Relief Design Considerations

A designer of relief systems must be familiar with governmental codes, industrial standards, and insurance requirements. This is particularly important because local government standards may vary. Codes of particular interest are published by the American Society of Mechanical Engineers, the American Petroleum Institute, and the National Board of Fire Underwriters. Specific references have already been cited. It is recommended that relief designers carefully consider all codes and, where feasible, select the one that is most suited to the particular installation. 

Pipe Orifice Area of a Single Safety Relief in Vapor Service/ Should Not Exceed 2X of the Cross Sectional Area of the Protected Line. 0 Multiple Valves with Staggered Settings May be Required. 

Process Lines Should Not be Connected to Safety Valve Inlet Piping. 

Another important consideration is the reaction forces generated when the relieved materials flow through the relief system at high speed. The API RP 5207 has some guidelines however, normal stress analysis is the recommended method. 

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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. 

When the control valve size is established by the designer, for example selecting a CV, it is assumed that this size valve is installed. If the control valve size is later increased it may be necessary to recheck the PR valve relief considerations. 

Unlike relief system sizing for non-reacting systems, a considerable amount of experimental information is normally required for the design of chemical reactor relief systems. It is necessary to assess all the credible maloperations and system failures that may occur on the process/ plant to determine the reaction runaway that requires the largest relief system. The Workbook also summarises the main steps necessary to do this. 

It is important that no part of the total relief system (including both the relief system hardware and the, chemical reaction system for which the relief system has been designed) is modified without consideration of the safety implications on the overall system. This is facilitated by good documentation, as described above. 

At present, there is no comprehensive gujdance on this topic However, it has been the subject "of much, research, particularly by the Design Institute for Emergency Relief Systems (DIERS) of the American Institute of Chemical Engineers (AlChE). As a result, the DIERS Project Manual[1] (a, record of the DIERS research) and a considerable number of papers have been published. This Workbook summarises the main hand calculation methods (which do not.need. the use of a computer) available as a result of this work and their limits of applicability. A number of worked examples are given to. help the reader understand their application. The experimental information required to size an emergency relief, system properly is also discussed.. ... 

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