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Environmental, Health & Safety :: Plant & Personnel Safety

January 13, 2009

Comments on Pressure Relief System Design

Discussions between one reader and the author of our November 2008 article, Pressure Relief System Design

Rebekkah Marshall

I would like to draw your attention to a few points on the November 2008 article, Pressure Relief System Design (pp. 40–45): 1. Pressure drop and pressure loss: The inlet pressure “drop” that the author refers to under Relief system piping...

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Comments (2) for Comments on Pressure Relief System Design
Dear author,

1/ On page 40 of your November 2008 article, you write: "Conventional pressure-relief valves are susceptible to back pressure. Such valves are not recommended when the TOTAL back pressure exceeds 10% of the set pressure". API Std 521 (Jan. 2007) states another criteria in section "... Where conventional safety relief valves are used, the relief manifold system should be sized to limit the BUILT-UP back pressure to approximately 10 percent of the set pressure of each relief valve that may be relieving concurrently".
Could you please clarify the confusion between total and built-up back-pressure?

2/ Also, on page 43, the k factor should be omitted from formula (2) for the Mach number calculation under ISOTHERMAL conditions. See API Std 521 (Jan. 2007).
Posted by Lionel Sheikboudhou on Thursday, February 12, 2009 @ 02:36 PM
The author's response follows here:

1. Conventional Pressure Relief Valves :

In reality, conventional pressure relief valves are recommended when the built-up back pressure is within 10% of the set pressure. However, in actual practice, the picture is sometimes a bit more conservative. In many cases, in process data sheets, the built-up back pressure and the superimposed back pressures are not separately reported. Instead, the total back pressure is reported. The vendor also selects the relief valve based on this total back pressure.

Take the case of a relief valve which has a set pressure of 5 bar(g). The superimposed back pressure is 0.4 bar(g) and the built-up back pressure in case of the contingency is 0.2 bar(g). Going by the built-up back pressure criterion, a conventional relief valve should satisfy the duty. It works out to 4% of the set pressure (0.2x100/5 = 4). However, if the total back pressure criterion is applied, it works out to 12% of the set pressure ((0.4+0.2)x100/5). A conventional relief valve would not be suitable in this case and we would need to go for balanced bellows.

The total back pressure criterion is followed in many cases and results in a more conservative design.

2. The 'k' factor in the Mach Number formula :

The previous edition of API 521 had this 'k' factor included in the formula for Mach Number. However, in the latest edition, it has been removed. In any case, k factors are in the range of 1.1 - 1.2 for hydrocarbons, and after taking the square root, the difference in Mach Numbers with k and without k would not be quite significant.

Hope this clarifies.

Best Regards,
Siddhartha Mukherjee
Posted by D. Lozowski on Thursday, September 10, 2009 @ 09:15 AM

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