Pressure relief design considerations

An accelerating rate calorimeter (ARC) can be used to provide design values for emergency pressure-relief flow requirements of runaway systems. The ARC is a device used to obtain runaway history of chemical reactions in a closed system (DeHaven, 1983 Huff, 1982,1984a Townsend and Tou, 1980). The experimental technique is fairly straightforward, but considerable engineering expertise is required to do the calculations needed to design a relief system from the ARC data. 

A pressure relief system is normally required on all bioreactors and pressurised tanks as a safety feature to comply with pressure vessel design regulations. In a few cases, some companies appear to have overcome the need to use a pressure relief system on the vessel either by ensuring pressure relief is provided on relevant pipework to the vessel and/or ensuring there are no pumps transferring material which could lead to a build up of pressure if the outlet pipework were closed. The absence of pressure relief on the vessels can considerably simplify the process plant. The safety and regulatory requirements, as well as insurance inspection requirements, for pressure relief is currently a confused area. 

Discharge of pressure relief systems and leaks . Although strictly not a category of waste distinct from those above, the design of a containment and treatment system for effluent must take into account the likelihood of the unexpected. Further consideration of these aspects is given later in this chapter. 

Explosion relief for vapor-air combustion within a closed top low-pressure tank cannot be accomplished by customary tank conservation vents or other Ught-weight protection. A well-written brief description of safety features of low-pressure tanks can be found in Volume 32 of the Institution of Chemical Engineers Loss Prevention Bulletin [5]. An excellent understanding of design considerations of the weak roof can be found under in section 1.3.5, from which the following paragraphs are quoted ... 

Safety Relief Valves Conventional safety relier valves (Fig. 26-14) are used in systems where built-up backpressures typically do not exceed 10 percent of the set pressure. The spring setting or the valve is reduced by the amount of superimposed backpressure expecied. Higher built-up backpressures can result in a complete loss of continuous valve capacity. The designer must examine the effects of other relieving devices connected to a common header on the performance of each valve. Some mechanical considerations of conventional relief valves are presented in the ASME code however, the manufacturer should be consulted for specific details. 

If casing limitations are fixed by user-supplied relief valves, this information should be conveyed to keep the vendor from rating the compressors on other data. Evaluations can be more of a problem if the same design basis isn t universal with all vendors. Startup and shutdown consideration influence various components, shaft end seals, seal system pressures, and even thrust bearings in some instances. The use of an alternate startup gas, or the desire to operate a gas compressor on air to aid in plant piping dryout should be covered. 

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