Safety relief systems

The maximum pressure from an explosion of a hydrocarbon and air is 7 x initial pressure, unless it occurs in a long pipe where a standing wave can be set up. It may be cheaper to design some small vessels to withstand an explosion than to provide a safety relief system. It is typical to specify %" as minimum plate thickness (for carbon steel only). 

Many columns have water removal trays designed into the column. Top or bottom temperatures may have to be changed to expel the water if the column isn t provided with water removal trays. In some instances, the water can be expelled by venting the column through the safety relief system. 

Safety relief systems are verified as part of PSM. This includes the PS Vs themselves and also flare system piping networks. Safety relief valves are covered in Section I—Fluid Flow. A good procedure for sizing the flare system piping is found in Section 19—Safety-Relief Manifolds. This method, first published in the Oil and Gas Journal, has been adopted by APl. I have also used... 

The DIERS Project Manual (published by the American Institute of Chemical Engineers) is a helpful compendium for experienced safety-relief system engineers. Extensive background and experience are required to properly understand and apply the methodology. Help is available from the DIERS contractors and the DIERS Users Group. 

Crowley, C. J., and J. A. Block. 1989. Safety relief system design and performance in a hydrogen peroxide system. International Symposium on Runaway Reactions, pp. 395-424. New York AIChE cited in Chem. Abstr. CA 111(4) 25668X. 

VCM was accidentally released to the atmosphere at the Dhopolis about 2 morrths prior to the 2004, fatal incident. An operator rrristakerrly routed the pressrrrized contents of an operating reactor to a stripper tank. When the reactor contents reached the stripper tank, which was designed for lower pressure, the safety relief system opened and VCM escaped. 

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. 

Balanced safety relief valves may be used in systems where built-up and/or superimposed backpressure is high or variable. In general, the capacity of a b anced valve is not signincantly affected by backpressures below 30 percent of set pressure. Most manufacturers recommend keeping tne backpressure on balanced valves below 45 to 50 percent of the set pressure. 

Since discharges of vapors from highly hazardous toxic materials cannot simply be released to the atmosphere, the use of a weak seam roof is not normally acceptable. It is best that tanks be designed and stamped for 15 psig to provide maximum safety, and pressure relief systems must be provided to vent to equipment that can collect, contain, and treat the effluent. 

Wilday, A.J. 1991. The Safe Design of Chemical Plants with No Need for Pressure Relief Systems. Elazards IX—New Directions in Process Safety. IChemE Symposium Series. No. 124, pp. 243-253. Institute of Chemical Engineers, IChemE, Rugby, U.K. 

Some method of pressure relief is required on all pressure vessels and for other proeess equipment where inereasing pressure might rupture the vessel. Mueh of the piping used in modern ehemieal operations also requires overpressure proteetion. Safety relief valves or rupture dises are employed for pressure relief. In many eases, either a rupture dise or a safety relief valve ean be used. Safety relief valves are usually used for proeess proteetion and rupture dises are used for vessel proteetion. The safety relief valve or rupture dise must be designed to operate at a known pressure and prevent the pressure within the system from inereasing. Therefore, it is important to eon-sider the flowrate the valve ean handle. 

Waldram, S. P, Toll Manufaeturing Rapid Assessment of Reaetor Relief Systems for Exothermie Bateh Reaetions, Hazards XII European Advanees in Proeess Safety, IChemE, Symp., Series No. 134, pp. 525-540, 1994. 

A. B. Smith, Safety Relief Valves on Pressure Systems, Reprint No. C454/001/93, Institution of Mechanical Engineers, London, 1993. 

Thus the maximum flow in a pipe occurs when the velocity at the exit becomes sonic. The sonic location may be other than the exit, can be at restrictive points in the system, or at control/safety relief valves. 

Integral safety relief valve works to protect system against excessive pressure. 

There are many design features and styles of safety relief valves, such as flanged ends, screwed ends, valves fitted internally for corrosive service, high temperature service, cryogenic service/low temperatures, with bonnet or without, nozzle entrance or orifice entrance, and resistance to discharge piping strains on body. Yet most of these variations have little, if anything to do with the actual performance to relieve overpressure in a system/vessel. 

WThen the pressure rise in a system is gradual and not explosive in nature, a safety or safety relief valve is the proper detdce, but when it is critical to completely depressure a system or the rate of pressure increase might be expected to be rapid, then a rupture disk is the proper device. Properly designed a pilot operated valve may be selected after checking its performance with the manufacturer. 

It must be emphasized, that the determination of the anticipated maximum overpressure volume at a specified pressure and temperature is vital to a proper protection of the process syste.m. The safety relief calculadons should be performed at the actual worst condidons of the system, for example, at the allowable accumulated pressure and its corresponding process temperature. These can be tedious and perhaps time-consuming calculations, but they must not be glossed over but developed in a manner that accounts for the seriousness of the effort. They must be documented carefully and preserved permanently. 

Accordingly, to emphasize the safety problems affecting all industrial process plants and laboratories, the American Institute of Chemical Engineers established the industry-supported Desigm Institute for Emergency Relief Systems. The purposes of the Institute are [51] ... 

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