Liquid phase in a compressor 2

[jhqpinto]

Why exactly a liquid cannot be present in a gas compressor? A liquid will certainly damage the compressor, but why and how?

 

[jamesbanda]

velocity.

The speed at which gas travells in suction pipework is very fast. Therefore the liquid is like a projectile in suction pipework so fast it will damange the blades.

 

[ash9144]

Exactly what James said.

I have seen a high speed compressor hit with 500 gallons of water. Not only was the blade destroyed, so was the shaft, the labyrinth seal, the bull gear assembly, the housing, all bearings including rear motor bearings.

When the manufacturer analyzed the metal of the shaft they were fairly amazed that the entire assembly hadn't come through the housing which would have seriously injured several people.

 

[Montemayor]

jhqpinto:

First - and foremost - just exactly what type of compressor are you talking about? A centrifugal? A reciprocating type?

It makes a big difference on what the correct answer is.

 

[jhqpinto]

Both centrifugal and reciprocating.

 

[123sen]

First upon compressors are designed not for liquids and we can not compress liquids. Its not a matter with centrifugal or reciprocating and both the compressors are damage either pistons in reciprocating and blads in centrifugal. If the liquids entered in to the compressors it induces heavy vibrations for all the parts of compressors.

 

[quark]

A simple yet elegant reply by Mr Montemayor deserves a star. His experience is undoubtedly of the greatest value and I admire him more for his back to basics attitude. I wish each of us would have such a mentor during our career startup.

The main cause for compressor damage is due to the incompressibility of liquids (or better, less compressibility) and valves are the things that get damaged first.

Each type of compressor has its own ability to handle certain percentage of liquids. Recips are worst prone, scrolls are a bit better, then comes screws and cetrifugals can handle it better. Refrigerating centrifugals(as far as my experience) can handle the accidental liquid flows (definitely not 100%) to great extent and you don't have problems.

 

[dcasto]

On reciprocating compressors designed for gas (I'll explain that one some day), the liquid cannot pass through the valves in sufficient quanity or rate and vertially no gas will enter too. As the piston reverses and starts the compression stroke the liquid will again not pass through the valves quickly and will stay in the cylinder filling up the volume of the cylinder that has clearance between the piston and end of the cylinder. After 1 or two stokes, there will be enough liquid that when the piston is nearing the end of the stroke the cylinder will be full of liquid AND because liquids do not compress the liquids will be just like a solid plug the the piston will push on. The piston will push on the liquid until something just breaks. The piston rod, the pin, the crank, the valves, the bolts holding the cylinder together, one of those items will fail.

What also happens is a liquid may enter the suction valve and because the valve is hot from the heat of compression, that liquid may vaporize (boil). The violent vaporization and localized cooling on the valve material can cause fatique in spots and the valve or parts of the valve will fail (typically the valve plates). All in all a recip can have a very very small drop of liquids.

If uou look at a reciprocating compressor and pump, there is no basic difference, suction valve, piston with sealing rings, back and forth, discharge valve, whats the difference. If you try to put liquids through a gas unit, it will fail. Put gas through a liquid unit, not a mechanical failure, it will just stop moving any thing. The diffence is in the valves and the area available to pass the fluid and open/close with the fluid.

The same is true for centifugals. Nothings really different except clearance areas. My favorite site had 3 3000 HP IR HHE compressors working in parallel on the same fluid as an 1000 HP Bingham splitcase pump I loved it.

 

[zdas04]

dcasto,
Gas in a plunger pump is not a trivial occurance either. Heat of compression causes really high temperatures that cause metal exapnsion that busts ceramic plungers in a couple of strokes.

David

 

[djack77494]

Also, recip pumps move as MUCH lower speeds than recip compressors. This gives the liquid time "to move outta the way".

 

[Jason5000]

There are, of course, exceptions to every rule. Liquid ring compressors seem to be able to handle about any fluid you throw at them. A colleague of mine occasionally tells a story about a set of liquid ring compressors in vac tower off gas service. One time during a severe tower upset the tower puked out the overhead. The off gas vacuum pumps remained in service, pumping reduced crude, and they were undamaged.

I wonder what would happen to flooded screw compressors in a liquid carryover? I've seen some trip a few times on high liquid level in the KO, but I've never seen a slug of liquid make it to the machine. I'd guess with enough liquid the twin screws wouldn't be able to shove it through fast enough, the case would pressure up, and something would break, but that's just my guess. Anyone ever see liquids make it to a flooded screw?

 

[Montemayor]

Jason:

First - and foremost - just exactly what type of compressor are you talking about? A positive displacement, rotary lobe of the Roots-Connersville type?

It makes a big difference. And what makes a difference also is what mechanical strength/tolerances/hp rating was built into the vacuum "pump" (actually a blower).