npshr

tim wheat

can someone explain npshr so that a ignorant fellow like me can understand...the question is -is the npshr figured with the pump running? is it the static weight with atmospheric pressure added?..im lost...thanks in advance

Uni R

npshr

http://www.mcnallyinstitute.com/CDweb/n-html/n002.htm

flange

i believe you are referring to net positive suction head. this is basically how high the column of water needs to be on the suction side of the pump to allow for correct performance. on most pumps you need this column of water to keep the pump primed.

DanHolohan

Yes,

it's figured with the pump running. You have to consider the positive pressure available, minus the pressure drop into the pump's suction. For instance, suppose you have a pump on a cooling tower and there's, let's say, 4.3 feet of water above the pump's suction. That would be a postive pressure of 2 psi. But there's also about five feet of piping through which the water has to flow on its way to the pump suction, so we have to subtract that pressure drop from the available static pressure. If there's a strainer in that line, we'll have to subtract its pressure trap as well.

Engineers call this NPSHA (Net Positive Suction Head Available). That's a job condition. NPSHR (Net Positive Suction Head Required) is determined by the folks who make the pump. NPSHA is what's on the job. NPSHA has to exceed NPSHR to avoid cavitation.

So keep your strainers clear. ;-)

How'd I do?

S Ebels

Simply put

NPSH is the "pressure" on the inlet side of the circulator. Any centrifugal circulator is happier and will live long and prosper with a positive pressure available on its inlet side. Circ's with different flow rates require different amounts of suction pressure. A circ moving 60 GPM into 70 ft of head requires a higher suction pressure than a little 15-58 Grundfos. Every manufacturer will have NPSH requirements for each individual circ. The requirement will also vary on each circ according to how much fluid it's moving.

This pressure comes from the static fill pressure of the system plus whatever positive pressure is there from the returning fluid.

Here's an example for a B&G 2X2X7 1 1/2HP circ we installed on a system last year. Minimum required NPSH is 5 ft of head or roughly 11.5 psi while delivering 50 GPM into 50 ft. of head. If you bump that to 90 GPM into 35 ft of head you then want to maintain 8 ft or 18.5 psi on the suction side of the pump.

All of this is calculated and done to prevent one thing and that is circulator cavitation. This occurs when the circ is trying to move more water out than it can be supplied with. When that happens your NPSH drops below spec or even goes negative. This is a bad thing, as Martha would say. Circ's will typically not live long in that condition.

NPSH is figured with the circ running. One way to measure it is to install a pressure gauge on the inlet side of the circ casting. Note your psi reading and multiply by 2.31 to convert to head. NPSH is the total pressure available at the inlet of the circ.

Jerry Boulanger_2

NPSH

NPSH is measured in psi absolute pressure. If you are drawing water from an open tank, you will have the pressure of the atmosphere, about 14.7 psia, pusing on top of the water. Added to that will be the static height of the water above the pump inlet. Deducted from that will be flow losses on the inlet side of the pump. The net result is the available NPSH, which should be higher than the pump required NPSH. Fluid temperature will have an impact as well - the higher the fluid temeprature the lower the vapor pressure.

hr

It stands for

Not Pumping So Hot!

http://www.pmmag.com/CDA/ArticleInformation/features/BNP__Features__Item/0,2379,4243,00.html

hot rod

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