Closed Loop Pumping Pressures

[TimboGMO]

I am having a brain fart here, this must be something easy. I don't ever deal with closed loop type systems, and any of the books I have found on pumps don't spend a lot of time on closed loop systems. Anyway, I have an existing closed loop chilled water system that is a constant flow system (no VSDs). The shutoff head of my pump based off the factory curve is 70'. The design point, or BEP of the pump is 400 gpm @ 55'. Currently the inlet pressure of the pump is 47 psig, or 108', and the discharge pressure is 71 psig, or 164' (a difference of 55'). My question is, why are my system pressures elevated so high, higher than pressures on the curve. I am assuming it would be due to the feedback loop created by a closed circuit situation and if so how can I predict how high my pressures will go? Or possibly due to an external pressure source such as a system make-up valve (which I haven't found or even see a need for since leaks would be the only reason the system needs refilled and that should be evident). Any help would be greatly appreciated. Thanks.

 

[TimboGMO]

Ok, I think I found it. There is a make-up to this system. Regulated to about 50 psig. It was hidden from view and an engineer who has been here longer than I pointed out.

 

[pmover]

Ok Good!

Typically, nearly all closed loop water systems (heating and cooling) have a make-up and expansion tank system. The make-up system ensures the piping system remain filled properly during operation/maintenance activities. If there is any leakage of fluid, the make-up system replaces the leaked fluid.

The expansion tank allows for thermal expansion of the fluid from initial fill temperature to operating temps.

Good Luck!
-pmover

 

[itdepends]

And the height of the expansion tank (relative to the pump) sets the pump suction pressure. There is no feed back from discharge to suction per se- the pump flow rate keeps increasing until the friction losses are sufficient to burn up all the pump head (minus that required to get the water/fluid back to the pump suction).