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.

 

[itsmoked]

What is power gas?

Doing these kinds of retrofits can get you into trouble if it isn't done correctly.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[ballenEEUK]

Power Gas is Natural gas taken off the discharge of the Compressor station to run any pneumatic devices, i.e. water pumps.

 

[roydm]

If the panel is within the hazardous area I think you will at least need to upgrade all the internal components to Class 1 Div 2, no arcing contacts etc.
If the panel is outside the hazardous area you will need seals on all the conduits to prevent migration of gas.
Roy

 

[itsmoked]

Tell me this is non flammable burnt combustion gases?

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

 

[ballenEEUK]

No it is raw natural gas that has been pressurized that is used to drive pneumatic devices, very common in remote Natural Gas Compressor stations.

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.

 

[ballenEEUK]

Just to clarify I'm not going to use "Power Gas" to pressurize the cabinet, I'm trying to seal the cabinet up so that it is Class 1 Div 2 rated, and doesn't need the air system to pressurize it.

 

[rcw retired EE]

I don't believe it is possible to modify a purged panel and make it suitable for Class 1 Div 2 without purge. The purging prevents ingress of combustible/explosive gas into the enclosure where hot surfaces or arcing contacts could ignite it.

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?

 

[waross]

I hope you are familiar with explosion proof panels and boxes. They are large and heavy an usually made of cast iron or cast aluminum. They are strong enough to contain an explosion of the rated gas. They have machined flanges wide enough to cool any escaping products of combustion below the ignition point. The flanges are held together by many large bolts. These panels are also expensive. It is often cheaper to use a less robust panel and pressurize it to prevent the ingress of potentially explosive gas. If you want to ditch the pressurized purge gas you should replace the panel with an explosion proof behemoth. And don't forget the conduit seals. Aren't the conduits already sealed?

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

 

[jraef]

Wait, am I the only one who sees this as strange? You want to pressurize the electrical enclosure with natural gas? Are you TRYING to make a bomb?


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

I guess if you can keep out the oxygen jraef. A novel concept even if it is a little outside the 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

 

[roydm]

Hold on guys, ballenEEUK is not planning to purge the cabinet with gas.
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

 

[itsmoked]

LOL. You two made the same assumption I did from, "remove the air system and use power gas to seal the control panels".

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

 

[roydm]

Keith, I agree, once the system has been approved one way it's pretty difficult to change. Even if all the internals are changed to comply it will be hard to get it past an inspector. The easiest way might be to re-locate the box outside the hazardous location, there must be a drawing defining the zone.
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

 

[ballenEEUK]

Thanks guys that what I was thinking. This is something the client requested, I've never heard of doing such a thing but I don't have the background in purged air systems, if it had been a new instalation I would have pushed for a new panel that is Class 1 Div 2 rated.

 

[PHovnanian]

One other thing to consider if you use natural gas where none was used before (I'm still not certain where ballenEEUK is planning on using 'power gas') is that some materials (plastics and rubber) are not suitable for contact with such gasses.

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.

 

[ballenEEUK]

The power gas, which is natural gas thats been pressurized will be used in various processes at the site to drive, water pumps, oil pumps, produced fluids pumps, and even the crane. The power gas will not be used to purge the pressureized panel, since it is flamable.

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.

 

[itsmoked]

A reasonable request based only on air compressor issues. Unreasonable in reality.

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

 

[waross]

If you drive the compressor with power gas, be sure that no gas exhaust is drawn into the purge air compressor.
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

 

[roydm]

Since you seem resolved to using a pressurized enclosure consider using a small blower fan rather than a compressor, you only need a fraction of an inch WC. Perhaps a computer cooling fan.
If you have enough room you could mount the blower inside the panel.
Roy