Relieving rates

Design for overpressure protection in most cases consists of providing pressure-rehef devices sized to handle the calcidated relieving rates necessary to prevent emergency pressures from rising above the design pressure (plus allowable accumulation). 

The required relieving rates for these PR valves are based upon the steam rate to the turbine As a rough guide, the appropriate size can be checked against the following tabulation, which are recommendations of the "Standards of the Heat Exchanger Institute, Surface Condenser Section."... 

Liquid service PR valves are characterized by progressively increased lift with increasing inlet pressure, rather than the "pop" action of vapor service valves. Liquid service valves are, therefore, less liable to chattering at low relieving rates, and they will modulate down to about 10% of design flow. 

The required relieving rate for a pressure relief valve is determined from consideration of the contingencies which can cause overpressure. Basic... 

In PR valve design, it is desirable to select a PR valve discharge location at a low enough pressure to permit designing for critical flow conditions, so that the relieving rate will be independent of minor back pressure fluctuations. 

The following is a procedure for caleulating the required relieving rates for a vessel that is exposed to an external fire. 

A vessel may be subject to more than one condition under different failure scenarios. For example, a low pressure separator may be subject to blocked discharge, gas blowby from the high pressure separator, and fire. Only one of these failures is assumed to happen at any time. The relief valve size needs to be calculated for each pertinent relieving rate... 

Relief valves are often seen to be undersized for the required relieving rate, due either to poor initial design or changes in the process conditions which occurred during design. The most common system problem is that the relief valve was adequately sized for blocked discharge but not sized for the flowrate that could occur as a result of a failure in the open position of an upstream control valve (i.e., gas blowby). See Chapter 13. 

Unfortunately, project schedules may require that SRV sizing be carried out long before the fabricator has finished the mechanical design and certified the MAWP of his vessel. The process engineer must then use some pressure on which to base the relieving rate calculations. Paragraph 1.2.3.2 (c), API 520 states that the design pressure may be used in place of the MAWP in all cases where the MAWP has not been established. 

Another area of confusion might involve the definition of capacity and how the term is used in ASME and API. Relieving rates are determined from what can go wrong scenarios and, if allowed to go unchecked, would overpressure the vessel. 

The computer program PROG34 determines the overall pressure drop for the 6-inch tail pipe having a relieving rate of 27,000 Ib/hr. Table 3-11 illustrates both the input data and computer output. The Mach number at inlet condition is 0.0169, and at the critical condition is 0.8874. The critical pressure is 7.985 psia and the overall pressure drop is 0.213 psi. The compressible fluid flow pattern through the pipe is SUBSONIC. 

This recommended practice provides guidelines for examining the principal causes of overpressure determining individual relieving rates and selecting and designing disposal systems, including such component parts as vessels, flares, and vent stacks... 

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