Conventional PR valves

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. 

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. 

Conventional PR valves and discharge systems should be designed such that built-up back pressure does not exceed 10% of set pressure (both measured in psig), to avoid chattering problems. In the case where a pressure relief valve system is sized for fire conditions, with 21 % overpressure, built-up back pressure up to 21 % of set pressure is permissible. However, the lower rates resulting from other contingencies still must meet the 10% limitation. 

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... 

Excessive Built-up Back Pressure - Built-up back pressure resulting from discharge flow through the outlet system of a conventional PR valve results in a force on the valve disc tending to return it to the closed position. If this returning force is sufficiently large, it may cause the valve to close, only to reopen immediately when the effect of built-up back pressure is removed. Chattering results from the rapid repetition of this cycle. 

Back Pressure - In the case of a conventional PR valve, the "maximum baek pressure" column should specify the maximum superimposed baek pressure (kPa gage) under nonflowing conditions. If the diseharge is routed to the atmosphere, or to a safety valve header where the pressure is essentially atmospherie under no flow, the maximum back pressure should be specified as zero. 

The operation and characteristics of a conventional safety relief valve are illustrated diagrammatically in Figure 3. The action of the valve as pressure rises from the initial normal operating pressure (assuming no back pressure) is described below. The effect of back pressure on PR valve operation is described later. 

Balanced bellows PR valves need not be restricted to the same built-up back pressure limit (10% of set pressure) as are conventional valves, since they are not subject to chattering from this cause. However, maximum back pressure is limited by capacity and in some cases by the mechanical design strength limitations of parts such as the outlet flange, bellows, or valve bonnet. 

It is important to note that back pressure affects balanced PR valve capacities in the same way as for conventional valves, and appropriate factors are included in the sizing procedures. They are subject to the same recommended limits of maximum total back pressure (superimposed plus built-up) as conventional valves. In the case of balanced bellows valves, mechanical considerations must also be evaluated, since they may limit the maximum permissible back pressure. 

Effect of Temperature on Back Pressure Limits of PR Valves - Maximum back pressure limits are specified by the valve vendor. Usually the vendor s specification is given to a reference temperature (normally 38 °C) for both conventional and bellows valves. These limits must be reduced for higher temperatures, as follows ... 

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