Restriction Orifice with High DP and % of liquid upstream

[Steve010]

I have a problem in a project i am working in now (still in the engineering phase).
The purge line for the flare is from the discharge of a compressor. so the design was to place a restriction orifice to drop the pressure from 1500 psi to 20 psi. I calculated the bore diameter of the RO and it was found to be 4 mm. The temperature downstream of the RO will drop to -35 C. The process engineer said that hydrates will form so decided to inject methanol upstream of the RO to prevent hydrate formation. The problem is that the required methanol injection for hydrate prevention made the stream a 2 phase flow (gas +2% methanol). I cannot place a drain hole because the drain hole should not exceed 1/10 of the RO bore which will yield a a drain hole diameter of 0.4 mm which is not practical. Also if i used a standard drain hole of 2 mm the flow will increase dramatically (because the drain hole diameter will be 1/2 the bore diameter). I am concerned about the accumulation of the liquid (methanol) at the RO upstream face. Note that this is the only source of purge for the flare and if it got plugged a flare flashback might occur.
I have searched in vendors catalogs for an eccentric bore orifice plate to be used as a restriction orifice but i couldn't find any. The vendors only state the eccentric bore for primary flow measurement elements. For restriction orifices they only provide concentric bores. I do not know why. There is no theoretical reason in my mind for not using an eccentric bore restriction orifice plate. Has any body purchased such an RO (with eccentric bore) ?
Does any body has a solution to my problem ?

 

[hacksaw]

Unless you have an established design that is tried and tested for that service, using the approach you outline described, then you need to use a control valve suited to the service.

To do otherwise is fool hardy and will get someone injured or worse.

 

[Steve010]

I agree with you.
The problem with the control valve is that if it fails open , the 1500 psig will have little resistance to the flare knockout drum, and if its fails close the purge gas will be cut. The problem is that the process doesn't want to take the purge from some low pressure source (100 psig for example) where i can use a control valve (fail open) without being concerned about transmitting very high pressure to the flare.

 

[roydm]

Just make your own plate with an eccentric bore, use the catalogue information from the concentric one to get the material & thickness.

 

[roydm]

To inject the methanol upstream of the orifice, you will need a 1500# pump. Why not inject it downstream of a conventional concentric plate and kill two birds with one stone?

 

[ironopolis]

would using 2 x RO's? And also insulation with some sort of heating meduim??

 

[ironopolis]

to stage the pressure let down.

 

[Steve010]

What do you think of using a pressure control loop with a single pressure control valve that reduces the pressure to 20 psig ?
Is such an installation reliable (provided that the flow at valve fail open will not make the downstream pressure exceed the design limits of the flare knock out drum) ?
I know the control valve bore diameter will be small as well, but i also know that control valves can be used for two phase flow.

 

[lacajun]

That's a hefty pressure drop, Steve. If I were you, I would call the valve manufacturers you use and discuss it with them. A simpler solution might be pressure regulators. I've used Fisher pressure regulators in the past.

Pamela K. Quillin, P.E.
Quillin Engineering, LLC