Positive Displacement Compressor Relief Valve Sizing

[ianmcq28]

Dear All

I am currently looking to replace an existing relief valve that is installed on the outlet of a positive displacement screw compressor. Unfortunately there is no existing sizing information and the compressor manufacturers' claim to "just install a 1/2" NPT relief device" doesn't stack up!

I am concerned that I am missing a trick with defining the required relief mass flow rate and would welcome some advise.

System description:

- The compressor is part of a chiller package to provide chilled water for our control room air conditioning
- The refrigerant is R407C
- The MAWP of the system is 28 bar
- The relief valve set-pressure is 28 bar
- Information regarding operating conditions is limited!
- Displaced volume through the compressor is 166 m³/hr

My approach:

- The relief case is for blocked / closed valve in compressor discharge (downstream of relief device)
- The volumetric displacement is constant
- Volumetric efficiency assumed to be 100%, therefore 166m³/hr is used
- The specific gravity is determined by using a mollier (pressure-enthalpy) diagram for R407C. In using this I have assumed that the compressor will compress the gas to 10% above the relief device set pressure and have taken the corresponding specific gravity of R407C at this pressure along the constant entropy line.
- In doing so the specific gravity at the relief condition is ~0.008 m³/kg
- I have therefore concluded that the relief rate is volumetric flow / specific gravity and this gives a mass flow of 20750 kg/hr
- I have then sized the relief valve and this gives a required relief device area that is greater than the diameter of the pipework and significantly greater than the device that is currently installed! (Our maintenance records quote the capacity of the current valve is ~2000 kg/hr (although these records are known to be misleading)).

Can anyone provide guidance on what to take the compressor suction temperature as please?

ASHRAE 15 suggests "Flow is to be calculated based on 50°F (10°C) saturated suction temperature at the compressor suction."

Can someone advise whether my basis is correct or advise otherwise please?

Best Regards

Ian

[RANGER1]

So are you going to relieve it to atmosphere , or back into system ?

Suggest you use danfoss POV main valve with a BSV8 pilot balanced port relief ( not effected by downstream pressure ) .
This is piped from discharge to suction and can handle high volume of gas .
POV setting is set below relief to atmosphere and higher than HP cut out .
Ask Danfoss for sizing ,you can be generous on POV main valve size .

[ianmcq28]

Thanks

Currently the relief valves vent to atmosphere and we wouldn't be looking to to modify the suction pipework to do otherwise for this application.

Regards

Ian

[nike123]

Check this catalogue for sizing!