CENTRIFUGAL PUMP SYSTEMS part 2

 13. Elevation changes between the suction tank surface and the discharge point 

can be disregarded 

Surprising statement! You can have the discharge pipe going up or down (within reason) 

as much as you like and this has no effect on the static head of the system. The fluid 

looses its elevation energy as it goes up but regains it without loss as it comes back 

down. If the discharge elevation of the system does not change then any changes prior 

to this point will cancel out as you reach the discharge point of the system. What does 

make a difference is the length of pipe between these two points, by going up and down 

you increase the length of the pipe which increases the friction head which will increase 

the total head but the static head will remain the same. 

There is a limit as to how high you can go in between the pump discharge and the pump 

system discharge point. This height will depend on the available shut-off head of the pump. Shut-off head is the maximum head that a pump can produce at zero flow. When 

you start the pump in a system with fluid that is going upwards, the velocity in the pipe 

will be low until the fluid completely fills the system. When the system is filled equilibrium 

is reached between the pump’s ability to push the fluid through the system and the 

resistance that the system offers, at this equilibrium the flow rate is established. 

If the shut-off head is insufficient then there will be insufficient energy to get the liquid 

past the high point. If the shut-of head is high enough to get the liquid past the high point 

then the system can be filled up which results in a lowering of the static head allowing 

the pump to operate at a lower total head. 

In the characteristic curve shown in Figure 42 the shut-off head is 125 feet. If your 

system has a high point higher than 125 feet between the suction surface and the high 

point then it will be impossible to fill this system up and operate it at the required flow 

rate. 

13. How to select a centrifugal pump 

It is unlikely that you can buy a centrifugal pump off the shelf, install it in an existing 

system and expect it to deliver exactly the flow rate you require. The flow rate that you 

obtain depends on the physical characteristics of your system such as friction which 

depends on the length and size of the pipes and elevation difference which depends on 

the building and location. The pump manufacturer has no means of knowing what these 

constraints will be. This is why buying a centrifugal pump is more complicated than 

buying a positive displacement pump which will provide its rated flow no matter what 

system you install it in. 

The main factors that affect the flow rate of a centrifugal pump are: 

- friction, which depends on the length of pipe and the diameter 

- static head, which depends on the difference of the pipe end discharge height 

vs. the suction tank fluid surface height 

- fluid viscosity, if the fluid is different than water. 

The steps to follow to select a centrifugal pump are: 

1. Determine the flow rate 

To size and select a centrifugal pump, first determine the flow rate. If you are a 

home owner, find out which of your uses for water is the biggest consumer. In 

many cases, this will be the bathtub which requires approximately 10 gpm (0.6 

L/s). In an industrial setting, the flow rate will often depend on the production 

level of the plant. Selecting the right flow rate may be as simple as determining 

that it takes 100 gpm (6.3 L/s) to fill a tank in a reasonable amount of time or the 

flow rate may depend on the interaction between processes.

2. Determine the static head 

This a matter of taking measurements of the height between the suction tank fluid 

surface and the discharge pipe end height or the discharge tank fluid surface 

elevation. 

3. Determine the friction head 

The friction head depends on the flow rate, the pipe size and the pipe length. 

This is calculated from the values in the tables presented here (see Table 2). For 

fluids different than water the viscosity will be an important factor and Table 1 is 

not applicable. 

4. Calculate the total head 

The total head is the sum of the static head (remember that the static head can 

be positive or negative) and the friction head. 

5. Select the pump 

You can select the pump based on the pump manufacturer’s catalogue 

information using the total head and flow required as well as suitability to the 

application. 

Example of total head calculation - Sizing a pump for a home owner application 

Experience tells me that to fill a bath up in a reasonable amount of time, a flow rate of 10 

gpm is required. According to Table 2, the copper tubing size should be somewhere 

between 1/2" and 3/4", I choose 3/4". I will design my system so that from the pump 

there is a 3/4" copper tube main distributor, there will be a 3/4" take-off from this 

distributor on the ground floor to the second floor level where the bath is located. On the 

suction, I will use a pipe diameter of 1”, the suction pipe is 30 ft long.

Friction loss on the suction side of the pump 

According to calculation or the use of tables which is not presented here the friction loss 

for a 1" tube is has a friction loss of 0.068 feet per feet of pipe. In this case, the distance 

is 30 feet. The friction loss in feet is then 30 x 0.068 = 2.4 feet. There is some friction 

loss in the fittings, let's assume that a conservative estimate is 30% of the pipe friction 

head loss, the fittings friction head loss is = 0.3 x 2.4 = 0.7 feet. If there is a check valve 

on the suction line the friction loss of the check valve will have to be added to the friction 

loss of the pipe. A typical value of friction loss for a check valve is 5 feet. A jet pump 

does not require a check valve therefore I will assume there is no check valve on the 

suction of this system. The total friction loss for the suction side is then 2.4 + 0.7 = 3.1 

feet. 

You can find the friction loss for a 1” pipe at 10 gpm in the Cameron Hydraulic data book 

of which the next figure is an extract:

Friction loss on the discharge side of the pump 

According to calculation or the use of tables which is not presented here the friction loss 

for a 3/4" tube is has a friction loss of 0.23 feet per feet of pipe. In this case, the 

distances are 10 feet of run on the main distributor and another 20 feet off of the main 

distributor up to the bath, for a total length of 30 feet. The friction loss in feet is then 30 x 

0.23 = 6.9 feet. There is some friction loss in the fittings, let's assume that a conservative 

estimate is 30% of the pipe friction head loss, the fittings friction head loss is = 0.3 x 6.9 

= 2.1 feet. The total friction loss for the  discharge side is then 6.9 + 2.1 = 9 feet. You can find the friction loss for a 0.75” pipe at 10 gpm in the Cameron Hydraulic data 

book of which the next figure is an extract:

The total friction loss for piping in the system is then 9 + 3.1 = 12.1 feet. 

The static head as per Figure 41 is 35 feet. Therefore the total head is 35 + 12.1 = 47 

feet. We can now go to the store and purchase a pump with at least 47 feet of total head 

at 10 gpm. Sometimes total head is called Total Dynamic Head (T.D.H.), it has the same 

meaning. The pump’s rating should be as close as possible to these two figures without 

splitting hairs. As a guideline, allow a variation of plus or minus 15% on total head. On 

the flow, you can also allow a variation but you may wind up paying for more than what 

you need.

What is the pump rating? The manufacturer will rate the pump at its optimum total 

head and flow, this point is also known as the best efficiency point or B.E.P. At that flow 

rate, the pump is at its most efficient and there will be minimal amount of vibration and 

noise. Of course, the pump can operate at other flow rates, higher or lower than the 

rating but the life of the pump will suffer if you operate too far away from its normal 

rating. As a guideline, aim for a variation of plus or minus 15% on total head.

14. Examples of common residential water systems

The following figures show various common water systems and indicates what the static 

head, the friction head and the pump total head.

15. Calculate the pump discharge pressure from the pump total head 

To calculate the pressure at the bottom of a pool, you need to know the height of the 

water above you. It doesn’t matter if it’s a pool or a lake, the height is what determines 

how much fluid weight is above and therefore the pressure. 

Pressure can be applied to a small object or a big object. For example if we take tanks of 

different sizes from small to big, the pressure will be the same everywhere at the bottom 

no matter how big the surface as long as the surface is at the same level.

Pressure is equal to a force divided by a surface. It is often expressed in pounds per square 

inch or psi. The force is the weight of water. The density of water is 62.3 pounds per 

cubic foot.

The weight of water in tank A is the density times it’s volume. 

                                 WA= dens.× volume

The volume of the tank is the cross-sectional area A times the height H. 

The cross-sectional area is pi or π times the diameter squared divided by 4. 

 

 A  =( π * D^2)/4

 

The cross-sectional area of tank A is: 

A =(3.1416 ×1^2)/ 4  = 0.78   ft^2

The volume V is A x H: 

 V = A×H =0.78×10=7.8 ft^3

The weight of the water WA is: 

WA  = dens. × V =62.3×7.8=485.9   Ib

Therefore the Pressure is:

  P = (WA /A) = ( 485.9/0.78) = 622.9  Ib / ft^2

 

This is the pressure in pounds per square feet, one more step is required to get the 

pressure in pounds per square inch or psi. There is 12 inches to a foot therefore there is 12x12 = 144 inches to a square foot. 

The pressure p at 

 

If you do the calculation for tanks B and C you will find exactly the same result, the 

pressure at the bottom of all these tanks is 4.3 psi. 

The general relationship for pressure vs. tank height is: 

 

SG or specific gravity is another way of expressing density, it is the ratio of a fluid’s density to that of water, so that water will have an SG =1. Denser liquids will have a 

value greater than 1 and lighter liquids a value less than 1. The usefulness of specific gravity is that it has no units since it is a comparative measure of density or a ratio of densities therefore specific gravity will have the same value no matter what system of units we are using, Imperial or metric. For those of you who would like to see how this general relationship is found go to Appendix E at the end of this article.

 

(We can measure head at the discharge side of the pump by connecting a 

tube and measuring the height of liquid in the tube. Since the tube is really 

only a narrow tank we can use the pressure vs. tank height equation

 

to determine the discharge pressure. Alternatively, if we put a pressure 

gauge at the pump discharge, we can then calculate the discharge head.)

We can calculate the discharge pressure of the pump based on the total head which we 

get from the characteristic curve of the pump. This calculation is useful if you want to 

troubleshoot your pump or verify if it is producing the amount of pressure energy that the 

manufacturer says it will at your operating 

flow 

For example if the characteristic curve of the pump is as shown in Figure 55 and the flow 

in the system is 20 gpm. The total head is then 100 feet. 

The installation is as shown in Figure 53, a domestic water system that takes its water 

from a shallow well 15 feet lower than the pump suction. 

The pump will have to generate lift to get the water up to its suction connection. This 

means that the pressure will be less than the atmosphere pressure at the pump suction. 

Why is this pressure less than atmospheric pressure or low? If you take a straw, fill it 

with water, cover one end with your fingertip and turn it upside down you will notice that 

the liquid does not come out of the straw, try it!. The liquid is pulled downward by gravity 

and creates a low pressure under your fingertip. The liquid is maintained in balance 

because the low pressure and the weight of the liquid is exactly balanced by the force of 

atmospheric pressure that is directed upwards. 

The same phenomenon occurs in the pump suction which is pulling up liquid from a low 

source. Like in the straw, the pressure close to the pump suction connection must be low 

for the liquid to be supported

To calculate the discharge head, we determine the total head from the characteristic 

curve and subtract that value from the pressure head at the suction, this gives the 

pressure head at the discharge which we then convert to pressure. 

We know that the pump must generate 15 feet of lift at the pump suction, lift is negative 

static head. It should in fact be slightly more than 15 feet because a higher suction lift 

will be required due to friction. But let’s assume that the pipe is generously sized and 

that the friction loss is small.

The total head is equal to the difference between the pressure head at the discharge HD

and the pressure head at the suction HS. HS is equal to –15 feet because it is a lift 

therefore

Note: you must be careful where you locate the pressure gauge, if it is much higher than 

the pump suction, say higher than 2 feet, you will read less pressure than actually is 

there at the pump. Also the difference in velocity head of the pump discharge vs. the 

suction should be accounted for but this is typically small.

APPENDIX A 

Flow rate and friction loss for different pipe sizes based on 5 ft/s velocity 

Flow rate and friction loss for different pipe sizes based on 15 ft/s velocity 

Flow rate and friction loss for different pipe sizes based on 4.5 m/s velocity 

Flow rate and friction loss for different pipe sizes based on 1.5 m/s velocity

APPENDIX B 

Formulas and an example of how to do pipe friction calculations

Example calculation 

Calculate the pipe friction loss of a 2 1/12” schedule 40 (2.469” internal pipe diameter) 

new steel pipe with a flow rate of 149 gpm for water at 60F and a pipe length of 50 feet. 

The roughness is 0.00015 ft and the viscosity is 1.13 cSt.

The average velocity v in the pipe is: