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Relief Valves: "What Can Go Wrong" Scenarios - Part 1

Nov 08 2010 01:30 PM | pleckner in Safety and Pressure Relief -----

Being Conservative

There are times when a failure may be obvious, cooling water stops to a condenser. Then there are times when it will take some stretching to find any, a pressure vessel operating in a nonflammable, low-pressure system with little fluid throughput. There are three approaches you can take when analyzing your process. You can be CONSERVATIVE. You can be conservative or, as I like to think of myself, you can be CONSERvative. I follow API 520 and 521 to the letter as a minimum. If my company or the client I am working for happens to have more stringent rules, these obviously supercede what is in API documents. For example, API 521, paragraph basically allows you to "ignore" heights above 25 feet when considering how much of a vessel to include in a fire zone calculation. I worked for a company that used 50 feet for its standard.

I will perform the necessary relieving load calculations for any scenario that cannot be rationally explained away, even if there is only a remote possibility that the failure will ever occur. The alternative is to perform fault tree analysis, risk assessment, etc. If it can be shown that a given scenario is indeed a 1 million to 1 long shot, then I'll label it as being not credible. Until then, it's only a relatively small amount of time that needs to be spent to perform the necessary calculations to be safe.

In the case of analyzing for Double Jeopardy, this is where I get my most grief; my conservatism tells me to error on the safe side, the client's money tells me to make the scenario go away. If I feel that even a Double Jeopardy failure can lead to a loss of life or major equipment damage, I might go ahead and do the relieving load calculations anyways (this does not usually go over too well with the powers-to-be because this usually results in larger relief systems).

API 521, paragraph 3.4 states that one can take credit for operator response after 10-30 minutes. I stick with the higher end at all times.

When analyzing a system for failures of control valves, I will always assume all my valves will fail as they are intended (fail close will indeed fail close, fail open will indeed fail open) except for the one control valve that will cause an overpressure hazard! This valve I assume to fail in the opposite direction (fails closed if it is intended to fail open).

When analyzing a system for "what can go wrong" scenarios, plant instrumentation sometimes may be used to justify the elimination of some scenarios. For example, if I have a hard wired (opposed to a Distributed Control System-DCS) pressure interlock that will shut steam off to the reboiler when the column pressure rises to some predetermined value, and there are redundant pressure switches, I might consider cooling water failure to the condenser as not being a credible scenario. On the other hand, once a credible scenario has been established, you are never to take into account the use of instrumentation as a means of reducing the relieving load.

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