Separator carryover

[ChemEng84]

Hi, currently we have a problem at the gas plant i work for.
we have a dehydration system it consists of the following:
a horizontal separator vessel to separat the free water up to 10 microns in the feed gas, after which the feed gas enters a filter coellescer to separate water particles up to 1 micron.
after that feed gas enters molecular sieve adsorption system to be dyhedrated.

what we noticed is that the valves at the bottom of the separator and the filter don't open which means that we don't separate any liquid from the feed gas so we invistigated it and realised that the separator is undersized, we installed a van pack and then a retrofit ( inlet deflector & Demister) and in both time the valves opened wide opens for a couple of days and then closed again.

we now realize that everytime the vessel is opened it has a high performance and then after couple of days something happens which decreases the performance, we are running a root case analysys and currently lookinf for any possible causes so could anybody help.
if any further information is required please feel free to ask, thanks in advance.

 

[chief]

Try using a cyclone separator in series with your Centrifuge. They do not have any moving parts and require zero maintenance. Place the cyclone after the centrifuge, as a back-up in case of problems with the centrifuge.
Sizing of the cyclone is of course dependent on flow rate and pressure .

Offshore Engineering&Design

 

[CJKruger]

ChemEng84, I would look at the level instrumentation. Maybe the connecting lines/nozzles block up or freeze up, and the vessel fills up with liquid.

 

[mbeychok]

chief:

Where did a "centrifuge" come into this? The original poster does not mention a centrifuge ... nor can I think of any application of centrifuging in a gas plant (which is what the original poster is talking about).

Milton Beychok
(Visit me at www.air-dispersion.com)
.

 

[tickle]

If the valves are opening for a couple of days, I am thinking that level controller may be the wrong way about, ie set to open on low level and then when a slug of water arrives in the separator, it trips the level controller. BUT this sounds too wrong to really be feasible, but I would check anyway.

Whilst the LCV is open, are you getting gas break through - if not your LCV is undersized.

If you are getting gas break through - you need to investigate the control system. If you are loosing gas to the drain, then the dehydrator performance may increase, because less gas is passing to the coalescer.

Is the flow to the separator subject to slugs of liquids? This could explain why the valve is remaining open for a long period.

You do not define performance - but if you are referring to the performance of the dehydrator, then it sounds as though that is undersized as well, and you are getting a reduced gas flow to the dehydrator when the LCV is open.

 

[chief]

Mbeychok,
You are of course correct. Typo error , should read separator in place of centrifuge.

Offshore Engineering&Design

 

[ChemEng84]

tickle, thanks alot for your contribution, what we are sure about is that there isn't anything wrong with the LCV.
also if you mean by gas break through gas blowby with the drained liquid then we ofcourse don't have that as gas pressure is 50 Bar while liquid drain is collected in a vesel of 3 bar pressure.
also can any one give me information on how to increase the efficiency of separators?!! and what are the factors that affect this effeciency?!! i know pressure and temprature does but what else?

 

[owg]

So it works for a couple of days and then the bottoms valve closes which means no liquid to draw. You open it up and it looks normal, no excessive fouling, no equipment damage. Could hydrates be forming?

HAZOP at

http://www.curryhydrocarbons.ca

 

[ChemEng84]

yea it works for a couple of days after any opening for the vessel.
ofcourse there is a big possibility for hydrate formation, could you please tell me how this could affect the separator efficiency?

 

[tickle]

By gas break through I mean that when all of the liquid has emptied from the separator, then gas starts passing to the drains. The gas pressure may pass to the drains system and over pressure it. But you seem to indicate that this is not happening.

Your last post indicates that the LCV does not work and then you open the vessel up. When you put the vessel into service again, the LCV and control system work adequately for a few days and then it stops working. Is this the case - or does the valve remain open for several days at a time?

What kind of level system do you use? - Is it a float that has a leak that fills up with liquid over a few days and then does not float? Is the float the correct size for the liquid phase?

You would only get hydrates forming if you get gas being cooled - maybe in external level instruments or through J-T cooling. It could be wax forming in the oil phase? (you do not mention if it is a two or three phase separator)

Is there a separate crash dump on the separator? You have not provided any information about what flows to the separator.

Another possibiity is the presence of foam in the separator, this would impact on the separation efficiency.

Your original post states that the filter drain valves were not opening either. Has this problem been resolved? If they are still not operating correctly then it suggests a problem caused by the liquid.

The drains vessel/system. Is this adequately sized? Does it have an interlock into the LCV's when it is at HLL?

From what you have said so far, it sounds like either the LCV is undersized (with it remaining open for several days), Or you are only getting a small flow of liquid. Or the vessel is receiving slugs of liquid.