Positive Pressure Control Box
Positive Pressure Control Box
(OP)
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.






RE: Positive Pressure Control Box
Doing these kinds of retrofits can get you into trouble if it isn't done correctly.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Positive Pressure Control Box
RE: Positive Pressure Control Box
If the panel is outside the hazardous area you will need seals on all the conduits to prevent migration of gas.
Roy
RE: Positive Pressure Control Box
That sounds like a really BAD idea to me. It would seem that you could have all sorts of other hard to define combustion products in that, that could damage electrical equipment. No?
Keith Cress
kcress - http://www.flaminsystems.com
RE: Positive Pressure Control Box
My plan was to put on conduit seals, seal up the cabinet, it should already be sealed except for the vent. It is in a classified area.
RE: Positive Pressure Control Box
RE: Positive Pressure Control Box
Without purging, the same enclosure has to be capable of keeping the gas out plus containing the explosion inside the enclosure if gas should get in and ignite.
Just sealing the conduits and entrances will minimize the gas leaks, but that will not make the typical purged enclosure suitable for containing burning and exploding gas.
Compressed air is not the only purge medium. Is it possible to use blower driven ventilation from a non-hazardous area to purge the panel?
RE: Positive Pressure Control Box
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Positive Pressure Control Box
"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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RE: Positive Pressure Control Box
Uh a scary thought. Do you have any pneumatic devices in the cabinet that vent instrument air in normal operation? Will these instruments now vent natural gas?
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Positive Pressure Control Box
Provided all the components inside the panel are approved for Class 1 Div 2 you don't need a purge.
He may have to change a lot of the components.
I don't believe it's possible to just "seal it up"
Another alternative would be to re-locate the panel outside the hazardous area e.g. in the switch-room or on the outside of the compressor room with seals on the conduits.
Roy
RE: Positive Pressure Control Box
ballenEEUK; They cannot do what is proposed. Not at all.
As we are saying you must then switch to explosion proof design which is very different from purged air.
When the original design of the system was done there was a choice. Use heavy expensive E X P enclosures or use inexpensive standard containers and maintain a compressor, dryer, and power bill for all eternity. The choice was made.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Positive Pressure Control Box
You don't need those heavy cast boxes any more, sheet metal is OK provided all the internals are approved.
There is an awful lot of confusion over Class 1 Div 2 requirements and we tend to over protect with explosion proof equipment when in fact, it's not required.
Roy
RE: Positive Pressure Control Box
RE: Positive Pressure Control Box
If the plan is to allow this 'power gas' to come in contact with equipment that was previously purged with fresh air, its compatibility must be verified.
RE: Positive Pressure Control Box
What we are trying to do is elimnate the air compressor system from the station due to reliablity issues. However it looks as though we will have to keep the air system. Originally our client proposed that we seal up the pressure panels to make them safe, such that we wouldn't need the air system.
RE: Positive Pressure Control Box
A key issue is the air tightness of the enclosures. If they have bad door seals, or poorly sealed conduits then the compressor energy, wear, and operating costs climb rapidly. They usually climb to the point of "Hey Joe theirs something wrong with the compressor..".
I'd use two compressors in tandem with alternate starting control. And I'd actually measure the air consumption. Do the math to make sure the compressors are sized correctly to prevent damaging short cycling of the starters and motors.
I would leave the aforementioned air consumption measurement system in-line after everything else, like dryers, and filters, and I would keep a log on the air flow. It should be reasonable and very low. And it shouldn't slowly increase over time!
Keith Cress
kcress - http://www.flaminsystems.com
RE: Positive Pressure Control Box
I am glad to hear that you are keeping the purge air. Sealing is not an option. Explosion proof enclosures are based on the principal of containing an internal explosion, not preventing it. Despite the best efforts at sealing no one can guarantee that an explosive atmosphere will never accumulate inside the enclosure without purging.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Positive Pressure Control Box
If you have enough room you could mount the blower inside the panel.
Roy
RE: Positive Pressure Control Box
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
RE: Positive Pressure Control Box
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
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies
RE: Positive Pressure Control Box
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
RE: Positive Pressure Control Box
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.