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Inlet Line Sizing Upstream of Blowdown Orifice

Inlet Line Sizing Upstream of Blowdown Orifice

Inlet Line Sizing Upstream of Blowdown Orifice

(OP)
I am currently sizing the blowdown orifices and associated inlet/outlet piping for an offshore oil separation process.

For sizing of the orifices I am using HYSYS's depressurisation utility.

For critical flow, HYSYS estimates the orifice size using the orifice equation stated in Perry's (Pg 10-15):

w = Cd.A.P1.(k.(M/(R*T1)).((2/(k+1))^((k+1)/(k-1))))^0.5

This equation is derived based on the assumption that the beta ratio (orifice diameter / upstream pipe diameter) is < 0.2.  When the beta ratio is < 0.2, i.e. the orifice diameter is small in comparison to the upstream pipe diameter, frictional effects can be ignored.

For the inlet piping upstream of a blowdown orifice, I normally size based on a rho.v2 criteria and ensure that the velocity and pressure drop are sensible.

I am concerned that this is not sufficient though.  For example, using the above criteria, I may end up with a 4" pipe for the maximum mass flowrate predicted.  However, for the orifice size calculated by HYSYS, a 4" pipe may mean that my beta ratio is a lot higher than 0.2.  This in turn means that frictional effects become a factor and in reality my orifice may not actually pass as much mass as HYSYS has predicted, thus the time to blowdown increases.

I would welcome any thoughts on the above and would also be interested on how you typically size the inlet lines upstream of blowdown orifices.

RE: Inlet Line Sizing Upstream of Blowdown Orifice

My thoughts on this topic-
The beta ratio of 0.2 is a very rough guideline. I would think that oftentimes when beta>0.2 the inlet friction loss is still small. Likewise, under some circumstances (e.g. long suction line) the friction loss might be large. Until you get a good feeling for when frictional losses are/are not important, analyze the entire system.
Doug

RE: Inlet Line Sizing Upstream of Blowdown Orifice

For AMSE Class 600 and higher, consider using fixed chokes for blowdown instead of orifice plates.  Try to limit the blowdown valves and associated inlet line around NPS 2 or NPS 3.  The full-port blowdown valve opens and with most drop taken via the choke.

I was slapped around a bit in another thread for this suggestion by one of the premier process engineers, perhaps Montemayor.

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