Pump head calculation/pump selection

[stan13]

i understand that this is a extensive topic, yet i hope someone could explain the following:

how do you calculate pump head and select booster pump based on that ?

thank you

 

[dcasto]

pump headis given in ft and is proportional to the differential pressure in pounds per square inch (PSI) based on the fluid being pumped density. So water with a 62.4 lb/ft^3 density when you divide 62.4 by 144 in^2 per ft^2, you get .433 psi per ft. Now multiply the pumps head from the chart at the flowrate you desire. Say the pump will put up 30 ft at 20 gallons per minute, that means 30 times .433 = 13 psi differential across the pump.

You can work backwards too. Say the system requires a pressure of 74 psi and the feed water has a pressure on 25 psi. so 73-25 = 48 psi. divide 48 by ,433 to get 111 feet of head. look for a pump that has 111 or more feet of head at your desired flowrate.

 

[PEDARRIN2]

You need to find where you require the most pressure. It is usually the toilet/urinal flush valve on the highest floor. But don't neglect the farthest fixture if you have a system with a long run.

Flush valves typically require 35 psi to work effectively.

Find out the static pressure at your inlet. Typically a fire department hydrant test will determine that.

With all that in mind, determine the pressure drop (friction losses_ in your system - don't forget the backflow preventor which is typically 10 psi at the flows you will see.

The difference will give you the total dynamic head the pump has to provide.

You will need to know what flow you will want. You will need to know a typical flowrate that you can size the pump to, but you will also want to determine the maximum flowrate so you can make sure you have enough pressure at that flow to flush the aforementioned toilet.

Think about constant speed pumps and variable speed pumps. I prefer the latter and their price is fairly competitive now.

After that, contact the pump manufacturer and see what he thinks.