Type X Purge

[nhee2]

I have an equipment mfr. proposing that their equipment to be mounted in Cl I Div 1 Group D location is protected by an X-Purge system. The purge system is supplied by the same mfr, and consists of a small blower (mounted in the non-haz area), an internal pressure switch and time delay relay which energizes the cabinet loads once pressure is present for a sufficient period of time, and de-energizes loads when pressure is lost.

I need to take a look at NFPA 496 (which I ordered today) to understand the technical requirements.

What NRTL listings would normally be expected for a Type X purge system? Would it generally apply to the entire system (cabinet + cabinet components+purge system components) or just the purge system components?

 

[rbalex]

I have an equipment mfr. proposing that their equipment to be mounted in Cl I Div 1 Group D location is protected by an X-Purge system. The purge system is supplied by the same mfr, and consists of a small blower (mounted in the non-haz area), an internal pressure switch and time delay relay which energizes the cabinet loads once pressure is present for a sufficient period of time, and de-energizes loads when pressure is lost.

I need to take a look at NFPA 496 (which I ordered today) to understand the technical requirements.

What NRTL listings would normally be expected for a Type X purge system? Would it generally apply to the entire system (cabinet + cabinet components+purge system components) or just the purge system components?

I confess I'm not a big fan of purged/pressurized systems in general and especially Type X. Once you read all the hoops you're required to jump through to maintain it you'll see why. Nevertheless, you need no special NRTL certification to identify this protection technique, you simply have to comply with NFPA 496.

 

[nhee2]

Thanks.

The proposed system as described sounds less complex than I envision it needing to be. Ultimately if I can convince myself it is acceptable somebody will still need to convince the AHJ as well. I'll be contacting the mfr. to get more details once I have reviewed NFPA 496 requirements.

 

[rbalex]

Thanks.

The proposed system as described sounds less complex than I envision it needing to be. ...

Read the whole document, it isn't that long. Then read Sections 4.10, 4.11 and 5.5 carefully. If that doesn't convince you that Type X is a PITA, then read them again very carefully.

Purged and pressurized schemes, unlike explosionproof, are active protection techniques. Type X has the extra added requirement of shutting things down immediately on loss of purge gas. Types Y and Z allow some delay in shutdowns in most cases.

 

[petersonra]

There are listed purge systems available from a number of manufacturers that include all the doodads you need.

I am less offended by them then Mr. Alexander but I will say that while I find them to be pretty straightforward for the most part, there tend to be more issues in the field than jump right out at you.

One of them is getting a source of compressed air that is not full of oil or water. Some places where these cabinets are located there is no compressed air line even available so people bring in nitrogen bottles. These have to be replaced now and then. If you forget, you will have an unscheduled shutdown.

I am not saying don't do it, but most times you can avoid the need to do so by locating things in unclassified areas, or using other techniques.

OTOH, if you are forced to put something in a classified area that does not lend itself to other protection techniques it is not a lot worse than having to use XP enclosures IMO.

One of the reasons people use these is because they think they can work on them by just defeating the shutdown and popping open the door with the fire watch guy standing there with his gas monitor. IMO, this is generally not a very good idea, but it seems to be a fairly common practice.

Equipment manufacturers like them because they can avoid doing a lot of redesign of proven systems, which is not a bad reason to do something like this, but it is just one aspect to consider.

 

[nhee2]

Bob and rbalex,

Thanks. I agree - the systems I have seen are compressed air, regulators, pressure switches, etc. which is why a system with a fan and some 2" PVC pipe, a pressure switch and a relay, at least on paper, seems like it might not meet the required standards. But as pointed out, I need to review/understand 496 better before critiquing the mfr. proposed system.

 

[petersonra]

Bob and rbalex,

Thanks. I agree - the systems I have seen are compressed air, regulators, pressure switches, etc. which is why a system with a fan and some 2" PVC pipe, a pressure switch and a relay, at least on paper, seems like it might not meet the required standards. But as pointed out, I need to review/understand 496 better before critiquing the mfr. proposed system.

I think there is some potential for the system they are proposing to work correctly, however I am not a fan of DIY purge systems.

 

[gadfly56]

There are listed purge systems available from a number of manufacturers that include all the doodads you need.

I am less offended by them then Mr. Alexander but I will say that while I find them to be pretty straightforward for the most part, there tend to be more issues in the field than jump right out at you.

One of them is getting a source of compressed air that is not full of oil or water. Some places where these cabinets are located there is no compressed air line even available so people bring in nitrogen bottles. These have to be replaced now and then. If you forget, you will have an unscheduled shutdown.

I am not saying don't do it, but most times you can avoid the need to do so by locating things in unclassified areas, or using other techniques.

OTOH, if you are forced to put something in a classified area that does not lend itself to other protection techniques it is not a lot worse than having to use XP enclosures IMO.

One of the reasons people use these is because they think they can work on them by just defeating the shutdown and popping open the door with the fire watch guy standing there with his gas monitor. IMO, this is generally not a very good idea, but it seems to be a fairly common practice.

Equipment manufacturers like them because they can avoid doing a lot of redesign of proven systems, which is not a bad reason to do something like this, but it is just one aspect to consider.

If there are reasonable utilities available, you can use pressure swing adsorption columns to produce nitrogen enhanced gas to feed your purge system. Come to that, a simple remote compressor should be doable. Something like an air-maintenance device for sprinkler systems could be perfect. Set up two for fail-safe(er) operation. They're cheap and reliable.

 

[rbalex]

Bob and rbalex,

Thanks. I agree - the systems I have seen are compressed air, regulators, pressure switches, etc. which is why a system with a fan and some 2" PVC pipe, a pressure switch and a relay, at least on paper, seems like it might not meet the required standards. But as pointed out, I need to review/understand 496 better before critiquing the mfr. proposed system.

While you're waiting for your copy of NFPA 496, you can read the online version. I note Section 4.10 no longer covers Type X; Sections 4.11 and 5.5 cover the unique Type X requirements.

The Standard permits a brief interruption for Type X if a shutdown introduces a greater hazard [Section 4.11.1 Exception]. I would caution that loss of production alone is not sufficient.

I haven't said you can't get a suitable Type X system; just that a compliant one for a stand alone application is a not particularly cost effective solution; especially by the time you do all the other things the defenders have suggested. "Standby nitrogen", " ...pressure swing adsorption columns to produce nitrogen enhanced gas to feed your purge system...", etc. - WOW all that to avoid a simple explosionproof enclosure.

On the positive side, while NRTL listed schemes may be available, they aren't required as long as the installation is NFPA 496 compliant. Also consider NEC Section 500.8(A)(3).

 

[petersonra]

WOW all that to avoid a simple explosionproof enclosure.

Not always that simple.

Redesigning an existing equipment skid to handle the extra weight and bulk of an XP box is not always real easy. Sometimes it is all but impossible, or at least impractical.

The other thing is that you can purge a much larger enclosure than you can get in XP designs, especially if you need depth.

But, there is merit in using XP boxes if you can make it work.

 

[rbalex]

Not always that simple.

Redesigning an existing equipment skid to handle the extra weight and bulk of an XP box is not always real easy. Sometimes it is all but impossible, or at least impractical.

The other thing is that you can purge a much larger enclosure than you can get in XP designs, especially if you need depth.

But, there is merit in using XP boxes if you can make it work.

Actually, it is always that simple - unless the specs weren't clear that it was a Class I, Division 1 application in the first place. Unless the facility already has an adequate purge/pressurize system in place, there is no cost effective way to accommodate a manufacture's design problems after the fact. The "extra weight and bulk" should have been factored in at the bid stage. The OP noted this was a manufacture's proposal - so it isn't too late to get it right in the first place.

Explosionproof may or may not be the only alternative; but, in my experience, Type X has never been cost effective unless, as I said, there was a purge system already in place.

 

[nhee2]

The equipment is a cng dispenser and the purge system appears to be the off - the - shelf method of installation of operator interface devices in the dispenser. Other components within the dispenser are XP and one component is IS. This dispenser is our customer's desired component for their station. It's been installed in other stations. I am just trying to convince myself it satisfies what is required.

 

[rbalex]

The equipment is a cng dispenser and the purge system appears to be the off - the - shelf method of installation of operator interface devices in the dispenser. Other components within the dispenser are XP and one component is IS. This dispenser is our customer's desired component for their station. It's been installed in other stations. I am just trying to convince myself it satisfies what is required.

Well this was a significant bit of undisclosed information. In this case, the whole dispenser must be listed and what the manufacturer puts inside is basically up to UL 87.

 

[nhee2]

Sorry to withhold critical info.

Does UL 87 apply to CNG? Table 514.3(B)(2) does not reference this standard, it is only referenced in 514.3(B)(1). A quick online search of 4 different CNG dispenser mfrs. and none reference 'UL 87' in their list of agency approvals. One listed 'NFPA 70' in its 'regulatory' listings.....

 

[rbalex]

Sorry to withhold critical info.

Does UL 87 apply to CNG? Table 514.3(B)(2) does not reference this standard, it is only referenced in 514.3(B)(1). A quick online search of 4 different CNG dispenser mfrs. and none reference 'UL 87' in their list of agency approvals. One listed 'NFPA 70' in its 'regulatory' listings.....

I honestly don't know; CNG dispensers are certainly within UL 87's scope and the scope of NFPA 30A which are the bases of Article 514. If it isn't, I'd like to know what your research revealed were the regulatory agencies. In any case, those agencies and neither the NEC nor NFPA 496 would be the bases for acceptance.

 

[petersonra]

That certainly took a turn for the strange.

 

[nhee2]

I honestly don't know; CNG dispensers are certainly within UL 87's scope and the scope of NFPA 30A which are the bases of Article 514. If it isn't, I'd like to know what your research revealed were the regulatory agencies. In any case, those agencies and neither the NEC nor NFPA 496 would be the bases for acceptance.

Following up on this post -

The specific purge system I was asking about is not 'listed' - it is 'manufactured to NFPA 496' and appears to meet all the requirements listed in NFPA 496. Evaluation/labeling by a NRTL at the factory is an option offered and I will likely recommend to our customer.

Looking at the websites for various mfrs - none listed compliance with or listing to UL 87.

One dispenser "Compliance"

NEC Class 1 Division 1 Group D hazardous locations compliant​

CEC CSA 221.1 compliant​

ASME B31.3 compliant​

NPFA 52 and CSA B108 compliant​

United States (NIST) and Canadian (Measurement Canada) Weights and Measures approval​

CTEP - California Weights and Measures approval
​

Another:

This dispenser is manufactured under the guidelines of:​

NFPA 52 NFPA 70 (NEC) ANSI/IAS NGV 4.0-1999 CSA 12.00-M99 ANSI B31.3 CENELEC (on request)​

And another:

"Regulatory"
NFPA 52
NFPA 70
Various metering approvals
​

ONe other references a methane sensor in the cabinet to kill power on >25% LEL.

IT does not appear UL 87 has been acknowledged by the CNG vendors or required by the AHJs to this point.

 

[rbalex]

Following up on this post -

The specific purge system I was asking about is not 'listed' - it is 'manufactured to NFPA 496' and appears to meet all the requirements listed in NFPA 496. Evaluation/labeling by a NRTL at the factory is an option offered and I will likely recommend to our customer.
...
.

Excellent research.

Since New Hampshire is not a State Plan State, with respect to listing, you may want to review the definitions in FedOSHA regulations 29 CFR 1910.399 with special emphasis on Approved, Accepted and Acceptable[especially Subpart (1)]. Basically, if you read Acceptable carefully, if a product can be listed, it must be.

Even if your client is not in a State Plan State their OSHA regulations must still be in substantial compliance with FedOSHA.

 

[petersonra]

While I am not in a position to discuss it at length, I have some experience of late on CNG systems. It appears to me that most of them use some kind of purge system on the dispensers. At least the ones I have run across.

 

[nhee2]

Looks like the 'dispenser' devices (flowmeter, solenoids, etc) are XP or IS. Looks like the purge systems are associated with the card reader/interface components designed to make the dispenser more customer friendly.

Do the dispensers you have dealt with have any listings/labels on the purge system? Given the growth of CNG, I am surprised that an overall dispenser listing/standard does not appear to be typical.