Constant backpressure

VARIABLE OR CONSTANT BACKPRESSURE SIZING FACTOR. K,. FOR BALANCED BELLOWS SAFETY RELIEF VALVES (VAPORS AND GASES)... 

Sizing for Gases or Vapors or Liquids for Conventional Valves with Constant Backpressure Only... 

Kv, = 1.0 (Figure 7-26), constant backpressure with variation not to exceed 10% of set pressure. 

Figure 7-27A, Variable or constant backpressure sizing factor, Ky, at 10% overpressure, BalanSeal valves only vapors and gases. By permission, Teledyne Farris Engineering Co.

Kh = Capacity correction factor due to backpressure. For standard valves with superimposed (constant) backpressure exceeding critical see Appendix B. 

Constant superimposed backpressure Usually backpressures that occur when a safety valve outlet is connected to a static pressure source and doesn t change appreciably under any conditions of operation. In this case, conventional valves may be used if the valve spring setting is reduced by the amount of the constant backpressure (Figure 3.8). 

In case of constant backpressure Actual set pressure = bench set + backpressure... 

Required set pressure 10 bars constant backpressure 2 bars Bench set pressure 10 - 2 = 8 bars... 

Kb = Capacity correction factor due to backpressure. For standard valves with superimposed (constant) backpressure exceeding critical values, see Appendix B for reference values, but consult the manufacturer s data also. EN 4126 requires physical testing of these values. 

For saturated steam, use Ksh = 1. For other values, see Appendix E Kn = Capacity correction factor for dry saturated steam at set pressures above 10,346 kPaA and up to 22,060 kPaA. See Appendix F Kb = Capacity correction factor due to backpressure. For standard valves with superimposed (constant) backpressure exceeding critical values, see Appendix B (Table B. 1) for reference values but consult the manufacturer s data. EN 4126 requires physical testing of these values... 

The approximate backpressure correction factor for vapours and gases, Kb for most conventional valves with constant backpressure and backpressure-compensated pilot-operated valves with backpressures exceeding critical pressure (generally taken as 53%-55% of accumulated inlet pressure absolute), is ... 

Practically speaking, problems may arise in both low- and high-viscosity solvent systems. In low-viscosity situations such as 99/1 v/v hexane/efliyl acetate, little backpressure is generated across the column even at high flow rates. This may lead to improper seating of the inlet and outlet check valves and result in erratic flow rates. To rectify this problem, a flow restrictor may be placed after the pump and before the injector to create a higher and constant backpressure on the check valves. 

Figure 5 A typical rupture pin pressure relief device (a) for constant backpressure, (b) for variable backpressure. fCouriesy of Rupture Pin Technology, inc,]...

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