Propane carryover into twin screw compressor

[fdomin]

We are planning a revamp of a gas plant and when checking the refrigeration system velocities in compressor suction scrubber are exceeded, then liquid carryover may be expected.
What could be the consequences of excessive propane carryover to the compressor?

Thanks,

 

[zdas04]

You don't say if this is a dry screw or an oil-flooded screw. It has been years since I've seen a dry screw in refrigeration service so I'll assume it is an oil-flooded screw.

Since the oil must be compatible with gaseous propane then the liquid propane (which is a terrible lubricant) will displace some of the screw oil and if this is a long-term thing then you risk some serious lubrication problems. A splash of liquid every so often is no big deal. There is a good chance that the oil is warm enough to flash the propane anyway, but that is not guaranteed.

How fast is the propane going through the scrubber anyway (and at what pressure)?

David

 

[fdomin]

zdas04,

As you stated, it is an oil-flooded screw. Maximum expected velocity in the scrubber is 4.9 ft/s, that is a K factor of 0.45 at suction P&T (30 psia & -11°F).
The scrubber has a two layer demister, the first one has a density of 5 lb/ft3 while the upper demister has a density of 12 lb/ft3. K factors for these demisters area 0.385 and 0.324 according to ACS Mistxpert.

 

[zdas04]

If it was my compressor I'd take quarterly oil samples (of the screw oil) and get extended analysis. As long as metals are not increasing I'd ignore it.

David

 

[fdomin]

zdas04,

Thank you very much for your answers. I think I´ll follow your advice.

 

[dcasto]

also, use an oil that has less propencity to absorb propane. A synthetic ice machine oil is nice, but expensive.

 

[maddocks]

I agree - oil flooded screws are generally tolerant of small amounts of carryover and you shouldn't see any big problems with this. Having said that, it's still important to have a high level shutdown on the scrubber to prevent massive liquids carryover. Synthetic oil is the way to go - especially if you're dealing with low temp propane systems. The solubility is much lower and if you can return the oil from the chiller boot, your consumption is almost nothing.