Relief valve design

In production facility design, the most common relieving conditions are (1) blocked discharge, (2) gas blowby, (3) regulator failure, (4) fire. (5) thermal, and (6) heat exchanger tube rupture. Relief valve design How rates are commonly determined as follows. 

A valve which automatically, without the assistance of any energy other than that of the fluid concerned, discharges a certified quantity of the fluid so as to prevent a predetermined safe pressure being exceeded, and which is designed to re-close and prevent further flow of fluid after normal pressure conditions of service have been Restored. See 5.2.2. In this Workbook, "safety valve" is used to mean any type of safety or relief valve (following the definition in BS 6759). The term is sometimes used to denote relief valves designed for gas/ vapour service only. 

Thermal relief valves are small, usually liquid relief valves designed for very small flows on incompressible fluids. They open in some proportion of the overpressure. Thermal expansion during the process only produces very small flows, and the array of orifices in thermal relief valves is usually under the API-lettered orifices, with a maximum orifice D or E. It is, however, recommended to use a standard thermal relief orifice (e.g. 0.049in2). Oversizing SRVs is never recommended since they will flow too much too short, which in turn will make them close too fast without evacuating the pressure. This will result in chattering of the oversized valve and possible water hammer in liquid applications. 

The safety valve is a pressure relief valve that is designed to open fully, or pop, with only a small amount of pressure over the rated limit. Where conventional safety valves are sensitive to downstream pressure and may have unsatisfactory operating characteristics in variable backpressure applications, pressure-balanced safety relief valve designs are available to minimize the effect of downstream pressure on performance. 

The DOT rules require that pressure cars have relief valves designed to limit pressure to 82.5 percent (with certain exceptions) of test pressure (110 percent of maximum operating pressure) when exposed to fire. Appendix A of AAR Specifications deals with the flow capacity of relief devices. The formulas apply to cars in the upright position with the device discharging vapor. They may not protect the car adequately when it is overturned and the device is discharging liquid. 

Figure 36-1 is a schematic of a commercially available closed digestion vessel designed for use in a microwave oven. It consists of a Teflon body, a cap, and a safety relief valve designed to operate at 120 10 psi. At this pressure, the safety valve opens and then reseals. 

A primary aspect of relief valve design is to provide an increase of fluid force on the disk to raise it against the spring. When this is achieved without an increase of reservoir pressure, but as a result solely of the changing flow structure in a lifting valve, it is known as lift-assistance. Without lift-assistance, valves will open smoothly and linearly with overpressure. It is the lift-assistance that causes the classic popping action of valves used with compressible fluids (i.e., sudden and rapid opening). 

Most companies have an accident/incident analysis process that identifies the proximal failures that led to an incident, for example, a flawed design of the pressure relief valve in a tank. Typical follow-up would include replacement of that valve with an improved design. On top of fixing the immediate problem, companies should have procedures to evaluate and potentially replace all the uses of that pressure relief valve design in tanks throughout the plant or company. Even better would be to reevaluate pressure relief valve design for all uses in the plant, not just in tanks. 

Three safety relief valves (designed for vapors) opened and relieved flammable liquid for about 6 min. The relief system routed liquid to the blowdown drum. The... 

Residual Heat Removal Relief Valve-Design Parameters Number... 

Process Design Responsibility for Relief Valve Design... 

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