Q&A | Compressor Relief Valves

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

Answer: ANSI/IIAR 2-2008 Addendum B §6.1.2.1 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 effected 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.
   
5. Determine the relief valve capacity required for the oil separator using the equation in ANSI/IIAR 2-2008 Addendum B §11.2.7.
   
6. 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 2-2008 Addendum B Appendix E for a more detailed explanation including an example problem with calculations.