Rotary Type Oil Flooded Compressor

[npf]

All:

Currently I am working on oil flooded rotary type compressor P&ID. It compresses propane vapor from 15psia at -42.5F to around 313 psia. The slide valve will operate based on suction pressure control. Could anyone expain to me this control scheme i.e what happens if pressure is too high or too low. Also does temperature has any bearings for this control scheme.
Also we have a recycle line with a throtling control valve.

Any help would be appreciated.

Regards,
npf

 

[zdas04]

This machine is generally known as an "Oil-Flooded Screw Compressor" to distinguish it from the Dry Screw which is a beast of a totally different hue.

In propane service, most of the big issues that I typically harp on (primarily keeping discharge temp high enough to cook off water) go away. There are Oil-flooded screws in refrigeration service that have been running for many years with minimal problems.

The slide valve is intended to keep the load on the driver constant. As demand increases, the screw will try to work harder which can cause problems during the transient, so the control scheme sees its control parameter (generally a surrogate for driver hp like engine manifold pressure or suction pressure) changing and opens the slide valve to bring it back down. In an ideal world, the slide valve would stay open for a little while and then drive slowly toward closed to load the driver without a significant transient. That doesn't always happen.

For suction control, decreasing pressure will cause the compressor to do more compression ratios (i.e., more load) and the slide valve will open to reduce the volume flow rate (i.e., less load) and let the suction drift up. On increasing pressure with the slide valve shut, nothing happens. On increasing pressure with the slide valve throttled it will drive toward shut.

A good design will have the slide valve shut in normal operations. A bad control scheme will have the slide valve in an intermediate position in normal operations. This is bad because it takes hp to compress gas from the inlet plenum to the slide valve port and that energy is wasted.

Nearly the only thing worse than a constantly throttled slide valve is a throttled suction controller. The only thing worse than a throttled suction controller is a recycle valve. Sometimes conditions change after installation and there is no good way to avoid these parasitic loads, but more often than not they came about through horrible design from the beginning.

Generally in refrig service, temperature is a second tier concern. You need to keep it high enough to keep the oil flowing and keep everything lubricated, but there really isn't a critical band like there is in wet-gas service.

David

 

[npf]

David:

Thanks for your response that helped me a lot in understanding the concept. As I mentioned we have a recycle line from the discharge of the compressor that ties back to the compressor KO Drum. I think that is the antisurge protection but it is more common on dynamic compressors. Do you have any comments?
After going thru your desciption, it means that high suction pressure is not a big concern as opposed to low suction pressure.
Also looking at the lubeoil scheme, I see thermal control valve that controls the performace of the oil.How does that work?
Also there is differential pressure transmitter beween the oil line and the inlet gas line to the comp. What is achieved by doing this?

Your help would be appreciated.
Regards,
npf

 

[zdas04]

Ok, I guess this turns into "Flooded Screw 101", but the questions you are asking come up often enough that it may be worthwhile.

You are right, anti-surge protection is a concept limited to dynamic compressors and isn't used in positive displacement machines (like flooded screws). People put recycle valves on PD compressors to limit hp by dumping "extra" pressure back to the suction. As I learned in Ariel school:

The best form of capacity control is to never compress the gas in the first place (vary rpm to match required load). Second best is to reduce volume early (with a slide valve or turn valve to dump it from near the bottom of the screw travel; the recip corollary is adding clearance). Third best is to restrict suction pressure (which can waste a huge amount of available energy). The worst form of capacity control is a discharge recycle because the pressure you are dumping has had a bunch of work done on it.

If your recycle valve is open very often, then you need to adjust your VI to lower the discharge pressure.

On suction controllers for capacity control, I've seen them double the driver hp requirement by taking a 90 psig stream down to 5 psig and then the compressor had to boost it back to 85 psig to get into the line--a bypass would have made a lot of sense

The thermal control valve is a 3-way valve with a "pill" in it that senses oil temperature into the process (oil pump suction if there is an oil pump) and increases or decreases the amount of cooler bypass that is allowed. In refrig service, I see a lot of 140F pills that will try to maintain the temperature into the process at 140F. For refrig service, this is a reasonable approach since the heat is not required to cook water off.

There are a lot of assumptions in the choice of pills (they are not adjustable, you can adjust the process by changing pills) since the pill is looking at the cold stream and the packager can assume a compression ratio (and therefore a heat of compression and temperature rise). This is fine in a very controlled environment with dry gas. In blood-guts-and-feathers well stream use, the suction and discharge pressure change continuously and the amount of water vapor in the gas is very high, so as the oil absorbs the water vapor and then never gets hot enough to cook it back out, the oil properties rapidly degrade. In wet gas service it is the single biggest cause of failure and oil loss because the bypass scheme rarely results in the desired 200-210F discharge temperature. I really hate 3-way valves in wet-gas service, but they are fine in a controlled environment like propane refrigeration

In refrig service, both the suction pressure and the discharge pressure tend to operate in fairly narrow bands. Packagers take advantage of this by using dP across the skid to drive oil flow and don't install an oil pump. That dP is typically quite adequate to allow the oil to perform all of its required functions (lubrication, seal the rotors, and carry away heat of compression), and with a reasonably constant dP they can control oil flow-rate with orifices instead of more expensive control valves. The dP sensor is probably tied to a Low-dP Kill.

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

The main difference beteen humans and apes is that we have cooler tools

 

[crjones]

Actually the 140degF thermal control point keeps things above the pressure dew point at 100psig