NPSHA-IN CLOSED LOOP

[moideen]

I know the calculation of NPSHA, that atmospheric pressure+/- static head-vapor pressure-friction head. My question is that how the same formula suited in closed chilled water circuit. The atmospheric pressure no influence in the closed circuit. Is my theory is correct?

 

[ashtree]

I believe you to be correct. What you are interested in is the pressure from the return leg of the loop as this in effect takes the role of the atmosphere pushing the liquid into the impeller.


Regards
Ashtree
"Any water can be made potable if you filter it through enough money"

 

[LittleInch]

You can do this a couple of ways.

Look at the min pressure at the highest point then add static head - friction - vapour pressure or look at there the pressurisation control is.

Closed liquid loops need to include a system whereby small changes in volume due to temperature don't affect the static system pressure and if you place the pressurisation vessel in the right place then it will maintain the pressure to > NPSHR.

Each system is different so you need to look at specifics.

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

 

[bimr]

Yes, the NPSHa is the pump discharge pressure minus all the losses in the loop back to the pump inlet.

 

[Compositepro]

Which came first, the chicken or the egg? It is a loop so all pressures are directly related to each other. The only point where pressure is actually known and controlled is usually the expansion tank. The expansion tank is usually close to the lowest pressure point in the loop, and that point is also usually close to the pump suction.

NPSHa is also measured in absolute pressure, not gauge pressure. To convert from gauge to absolute pressure you still need to add one atmosphere.

 

[moideen]

Thanks all response, so the static head and the atmospheric pressure have no role in closed loop.
Can rearrange this way that NPSHA= vessel pressure +absolute pressure –vapor pressure-friction head.

 

[LittleInch]

Static head is still needed in terms of any head difference between your "vessel pressure" and pump inlet.

If you're going to put friction head in they then you also need differential head somewhere.

Also all should be head not pressure.

Still not sure in a loop that that formula will work for all situations.

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

 

[moideen]

LittleInch" do you mean that the static head should also be considered in the head calculation in closed loop?

 

[LittleInch]

I'm saying the static head from the pressure source equivalent to atmospheric pressure ( the pressurisation vessel/tank) to the pump needs to be considered. This might be zero if it's next tot he pump inlet line ( common location) or could be the height of the building if its on the roof and the pumps are in the basement.

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

 

[ashtree]

Will it matter though if it is truly a closed loop, with no pressure break tank?

Regards
Ashtree
"Any water can be made potable if you filter it through enough money"

 

[LittleInch]

In that case (unrealistic), it would be the location/ height of the pressure guage/ sensor relative to the pump inlet.

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

 

[Compositepro]

Ashtree, your thinking is too simplistic. Every "closed loop" must have some way to allow for thermal expansion of the fluid. Otherwise a few degrees change in temperature can cause pressure to go from vacuum to hundreds of psi. Random gas bubbles may mitigate the problem, but a tightly closed loop with no expansion tank (or equivalent) would be a very poor engineering design.