Effects of Back-Pressure

Safety Relief Valve - A safety relief valve is an automatic pressure-relieving device suitable for use as either a safety valve or relief valve, depending on application. (In the petroleum industry it is normally used in gas and vapor service or for liquid.) Safety relief valves are classified as "Conventional" or "Balanced", depending upon the effect of back pressure on their performance. 

Balanced Bellows Safety Relief Valve - A balanced safety relief valve incorporates means for minimizing the effect of back pressure on the performance characteristics opening pressure, closing pressure, lift and relieving capacity. This is usually achieved by the installation of a bellows. 

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. 

Figures 17. Effect of back pressure on (A) conventional safety valve and (B) piston type valve.

Back-pressure can affect either the set pressure or the capacity of a relief valve. The set pressure is the pressure at which the relief valve begins to open. Capacity is the maximum flow rate that the relief valve will relieve. The set pressure for a conventional relief valve increases directly with back-pressure. Conventional valves can be compensated for constant back-pressure by lowering the set pressure. For self-imposed back-pressure—back-pressure due to the valve itself relieving—-there is no way to compensate. In production facility design, the back-pressure is usually not constant. It is due to the relief valve or other relief valves relieving into the header. Conventional relief valves should be limited to 10% back-pressure due to the effect of back-pressure on the set point. 

Figure 6-15. Effect of back-pressure on singie-stage exhausters. By permission, Ketema, Schutte Koerting Div. and Ketterer, S. G. and Biatchiey, C. G. [8],...

Figure 15.23 The effect of back-pressure upon steam turbine performance...

Air/water vapor mixture, chart, 364,365 Air/water vapor, 359 Capacity at ejector suction, 369 Capacity for process vapor, 362 Evacuation time, 371, 380 Load for steam surface condenser, 367 Non-condensables, 362, 363 Size selection, 371 Steam pressure factor, 373 Steam requirements, 372 Steain/air mixture temperature, 361 Total weight saturated mixture, 362 Capacity, 358 Discharge, pressure, 358 Effect of excess steam pressure, 358 Effects of back pressure, 359 Effects of wet steam, 356 Inter-and-after condenser, 351 Load variation, 370 Materials of construction, 347 Molecular weight entrainment, chart, 360 Performance, 358, 370, 375 Relative comparison, 357... 

Effect of back pressure on set pressure (AN = nozzle area AD = disk area AP = piston area Ab = bellows area). 

Effect of Back Pressure on Set Pressure of Conventional Safety-Relief Valves... 

These valves are classified as conventional or balanced depending on the effects of back pressure on their performance. The conventional type pop valve is used where pressure is released to the atmosphere or to a low-pressure relief header. The balanced-type pop valve is used where a relatively high pressure exists or is built up in the relief header. 

In this section, we consider the effects of back pressure on the flow behavior in micro-/... 

This is a PRV designed to minimize the effect of back pressure on its performance characteristics. 

Use of high back pressure during injection molding is usually associated with better mixing in the barrel. This condition should either further improve dispersion of the elastomer or at least prevent its agglomeration. In this study, the effect of back pressure was examined only for the blend with the higher viscosity ratio (TPO A). 

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