Linde cryogenic hydrogen recovery unit maintenance

[Siraj1986]

Hey all,
We have a hydrogen recovery unit working on the basis of cryogenic separation. It has a pressure regulating valve at hydrogen product line. That valve is installed with in the cold box and it's actuator assembly is outside for easy maintenance.
We have to remove that actuator along with valve plug for maintenance. This means that cold box internals will be exposed to outside atmosphere.
Do we need to derime the cold box for this activity or simply remove tha actuator plug assembly with positive nitrogen flow through the cold box.
Is it also necessary that we have to derime the cold box for even minor maintenance or not?
Thanks

 

[StoneCold]

Siraj1986
You are not getting any answers to this question I assume is because the translator program you used mis translated the word "derime" We don't know what that word means. Can you try again?


Regards
StoneCold

 

[iainuts]

Actually, deriming is a common term for air separation units (ASU's). There was a thread here regarding deriming:

http://www.eng-tips.com/viewthread.cfm?qid=115300

I don't see why the plant would need to be derimed if you can ensure there's little or no influx of air when the valve's top works have been removed. Keeping a purge on it should work. Not sure nitrogen is the best solution since it will liquify or solidify at LH2 temperature, but it isn't clear from the OP if that's an issue or not. Is the nitrogen going to get into any area of the plant where it would need to be removed? For liquid hydrogen systems, we've always used helium as a purge gas.

 

[Siraj1986]

Actually we have a system as similar to ASU, the system installed at our plant is used to separate H2 from gases containing N2/Ar and Methane in it. The temperature and pressure conditions are -185 oC and 86 Kg/cm2. At this pressure all the gases except H2 liquefy, thus causing separation.
My question is that we have to remove the pressure control valve plug for maintenance. We have 02 options
1) Derime the plant which means we have to heat up the temperature from -185 to 25 oC for hand over which is a time consuming activity.
2) We can hand over the valve just after depressurizing the cold box with out heating it to positive temperature.
My question is related to 2nd option, is it possible to remove the plug from the system having that low temperature or we have to take any precautions for that.

Hope you can understand what i need to know.
Thanks

 

[StoneCold]

Siraj1986
My apologies. I guess it was just my ignorance.
Best of luck on your repair.

StoneCold

 

[iainuts]

I don't think you need to derime if you have a purge as you've suggested and don't leave the valve disassembled for long. I would also put something over the opening to prevent air ingress such as a plastic bag or a special made plug.

 

[Siraj1986]

But if we will not derime it, air ingress at that low temperature can cause problem to the system. It can liquefy or some thing else. Do we need to take that in account or not?

 

[iainuts]

How much air do you think will get in? Consider the worst case given the precautions you plan on implementing, and see if that amount will cause a problem.

 

[Siraj1986]

Ok if air ingress is not the big issue ... One more problem which we are expecting is that when we will remove the valve along plug from that low temperature to ambient, the valve can bear that temperature change or not ?
What is your opinion

 

[iainuts]

Not a problem.

 

[Siraj1986]

How can you be so sure that this temperature change will not effect the valve as the difference of temperature is almost 200oC.
[tt][/tt]

 

[iainuts]

Only from experience. I've warmed up SS valves and pumps from -420 and below to ambient by using a garden hose on them to warm them up quickly. Never had a problem with warpage or anything like that.

 

[Siraj1986]

Hmm now that's interesting. Let's see what we go for ... Fingers crossed .. In between thanks for the assistance ..