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

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

The Concept of Double Jeopardy

I can't begin to tell you how many people still don't understand this concept. API allows you to "ignore" failures that fall under the "Double Jeopardy" principle (See API 521, March 1997, 4th edition, paragraph 2.2). Double Jeopardy basically means two unrelated failures occurring at the exact same time, i.e. simultaneous. This does not mean the failures occurred one minute, one second or even one millisecond apart. It means at exactly the same instant in time! Let's look at some examples.

There is a loss of power to a pump causing stoppage of cooling water to a condenser on a distillation column. Because vapor from the distillation column can no longer be condensed, pressure builds up to the point of popping the relief valve, i.e. the system goes into relief. At the same time, the control room operator strokes open a steam flow control valve to the reboiler on that same distillation column causing the generation of an excessive amount of vapor. When calculating the total amount of vapor that must be relieved to prevent damage, should one take into account the excessive vapor produced by the wide-opened steam valve? Or, should we consider only the normal amount of vapor exiting the column; API 521, paragraph 2.3.2 says that the control valve should be considered to be in its normal operating position unless its function is affected by the primary cause of failure, this being loss of power to a pump.

The answer is, this is a Double Jeopardy failure, two unrelated events occurring at the same time. One has nothing to do with the other. Therefore, you only need to calculate the relief load for one scenario at a time. For the loss of power to a pump scenario, the relief load would be based on the amount of vapor generated at the "normal" rate of steam to the reboiler.

For the steam control valve failure scenario, the relief load would be based on the amount of vapor generated by the heat provided by a wide opened steam valve (possibly limited by heat transfer constraints) with credit taken for the amount of vapor that can be condensed! Remember that for this failure, the condenser would still be in operation.

Let's look at the situation again. With the pump stopped, cooling water is lost to the condenser causing the distillation column to go into relief. However, this time the control room operator realizes that the relief valve has opened and attempts to stop steam flow to the reboiler. The operator puts the steam control valve in manual and tries to close it but it won't respond because it is stuck. To free it, he strokes it wide-open, shooting steam into the reboiler and causing the generation of an excessive amount of vapor. Now we have two failures occurring at the exact same time but are now related. The power failure stops the pump and thus the cooling water to the column condenser. This causes the column to go into relief, which then causes the operator to react, initiating the second failure.

This is a perfectly credible relieving scenario and the calculation of relieving load should be based on the amount of vapor generated by the heat provided by a wide opened steam valve (possibly limited by heat transfer constraints) without credit taken for the amount of vapor that can be condensed! Remember that for this failure, the condenser will NOT be in operation.

A very obvious example of a Double Jeopardy failure would be a tube rupture in the reboiler occurring at the same time cooling water flow was lost to the condenser. Two very unrelated failures occurring at exactly the same time.

By the way, stuck opened control valves occurring simultaneously with a second failure does NOT constitute Double Jeopardy. That valve may have been stuck in its operating position for a significant amount of time before the second failure has occurred. The first failure was the mechanical failure of the valve (sticking) and that did not happen at the same time as the second failure. These are unrelated failures but they do not occur simultaneously!