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Two Restricive Orifices In Series - Psv Design Limitation


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#1 blbrooks

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Posted 07 December 2007 - 08:23 AM

Hello,

Are there any guidelines for the distance required between two restrictive orifices in series used for flow limitation?

I am trying to limit the flow of a liquid mixture of isobutene and ammonia at 200 barg in such that an exisitng system PSV is still adequately sized. This could be done with one orifice but the bore diameter would need to be very small, which may lead to operational problems. Any help is appreciated. Thanks!

#2 pawan

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Posted 08 December 2007 - 03:05 AM

I wonder if U can attach a simple line sketch of ur proposal to understand the system better. Then only we can input our suggestions.

#3 blbrooks

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Posted 10 December 2007 - 09:41 AM

QUOTE (pawan @ Dec 8 2007, 04:05 AM) <{POST_SNAPBACK}>
I wonder if U can attach a simple line sketch of ur proposal to understand the system better. Then only we can input our suggestions.



Pawan,

Here is a simplified diagram of the situation. As it stands, the pressure downstream of the series of restrictive orifices (upstream of the pressure control valve "PV") is ~120 barg, when it should be more like 60 barg based on design. These two orifices are only ~6" apart in the field, so I believe they are too close for the second RO to take any pressure drop. I am looking for seperation guidelines for the placement of ROs to validate my idea. I believe that the pressure has to have time to completely recover after the first RO before going through the second orifice. Any ideas would be appreciated.

Thanks!

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#4 pawan

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Posted 14 December 2007 - 05:03 AM

Hi Blbrooks,

QUOTE
As it stands, the pressure downstream of the series of restrictive orifices (upstream of the pressure control valve "PV") is ~120 barg, when it should be more like 60 barg based on design.


Not clear to me whether orifices are supposed to reduce it to 60 as per design & u r getting 120 bar OR something else.


QUOTE
These two orifices are only ~6" apart in the field, so I believe they are too close for the second RO to take any pressure drop. I am looking for seperation guidelines for the placement of ROs to validate my idea. I believe that the pressure has to have time to completely recover after the first RO before going through the second orifice. Any ideas would be appreciated.


Definitely if spacing is not proper the profile thru them will change & they shud be as per std guidelines.

However the most important part of ur problem is that U want to restrict the flow to column to 10500 Kg/hr in case PV fails. That means U expect flow to go up in case of failure just bcoz of suddent depressurization of rxr or something else.

If it is so, What about the safety of column bcoz ur pressure (If it is as per design is 60 barg) of 60 barg will go to column & so I dont know what is the design condition of ur column.

What we do in such case is that we provide an RO downstream of PV......May be with another PSV for line safety (Not always......depending on fluids or layout). This ensures Max flow & pressure always to the column.

For column safety & cost -Design P is more important than flow.




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