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How to purge a panel with orifice and vent

  1. Feb 6, 2014 #1
    Hi, I have a stainless steel panel that I would like to purge with GN2. The panel is roughly 5' x 4' x 2'. Nearby, I have a 1/4" steel line that contains 90 psig of GN2 that I can use. My thought was to run a small line (1/4" or 1/8") to the panel, and insert a small orifice to restrict the flow going in. I do not want the pressure to build up much because the door has a large surface area and I don't want to apply a great force to that (or to the components in the panel), so I'd need some sort of a vent, check valve, or relief device. However, I am having trouble thinking of the best way to set this up intelligently. Obviously I don't want to flow more out than in because I wouldn't be keeping a positive purge inside the panel, but too much and I could slowly build up too much pressure. Any help would be tremendous. Thanks
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  3. Feb 6, 2014 #2


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  4. Feb 7, 2014 #3
    Hi Q_Goest, thank you for your reply. The enclosure is required by design to conform to OSHA Class 1 Div 2 regulations for hazardous locations. It currently does not conform to this requirement.
  5. Feb 7, 2014 #4


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    Then you probably want something like a class Z purge. Take a look at the link I provided for a description.

    I've seen that done in different ways. It needs some kind of indication on it such as a pressure switch or a flow switch on the purge. That indication doesn't require shut down but it does require a minimum of local indication as I understand it.

    The purge is generally dry air or nitrogen. Flow rate is typically regulated using a needle valve, rotometer or orifice. Having something adjustable such as the needle valve or rotometer valve to compensate for panel leakage is good because it allows you to set a flow rate that is slightly higher than what is required to maintain whatever means of purge indication you have.

    I've also seen low pressure relief valves put on the box to ensure pressure doesn't exceed the limits of the box or any instrumentation installed, especially where you have a cut out on the door for example, and an HMI screen installed that's seeing the pressure differential. Relief valves or check valves with a low cracking pressure can both be used. I'd suggest something around 1/4 to 1/2 psi.

    You should also consider doing a quick stress analysis on the box if it's large (more than a foot or 2 in any dimension) to verify you don't need reinforcement. One company I worked for always did a stress analysis and we ended up modifying a lot of those NEMA enclosures. The box needs to be some kind of sealed enclosure of course. A NEMA 3R or 4 box is typically used for outdoor use and a NEMA 12 box can be used indoors.

    I'd suggest going through the OSHA requirements and make sure you understand what type of purge is needed for the electrically classified area you will be operating in and that you meet those requirements. Also, might want to have this thread moved to the electrical engineering forum for further advise. If you want to move your post, click on the REPORT button on your initial post and just request the moderator move it. Understandably, this stradles both mechanical and electrical engineering but I'd say it's primarily an electrical engineering issue.
  6. Feb 7, 2014 #5
    Thanks, I'll look into the specific requirements per our facility (or better yet, I'll have our electrical engineer do that), as well as an analysis of the panel integrity. There are two similar panels nearby that are purged with 0.5 inH20 of nitrogen, so I was just going to try to mimic that under the assumption that the requirements are the same. I am primarily trying to figure out the "how" more than the "why", so this is more of a mechanical question than electrical. I've been looking into some variable flow meters that are rated for extremely low flow rates, but still have questions on the flow in versus out. I am essentially just trying to find an "equilibrium" between the inward flow and the outward flow without over-pressurizing the panel. I will look into relief devices rated for very low pressures.
  7. Feb 7, 2014 #6
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