Q&A | Relief Valves Protecting Multiple Vessels

Question: Can a single relief valve assembly be used to protect more than one ammonia refrigeration pressure vessel?

Answer: While it is definitely not common-place, it is compliant to utilize one relief assembly to protect multiple pressure vessels, provided that the relief valves are sized correctly and the vessels are interconnected without allowance for independent isolation. The two most relevant codes that address this issue can be seen below:

2013 B&PV Code Section VIII UG-133(c)

Vessels connected together by a system of adequate piping not containing valves which can isolate any vessel, and those containing valves in compliance with Nonmandatory Appendix M, M-5, may be considered as one unit in figuring the required relieving capacity of pressure relief devices to be furnished.

ANSI/IIAR 2-2008 Addendum B §11.2.7
…When one pressure relief device is used to protect more than one pressure vessel, the required capacity shall be the sum of the capacities required for each pressure vessel. In the case of a plate heat exchanger, replace the DxL portion of the equation with a term equal to half the overall external surface area in square feet (m²). In the case of a double-pipe condenser, replace the DxL portion of the equation with a term equal to half the overall external surface area in square feet (m²).

Furthermore, IIAR 2 has  had similar allowances since 1974:

ANSI/IIAR 74-2 – 1978 §3.11.4.6
…When one pressure-relief device is used to protect more than one pressure vessel the required capacity shall be the sum of the capacities required for each pressure vessel.

As the above references clearly demonstrate, one relief valve assembly can be used to protect multiple vessels provided the following:

1. The installed relief valves have adequate capacity to protect all vessels being protected.
2. Means are in place to prevent the vessels from being independently isolated. Acceptable means include: a) No valves are installed which allow the vessels to be isolated independently; b) Valves are installed which allow for independent isolation, but the valves comply with 2013 B&PV Code Section VIII Appendix M-5. If you are interested to learn more about the content of Appendix M-5, we have written about it in great detail here.