Crankcase Explosion 1

[TangoCleveland]

We have two Cooper Bessemer reciprocating vacuum exhausters, 8 cylinders each, 40" bore, 14" stroke, 327 rpm. Cooper recently informed us of a 1995 product bulletin advising us to install BICERA explosion relief devices and replace access covers because of crankcase explosion hazards. Hazard results from air in crankcase, possible lube oil mist generation, and possible mechanical friction producing heat/ignition energy.

I know that this is a big issue in marine diesels, and possibly flammable gas compressors, but we're evacuating air from research facilities. Does anyone have crankcase explosion vents on reciprocating air compressors?

Larry

 

[zdas04]

When we started installing GasJack compressors (for those not familiar with this machine, it is a conventional industrial V-8 engine with one bank of cylinders converted to compressor cylinders) everyone was concerned about the chance of crankcase explosions because there was not a distance piece.

After exhausting (and hopefully exhaustive) research we found many cases of explosions and fires within the distance pieces of integral compressors, but no crankcase fires or explosions could we find. They may be out there, but they are tough to find. What we did find was that the temperature of the crankcase tended to cause it to be very slightly over atmospheric pressure and there was no way to introduce air (on machines compressing natural gas) into the overpressured space.

On an air compressor I don't see how you could get either fuel or an ignition source to the fire. Engine lube oil is not the most flamable of substances (I'm not sure what the autoignition temp is, but I think it is higher than Methane which is 1,300F), but it will burn. Where is the ignition source? If you have a bearing hot enough to ignite lube oil you have far bigger worries than a fire.

The cylinder rings will block a flame front just like an explosion-proof box will block one. I suppose you could look at an entire ring-set failing on a power cylinder, but in a diesel that would prevent ignition.

In a gasoline or natural-gas engine with all the rings gone I have a hard time developing a scenario that an explosive mixture gets ignited by the spark plug. It's not impossible, but it is somewhat implausible.

On the other hand, you can apparently get enough friction with the crosshead rubbing within the distance piece to get to an ignition point, so if you have leakage past the compression cylinder rings (on an explosive-gas compressor) and trapped air in the chamber from the last maintenance then I suppose it is an unpleasent mixture. With an air compressor I can't even make this problem work out.

If I was you I'd get a copy of the 1995 product bulletin and make sure I truly understood the equipment in question, the potential risks (and the service where those risks are manifested), and the potential benefits. You might find that the document is limited to natural gas compressors.

Good luck with this.

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

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