IIAR 2-2021: Hydrostatic Overpressure Protection Requirements

The 2021 edition of IIAR Standard 2 has improved clarity on the minimum requirements for protecting equipment and piping from hydrostatic overpressure. The term “hydrostatic overpressure” refers to the pressure exerted when trapped liquid is subject to thermal expansion. Common components in ammonia refrigeration systems that may be subject to hydrostatic overpressure include:

• Oil coolers
• Oil pots
• Heat exchangers

The following list provides a bird’s eye view of the hydrostatic overpressure requirements in ANSI/IIAR 2-2021:

• Any ASME-stamped piece of equipment expected to operate filled with liquid ammonia must be equipped with a certified liquid-rated ASME relief valve. [ANSI/IIAR 2-2021 §15.3.10, ANSI/IIAR 2-2021 §15.3.10.4]
• Equipment that is not ASME-stamped does not require protection against hydrostatic overpressure if trapping of liquid and subsequent thermal expansion is only possible during maintenance or service operations. When this is the case, engineering or administrative controls are adequate as a means of protection. [ANSI/IIAR 2-2021 §15.3.10.1]
• If liquid can be trapped in non ASME-equipment automatically during normal operation, during shutdown, during planned isolation, due to equipment fault, or during shipping, then hydrostatic protection is required. [ANSI/IIAR 2-2021 §15.3.10.1]
• The following device types are approved for mitigating hydrostatic overpressure in non ASME-stamped equipment:
  • A hydrostatic relief device, or check valve, relieving to another part of the closed-circuit system;
  • An expansion compensation device;
  • A control valve with an internal design that will relieve downstream pressure to the upstream side of the control valve.
    [ANSI/IIAR 2-2021 §15.3.10.2]
• Full area shut-off valves can be used on the outlet of any hydrostatic relief device. Full area shut-off valves can be used on the inlet of non ASME-stamped equipment only. [ANSI/IIAR 2-2021 §15.3.10.5]
• Any ASME-stamped equipment that is connected to the ammonia refrigeration system and designed to be filled with a non-volatile liquid (e.g. oil, water, glycol solution) must be protected with a certified liquid-rated ASME relief valve. Full area stop valves are permitted upstream and downstream so long as controls are implemented to ensure that the valves are not closed and that protection is provided during isolation. [ANSI/IIAR 2-2021 §15.3.10.6.1]
• ANSI/IIAR 2-2021 Appendix G provides informational guidance for selecting a hydrostatic relief valve.