Relief Valve on Chilled Water System???

[BronYrAur]

I am looking at some drawings that show a Pressure Relief Valve on a chilled water system. The valve is on the water line leaving the chiller, and no further information is given on its size or rating. Is this common? I don't recall ever seeing a PRV on chilled water. If so, any idea on how to size it?

 

[BigInch]

Don't believe that it matters what kind of system it is. If there is a possibility for overpressures to develop, you must have a PSV.

Without any additional info, its usually safe to size the PSV for the full flow possible in that pipe when flowing at its max operating pressure and temperature.

http://virtualpipeline.spaces.msn.com

 

[dcasto]

Based on your brief description, the relief was sized for a tube failure and the "stuff" being cooled operates at a pressure higher (or can be higher) than the rateing on the chilled water side of the exchanger.

 

[BronYrAur]

I understand what you are both saying, but this is just a run-of-the-mill chiller for an HVAC application. It serves fan coils in a 2 story building and has a pump with max head of 81' at design flow. I see no way for the pressure to get too high.

 

[dcasto]

If there were a fire at the chiller and the chilled water boiled, then you'd have a big problem.

 

[lizking]

bloced in-thermal expansion.

 

[lizking]

Sorry: blocked in, thermal expansion.

 

[marcoh]

Chilled water systems require some system to account for expansion/contraction of the water volume (expansion tanks, vessels etc)which in most design would act a over pressure relief by default.

 

[Newmanite]

?High/low pressure pressure switch?

 

[BigInch]

Pressure switch doesn't exactly provide the same function as a PSV.

http://virtualpipeline.spaces.msn.com

 

[dcasto]

unless the switch stops the sunshine and puts out a fire and restores the chilling ..... Nope, no substitute device on a close system.

 

[BronYrAur]

I guess the reason I ask this question is because I have never seen one on a chilled water system. Standard setup is a chiller, pump, air seperator, and expansion tank. Never a relief valve. Now, a boiler system is a different story.

 

[dcasto]

If the system is open to the atmoshere and that opening is close enough to the chiller exchanger and all, then you could get by without one.

 

[TBP]

Just as an example, I know of a line that was being hydrotested with glycol outside, on a sunny day in late winter. The crew filled the line with cold glycol, closed the valves, and went to lunch. There was, at most, 10 PSIG in that line when they left. When they came back a 1/2 hour later, there was over 400 PSIG on the certified test gauges. It so happens that that pressure was just under the required test pressure. As they were getting the paperwork ready for the test, they noticed that the pressure kept rising over that few minutes, and they had to crack a drain valve to keep it below 425.

Relief valves for chilled water systems are relatively cheap. Weigh that cost against the potential damage from a blown gasket in an unoccupied area, over a weekend.