(87a) Overlooked Factors in Pressure Relief Systems Design

Typically when engineers refer to a pressure relief study, the first idea that comes to their minds is relief valves and pressure drop calculations for the pressure relief system network. While it is true that sizing relief valves, calculating relief loads, pressures and velocities through the system represent the core of the study, usually other important design aspects of the system are left out and should be considered according to industry standards, codes and recommended practices.

Since a pressure relief system represents the way a plant controls regulated vent streams and handles non routine emissions (e.g., purges, emergency releases) its proper design is crucial to the plant’s overall safety and the environment.  Unfortunately, since this system does not produce revenue, its importance is often under estimated and its design under-considered. Companies often under-budget projects that involve pressure relief system projects and usually rely exclusively on the capability of design software.

A comprehensive pressure relief system evaluation requires a high level of technical knowledge and deep understanding of the importance of this system to the integrity of the unit and the environment. Some factors that are often neglected involve the following:

• Sizing the PSV outlet lines based on the relief devices relieving independently. It is common to perform a pressure relief study by evaluating only the global relieving scenarios that overpressure the system and produce multiple relieving at a time. While this consideration is good for the header and sub-header sizing, this evaluation may undersize the individual PSV outlet pipes. The evaluation of PSVs relieving independently will give the most conservative size of the PSV outlet pipe, as referenced in API 521.

• Piping and equipment design temperature. In some relieving scenarios the relieving temperature  could exceed the design temperature of piping and equipment that could result in damage of the system in a relieving event.

• Noise evaluation. High noise level due to a undersized piping system and a poor disposal stack design represents environmental and safety concerns.

• Emissions dispersion and thermal radiation. Design, height and location of the flare stack is based on dispersion and thermal radiation evaluation. If these factors are not considered, it will cause environmental, personnel and equipments problems.

• Other factors such as flame properties, smoke production and volatile emissions.  Exceeding these factors’ limits will cause environmental pollution and could be associated with fuel properties, mix of combustible gas - air properties and design of the pressure relief or vent stack.

This paper will identify those factors required in basic industry standards, codes and recommended practices which are often overlooked in pressure relief system design. Industry standards, codes and recommended practices such as the API 520, API 521, ASME B31.3, ASME B31.4, ASME B31.8 and ASME Section VIII Division 1 are going to be discussed, as well as some federal and environmental codes. Incorporating these factors will result in a safer and more effective pressure relief systems.