Superimposed back pressure

This is fhe pressure at the outlet of the PRV and is the sum of built-up and superimposed back pressures. The impact of back pressure to the selection of the type of PRV can be summarized as under  

Process engineering and design using Visual Basic 

This is the pressure at the outlet of tiie PRV when the valve is in a closed position. The superimposed back pressure arises from other sources in the discharge system and it may be eitiier constant or variable. 

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Spring Pressure - The spring pressure is equal to the set pressure minus the superimposed back pressure for a conventional PR valve. For a balanced bellows safety relief valve, the spring pressure equals the set pressure. 

Superimposed back pressures are not constant. (Where back pressures fluctuate on a conventional valve, the valve may open at too low a pressure or permit the vessel pressure to exceed the equipment rating, depending upon back pressure fluctuation.)... 

Although venting to the atmosphere as described above is preferred, an alternative is to tie into a closed low pressure system, if available. This method may be used in the case of severely toxic fluids. Minimum length vent piping should be used. The effects of any back pressure must be thoroughly examined, since in such a case, superimposed back pressure is additive to the spring force. 

For applications involving unusually high superimposed back pressure, a pilot operated valve may be the only possible balanced valve that is commercially available, because of the mechanical limitations which apply to bellows. 

Conventional PR valves subject to a constant superimposed back pressure are designed so that they will open at the required set pressure, by appropriate reduction in spring pressure. 

Conventional PR valves that are exposed to variable superimposed back pressure will open at correspondingly variable pressure, since the superimposed back pressure is additive to the spring force. 

Balanced bellows valves need no reduction in spring pressure to compensate for superimposed back pressure, and they can tolerate variable superimposed back pressure without an effect on opening pressure. 

If the superimposed back pressure is less than the calculated critical flow pressure, the capacity of a conventional PR valve in vapor service is unaffected and back pressure is not a factor. However, builtup back pressure on a conventional pressure relief valve will affect its flow capacity and operating characteristics, and should not exceed 100% of its set pressure. If total back pressure (superimposed plus built-up) is greater than the calculated critical flow pressure, the capacity of a conventional PR valve in vapor service is affected, and total back pressure is incorporated into the sizing procedure. Any back pressure reduces the capacity of a conventional PR valve in liquid service, and... 

Differential Spring Pressure - For conventional valves, the differential spring pressure equals the set pressure minus the maximum superimposed back pressure under non-flowing conditions. For balanced valves, the opening pressure is not affected by baek pressure, and the differential spring pressure equals the set pressure. 

As an alternative, a balanced bellows valve could be used since it is not affected by a variable superimposed back pressure. 

Back Pressure - The combined atmosphere discharge system must be designed to comply with the superimposed back pressure limitations. 

Superimposed back pressure The static pressure existing at the outlet of a pressure relief device at the time the device is required to operate. It is the result of pressure in the discharge system from other sources. 

Superimposed Back Pressure Units 51. Orifice Designation (letter) ... 

Superimposed Back Pressure Units 59. Calculated Orifice Area (in square in.) ... 

A V. It may be noted that the flat EO flow profile in Figure 6.10 now has a parabolic dent due to the superimposed back-pressure-driven Poiseuille flow profile Up... 

For pilot-operated PRVs, the variation in superimposed back pressure does not affect the opening pressure. 

This is the pressure at which the PRV is adjusted to open the test stand. The cold differential test pressure includes corrections for the service conditions of superimposed back pressure, temperature, or both. 

Pg = superimposed back pressure in pounds per square inch gauge,... 

Conventional PRVs are the most widely used relief devices in chemical industries, particularly in a system in which the superimposed back pressure is relatively low. Conventional PRVs are not very suitable for high back pressure. 

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