CH-09-061 — Pressure Relief Device Capacity Determination

Pressure relief valves are the primary component in an engineered safety system intended to prevent the catastrophic failure of refrigeration equipment due to overpressure conditions that can occur during abnormal standby or operating excursions. ASHRAE Standard 15 (ASHRAE 2007) prescribes methods for determining the mass flow rate (capacity) requirements for pressure relief devices aimed at protecting the pressure vessels and positive displacement compressors used in refrigeration systems. With the good intention of enhancing system safety, designers, contractors, or owners often install relief devices on other types of refrigeration equipment that may include: heat exchangers, piping and pumps. Unfortunately, the sizing and selection of pressure relief devices in these situations often occurs without the use of a clear and consistent basis; thereby, decreasing rather than increasing system safety.

This paper reviews the pressure relief valve sizing methodology for vessels and presents approaches for relief capacity determination applicable for other types of refrigeration equipment not explicitly covered in ASHRAE 15 including oil separators, shell-and-tube heat exchangers, plate-and-frame heat exchangers, oil cooling heat exchangers, and product storage tanks. One of the principle aims of this paper is to document a basis for relief device capacity determination to ensure these types of protected components remain safe during abnormal excursions that can lead to high pressures. Although the methods presented in this paper are intended to apply across a wide range of refrigeration equipment and operating conditions, it is not possible to neatly prescribe relief device sizing and selection criteria to cover all situations. As such, the use of sound engineering principles and the application of engineering judgment should be always expected.