NPSHA in a closed loop system

[mech711]

How would I go about calculating the NPSHA for a pump in a closed loop chilled water system? The fluid is a 40% propylene glycol water solution @ 54 degrees 65 ft head and 137 gpm. Do I need any other information?

 

[KiwiMace]

This is usually not a big problem:

The pumped head in a closed system doesn't count, as it acts back on itself at the suction, negating any suction head.

Pressurisation systems (head tanks or diaphragm tanks) are used to increase the total system static pressure to a point where no close-to-negative suction occurs.

If in doubt - plot out your system static and total pressure diagram by laying out your system linearly.

 

[BigInch]

Cinci, it sounds like you probably have a lot of cavitation problems.

Are you really saying that a return pressure to a pump suction in a closed system CANNOT be below npsha???

Going the Big Inch!

http://virtualpipeline.spaces.msn.com

 

[KiwiMace]

Nope, just that it's not usually a big problem if you implement the measures stated, compared to open systems.

 

[BigInch]

Then you mean its not a problem in a well designed system?

Going the Big Inch!

http://virtualpipeline.spaces.msn.com

 

[25362]

To mech711, has the loop been deareated ? Any dissolved gas that may be evolved in large volumes in the pump, may invalidate the conventional NPSHA (generally interpreted as pressure at suction less vapor pressure of the fluid).