Positive Pressure Control Box

[ballenEEUK]

Hello everyone. I have a client who has a Natural Gas liquids facility at their compressor station. The control box for the NGL plant is a postive pressure box, fresh air is pumped in to prevent flammable gas from entering. They have decided to remove the air system and use power gas and to seal the control panels. What I would like to know is there any product out there that could seal the box sufficiently for a Class 1 Div 2 area, and is there any code violations for doing this.

 

[waross]

Hi roydm;
You can't pressurize with a potentially explosive atmosphere, so the clean air supply often has to be some distance away. If long distances are involved, compressed air lines are often cheaper and more dependable than blower piping. There is also the issue of proving pressurization. Pressure switches are more dependable at higher pressures. There may also be a nearby source of plant air which may be tapped for pressurization.
BUT, if the application is suppling cooling air for a large motor in a hazardous atmosphere, the economics change and your suggestion of a blower may be used.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

I think I get it now. Usually a purged air system is a viable solution when either the air is a required part of the process operation anyway, and/or the system in question is too large to consider using EXP boxes. So now that I think about it, I believe that what you were saying is that you are getting rid of the OTHER requirements for compressed air by using the Power Gas for them, so the only thing left is the purged air enclosure, and you were wondering if you could do it some other way. Right?

OK, so then the answer(IMHO) is ...
.
.
.
.
No.


Just sealing is not good enough for classified areas, unless as roydm said, every single component inside is rate for Class 1 Div2. Move it or purge it. keep in mind though that now you may need a lot less air volume for your purging, so maybe a smaller more reliable compressor system would be worth the investment.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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[roydm]

Waross, I think this is a remote compressor station, not part of a refinery hense the use of gas to power the instruments instead of compressed air.
The hazardous zone usually only extends a few meters from the possible source of gas, A typical Z purge pressure switch is set about 0.5" I think. I have seen a blower used to pressurise a lab in the middle of a refinery. It had a 30 ft stack to suck air from above the hazardous sphere. I agree it's probably easier to bite the bullet and provide a small compressor, I was just suggesting some possible alternatives thats all.
Cheers
Roy

 

[LionelHutz]

In theory you could change to Class 1 Div 2 components inside the box and be OK. However, you really should get that field inspected and also have the insurance company on-board with the change and if you're not going to involve an engineering firm with the work though it's not likely to pass.

But then, an engineer might probably won't give their stamp of approval because it's a retrofit, unless it's the same engineer that did the initial station design. Professional engineers are not to approve work done by others and approving (re-approving?) the existing enclosure to be suitable for use without purging air does seem to fall into that category.


Roydm - A fan could work but I think a computer fan is a little small. We've got a few 10" axial fans here that could likely do it though.