Hello everyone,

I am confused by the API criteria. It says : For conventional soupape, the back pressure can't exceed 10% of set pressure, and for balanced type, the back pressure can't exceed 50% of set pressure. My question is that which type of percent we use, gauge pressure/gauge pressure or absolute pressure/absolute pression?

I saw that in API RP 520 there is a figure about Kb which is the ratio of gauge pressures. And I fonud some ones used absolute pressure ratio for conventional soupape. Could you help me to find the right ratio and tell me why? Thanks in advance.

Tony

Tony,
Known issue.

Answer to your question.



Generally Gauge / gauge.



Kb for conventional PSV (vap & gas), fig 35 in API520 PI ==> abs /abs
Kb for Balanced-bellow PSV (vap & gas), fig 30 in API520 PI ==> gauge / gauge.
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They are mixed...when you use the value, just needs to pay more attention...Extra attention to unit used and ability for unit conversion is one of the nature & requirement to be a good Chemical & Process engineer.

JoeWong

The figures you site in API RP520, 7th Edition, January 2000 are very clear in what to use.

Figure 30—Back Pressure Correction Factor, Kb, for Balanced-Bellows Pressure Relief Valve (Vapors and Gases)

AND

Figure 31—Capacity Correction Factor, Kw, Due to Back Pressure on Balanced-Bellows Pressure Relief Valves in Liquid Service

both use the ratio of gauge pressures.

Figure 35—Constant Back Pressure Correction Factor, Kb, for Conventional Pressure Relief Valves (Vapors and Gases Only)

does indeed use ratios of absolute pressures.

Even the labels for the x-axis are different for Figures 30 and 31 even though they use the same units in the ratio. Figure 30 is labeled, "Percent of Gauge Pressure" and Figure 31 is labeled, "Percent of Gauge backpressure"

Note that Figure 35 x-axis label is "Percent of backpressure" but the term is also caculated differently from the others.

Why is it this way? You would have to ask the API committee. Confusing? Perhaps. But this is why we always need to look at what we are doing, read the fine print and make no assumptions.

By the way, API does NOT say the backpressure can't exceed 10% of set pressure for a conventional style relief valve. This is an old misconception. Please re-read the section 3.3 in API RP520.

Thanks so much for your rapid and helpful answer. I wasn't careful enough as a fresh engineer. I appriciate your suggestions and I will do better and better with your helps.

I checked the API and found that for conventional valve, the back pressure can't exceed 10% of the set pressure at 10% allowable overpressure. It seems that THE RATIO is ( back pressure / set pressure *110%).

Best regards,

Tony

@tonywen:

I have to repeat that API does not say CAN'T, it says SHOULD NOT. Am I being nit-picky? Yes but when you deal with the Codes and guides, you have to be. You must be careful on how it is worded as different words mean different things (stating the obvious...).

To pleckner,

Yes, you looked for bones in my egg. I am so happy to meet a excellant engineer and at the same time I accept your precious comment.

regards,

Tony

Tony,
Phil is NOT looked for bones in egg. This is working manner and conduct in dealing with any engineering issue specifically with SAFETY related issue.

Have you ever asked yourself why API stated SHOULD NOT instead of CAN NOT or SHALL NOT ?
You may read

• Back pressure effects on safety valves operating with compressible flow
  
• Use of conventional type PSV with back pressure exceeded 10% set pressure

JoeWong