Determining Compressor Relief Valve Required Capacity

This is an update to a blog that was originally posted on 10/14/14. You can read the original blog here.

Question: What is the proper procedure for selecting the pressure relief valve capacity for a positive displacement compressor?

Answer: ANSI/IIAR 2-2021 §8.2.2.2 addresses this very question. Furthermore, Appendix E (Informative) of that same document gives a detailed example of one acceptable method for determining the proper capacity. A summary of the steps in Appendix E are included below:

1. Determine the swept volume flow rate (displacement rate) of the compressor in ft3/min. This information is typically readily available in compressor operating manuals.
2. Determine the unloading capabilities of the compressor as a percent of full capacity. This information may be available in the compressor operating manual, but is also affected by compressor control schemes.
3. Determine the volumetric efficiency of the compressor when operating at the pressure relief valve set pressure. If this is unknown, assume an efficiency of 0.9.
4. Using the information obtained in Steps 1-3 calculate the capacity using Equations E.1 and E.2 in ANSI/IIAR 2-2008 Addendum B Appendix E.
   

Where

Wr = mass flow of refrigerant, lb/min (kg/s)

Q   = swept volume flow rate of compressor, ft3/min (m3/s)

PL  = fraction of compressor capacity at minimum regulated flow

v   = volumetric efficiency (assume 0.9 actual volumetric efficiency at relieving pressure is known)

vg    = specific volume of refrigerant vapor (rated at 50ºF [10ºC] saturated suction temperature),  ft3/lb (m3/kg)

Where

Wa = relieving capacity in mass flow of air, lb/min (kg/s)

Wr = mass flow of refrigerant, lb/min (kg/s)

rw  = refrigerant-to-standard-air-mass-flow conversion factor 

Constants for ammonia are listed below:

vg = 3.2997 ft3/lbm (m3/kg)

 rw  = 1.28

6. Determine the relief valve capacity required for the oil separator using the equation in ANSI/IIAR 2-2008                  Addendum B §11.2.7.

Where

C = required discharge capacity of the relief device, lb air/min (kg/s).

f = capacity factor of the relief device, which is 0.5 (0.04) for ammonia.

[0.5 is in inch-pounds (IP), 0.04 is in International System of Units (SI)]

D = outside diameter of vessel, ft (m).

L = length of vessel, ft (m).

7. The required relief valve capacity is the larger of Step #4 and Step #5.

For more information on this topic, I would refer you to ANSI/IIAR 2-2021 Appendix E for a more detailed explanation including an example problem with calculations.