Fail mode of blanketing gas

[Jack Nicholson]

Hi.
My last post has been deleted!!!
Here is my question.
N2 fail mode is fail open. Suppose the power unit fails, so all control valves fail to safe mode and n2 control valve open completely and drum pressure reaches 8 barg.
Could we change fail mode of n2 control valve to close? (In order to prevent drun overpressurizing)

https://pasteboard.co/2UUJO52tDw7a.png

 

[1503-44]

That does not look correct.

First, the drum should not have the possibility to experience overpressure on failure of that control valve, or anything else. It looks possible to get overpressure on the drum, if the control valve fails open, so that is not correct. The N2 system should not have any chance to deliver more than 6.5 bars to the drum.

If there is any possibility of overpressure from any source, you should try to eliminate that chance, plus the drum should have its own relief valve set at 6.5 barg. That would prohibit any overpressure and it would also hold any existing blanketing inside the drum.

N2 failing closed looks like it could be a solution when combined with a drum relief valve to hold blanketing and the drum pressure below its design pressure. Better if n2 overpressure to the drum was not possible at all. Consider a relief valve and a fail closed block valve downstream of the N2 CV on piping leading to the drum.


A black swan to a turkey is a white swan to the butcher ... and to Boeing.

 

[Jack Nicholson]

I couldn't get your point! You are saying that n2 can not deliver overpressure? Well, max. n2 header pressure is 8 barg, so it can overpressure the drum!

 

[1503-44]

Overpressure is not allowed.

I am saying that IMO it is not allowed to have any means of overpressuring the drum.
IMO it is NOT CORRECT DESIGN. At the very least, You must have a relief valve on the drum set to 6.5 barg and the minimum relief capacity of that valve must be the maximum flow rate of the N2 feeding it. A control valve is not considered protection, because control valves should always assumed to be a potential pressure leak, whether it is considered failed, or not. A control valve that can cause overpressure downstream should always have an associated ESD valve and a relief valve, both set to the lowest allowable downstream pressure.

A black swan to a turkey is a white swan to the butcher ... and to Boeing.

 

[georgeverghese]

There should be a check valve next to PCV on line to flare.

 

[IheomaS]

Hi
Valve fail safe position is usually defined considering the consequences on the process system in the event of instrument air failure such that plant safety is guaranteed on valve failure position.
With the current configuration, there is potential of vessel over pressurization on instrument air failure.
In my opinion, the control valve on the N2 line should be fail close. This is to ensure that N2 supply is shutoff and the vessel is not pressurized in the event of instrument air failure. Additionally the PSV on the vessel should be sized/ verified for control valve on the N2 line failing wide opening.
Hope this helps

 

[EdStainless]

Why not use a pressure regulator to assure that you reduce the N2 supply pressure?

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P.E. Metallurgy, consulting work welcomed

 

[1503-44]

Leo, most of us here like to read the thread before commenting.

Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."