Purpose of Explosion proof seals and enclosures 11

[TheGanner]

I understand how to use explosion proof seals and enclosures, I`ve installed them many times. What I can`t find an answer to [even after reading the NEC and searching several websites] is what exactly is the purpose. Is it to prevent an explosion from entering electrical systems or is it to CONTAIN an explosion and keep it from exiting an explosion proof enclosure.
My thought is that it is to contain an explosion. Simply because it is near imposible to prevent vapors from entering sparking areas and why else would there be a hundred bolts on an enclosure? To keep it from blowing in? I think not. To keep the explosion from blowing out makes more sense.
Any clarification would be appreciated.

 

[BJC]

Download this and read it. A very good source.

http://www.crouse-hinds.com/crousehinds/resources/codedigest.cfm

Appelton has a similar book, but I don't think it's on line.

 

[Comcokid]

My understanding is that with explosion proof equipment, the assumption is that the electrical equipment in the enclosure is ignition source, and that vapor that has crept into the enclosure is ignited. This explosive ignition must be contained within the enclosure.

When explosion proof enclosures are tested for approvals they put explosive gas in the enclosure and ignite it and make sure explosive vapors outside the enclosure are NOT ignited. Other things that are taken into account within the testing is the air volume within the enclosure. If this volume is explosive, and ignited, you must contain it. Less volume means less potential explosive energy.

Once you have an explosion at a facility, what happens to the equipment is a very minor concern to the potential injuries and death that may occur.

 

[davidbeach]

The explosion proof enclosure does not even have to fully contain the results of the internal explosion, it just has to cool the escaping gasses sufficiently that they cannot ignite gasses outside the enclosure. Large flat surfaces or many threads provide the heat dissipation during venting.

 

[rthiyagu00]

1 The Seals are provided to contain the gases/flames within the enclosure to prevent creation of possible hazzardous condition/Fire in the suroundins as the surrounding atmosphere (which may contain explosive gases) by the leakage of hot gases /flame.

2 The number of bolts are provided to ensure that enclosure do not break due to the pressure/force generated by the explosion within the enclosure.

R.Thiyagarajan

 

[rovineye]

That's right. The enclosure ignites the fuel, and gasses that escape must be cool enough that they won't ignite the fuel outside the enclosure. Motor use labyrinth seals to make it happen. By the time the hot gasses race around the labyrinth, they lose enough heat to be safe.

It also explains why the group and fuel type is important. The enclosure design has to take into account the molecule size and the flash point of the fuel to design the seal of labyrinth.

It also explains why an explosion proof enclosure may not have a high enough watertight integrity rating for the application. For instance a Class 1 Div 1 Group D motor may not have an IP rating of 67, and could fail due to water ingress.

 

[itsmoked]

davidbeach; Has it totally correct. The only point is to cool the exploding gases to below ignition temperatures. This of course requires the enclosure to contain the initial shock wave and not rupture or its gas cooling aspect is, of course, lost. It is most assuredly NOT to prevent explosive gases from "getting out" of the enclosure. This is why they all have exaggerated mating surfaces for gas cooling.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[jraef]

And hence, the conduit seals are providing the same functionality. They don't "prevent" the gasses from entering or the explosion from escaping. They just have to slow it down long enough to cool the gasses and keep the explosion from spreading outside or to other equipment sharing that conduit.

JRaef.com
"Engineers like to solve problems. If there are no problems handily available, they will create their own problems." Scott Adams
For the best use of Eng-Tips, please click here -> faq731-376

 

[waross]

I think that the word "contain" is misleading. Explosion proof devices are not intended to "contain" an explosion. They are intended to survive an explosion, and at the same time to cool any escaping gases to a safe temperature.
Not so sure about the seals. The ones that I am familiar with may allow cooled gases to escape, but prevent the passage of hot gases into the conduit. The casting compound is pretty tough and impervious. I think that to pass any gases the potting or casting compound would have to rupture, at which time there would be no controlled cooling. Any chance of ignition of gases in a conduit is to be avoided because of "Pressure Piling". As an explosion propagates down a pipe, the pressure generated by the combustion/explosion increases both the strength of the explosion and the speed of propagation.
There may be some leakage past the threads of the filling plug which will provide the required cooling, but no passage past the potting compound into the conduit.

 

[waross]

Sorry, no disrespect or testiness intended. I finger fumbled and submitted my post before adding my tag-line.
respectfully

 

[ScottyUK]

The main design principle of flameproof (explosion proof) protection is to direct the explosion through narrow flame paths of carefully controlled width which act to slow and cool the flamefront below the ignition temperature of the surrounding atmosphere. Compound seals provide no such cooling path and their function is to remain prevent the flame front from propagating down conduit. Glands have extended threaded portions which behave as cooling paths so some gas leakage past gland threads is acceptable.


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[Loopee]

I still like to think of the explosion being contained. An explosion can occur, and when it does the design of the "explosion proof" enclosure contains it and prevents its propagation to the outside world. It is explosion proof if your reference is the outside world that is being protected, but the European term "flameproof" is more descriptive because it describes the means to prevent the explosion.
Its also interesting that as the volume of the enclosure increases the potential explosion pressure increases and the enclosure walls have to be made much thicker. It was much less expensive for me once to use two small enclosures once than one larger one for that reason.
The Europeans have the best idea in all this: Intrinsic Safey design.

 

[bogeyman]

My previous emlpleyer was an explosives maker and I was told that "Proof" in explosives and arms manufacturing means that the items have been tested (ie proved) in the environment that they are proven for, it does not imply resistance to explosions just that they will not degrade the safety of their environment.

 

[TheGanner]

Thank you everyone for your response. Very helpful.

 

[sslobodan]

I am working with this gear all the time because I am building gas power plants. So in short:
Fire protected equipment is for stopping flammable gaseous to go inside of electrical equipment. What ever happen inside must stay inside. What ever is outside must stay outside.
Depending on type of hazardous area you have different types of protection.

 

[ScottyUK]

Fire protected equipment is for stopping flammable gaseous to go inside of electrical equipment.

Where is this principle applied? This is not the basis of explosion proof (US) or flameproof (European) protection. Flammable gases may enter the enclosure, but if ignited the flame is not allowed to propagate to the outside of the enclosure. See my earlier post regarding the function of flameproof equipment.


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[itsmoked]

Where is it applied? Where you pressurize the enclosures with air to keep the hazardous atmosphere out.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[ScottyUK]

Hi Keith,

It is the name 'fire protected equipment' that has me puzzled. Maybe it is how the phrase translates into English. I agree: it sounds like a pressurised enclosure, Ex 'P' in Europe.


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