Liquid pressure relief device sizing

V11.2 Liquid pressure relief device sizing There are two different type of sizing depending on whether the PRD to be certified or not (by ASME code). 

V1L2.1 Certified liquid pressure relief device sizing ASME code requires that PRV used for liquid relief should have capacity certification, guaranty PRV full open at 10% overpressure. Following equations should be used for liquid PRD sizing ... 

VIT.2,2 Non-cetlified liquid pressure relief device sizing This applies to liquid PRV before ASME code set capacity certification requirement. Interested reader can find its sizing requirement in API 520, section 5.9. [1]... 

The method used for the safe installation of pressure relief devices is illustrated in Figure 8-1. The first step in the procedure is to specify where relief devices must be installed. Definitive guidelines are available. Second, the appropriate relief device type must be selected. The type depends mostly on the nature of the material relieved and the relief characteristics required. Third, scenarios are developed that describe the various ways in which a relief can occur. The motivation is to determine the material mass flow rate through the relief and the physical state of the material (liquid, vapor, or two phases). Next, data are collected on the relief process, including physical properties of the ejected material, and the relief is sized. Finally, the worst-case scenario is selected and the final relief design is achieved. 

The most important safety devices in a production facility are the pressure relief valves, which ensure that pipes, valves, fittings, and pressure vessels can never be subjected to pressures higher than their design pressures. Relief valves must be designed to open rapidly and fully, and be adequately sized to handle the total flow of gas and liquids that could potentially cause an overpressure situation. They relieve the pressure by routing this stream to a safe location where it can be vented to atmosphere or burned. 

Figure 5-7. Capacity correction factor, K, due to back pressure on balanced-bellows pressure relief valves in liquid service. (API Recommended Practice 520, Sizing, Selection and Installation of Pressure Relieving Devices in Refineries, Part 1, 5th ed., 1990. Reprinted courtesy of the American Petroleum Institute.)...

For liquid discharges, the design of the pipework may be complex. If the liquid is subcooled, the discharge pipework is sized by standard liquid pressure formulae. If the liquid in the equipment is at or near its saturation pressure, the relief device and discharge pipework should be sized for two-phase flow. As the liquid flows through the relief valve. 

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