EExd Motor - Surface Corrosion

[Ahr35181]

I’d like to know the opinions of those with specific knowledge of EX Hazardous Area Equipment on the following:-

How bad should surface corrosion on an EExd motor operating in an outdoors chemical enviroment be allowed to get before it affects its ability to operate safely?

We have several cast iron EExd motors which don’t have a lick of paint left on them & are covered in surface corrosion, although the surface corrosion does not appear to be deep. These motors are old & will probably be replaced with new ones rather than overhauled, my personal opinion is that this should be done sooner rather than later as the motors are out of compliance with BS EN 60079.

Thanks,

Alan

 

[Ahr35181]

Surely there must be some EX experts out there who can advise me on this?

My concerns are with the affect on mechanical strentgh to the enclosure. Allso, will the temperature classification be infringed?

Thanks,

Alan

 

[vanskat]

See the site below. They have some good options for motor covers which protect the motor surface as well as limit operating temperatures.

http://www.themotorcover.com

 

[Ahr35181]

vanskat,

Thanks for your reply, but with flameproof equipment we are not allowed to obstruct the flanges.

Alan

 

[castera]

Surface corrosion should be picked up as part of a visual inspection, which should lead on to a closer inspection during the detailed inspection. I would have thought that the surface rust in itself is not a problem so long as the corrotion has not penetrated the metal so far as to make the enclosure not able to withstand an internal explosion (though how you determine this is behond me), or effected or penetrated the flange gaps. If the EX'd markings are not ledgible i think this in itself make the equipment out of compliance.

 

[jbartos]

Suggestion: Perform a cathodic protection study. Measure dc voltages between the motor and soil. If there are dc voltages above ~0.3VDC, then the cathodic protection dc source, and controller might be the solution

 

[Ahr35181]

jbartos,

I notice your high input to threads & maybe your looking for a world record??

You obviously have limited knowledge on hazardous area electrical equipment,so my advice to you is GET REAL!!

I'm sure your advice in the past has been apreciated by a few.

Best Regards,

Alan

 

[rshoultz]

Being a motor guy, I keep reminding myself that insulation, bearings and air gap (and magnets if used) make a motor. If it were me, if oxide is getting inside the motor, then I would imagine failure is close. If the oxidation is heavy, but over the lamination, there is likely still a lot of inherent structure there to keep things from falling apart. I think if you lay your hand on it and feel for changes in temperature, sound and vibration on a periodic basis, you're on the right track. What's the lead time on a new one? Try getting one Teflon coated next time. That's working for us.

 

[prewrathrap]

I am not expert on EExd motors or standard that is affecting them. I worked mainly with instrumentation but basic concepts are the same. In EExd which is explosion proof housing most severe case. Not sure whether you are in zone 0 or 1 area or temperature class. The main issue is to heat generation. With an EExd motor it is totally enclosed and should have very high heat insulation class and long paths for limiting an explosion within unit if gas were to get inside. I do not believe that the heavy oxidation will affect this rating unless it is so heavy that you have breached the motor enclosure. I think you might repair motor with good blast and paint job - remember to cover the motor nameplate. Again we are trying to limit the amount of surface temperature and have long flame paths if ignition occurs within unit.

Hope this helps
Mark

 

[acmotorengineer]

ahr35181

don't worry about surface corrosion on cast iron motors UNTIL you can see electrical cores or rotating parts or violent vibration.

I have a picture of a tin can type motor that was operating with half of the frame rusted away - running quite nicely. You could actually touch the rotor -- not recommended.


Keep the bearing lubrication clean and you should be OK.

PS: make sure the condenstate drain holes are free and properly placed.

 

[jbartos]

Suggestion to Ahr35181 (Electrical) Oct 18, 2003 marked ///\\jbartos,
I notice your high input to threads & maybe your looking for a world record??
///This deviates from the eng-tips intent of this Forum.\\You obviously have limited knowledge on hazardous area electrical equipment,so my advice to you is GET REAL!!
///Please, notice that some threads are so demanding beyond eng-tips that they remain unanswered.\\I'm sure your advice in the past has been apreciated by a few.
///Some original postings seeking complete and perfect solutions in this Forum may be so-so satisfied with eng-tips only. I understand your frustrations.\\Best Regards,

Alan

 

[Ahr35181]

acmotorengineer,

I think you missed the key phrase here 'HAZARDOUS AREA', my concerns are with the possiblity of a motor causing an explosion!!!

Thanks anyway for your interesting point.

Alan

 

[Ahr35181]

jbartos,

My conclusion............

You ARE a MAD Professor, have a beer & chill out!!

Regards,

Alan

 

[bigamp]

Ahr35181,

I think that your responses to jbartos are inappropriate and uncalled for. I like your advice though (but not in the context in which it was applied) and intend to follow it myself (in part at least) after work tonight.

As far as EEx d motors go, you have a couple of issues I guess. They always need to be strong enough to be able to withstand the forces generated from an internal explosion and most importantly, the integrity of the flame paths must be preserved. By the sound of it, if you have surface corrosion only, the strength may not be affected but it is possible that the flame paths are compromised. You would need to check this. The best thing would be to engage the services of a competent inspector (or the original motor manufacturer if possible) and have them check out the motors and give an opinion.

Personally, I do not like EEx d. If you are in a Zone 1 hazardous area, you could consider fitting EEx e replacement motors. Better still, if you are in a Zone 2 area you could go with EEx N motors.

 

[jbartos]

Suggestion: Reference:
1. Donald G. Fink, H. Wayne "Standard Handbook for Electrical Engineers," 13th Edition (and perhaps some others too), McGraw-Hill, 1993, Section: Corrosion of Iron and Steel
on page 4-175
Electrochemical potentials may be hazardous since day one or minute one in in EX Hazardous Area.