Page 433 - APPLIED PROCESS DESIGN FOR CHEMICAL AND PETROCHEMICAL PLANTS, Volume 1, 3rd Edition

Chapter

7

Process Safety and

Pressure-Relieving Devices

The subject of process safety is so broad in scope that possible injury to personnel, the loss or equipment can be
this chapter must be limited to the application design, rat- serious and an economic setback.
ing, and specifications for process over-pressure relieving Most states have laws specifying the requirements
devices for flammable vapors and dusts; process explo- regarding application of pressure-relieving devices in
sions and external fires on equipment; and the venting or
flaring of emergency or excess discharge of gases to a vent process and steam power plants. In essentially every
instance at least part or the reference includes the
flare stack. The subject of fire protection per se cannot be A.S.1\;1.E. Boiler and Pressure Vessel Code, Section VIII, Divi-
adequately covered because partial treatment would be sion 1 (Pressure Vessels) and/or Division 2 [1]; and Sec-
"worse" than no treatment; therefore, the engineer is tion VII, Recommended Rules for Care of Power Boilers [2]. In
referred to texts dealing with the subject in a thorough addition the publications of the American Petroleum
manner [l, 30, 31, 32, 33, 34]. The important topic or Institute are helpful in evaluation and design. These are
steam boiler safety protection is not treated here for the
sarne reason. API-RP-520 [ 10 J, Design and Installation of Pressure-Relieving
Systems in Refineries; Part 1-Desig;a; Part !I-Installation;
and API-RP-521 [13], Guide for Pressure Relief and Depres-
The possibilities for development of excess pressure surizing Systems, Ai"JSI/ASME B31.l Power Piping; Bl6.34;
exists in nearly every process plant. Due to the rapidly and NFPA [27], Sections 30, 68, and 69.
changing and improved data, codes, regulations, recom-
mendations, and design methods, it is recommended that The ASME Code requires that all pressure vessels be
reference be made to the latest editions of the literature protected by a pressure-relieving device that shall prevent
listed in this chapter. While attempting to be reliable in the internal pressure from increasing more than 10%
the information presented, this author cannot be respon- above the maximum allowable working pressures of the
sible or liable for interpretation or the handling of the vessel (MA',,VP) lo be discussed later. Except where multi-
information by experienced or inexperienced engineers. ple relieving devices are used, the pressure shall not
This chapter's subject matter is vital to the safety of plants' increase more than 16% above the Mi-\WP or, where addi-
personnel and facilities. tional pressure hazard is created by the vessel being
exposed to external heat (not process related) or fire,
It is important to understand how the over-pressure supplemented pressure relieving devices must be installed
can develop (source) and what might be the eventual to prevent the internal pressure from rising more than
results. The mere solving of a formula to obtain an orifice 21 % above the MAWP. See Ref. [l] sections U-125 and
area is secondary to an analysis and understanding of the UG-126. The best practice in industrial design recom-
pressure system. Excess pressure can develop from explo- mends that (a) all pressure vessels of any pressure be
sion, chemical reaction, reciprocating pumps or com- designed, fabricated, tested and code stamped per the
pressors, external fire around equipment, and an endless applicable ASME code [ 1] or American Petroleum Insti-
list of related and unrelated situations. In addition to the tute (API) Codes and Standards, Ref. [33] and (b) that

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