PLC Positive Pressure Cabinet

Hey Guys,

I Just want to know how to build a positive pressure PLC Cabinet to eliminate humidity, dust, and Hydrogen Sulfide in the atmosphere
near the cabinet which is already enclosed with glass windows and doors and a portable humidifier .....

The Plc's get crazy when the humidity rise to 90 % ...... and also the H2S had corroded most of the copper grounding busbar and some signal wiring.....

Hope to hear from you,

Cheers
Fidel
Macau - Taipa Waste Water Treatment Plant
China

Fidel,

Attach a clean compressed air line to the cabinet. The air supply must be free of water and oil. A filter may be necessary. Install a pressure regulator on the air line. Set the regulator to 5 pounds per square inch.

Also see if you can find an overpressure relief device. If for some reason the regulator fails you will have a high pressure cabinet, or a bomb.

Lancie1 said:

Fidel,

Set the regulator to 5 pounds per square inch.

Click to expand...

NO NO NO. That will kill someone when something lets loose.

NEVER have the enclosure pressure to be more than 1/2 INCH of WATER !!! Yes, that's one half inch of water column pressure, NOT psi.

As Gerry M mentions, an overpressure relief is needed, can be had from companies that make purge controls, like Expo Purge, Bebco, & others.

Just to put theDave2's comment into perspective.

If you were to pressurize a cabinet to 5 PSIG, and the cabinet door is 2 feet wide by 5 feet tall, there is 7200 pounds of force trying to open that door. Do you want to be standing in front of it when it opens?

Sorry about that, Fidel. I had a CRS moment. I hope you didn't blow yourself up!

The title of NFPA 496 is Standard for Purged and Pressurized Enclosures for Electrical Equipment.

The IEC standard differs from NFPA 496: it increase the minimum overpressure from 0.1 to 0.2 inches of water column (for Div 1).
http://www.findarticles.com/p/articles/mi_qa3739/is_199906/ai_n8845766

Although the thread request was for a pressurized panel to resist corrosion (not EXP rating), the IEC pressure value gives you some idea of of the magnitude of pressurize that pressurized enclosures deal with. fractions of an inch of water column, not psi.

The NFPA standard is available for $35
http://www.nfpa.org/aboutthecodes/AboutTheCodes.asp?DocNum=496&cookie%5Ftest=1

Pepperl & Fuchs has a web page on purging and pressurizing at
http://www.bebcoeps.com/purge/standards.html

There's even a wizard, "Build your purge system". (Corrosion protection would be class 2, div 1; general purpose components)

I heard about a plant where their shop mounted a low pressure, gauge pressure switch inside the enclosure, purportedly to prevent overpressure by dropping out a solenoid valve in the air line. This plant had no relief valve installed.

They didn't understand why the pressure switch did not drop out the solenoid valve in the air line before the pressure got high enough to burst the panel.

A gauge pressure switch would have to be mounted outside the enclosure with its pressure access port plumbed to the inside the cabinet, because a gauge pressure switch references the surrounding 'atmosphere'. If the surrounding atmosphere pressure is increasing at the same rate as the pressure port, a gauge pressure switch 'sees' no change, the differential is zero.

Dan

Just thought I'd toss in a little tidbit I came across regarding this issue. Be wary of any components in your panel that have integrated cooling fans. I had a situation where I was pressurizing a cabinet that had an industrial PC. When the cabinet door was open, everything was fine. Within minutes of closing the cabinet door and tightening the hasps, the PC would fault out with Windows hard errors, etc. It took me a while to figure out that the pressure in the cabinet was probably slowing down or stopping the cooling fans on the PC. An adjustment to the air pressure and a small vent hole did the trick.