Liquidtight use for adjacent enclosure purge flow?
- Thread starter mabell
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ryan_618
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- Salt Lake City, Utah
Re: Liquidtight use for adjacent enclosure purge flow?
What is a "purged raceway connection"?
What is a "purged raceway connection"?
Re: Liquidtight use for adjacent enclosure purge flow?
There are three enclosures supplied by one purge system for a hazardous environment. The rapid exchange of air is supplied by a 1/2' diameter ss tube to the first enclosure, the second enclosure is connected to the first one by some means to keep the air flow until it reaches the last one wher the vent is. The purged raceway is the means to connect each enclosure to achieve flow.
There are three enclosures supplied by one purge system for a hazardous environment. The rapid exchange of air is supplied by a 1/2' diameter ss tube to the first enclosure, the second enclosure is connected to the first one by some means to keep the air flow until it reaches the last one wher the vent is. The purged raceway is the means to connect each enclosure to achieve flow.
- Location
- Mission Viejo, CA
- Occupation
- Professional Electrical Engineer
Re: Liquidtight use for adjacent enclosure purge flow?
Is this an "X," "Y," or "Z" pressurization?
[ July 16, 2004, 03:24 PM: Message edited by: rbalex ]
Is this an "X," "Y," or "Z" pressurization?
[ July 16, 2004, 03:24 PM: Message edited by: rbalex ]
- Location
- Mission Viejo, CA
- Occupation
- Professional Electrical Engineer
Re: Liquidtight use for adjacent enclosure purge flow?
Oops
I should have also asked what the enclosures contain.
[ July 16, 2004, 03:45 PM: Message edited by: rbalex ]
Oops
I should have also asked what the enclosures contain.[ July 16, 2004, 03:45 PM: Message edited by: rbalex ]
- Location
- Mission Viejo, CA
- Occupation
- Professional Electrical Engineer
Re: Liquidtight use for adjacent enclosure purge flow?
Since the potential source of ignitible material is outside the enclosures, you are using a "pressurized" rather than "purged" system. This is a good thing.
Personally, if it had been up to me, I would not have required venting the system at all, since all that is needed is to maintain a positive pressure of 25pa (0.1 in. water) in an area where no gas is supposed to be in the first place (Definition of Div 2). 25 pa is piddlely. But it isn't up to me.
Pressurization is an identified protection technique for Class I locations (Section 500.7 via 500.2 and NFPA 496). The "catch" is that the installation is still in a Division 2 location and nothing in 501.4 recognizes pressurization for wiring methods as it does for enclosures in 501.3(A), for example.
Since you haven't yet demonstrated a need for flexibility [501.4(B)(2)], technically liquid tight would still be a violation. Here is where reason needs to step in. With the additional proviso of installing the liquid tight per 501.16(B), I think this would be a good application of 90.4.
Since the potential source of ignitible material is outside the enclosures, you are using a "pressurized" rather than "purged" system. This is a good thing.
Personally, if it had been up to me, I would not have required venting the system at all, since all that is needed is to maintain a positive pressure of 25pa (0.1 in. water) in an area where no gas is supposed to be in the first place (Definition of Div 2). 25 pa is piddlely. But it isn't up to me.
Pressurization is an identified protection technique for Class I locations (Section 500.7 via 500.2 and NFPA 496). The "catch" is that the installation is still in a Division 2 location and nothing in 501.4 recognizes pressurization for wiring methods as it does for enclosures in 501.3(A), for example.
Since you haven't yet demonstrated a need for flexibility [501.4(B)(2)], technically liquid tight would still be a violation. Here is where reason needs to step in. With the additional proviso of installing the liquid tight per 501.16(B), I think this would be a good application of 90.4.
Re: Liquidtight use for adjacent enclosure purge flow?
The ventilation is what started this whole thing; it's easier said to "balance" the ventilation than do in some applications. Pressurizing each only may have been the easier way to go.
There is not a need for flexibility, other than it's easier to install. Does 501.16(B) mean I should probably run a ground wire through the conduit? Looking to install approxiamately 5' of rigid with a liquid tight whip (less than 18") on each end to connect the second and third enclosures.
The ventilation is what started this whole thing; it's easier said to "balance" the ventilation than do in some applications. Pressurizing each only may have been the easier way to go.
There is not a need for flexibility, other than it's easier to install. Does 501.16(B) mean I should probably run a ground wire through the conduit? Looking to install approxiamately 5' of rigid with a liquid tight whip (less than 18") on each end to connect the second and third enclosures.
- Location
- Mission Viejo, CA
- Occupation
- Professional Electrical Engineer
Re: Liquidtight use for adjacent enclosure purge flow?
The maintained pressure requirement is so small, ?balance? shouldn?t even be an issue. The venting requirement is based on the idea that you first need to ?purge? the system of possible gases before energizing it.
With regard to 501.16(B), I prefer to use an external bonding jumper. It has been demonstrated to CMP14, that a ground fault can create an arc between the turns of flexible metallic raceways ? both standard and liquid tight. The arc is definitely capable of igniting flammable gases. The internal copper strip included in listed liquid tight has also been demonstrated to not be adequate to prevent the arc. An internal, properly sized and bonded, EGC has been found to be sufficient though. My preference is just based on the fact that I like to see it.
I guess an interesting side question would be whether pressurized liquid tight could do without the auxiliary bond if it were an otherwise acceptable EGC. I believe you would have to get pressurized raceways recognized first .
The maintained pressure requirement is so small, ?balance? shouldn?t even be an issue. The venting requirement is based on the idea that you first need to ?purge? the system of possible gases before energizing it.
With regard to 501.16(B), I prefer to use an external bonding jumper. It has been demonstrated to CMP14, that a ground fault can create an arc between the turns of flexible metallic raceways ? both standard and liquid tight. The arc is definitely capable of igniting flammable gases. The internal copper strip included in listed liquid tight has also been demonstrated to not be adequate to prevent the arc. An internal, properly sized and bonded, EGC has been found to be sufficient though. My preference is just based on the fact that I like to see it.
I guess an interesting side question would be whether pressurized liquid tight could do without the auxiliary bond if it were an otherwise acceptable EGC. I believe you would have to get pressurized raceways recognized first
.Re: Liquidtight use for adjacent enclosure purge flow?
The 1" liquidtight connection is actually between the first and second enclosures adding to a 1" ss tube since the 1"ss didn't allow enough flow and caused the first enclosure to flex considerably. Side note is the enclosure mfg. had changed their design eliminating the door stiffener and panel brace; looked like it was going to blow a seal at 5 inches of water during purge and fine under regular pressure.
The bond at this time is the welded stainless steel tube between the enclosures.
The 1" liquidtight connection is actually between the first and second enclosures adding to a 1" ss tube since the 1"ss didn't allow enough flow and caused the first enclosure to flex considerably. Side note is the enclosure mfg. had changed their design eliminating the door stiffener and panel brace; looked like it was going to blow a seal at 5 inches of water during purge and fine under regular pressure.
The bond at this time is the welded stainless steel tube between the enclosures.
- Location
- Mission Viejo, CA
- Occupation
- Professional Electrical Engineer
Re: Liquidtight use for adjacent enclosure purge flow?
I think we?ve pretty much exhausted the discussion of your original query ? now we?re left with my curiosity. Why in the world would 50 times the necessary pressure ever be applied - even during purge? That might blow the liquid tight too. (I have absolutely no idea if it would or not; I don't know the pressure rating - if it has one )
As I said in my earlier post, if it were up to me, there would be no pre-energization purge requirement for Type Z systems at all. Few people realize how infrequently Division 2 locations are actually hazardous. In many cases, it could go years without seeing an ignitible mixture. This is not to say Division 2 doesn?t warrant respect. Exercising proper caution with constant or frequent ignition sources is definitely necessary.
Personally, I would ?sniff? the enclosures with a properly calibrated gas detector, close them while continuing to monitor the area, turn on the pressurizing system immediately, recheck the gas detector's calibration and energize at will. NFPA 496, Section 3.4, Exception would permit this. Most modern portable gas detectors are self-calibrating and it only takes a few seconds to check. I?d also get rid of the vent too to reduce flow rates.
I think we?ve pretty much exhausted the discussion of your original query ? now we?re left with my curiosity. Why in the world would 50 times the necessary pressure ever be applied - even during purge? That might blow the liquid tight too. (I have absolutely no idea if it would or not; I don't know the pressure rating - if it has one
)As I said in my earlier post, if it were up to me, there would be no pre-energization purge requirement for Type Z systems at all. Few people realize how infrequently Division 2 locations are actually hazardous. In many cases, it could go years without seeing an ignitible mixture. This is not to say Division 2 doesn?t warrant respect. Exercising proper caution with constant or frequent ignition sources is definitely necessary.
Personally, I would ?sniff? the enclosures with a properly calibrated gas detector, close them while continuing to monitor the area, turn on the pressurizing system immediately, recheck the gas detector's calibration and energize at will. NFPA 496, Section 3.4, Exception would permit this. Most modern portable gas detectors are self-calibrating and it only takes a few seconds to check. I?d also get rid of the vent too to reduce flow rates.
Re: Liquidtight use for adjacent enclosure purge flow?
it has come down to curiosity, thanks for the discussion. I recall seeing a defined PSI as requirement for NEMA enclosure ratings, didn't really see one for LT. This mfg's purge system rated pressure is 0.25"WC for normal operation and 5.0"WC for rapid purge.
it has come down to curiosity, thanks for the discussion. I recall seeing a defined PSI as requirement for NEMA enclosure ratings, didn't really see one for LT. This mfg's purge system rated pressure is 0.25"WC for normal operation and 5.0"WC for rapid purge.
- Location
- Mission Viejo, CA
- Occupation
- Professional Electrical Engineer
Re: Liquidtight use for adjacent enclosure purge flow?
I just reviewed NEMA 250. The only standard enclosures that are not already intended for classified locations that go through any pressure tests are NEMA 6 &6P (submersibles).
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