Net Positive Suction head NPSH

[den23]

I'm going to design a water pipe system onsite fire hydrants for our project site and it requires a booster pump to meet a minimum 20 psi on the pipe system. I'm using waterCAD for analyzing the pipe system. Everytime I run it, it gives me a warning that -20 psi gage pressure on the suction side of the pump and pipe junction upstream or on the side of the pump. The same junction has a positive available pressure, although not enough, when the system is analyzed without the pump. Is the -20 psi gage pressure theoretically caused by the impeller? Is the positive pressure analyzed without the pump (which is gage pressure minus pressure loss due to friction) what I should account for the Net Positive Suction Head Available NPSH (A)? Or Is it the -20 psi gage pressure? Thanks in advance.

 

[DubMac]

Incomprehensible.

 

[LittleInch]

What exactly is the "positive" pressure in psia or psig? Before you add the pump. Does the flowrate change when you add the pump?

If no then nothing should change. If yes then your friction losses will increase and hence inlet pressure might fall.

Look up NPSH and there are many good explainatons of what this is and how to calculate.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Artisi]

Dump the waterCAD to start with and go back to pencil and paper - then you might understand some of the reasons why you are getting a nonsense answer. Calculating a fire pump system couldn't be much easier, fixed conditions with all known data in front of you.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[QualityTime]

I agree with Artisi. Use a piece of paper and a pencil to understand the fundamentals of your system before using the computer in the first place. That way you can interpret what the results are telling you. I would assume that the results are telling you the data you have entered is incorrect and because of that it will not work. It is good that you took the time to ask questions in this forum. This is a good lesson for you to find out that the computer is just a mindless tool that can do calculations quickly. If you, as an engineer, do not know the fundamentals, then garbage in and garbage out. Until you get experienced in solving a problem on a piece of paper I would avoid depending on the computer for an answer

 

[chicopee]

It seems that you are "sucking" water from either a well or a lake. Hand calculate your NPSHR as you computer program seems to be giving you a warning.

 

[hbw]

NPSH available = Atmospheric pressure + static head - liquid vapor pressure - suction piping losses
NPSH required is published by Pump Manufacturer
NPSH available must exceed NPSH required.