Relief Valves Discharging Back into a Refrigeration System

This is an update to a blog that was originally posted on 4/19/16. You can read the original blog here.

ANSI/IIAR 2-2021 §15.3.10 outlines the requirements for providing Hydrostatic Overpressure Protection. In situations where thermal hydrostatic expansion of trapped liquid ammonia could occur, hydrostatic protection must be provided. Per ANSI/IIAR 2-2021 §15.3.10.1,

Protection against overpressure due to thermal hydrostatic expansion of trapped liquid ammonia shall be provided for equipment that is not ASME stamped and piping sections that can be isolated and can trap liquid ammonia in an isolated section in any of the following situations:

1. Automatically during normal operation;
2. Automatically during shutdown by any means, including alarm or power failure;
3. During planned isolation for standby or seasonal conditions;
4. Due to an equipment or device fault; or
5. During the shipping of any pre-charged equipment.

EXCEPTION: If trapping of liquid with subsequent thermal hydrostatic expansion is only possible during maintenance or service operations, engineering or administrative controls, or both, shall be permitted as the means of relieving or preventing overpressure.

The protection required in §15.3.10.1 can be provided in one of three ways:

1. Provide a hydrostatic relief device or check valve relieving to another part of the closed-circuit system.
2. Provide an expansion compensation device.
3. Provide a control valve with an internal design that will relieve downstream pressure to the upstream side of the control valve.

It is well known that ammonia refrigeration relief valves must be replaced or tested at least every five (5) years. The question often arises as to whether this requirement applies to liquid-rated relief valves (e.g. Cyrus Shank 803 LQ) which discharge back into the system. Hydrostatic relief valves are not required to be replaced every five (5) years according to IIAR standards. 

ANSI/IIAR 6-2019 Table 13.1 states that hydrostatic/internal relief valves should be replaced “When an operational indication occurs”. An example of this circumstance could be a relief valve that no longer closes after opening. In this instance, replacing the hydrostatic relief valve would be wise, but not explicitly required.

However, some manufacturers recommend replacing relief valves every five (5) years, regardless of the valve application. For example, Page 10 of the 2018 Cyrus Shank Catalogue Volume VII has the following recommendation which does not discriminate between hydrostatic and atmospheric relief valves:

At least every 5 years, Cyrus Shank Safety Relief Valves (or cartridges) shall be removed and replaced with new safety relief valves or cartridges.

Regarding the maintenance of hydrostatic relief valves, ANSI/IIAR 6-2019 §13.1.4 states that “Hydrostatic/Internal PRVs that relieve internal to another portion of the closed-circuit refrigeration system shall be maintained in accordance with the manufacturer’s recommendations.”

In summary, overpressure protection must be installed where liquid ammonia can be trapped. Often, this is addressed with a hydrostatic relief valve. Where hydrostatic relief valves are installed, they must be maintained according to the manufacturer’s recommendations and are not required to be replaced every five (5) years, unless recommended by the manufacturer.