Relieving pressure

The autoclave is not the only component of an LDPE plant which may be exposed to a decomposition. Local hot spots in a secondary compressor may initiate a decomposition reaction consequendy it is necessary to protect these units from serious overpressure by pressure relieving devices and to release the products of the decomposition reactions safely. The problem of the aerial decomposition referred to eadier has been largely overcome by rapidly quenching the decomposition products as they enter the vent stack. 

Guide fior Pressure Relieving and Depressuring Systems, RP 521, 2nd ed., 1982 Safie Maintenance Practices in Refineries, RP 2007, 2nd ed., 1983. 

Pressure Relief Valves Definitions for pressure relief valves, relief valves, pilot-operated pressure relief valves and safety v alves, are found in the ASVIE Boiler and Pressure Assel Code, Section Division 1, Rules for Construction of Pressure Assels, Paragraphs UC-125 and UC-126, The pressure-relief valve is an automatic pressure relieving device designed to open when normal conditions are exceeded and to close again when normal conditions are restored. Within this class there are relief valves, pilot operated pressure relief valves, and safety valves. 

API RP 520. Sizing, Selection, and Installation of Pressure-Relieving Devices in Refineries. Part I, Sizing and Selection, 5th ed., July 1990, and Part II, InstaUation, 3d ed., November 1988. American Petroleum Institute, Washington, D.C. 

API RP 521. 1990. Guidefor Pressure-Relieving and Depressuring Sy.stems, 3d ed., American Petroleum Institute, Washington, D.C. 

National Board of Boiler and Pressure Vessel Inspectors. 1992. Pressure Relieving Device Cei tification (Red Book), National Board of Boiler and Pressure Vessel Inspectors, Columbus, Ohio. 

The time-dependent nature of the emergency pressure relieving event is obtained by the simultaneous solution of Eqs. (26-27) and (26-28). Generally, the only unknown parameters in these two equations are the venting rate W and the vent stream quahty (Xo). The vent rate W at any instant is a func tion of the upstream conditions and the relief system geometry. 

General References API Report 521, Guide for Pressure Relieving and... 

The OREDA Offshore Reliability Data Handbooks covers a variety of components used in offshore oil drilling and platforms, including gas/fire detection systems, process alarm systems, firefighting systems, pressure relieving systems, general alarm and communication systems, evacuation systems, process systems (vessels, valves, pumps, heat exchangers, and compressors), electrical and utility systems, and drilling equipment. 

Recommended Practice for the Design and Installation of Pressure Relieving Systems in Refineries, Part I— Design, latest edition. Part II— Installation, latest edition RP 520 American Petroleum Institute. 

API Recommended Practice 520 , Sizing, Selection, and Installation of Pressure Relieving Devices in Refineries, 1993. 

API RP 520—Recommended Practice for the Design (Part 1) and Installation (Part II) of Pressure Relieving Systems in Refineries, 1976(1) and 1973(11), American Petroleum Institute, 1220 L Street, NW., Washington, D.C., 20005. 

Rupture discs on pressure relieve valves and plugs in open ended valves can reduce fugitive emissions. 

This chapter describes the basic principles and procedures for the evaluation of overpressure potential in plant equipment, and for the selection, design and specification of appropriate pressure relieving facilities. The design of closed safety valves and flare headers is included in this chapter, but blowdown drums and flares are covered separately. To properly discuss this subject, the reader should become familiar with the following terminology. 

Relief Valve - A relief valve is an automatic pressure-relieving device actuated by the static pressure upstream of the valve, and whieh opens in proportion to the increase in pressure over the opening pressure. It is used primarily for liquid service. 

Safety Valve - A safety valve is an automatie pressure-relieving deviee actuated by the static pressure upstream of the valve and characterized by rapid full opening or pop action. It is used for gas or vapor service. (In the petroleum industry it is used normally for steam or air.)... 

Safety Relief Valve - A safety relief valve is an automatic pressure-relieving device suitable for use as either a safety valve or relief valve, depending on application. (In the petroleum industry it is normally used in gas and vapor service or for liquid.) Safety relief valves are classified as "Conventional" or "Balanced", depending upon the effect of back pressure on their performance. 

Consideration of Plant-wide or Refinery-wide Power Failure - Although not normally used as a basis for sizing pressure relieving facilities, the following general power failures on a plant-wide scale must be considered. 

When analyzing such individual control valve failures, one should consider the action of other control valves in the system. In the first two cases above, credit may be taken, where applicable for the reduction in pressure of a high-pressure source due to net inventory depletion during the period that the downstream equipment pressure is rising to relieving pressure. However, the pressure relieving facilities must be sized to handle the calculated peak flow conditions. 

Also, the vapor load at the time of relief may be reduced below the normal design rate, due to the higher pressure, which may suppress vaporization at the time of the overpressure. Pinchout of a reboiler is such a situation. In such a case, steam pressure design conditions may be used, rather than the maximum steam pressure which could exist under pressure relieving conditions of the steam system. These changes can be taken into account, where appropriate, both for the equipment involved and for downstream equipment. 

Increased Plant Capacity - When an existing plant capacity is increased, the entire pressure relieving system should be reevaluated, even though new equipment has been added. For example, a unit operating at 120 % of design capacity may require additional pressure relieving capacity. 

A three-way control valve which sphts or combines two flows in a pressure relieving path is acceptable, provided that the total open cross-sectional area is at least equal to that of the adjacent piping in all valve positions. Also, the valve should be of the type that never has a fully closed port. 

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