discharge stat ic 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



Submerged Pump Discharge Pipe

Discharge Pipe Enters The BottomOf The Vessel

Suction Static Head

The suction static head is sumof 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 centr e line of the pump.

Suc tion static head = Suc tion vessel gas pressure head + elevation of suc tion vessel liquid surface

– elevation of pump c entre line

Note: gas pressure can be converted to head using:

Gas head = gas pressure ÷ (liquid density x acceleration due to gravity)



Pump Suction

Frictional Head Losses

The total frictionalhead losses in a system are comprised of the frictional losses in t he suction piping

system and the frictionallosses in the discharge piping system.

Frictional head losses = frictional losses in suc tion piping system + frictional losses in discharge

piping system

The frict ional losses in the suction and discharge piping systems are the sumof 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 frict ion factors and fittings factors to calculate the

pressure loss in pipes and fittings.

Darcy-Weisbach equation:

In order to calculate the frict ional head losses you therefore need to know the lengths and diameters of

the piping in the systemand 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 t he

pump suct ion and the vapour pressure of the pumped liquid at the pumping temperature.

It is important because for the pump to operate proper ly, the pressure at the pump suction must exceed

the vapour pressur e for the pumped fluid to remain liquid in the pump. If the vapour pres sure 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 t he pump operates correc tly the net positive suction head available (NPSHa) must exceed



To ensure that t he pump operates correc tly the net positive suction head available (NPSHa) must exceed

the net positive suction head required (NPSHr) for that par ticular 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 suc tion – liquid vapour

pressure head

Pump Power

Pumps are usually driven by electr ic motors, dieselengines or steam turbines. Determining the power

required is essential to sizing the pump driver .

Pump pow er = flow rate x total differential head x liquid density x acc eleration due to gravity ÷

pump efficiency

How To Size A Pump Example

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

30000 kg/hr of water needs to be pumped from one vessel to another through the syst emshown 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

viscos ity of 1cP. We’llassume that the pump efficiency is 70%.



Calculation

The calculation is presented below:





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

Tags: How To, pump



Tags: How To, pump

Leave a Reply

Name (r equired)

Mail (willnot be published) (required)

Website

Submit Comment

© 2012 Blackmonk Engineering - Website Terms & Conditions. OverEasy by

Documents

How To Size A Pump Blackmonk Engineering.pdf