Quick method for sizing a Slug Catcher/Separator on a Nat Gas Line

[Juujee12]

Does anyone have a quick method/rule of thumb that can be used as the sizing basis for a slug catcher/separator located on the inlet to a compressor on a natural gas line.

The intent of the vessel is to just avoid any liquid carry-over into the compressor. So far, the idea is to place a control valve on the inlet to the vessel to minimize the slug flow into the vessel. If I have this control valve in place does this mean that any size vessel (16" above) will work?

Any help would be much appreciated.

thank you.

 

[SNORGY]

Juujee12:

(1) See GPSA Chapter 7.

(2) In answer to your last question...no. Consideration of settling velocity, liquid level, droplet size and other parameters in combination will show what size works and what size does not work. The slug-buster valve on the separator inlet serves mainly to optimally size the dump valves on the liquid outlet, with some corollary benefit of simultaneously throttling the gas rate somewhat. I am not sure that I would count on that valve to help downsize the separator much, beyond its raw slug capacity.

Regards,

SNORGY.

 

[zdas04]

The GPSA stuff works well above about 130 psia line pressure. It is OK down to around 50 psia. Below that it falls apart pretty badly.

I don't have a decent quantative solution below 50 psia. As a stopgap I generally use 100 pipe diameters of fluid hitting the slug catcher at 100 ft/sec. I've been using this "technique" for nearly 10 years without an upset that caused me to rethink it. You're welcome to it, but know that it is just about as arbitrary as any "rule of thumb" you'll ever see. The problem is that there really isn't any published experimental data for low-pressure gas.

David

 

[BigInch]

"The intent of the vessel is to just avoid any liquid carry-over into the compressor. So far, the idea is to place a control valve on the inlet to the vessel to minimize the slug flow into the vessel. " WHY BUILD IT THEN?

I've not seen a control valve in front of a slug catcher.. ever. If I had seen one, I would have expected to see someone removing the pieces after it had been destroyed by a slug impact. You normally don't want control valves before slug catchers in lines with enough liquids in them to need a slug catcher. You want to have as much of a constant flow operation as possible to minimize slug formations.

I think that FIRST you must decide if this is going to be a simple KNOCKOUT POT or a real SLUG catcher.

We will design everything from now on using only S.I. units ... except for the pipe diameter. Unk. British engineer

 

[Juujee12]

Thanks for the feedback.
I looked in GPSA 12th edition - chapter 7 and there is only a couple of lines on slug catchers. No formulas or methodology presented. Perhaps you all are referring to an older version?
Operating pressure is around 750 psi.

"BigInch" the idea for the inlet control valve was only to protect against a high level shutdown if the valving downstream of the vessel was overloaded. It will be open most of the time. Does this still seem like a bad idea to you?

thanks again.
Juujee12

 

[dcasto]

I cheat and use software that calculates hold up based on lockhart-martinelli which takes into account elevation changes for the hold up volume that could come see you eventually, hence the slug catcher.

Put the level control valve on the slug catcher, downstream on the gas side. If the liquid volume coming in is exceeding your ability to handle the liquids through the liquid system, pinch the gas outlet flow from the slug catcher.

 

[BigInch]

Juujee, It would not be a control valve, that would be a shut down valve. It is a very bad idea to use a flow or pressure control valve for a liquid level control on a slug catcher. There is too much pressure and flow potential variation as slugs arrive into the station area. A control valve would likely swing from one side to the other and back several times, before winding up controlling nothing.

When the slug catcher is full, I would think that you should shut down the compressor station, as any further liquid entry would presumedly carry a very high risk of flow through the slug catcher and into the compressor. Put that level control valve dcasto mentioned on the slug catcher and dump the accumulated liquids. If for some reason you can't dump the liquid to the normal disposal route, plan on completely closing a valve between the pipeline and slug catcher when the slug catcher goes to HHi level and shutting off the compressor until the slug catcher has returned to operating levels. Then, and only then, reopen the pipeline valve and start the compressor again.

We will design everything from now on using only S.I. units ... except for the pipe diameter. Unk. British engineer

 

[1969grad]

One of my pet peeves is people who use the term slug catcher when they are really refering to separators.

First thing you have to do is to figure out how big the slug is. If it is the result of pigging then it will be of considerable size.

If you want to see what the pipe or finger type slug catcher looks like then check the Taylor Forge website.

The design steps for a real pipe type slug catcher is far more complex than can be discussed in this forum.

 

[SNORGY]

They are all slug catchers. It's just that some of them aren't big enough.

Regards,

SNORGY.

 

[Juujee12]

Agreed SNORGY!

Thank you all so much for your input.

Regards,
Juujee12

 

[bcs5274]

if you are going to be receiving slugs frequently look into a 2 or 3 phase seperator/free water knockout , just make sure the trim size in the dumps are big enough to get rid of it quickly