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Chilled water Pressure Relief VLV setting and Discharge Pressure question

Chilled water Pressure Relief VLV setting and Discharge Pressure question

Chilled water Pressure Relief VLV setting and Discharge Pressure question

(OP)
<http://www.adoptionmultimedia.com/?attachment_id=7...;

I am designing a chilled water system that will use two 150 ton chillers and two pumps, this system will be a closed loop system. See the diagram for what the chillers and pump set up will look like. Only one pump and one chiller will be used at any one time. The chillers, pumps and expansion tank are located at the top of the system, with the air handlers spread across on the same level and the two floors below them. The NPSr of the pump is about 8 psi and we are setting the expansion tank to 22 psi to keep about 1 ATM of pressure extra so we don’t lose suction. My calculated head loss for the entire system was 305 feet. The total flow with all air handlers demanding water is 500 GPM.
Couple Questions based on this information:

First, we are looking at setting the relief valve at 55 PSIG, my understanding is that this is based on the weakest point in the system as well as the pressure losses from that point to the relief valve. Is that how you recommend setting the relief valve? We could set it much higher as the system components can take higher pressures, but I don’t see a need to.

Second: I am curious to know how the discharge pressure and feet of head of the pump are related? I know the pump can produce 500 GPM at 300 feet of head. But does that mean that the discharge pressure would be the standard 0.434 x Ft head? This would mean the discharge pressure is 217 psig, which seems way to high?

Finally, we are adding a small 1-1/4” recirculation line for the pump in case the discharge and suction valves are shut and the pump is turned on. These lines would give the operator some time to shut the pump off without the pump immediately going to shut off head. Is this a standard practice and what % of the overall flow is recommended for this type of orifice line?
Thanks

RE: Chilled water Pressure Relief VLV setting and Discharge Pressure question

I'd suggest reading ASHRAE's Hydronic System Design manual. It answers most all of your questions.

RE: Chilled water Pressure Relief VLV setting and Discharge Pressure question

You should find someone in your office to help you with these questions......you need some very basic Engineering training. ASHRAE literature will help.

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