VRU-Liquid Ring Compressors

[Glenfiddich]

Would you have experience with using liquid ring compressors/vacuum pumps for a VRU? The water seal causes quenching and condensation of some HC and water vapors and the discharge gas from the separator is saturated vapour. In cold weather there's 2 phase flow on the discharge piping from HC/water condensation.

Wouldn't there also be some scaling issues from the (make-up) water?

 

[Glenfiddich]

I forgot to add the following concern: If the composition and flow of the inlet gas changes, there's a potential of more condensation at one moment and less at another moment. If there's more condensation, the discharge pressure rises. So wouldn't the discharge be up and down all day if the feed composition and rate changes?

 

[sms]

We are beginning to look at using them for VRU, but I can't say I like the idea.. One of my main concerns is that my experience has been that they don't much like entrained solids. I am concerned that if you have gunk in the gas stream, which you always do on VRU, then you may wreck the machine.

I have seen a few wreck on pipe scale and sand,along with coke and other stuff in flare gas recovery service and vacuume service. I can just imagine what would happen in VRU service.

Make up water scaling in not something I have really experienced, but I can assure you liquid ring machines won't like scale...

"Why don't you knock it off with them negative waves? Why don't you dig how beautiful it is out here? Why don't you say something righteous and hopeful for a change?" Oddball, "Kelly's Heros" 1970

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

 

[Glenfiddich]

Actually, the vendor said that if any compressor can tolerate solids best it would be a liquid ring. It's used in pulp and paper alot.

 

[zdas04]

Compressor salesmen are ... well, salesmen. Some are really good technical assets. Some are polyester-wearing slime. Most are pretty honest guys doing the best they can with the tools they've had provided to them. I'm always leery of relying too heavily on their technical statements. Better to go to a free web site with a bunch of self-styled engineers using funny handles to hide their true identities.

To me the big issue with liquid ring machines in VRU service would be compatibility between the gases/vapors and the seal liquid. In moderately heavy hydrocarbon service you often end up with stuff that is a vapor at the suction pressure and a liquid at the discharge pressure (and then a vapor again when the mixed liquid makes the next loop). There have been a lot of cases where this near-vapor liquid has been very drying and caused seals/wear points to fail. If you're going to use liquid-ring machines, you need to be really careful to select a seal liquid that will provide the necessary properties at the required pressures and temperatures to work with the vapors you are recovering in your unit.

Your question about scaling issues with the makeup water indicates that you have already decided that the seal liquid must be water. If your process analysis indicates that water is the best choice, then fine. I've seldom seen that to be the case in a VRU.

David

 

[sms]

If it is any consolation I have had a hell of a time with screw compressors in VRU service as well... Cracked silencers, blown seals, other nasty problems.

And then there are the standard recip compressor issues in VRU service, failed valves, rings and riders...

So maybe a liquid ring is no worse.. We use them in flare gas recovery service in refineries a lot.. For the most part they work.

I know one of the projects we have going is set up with a liquid ring on the first stage, and then a recip on the second. The project I am currently working uses screw compressors for both the first and second stage...

"Why don't you knock it off with them negative waves? Why don't you dig how beautiful it is out here? Why don't you say something righteous and hopeful for a change?" Oddball, "Kelly's Heros" 1970

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

 

[dcasto]

They use liquid ring compressors in gas gathering near Pampa Texas. They pull 10" Hg vacuum and discharge at 10 psig. They have average runtime, not any more or less than screw or ripcips.

 

[green1959]

Have you considered using Ejectors (Also known as Eductors, Jet Pumps) for vapour recovery. They have been succesfully used to eliminateand recover flare gasses as well as waste gas, both offshore and on-shore.

 

[Glenfiddich]

My personal preference would be to use a screw compressor or sliding vane so that the vapour leaves the compressor superheated. With an eductor or liquid ring, the vapour leaving the system is saturated. I'm considering the winter ambient temperatures and the long discharge line.

 

[green1959]

Hi Glenfiddich,
The discharge from an Ejector is not necessarily saturated. This depends on a number of things, including the condition and quality of the motive gas.
I do take your point however about winter ambient and the length of any discharge line.

 

[Glenfiddich]

The motive gas I have available isn't that hot.

 

[green1959]

If you have an application, I could have a look at it for you? I realise I have to be careful here as I realise I must not promote or sell on this site.

 

[Glenfiddich]

It's ok-thanks though. It's actually out of my hands. Someone higher up is going to decide the type of compressor to be used. I think they're aware of the potential of the:
-variation of discharge pressure due to the degree of condensation wrt to the weather and wind
-variation in discharge pressure due to variation in suction condensibles concentration