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nightware(at)sasktel.net
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 Posted: Wed Oct 25, 2006 9:16 pm Post subject: explosion proof fans - intrinsically safe vs explosion pro |
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Doug, I basically agree with your statement about ratings for equipment in
the hazardous area, but I would go on a bit further.
Please excuse me for being picky about the definitions.
There are two ways to do electrical things in hazardous areas -
intrinsically safe (I.S.), and explosion-proof.
Both attempt to avoid igniting the hazardous atmosphere around the device.
I.S. systems limit available energy in the system (total connected
capacitance, allowed voltage, non-incendive barriers etc) and I've used IS
with instrumentation - switches, transmitters, communication networks, etc.
There are many constraints here, including barriers in the non-hazardous
area, and segregation of wiring; if I was instrumenting an industrial paint
booth with pressure, airflow, temperature and other instrumentation, I would
consider IS equipment. As far as I know, there are no IS motors
commercially available. A single device wouldn't be IS by itself, but
would have to be part of a designe IS system.
Explosion-proof equipment allows for higher energy levels (both voltage and
current) and prevents ignition by using seals or wide machined flanges on
doors and housings to allow for cooling of the flame front before it exits
the enclosure. The enclosures are usually built quite heavy, as they may
have an ignition internally to survive, without allowing the flame front to
ignite the surrounding environment.
To select motors for use in a hazardous location, I look for TEXP (totally
enclosed, explosion proof). In a fan application, air friction may be
generating static electricity on the fan blades and the ducting material.
When I was initially looking for a rated fan, I found words like
"non-igniting fan and housing" along with spectacular price tags.
I believe that you are partially correct regarding placement of the fan on
the supply side of the booth. This placement could reduce the hazard if:
1. you do not dead-head the fan (any in-line filters always allow
significant flow), which would allow booth contents to migrate to the fan.
2. you never have the fan stop while the hazard is present. If it
stops, you shouldn't restart it, because the motor can be considered to be
in the hazardous area once the flow stops.
Mitigation of the risk may be possible by using long supply ducting, and
your suggestion about using the fan on the supply side - this is a question
for an insurance inspector; I don't know, myself. The fan would have to be
up-sized a bit to address the additional pressure drop from the longer
ductwork. In an industrial plant, I wouldn't use a fan that wasn't rated
(fan, motor, and ductwork) for the specific hazardous area, and my
experience is that the engineer would be pretty clear about checking that.
There would also almost certainly be explosion doors and anti-backdraft
dampers in the ducting system as well.
Another method is to use a belt or chain driven fan, with the motor outside
the duct in the non-hazardous environment. I think I saw that in a Sam
James painting video, but I may be remembering incorrectly. If using a belt
drive, I would pay some extra attention to safety grounding to avoid static
and sparks.
I think that your comment on inlet and outlet placement was good. I went
with a full height inlet (three furnace filters high on the door) and a
floor-level exhaust, and an approximately 8x8 ft temporary booth. I am
still looking for a reasonably priced TEXP motor.
For further information on IS equipment, there are good references on a
number of system vendor websites. Here are a couple:
http://www.omega.com/techref/intrinsic.html
http://www.setra.com/tra/app/app_exp.htm
The second reference has brief discussion of IS vs explosion-proof.
My comments do not constitute advocacy or approval of a particular design.
I strongly suggest that anyone considering their own hazardous area
electrical design should seek the opinion of a professional engineer
licenced to practice in their area, and consider whether your insurance
would cover your loss in the event of an incident and related claim.
Howard McKay
RV8A, empennage
Saskatoon, Canada
---
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LarryRosen
Joined: 16 Jan 2006
Posts: 415
Location: Medford, NJ
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| Posted: Thu Oct 26, 2006 8:36 am Post subject: explosion proof fans - intrinsically safe vs explosion pro |
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If you believe you have or could have an explosive atmosphere (above the LEL and below UEL for the solvents) there is a lot more you need to be concerned about besides just the fan. All the electrical switches, outlets, lights, radio and any other electrical device in the area need to be either explosion-proof or intrinsically safe. Not a trivial task and not inexpensive.
Larry Rosen
-------------- Original message ----------------------
From: user01 <nightware(at)sasktel.net>
[quote]
Doug, I basically agree with your statement about ratings for equipment in
the hazardous area, but I would go on a bit further.
Please excuse me for being picky about the definitions.
There are two ways to do electrical things in hazardous areas -
intrinsically safe (I.S.), and explosion-proof.
Both attempt to avoid igniting the hazardous atmosphere around the device.
I.S. systems limit available energy in the system (total connected
capacitance, allowed voltage, non-incendive barriers etc) and I've used IS
with instrumentation - switches, transmitters, communication networks, etc.
There are many constraints here, including barriers in the non-hazardous
area, and segregation of wiring; if I was instrumenting an industrial paint
booth with pressure, airflow, temperature and other instrumentation, I would
consider IS equipment. As far as I know, there are no IS motors
commercially available. A single device wouldn't be IS by itself, but
would have to be part of a designe IS system.
Explosion-proof equipment allows for higher energy levels (both voltage and
current) and prevents ignition by using seals or wide machined flanges on
doors and housings to allow for cooling of the flame front before it exits
the enclosure. The enclosures are usually built quite heavy, as they may
have an ignition internally to survive, without allowing the flame front to
ignite the surrounding environment.
To select motors for use in a hazardous location, I look for TEXP (totally
enclosed, explosion proof). In a fan application, air friction may be
generating static electricity on the fan blades and the ducting material.
When I was initially looking for a rated fan, I found words like
"non-igniting fan and housing" along with spectacular price tags.
I believe that you are partially correct regarding placement of the fan on
the supply side of the booth. This placement could reduce the hazard if:
1. you do not dead-head the fan (any in-line filters always allow
significant flow), which would allow booth contents to migrate to the fan.
2. you never have the fan stop while the hazard is present. If it
stops, you shouldn't restart it, because the motor can be considered to be
in the hazardous area once the flow stops.
Mitigation of the risk may be possible by using long supply ducting, and
your suggestion about using the fan on the supply side - this is a question
for an insurance inspector; I don't know, myself. The fan would have to be
up-sized a bit to address the additional pressure drop from the longer
ductwork. In an industrial plant, I wouldn't use a fan that wasn't rated
(fan, motor, and ductwork) for the specific hazardous area, and my
experience is that the engineer would be pretty clear about checking that.
There would also almost certainly be explosion doors and anti-backdraft
dampers in the ducting system as well.
Another method is to use a belt or chain driven fan, with the motor outside
the duct in the non-hazardous environment. I think I saw that in a Sam
James painting video, but I may be remembering incorrectly. If using a belt
drive, I would pay some extra attention to safety grounding to avoid static
and sparks.
I think that your comment on inlet and outlet placement was good. I went
with a full height inlet (three furnace filters high on the door) and a
floor-level exhaust, and an approximately 8x8 ft temporary booth. I am
still looking for a reasonably priced TEXP motor.
For further information on IS equipment, there are good references on a
number of system vendor websites. Here are a couple:
http://www.omega.com/techref/intrinsic.html
http://www.setra.com/tra/app/app_exp.htm
The second reference has brief discussion of IS vs explosion-proof.
My comments do not constitute advocacy or approval of a particular design.
I strongly suggest that anyone considering their own hazardous area
electrical design should seek the opinion of a professional engineer
licenced to practice in their area, and consider whether your insurance
would cover your loss in the event of an incident and related claim.
Howard McKay
RV8A, empennage
Saskatoon, Canada
---
| | - The Matronics AeroElectric-List Email Forum - | | | Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:
http://www.matronics.com/Navigator?AeroElectric-List |
|
_________________
Larry Rosen
#40356
N205EN (reserved)
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