Backpressure correction factor

When using a balanced/bellows relief valve in the sub-critical, use Equations 7-18 through 7-22 how ever, the backpressure correction factor for this condition should be supplied by the valve manufacturer [33A]. For sub-critical, conventional valve ... 

Therefore, an SRV has a backpressure correction factor which is applied in the calculation and which is different for each manufacturer depending on the design of the valve and, in particular, the nozzle, huddling chamber arrangement and the shape of the body bowl. 

Per EN 4126 (but not per API), it is required that manufacturers demonstrate the correctness of its backpressure correction factors on the basis of effective... 

Volumetric flow rates of different gases are often compared to equivalent volumes of air at standard atmospheric temperature and pressure. The ideal gas law works well when used to size fans or compressors. Unfortunately, the gas law relationship, PV/T = constant, is frequently applied to choked gas streams flowing at sonic velocity. A typical misapplication could then be the conversion to standard cubic feet per minute in sizing SRVs. Whether the flow is sonic or subsonic depends mainly on the backpressure on the SRV outlet. In the API calculations, this is taken into account by the backpressure correction factor. 

Backpressure correction factors highly depend on the design of the valve it is impossible to provide generalized figures. In principle, manufacturers should provide their own data, and per EN 4126, they should be able to demonstrate them by tests. 

Unlike European norms, API 520 has always published typical backpressure correction factors in its code. These curves serve only as a guide and represent a sort of average for a number of manufacturers. API states that they can be used when the make of the valve is unknown (which is rather unlikely) or for gases and vapours when the critical flow pressure point is unknown. 

When contacting the manufacturer, it is also recommended you ask for the test data supporting the numbers being published or issued. Interestingly, when you start comparing backpressure correction factors in several manufacturers catalogues, you will notice that many just use the numbers given in the... 

Kw = Backpressure correction factor (depends on manufacturer but for reference, see Appendix B)... 

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 ... 

As a side note, it needs to be noticed that the backpressure correction factors given by API 520 are for pressures above 3.45 Barg (50 psig) only and that they are limited to backpressures below critical flow pressure for a given pressure. For everything below 3.45 Barg, the manufacturer should in any case be contacted. 

Figure B.l shows the backpressure correction factor for balanced bellows safety relief valves (SRVs) on gases and vapours as published in API.

A is required effective PRV oriflce area or rupture disk area, in inch W is vapor relief load, in Ib/hr T is relief temperature, in R Z is vapor compressibility factor at relief conditions. Let Z=1.0 for ideal gas. (see note 5 at end of this section) M is relief vapor molecular weight C is a coefficient calculated by q. (7b) or let it be 315 Kd is effective discharge coefficient Kb is backpressure correction factor Kc is a correction factor for install a rupture disk at PRV inlet, set kc to 1.0 for rupture disk sizing only Pi is PRD inlet pressure, in psia,... 

For backpressure correction factor (Kb), estimate the backptessure at PRV outlet Then, find Kb from manufacturer s catalog. Kb depends on the type of PRV (conventional or balanced-bellows) used. Forpiloted-operated PRV and rupture disk, Kb equals to LO. Kb needs to be rechecked once the PRV outlet piping is designed, and its backpressure for rated flow case is calculated by a hydraulic analysis,... 

Figure 9 Typical backpressure correction factor Kw for balanced-bellows PRV liquid relief. [Courtesy of Tyco Valves and Controls LP]...

Backpressure correction factor Combination conection factor Required dbcharge area Type of oriice reqived... 

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