Net Positive Suction Head - what does the pump "see"?

RonnieJ

Hello Wallies,

I've read Dan's post "Understanding primary-secondary pumping", and many others, which have been really helpful. Pertinent take-aways are to pump away from the expansion tank, and to place the air separator/expansion tank on the primary loop - in my case, a heat pump used for cooling. I've got one nagging area of doubt: I'm installing a four-port buffer tank (Heat-Flo HF-22BT44-1.25) to re-pipe my chiller and air handlers in a primary/secondary fashion, as illustrated in the attached diagram. The tank has four connections - two on the left, one on the right, and the fourth at the top. To achieve air extraction, I need to place the spirovent on that top connection - shown with green colored lines. I'm worried about pump cavitation - the tank is 27" tall - if I pipe down 24" and place the pump at floor level, will the pump see the 24" of water column? Is there any difference between this arrangement, versus connecting the pump to one of the side-wall connections, in terms of the pressure of the water column presented to the pump? I would appreciate your comments and suggestions.

Thanks,

Ron

Alan (California Radiant) Forbes

In a closed system, whatever pressure you lose in an elevation gain is offset by an increase in pressure with the drop in elevation. The pump will not notice.

RonnieJ

Thanks Alan, My concern was that if I placed the pump at the same elevation as the top of the tank, there might be air pockets or low NPSH-Available, leading to cavitation - so it sounds like I don't need to worry about that in a closed loop.

Cheers,

Ron

EBEBRATT-Ed

What @Alan (California Radiant) Forbes said is correct. Just make sure you don't undersize the pump suction line or any components

RonnieJ

great - thank you both!

jumper

Alan (California Radiant) Forbes said:

In a closed system, whatever pressure you lose in an elevation gain is offset by an increase in pressure with the drop in elevation. The pump will not notice.

Correct but there can still be an NPSH issue. As long as the system is properly pressurized then there is no issue. You see boilers and chillers along with their circulators on top of buildings. Work fine for years but then somebody doesn't add fluid correctly and pump destroys itself. It's prudent to make certain that there is sufficient NPSH even if the system is opened.

RonnieJ

Thanks Jumper. I'm running 15 psi at rest - the pump will be about 11 feet above the lowest point in the circuit, with about two feet of water column above it.