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Chemical Process Technology

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Tuesday, March 25, 2008

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A vapor-liquid-liquid separation is rather common in offshore oil and gas facilities. Generally separator with boot is preferred for low heavy liquid percentage in the mixtures. What are the factors you shall consider so that you select a separator with boot.

i) Material cost for separator with boot compare to separator with weir is lower. This is in generally perspective but there are exceptional case. It is pretty hard to provides a simple rule of thumb in general for all services. Nevertheless, Bill Svrcek et. al (1994) has indicated that a boot may be selected if the heavy phase is 15-20wt%. This could be a quick goby for a pre-selection during early engineering phase, however, it is always encourage optimization.

ii) Boot fabrication required hole drilling at the bottom of horizontal vessel, installation of reinforcement ring around the the boot-vessel connection and additional weldings around the connection. This signify the fabrication is rather complicated and increases the fabrication cost. Thus, combination of material and fabrication cost, total cost for a separator with boot may be higher than a separator with internal weir.

iii) Overall separator size would be one of the factor to be considered. It is not much cost benefits by selecting a separator with boot for rather small separator.

iv) Cost for space or overhead. Separator with boot may results small vessel and save installation cross section area. This potentially save cost on the foot print. However, it impose additional cost for overhead. For onshore facilities, normally head room is not a an issue in most cases, however head room could be major cost factor for offshore installation.

v) Separator with boot will result lesser inventory in the separator and this will reduce risk (inventory associated) in this installation.

vi) Separator with boot is having lesser weight (material and inventory) and this will reduce cost for vessel support. This is pretty attractive for large separator.

vii) For light and heavy liquid separation with very similar density e.g. heavy crude (typical ~900 kg/m3) and produced water (~1030 kg/m3), it could leads interface controllability problem if insufficient margin is provided. Separator with boot would increases the vertical length and increases the controllability of interface.

viii) Normally there is a optimum diameter for Separator with boot installation (i.e. boot diameter is 2/3 maximum of separator diameter). The limitation of the boot diameter over separator diameter may be a major determining factor. One process engineer shall not ignore this mechanical constraints.

ix) As there is boot diameter limitation, increase boot length is the only direction to increase interface controllability. Extended boot length is not really cost effective as all other cost factor increase.

x) Boot installation will results more vessel cutting and more welding compare to internal weir. This will induce higher risk to be managed.

xi) Boot once it is constructed, it is fixed and you have no way to revamp if you wish to. However, a weir installation still can be changed (especially removable weir) after it's fabricated or in operation. The weir height can be changed/adjusted as operator wish. This increases flexibility on made-good for uncertain fluid characteristic.
There is no fix rule for the selection of separator with boot in generally. What do you think ?

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posted by Webworm, 6:32 PM


Anonymous Anonymous said...

Dear JoeWong,
In the provided sketch it seems that the "Inlet Liquid(Inlet)" should be replaced with "Light Liquid(Outlet)". Am I right?
Also in point ii it seems that the last sentence should be corrected as follows:
"Thus, combination of material and fabrication .... may be HIGHER(INSTEAD OF LOWER)...weir. Yes?

March 31, 2008 at 2:11 AM  
Blogger Webworm said...

Thanks for highlight. Revised as noted...

March 31, 2008 at 3:32 PM  
Anonymous Roberto Martínez Zárate said...

I want to know the equation to calculate a Separator Gas-oil-water, because I need to know the the boot size in a Separator Gas-oil-water with weir. Would you help me?

I know the way to evaluate a Separator Gas-oil-water with weir, but no with boot.

Thank you

April 9, 2010 at 1:35 PM  
Blogger Webworm said...

Oil water separation will occur in the boot. You must ensure HC droplet will not under carry to water outlet.

April 10, 2010 at 3:03 AM  

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