Maximum backing pressure

Pilot-operated valves may be satisfactorily used in vapor or liquid services up to a maximum back pressure (superimposed plus built-up) of 50% of set pressure, provided that the back pressure is incorporated into the sizing calculation. At higher back pressures, capacity becomes increasingly sensitive to small changes in back pressure. As an exception, back pressure up to 7% of set pressure may be used, provided that this disadvantage is recognized. 

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. 

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. 

For balanced bellows pressure relief valves, the maximum superimposed baek pressure should be specified. (For balanced bellows valves discharging to a safety valve header which is at atmospheric pressure under non-flowing conditions, the maximum back pressure is zero.)... 

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 ... 

In the case of a balanced bellows pressure relief valve, to the maximum pressure permitted by considerations of bellows and bellows bonnet flange mechanical strength. This maximum pressure may be obtained by applying the following correction factor to the maximum back pressure listed for 38 °C. 

The sum of all the pressure losses starting from the flare staek up to the safety valve yields the total back pressure in psig. This back pressure must be lower than the maximum back pressure allowed in the system and corresponding to the lowest set pressure of the safety valve. 

For non-critical flow the maximum back pressure must be set and pressure drop calculated by the usual friction equations. 

II BalanSeal valve construction is used and variable back pressure conditions exist, use the maximum back pressure to determine Pn in equation and correct K factor (see graph page 44)... 

The forevacuum pressure also influences the vapor jet and becomes detrimental if its value exceeds a certain critical limit. This limit is called maximum backing pressure or critical forepressure. The capacity of the... 

Assume that the back pressure on the safety, valve will be limited to 10% of the gauge set pressure. (The back pressure required is the sum of the constant and built-up back pressure. If the relief system discharges direct to the atmosphere, then the constant back pressure will be zero.) If the relief pressure is 4.4 barg, the set pressure is 4.4/1.1 = 4.0 barg, and the maximum back pressure is 0.1 x 4.0 = 0.4 barg. Thus 4... 

Table 9.2 Maximum back pressure percentage on liquid applications...

The equation for A is valid for a maximum back pressure of 0.5Pi for higher back pressure a correction can be made by means of Figure 6-12. 

Fig. 2. Relationship between the specified maximum back pressure and the allowable heat sink temperature rise.

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