Overflow or Submerged Weir in 3-phase Sep

[bth1]

Can anyone tell me when an overflow weir should be used, or a submerged weir, in a 3-phase horizontal separator.

What are the advantages and disadvantages of the two?

 

[hanon]

I have the rough idea (not confirmed) that an overflow weir is used to get a better water-hydrocarbon separation because the residence time is bigger in the water part and both liquid have more time to separte. So, an overflow weir is used (I think) when you need a high residence time in the water to improve decanting. Also if the overflow can be installed in case of needing a minimun level of liquid :the overflow assure you to have always that level for example if you want to make a seal leg (barometric) or level in a ketlle reboiler. Anyway I don´t have either a clear idea

 

[mbeychok]

tbushe1:

In my experience, most horizontal 3-phase separators (i.e., gas, hydrocarbon liquid and water) do not use either an overflow weir or a submerged weir. Such separators most usually have a vertical, cylindrical "boot" welded to the bottom of the horizontal cylindrical section.

The interface between the gas and the top of the hydrocarbon liquid is controlled at about the middle of the horizontal section. (Controlled by a liquid level control valve)

The interface between the bottom of the hydrocarbon layer and the top of the water layer is controlled at about the middle of the vertical boot. (Controlled by a liquid interface level control valve)

Every large petroleum refinery would have many such vessels. They are also used at the wellheads of oil and gas wells to separate gas, oil and produced water.

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

 

[25362]

I think the submerged vertical weir is used when the dispersed liquid phase volume is larger than 15%, and when the liquid phases are further processed as in mixer-settlers, or when collecting heterogeneous azeotropic condensates. I've seen these used in lube extraction and solvent dewaxing units.

 

[bth1]

My question was primarily related to offshore production separators.

In reply to Hanon and 25362's statement, I agree - I think that an overflow weir is appropriate if it is necessary to thoroughly dehydrate the oil. If a 10% or more BS&W can be tolerated on the oil outlet, then a submerged weir is likely to be more suitable.