6 replies to this topic

[Choppy]

Hi, I'm an intern from INTECSEA. I would just like to ask, why is vertical separator more preferred when the inlet fluid has a high gas/liquid ratio? On the other hand, why is horizontal separator preferred for fluid with high liquid volume or sluggish flow? I noticed that many standards only give these recommendations without providing any reason. Thanks alot for your help!

[Dacs]

Horizontal separators tend to have a larger liquid holdup and this is preferred if you expect a higher volume of liquid in your feed whereas a vertical separator is preferred if you have a smaller liquid holdup expected.

It's not really set in stone whenever choosing a vertical or horizontal drum since both can be used (if properly designed as such).

[Choppy]

Hi Dacs, thanks for your reply!

I get what you mean. Is there any possibility that the dimension/shape of a vertical separator actually eases the G/L separation?

Btw, I've read somewhere that in a vertical separator, the large volume of gas rising up actually hinders the liquid droplets from falling down. Now wouldn't that kinda contradict the recommendation?

[Dacs]

You have to look at these considerations independently: (1) Vapor-Liquid Disengagement Velocity and (2) Liquid Holdup.

For (1), you have to consider how the vessel is being sized. What drives your diameter size is the requirement of meeting the disengagement velocity of the particle of a certain size (depending on your service, may range from 150 to 600 mm). For (2), in case of a vertical vessel, it's good engineering practice to use a length/diameter ratio of 3 to 5.

If you look at this, your velocity requirement will drive your vessel diameter, in which will fix the vessel height, which also fix the maximum liquid holdup your vessel can hold.

So in cases you have a larger volume of liquid flow such that your resulting vessel L/D may exceed the usual design practice (larger than 5), then the consideration of a horizontal vessel is warranted.

Btw, I've read somewhere that in a vertical separator, the large volume of gas rising up actually hinders the liquid droplets from falling down.

This will hold true if the volume of gas will cause the velocity inside the drum to increase beyond the disengagement velocity. In this case you either (1) relax the minimum particle diameter disengaged by increasing it, which basically means more liquid entrainment in vapor stream (2) lower the feed flowrate or (3) replace the vessel with a bigger diameter.

Hope this clarifies things up

[neha35]

What exactly is the meaning of gas/liquid ratio high?

Actually i read that vertical separator is to be used when liquid-gas ratio is high or total gas volumes are low.

Is that not a contradictory statement. HELP!!!!

[Art Montemayor]

neha35:

 

Where did you read that "vertical separator is to be used when liquid-gas ratio is high or total gas volumes are low"?

 

As Dacs states, it all depends on the internal allowable vapor velocity.   As long as you don't exceed it, you're safe.  Horizontal separators are selected, in my experience, when you have excessive liquid loading - the ratio of liquid to gas is high.  The total gas volumes may be low (but, so what?) - however, you have sufficient internal volume to render residence time and slug mitigation to the large amount of liquid.

 

This makes sense from a practical point of view (which is what engineers are paid to furnish) because a horizontal vessel has an ability to slow down the RATE of level increase inside as compared to a vertical vessel.  And this is the important feature you desire if you are to instrument and control the liquid flowing into the vessel.  You need the ability to react and drain the slugs or large flows ingressing and the horizontal vessel provides more of this than the vertical one.

[Propacket]

Apart from what Art and Dacs mentioned, i have a different example for selection of a horizontal separator handling large liquid flow rates. On a recent project, due to space limitation, we selected a vertical separator although the gas had large amount of liquid. Separator sizing showed that height required to accommodate the large liquid volume is 10 ft. It had following implications:

 

A large level gauge glass length. 

A large capillary tube for a pressure differential type level transmitter.

A ladder for access e.g. to PSV for maintenance. 

 

A horizontal separator would have saved the above additional costs.