Back pressure sizing factor

Figure 12. Variable or constant back pressure sizing factor Kb for balanced bellows safety relief valves (vapors and gases).

Figure 17. Variable or constant back pressure sizing factor, Kw for 25% overpressure on balanced bellows safety relief valves (liquids only). The curve represents conqiromise of the valves reconunended by a number of relief valve manufacturers. This curve may be used wiien the make of dw valve is not known. When the make is known, the manufacturer should be consulted for the correction factor.

VARIABLE OR CONSTANT BACK PRESSURE SIZING FACTOR... 

Figure 5-5. Constant back pressure sizing factor, for conventional safety relief valves in vapor or gas service. (API Recommended Practice 520, Sizing, Selection and Installation of Pressure Relieving Devices in Refineries, Part 1, 5th ed., 1990. Reprinted courtesy of the American Petroleum Institute.)...

All relief valves are affected by reaching critical flow, which corre-spond.s to a back-pressure of about 50% of the set pressure. Pilot-operated relief valves can handle up to 50% back-pressure without any significant effect on valve capacity. Back-pressure correction factors can be obtained from the relief valve manufacturers for back-pre.ssures above 50%. API RP 520 gives a generic method for sizing a pilot-operated relief valve for sub-critical flow. 

Back pressure is included as a factor in PR valve selection and sizing accordingly ... 

In general, the total back pressure on a balanced bellows pressure relief valve (superimposed plus built-up) should be limited to 50% of set pressure, because of the marked effect of higher back pressures on valve capacity, even when appropriate correction factors are used in sizing. In exceptional cases, such as a balanced bellows PR valve discharging into another vessel, total pressure up to 70% of set pressure may be used. 

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... 

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. 

Calculations of Orifice Flow Area using Pressure Relieving Balanced Bellows Valves, with Variable or Constant Back Pressure. Must be used when backpressure variation exceeds 10% of the set pressure of the valve. Flow may be critical or non-critical for balanced valves. All orifice areas. A, in sq in. [68]. The sizing procedure is the same as for conventional valves listed above (Equations 7-10 ff), but uses equations given below incorporating the correction factors K, and K,, . With variable backpressure, use maximum value for P9 [33a, 68]. 

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... 

The size of a catalyst support depends on many factors. Predominant among these are flow rate, light-off performance, conversion efficiency, space velocity, back pressure, space availability, and thermal durability. Other factors, such as washcoat formulation, catalyst loading, inlet gas temperature, and fuel management, can also have an impact on the size of a catalyst support. 

Form and size of support The particle size will have an influence on filtration times from stirred tank reactors in repeated batch mode. Furthermore, this factor is important for the performance in column reactors regarding back pressure and flowrates, which of course are correlated. For this purpose a size of spherical particles in the range of 150-300 pm is preferred. 

A bellow style pressure relief valve is required to protect a vessel containing an organic liquid. The required relieving capacity is 310 gpm. Inlet temperature is 170°F. Set pressure is 100 psig. Allowable overpressure is 25%. Built-up back pressure is 25 psig. Specific gravity is 1.45 and viscosity is 3,200 cP, Determine the orifice size of the valve. The correction factors are ... 

The absorber is a simple splash-deck tower. Because the back pressure of hydrogen sulfide over Stretford solution is negligible, the absorber can be sized to reduce the inlet hydrogen sulfide concentration by a factor of 100,000. Commercial absorbers have met this design criterion. 

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