Fundamentals of safety relief systems
ASHRAE Journal, Feb, 2008 by Douglas T. Reindl, Todd B. Jekel
Ashrae made significant changes in 2001 to the calculations required for sizing vent lines for the safety relief systems applied to refrigerating systems in its Standard 15, Safety Standard for Refrigeration Systems. This article provides a review of the fundamental requirements for properly engineering and installing safety relief systems. In addition, the authors review and clarify some of the newly implemented changes to safety relief systems.
Pressure relief devices are an engineering control designed to self-actuate and permit flow as a means of protecting system components by preventing their catastrophic failure during operating excursions that create overpressure. ANSI/ASHRAE Standard 15-2007 defines a pressure relief valve as “a pressure-actuated valve held closed by a spring or other means and designed to automatically relieve pressure in excess of its setting.”
By permitting vapor to flow out of the protected component through a pressure relief valve, the pressure in the protected component is reduced, preventing its catastrophic failure. For pressure relief devices to be effective at achieving their intended purpose of protecting components in a safe manner, a properly engineered safety relief system is essential. A safety relief system is comprised of the necessary pressure relief devices (pressure relief valves, rupture disks, fusible plugs, etc.), valves (three-way, stop), fittings (elbows, tees), piping (inlet and outlet), and treatment subsystems (atmospheric diffuser, water tank, flare) arranged to direct the refrigerant vapor causing the overpressure to a safe terminal location.
In this article, we review requirements for the application of pressure relief protection on refrigeration systems and outline some of the basic steps to properly engineer a safety relief system.
Equipment Requiring Relief Protection
Section 9.4 of Standard 15-2007 prescribes equipment requiring pressure relief protection, limitations on the use of isolation valves in a relief vent piping system, materials of construction for relief valves, and maximum changes in the relief valve’s set pressure over time. The equipment requiring pressure relief protection includes:
* Pressure vessels ([section]9.4.2 and [section]9.7.2);
* Liquid containing parts of a system capable of being isolated ([section]9.4.3);
* Evaporators located within 18 in. (457 mm) upstream or downstream of a heating coil ([section]9.4.4); and
* Positive displacement compressors ([section]9.8).
Table 1 summarizes pressure relief protection requirements.
Although permitted by the ASME Boiler and Pressure Vessel Code, (1) intervening stop valves between the pressure relief device and the component being protected are not permitted by Standard 15-2007 ([section]9.4.6). In situations where more than one relief device discharges into a common manifold or header, full area stop valves are permitted at the outlet of individual relief devices if the stop valve is car-sealed (locked) open during normal operation ([section]9.4.6). In practice, these stop valves are rarely used.
Section [section]9.4.9 of Standard 15-2007 requires that the materials of construction used in the relief valve be compatible and robust such that the valve’s set pressure does not change by more than 5% over a five-year span. Additional discussion on materials of construction for vent piping is provided in the “Relief Piping” section of this article.
Pressure Relief Device–Certification
Pressure relief valves selected for protecting ASME-stamped vessels and equipment must be designed and constructed in accordance with the ASME Boiler and Pressure Vessel (B&PV;) Code Section VIII Division 1 (UG-131) per Standard 15-2007, Section 9.4.2. The capacities of these pressure relief devices are then certified by the National Board of Boiler and Pressure Vessel Inspectors
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