23 replies to this topic

[sukanta87]

Dear All,

 

 

To size restriction orifices (RO) in this scenario (refer to the attached file), how can I proceed? Need to decrease the pressure from 1000 psig to 85 psig. One RO is in the series of the control valve and another one is at bypass.

 

 

Regards

Attached Files

•  RO.pdf   1.18MB   477 downloads

[AlertO]

Dear Sukanta87

Normally, we can use only control valve to let down pressure as you mentioned above ( the sizing is performed as typical calculation method) however, due to the the high diff pressure between P1 and P2, RO is installed to protect the down stream pipe (lower pipe rating)in case of control valve wide open in stead of PSV installation.

The RO sizing needs the flow and pressure drop. the flow rate is from the control valve during fully open while the diff pressure is from the different pressure which the control valve has ever done (the control valve may still has some pressure drops). Now you can sizing your RO!!

Anyway, your additional RO will creat pressure drop to your system. Please check the hydraulic again to ensure that the selected control valve is still proper size.

Hope this may help you.

[paulhorth]

Sukanta87,

A restriction orifice reduces pressure only when there is flow. It is just like a valve with a fixed opening.

If your pipe is blocked in downstream of your diagram, then all of that pipe will be exposed to the 1000 psig pressure, to the furthest isolation valve. Therefore the pipe should be designed for the max upstream pressure (the PSV setpoint of the upstream vessel) all the way to the furthest valve.

The control valve can perform all the pressure reduction that you need, on its own, if it is correctly sized. Why do you need to use a RO in series with the control valve, at all? 

 

Paul

[sukanta87]

Hi AlertO,

 

Thanks. In this case:

1. If we don't know the DP of the control valve (CV) (position 1), what should be the allowable DP for the RO (position 2)?

2. If we know the DP for the CV is 1000 psig to 85 psig, what should be the allowable DP for the RO?

3. Should the RO at bypass (position 3) is same as the DP of CV (or CV's orifice)? where DP of CV is (1000-85) psig.

4. If the DP of CV is 1000 to 85 psig, is there any need of the orifice (position 2) after CV?

5. Minimum beta ratio of RO? Can it be < 0.1?

6. The maximum allowable cavitation index. can it be higher than 1.0?

 

 

Regards

[fallah]

 

To size restriction orifices (RO) in this scenario (refer to the attached file), how can I proceed? Need to decrease the pressure from 1000 psig to 85 psig. One RO is in the series of the control valve and another one is at bypass.

 

 

Hi,

 

Please specify the function of the control valve. Is it a FCV or a PCV, and if it is a PCV it would control the pressure of which point?...

Edited by fallah, 22 January 2014 - 05:42 AM.

[sukanta87]

Hello Paul,

 

To optimize the CV sizing, RO can be used in series.

[sukanta87]

Hello fallah,

 

It's actually LCV here.

[fallah]

Hello fallah,

 

It's actually LCV here.

 

Hi,

 

The main RO function in your case mostly is: to prevent the gas breakthrough case during LCV wide open. Then you should size the RO based on upstream pressure equal to 1000 psig minus delta P across the LCV in wide open position, downstream pressure equal to 85 psig and a maximum flow rate based on which the gas breakthrough beween high pressure upstream and low pressure downstream couldn't be occured at LCV wide open...

Edited by fallah, 22 January 2014 - 06:08 AM.

[AlertO]

Sukanta87

No1&2 : you have P1&P2; anyway, you may think that you have to do the hydraulic calculation two times: one during normal operation for control valve sizing + one for during LCV wide open for RO sizing. This might be two or more rounds for the calculation based on an assumption at the begining because you don't have the size of both items now. For RO sizing, fallah has clearly explained.

No.3: you may think the bypass valve is another control valve. if the bypass valve body has the same CV as the LCV, the RO is also the same size.

No.4: the pressure drop for LCV is not exactly equal to 1000-85 psig because the RO will generate some dP (although the RO is size based on another scenario, but you can estimate dP for your normal operation)

No.5&6: i'm not sure about the instrument term. Please consult with instrument guys.

[sukanta87]

No.3: you may think the bypass valve is another control valve. if the bypass valve body has the same CV as the LCV, the RO is also the same size.
 

Hi,

 

If the bypass has NOT the same CV as the LCV due to the smaller orifice size of LCV, then should we rely ONLY on the RO for this kind of DP? Is it better to think that the bypass manual valve (position 4) will be fully open in time of operation and the whole pressure drop must be done by the RO?

Another thing is if we allow a higher DP like 1000 to 85 psig to a single RO, the beta ratio become < 0.1. Though it can be overcome by other considerations.

[AlertO]

Hi

yes, RO will be sized when the bypass valve is fully opened. the pressure drop depends on you valve type (full bore or reduced bore). If it's a reduced bore valve, there is still some pressure drop although at fully open position.

DP is too high, isn't it? let you consider multistage RO type.

[sukanta87]

Yes, required DP seems too high!

[sukanta87]

If we design the restricted orifice for pressure drop of 1000 to 85 psig then orifice bore diameter becomes too small (1-3 mm).
We have considered multiphase orifice also.

[PaoloPemi]

assuming the LCV is a control (not on-off)  valve,

a variable to consider would be fluid vapor pressure,

to avoid flashing a criteria could be to design the RO

to maintain a backpressure (valve out) > vapor pressure

in the required interval of flows,

in practice that could be not easy (for liquids, with constant inlet density, a RO typically allows only a limited control over flow),

another possibility could be to convert LCV to on-off and leave the flow control to RO (critical condition)

[dangthieugia]

Sukanta87

No1&2 : you have P1&P2; anyway, you may think that you have to do the hydraulic calculation two times: one during normal operation for control valve sizing + one for during LCV wide open for RO sizing. This might be two or more rounds for the calculation based on an assumption at the begining because you don't have the size of both items now. For RO sizing, fallah has clearly explained.

Hi AlertO,

 

I am in the same situation with sukanta87. My question is how do you size the LCV if you do not know the RO- pressure drop? Also, the liquid will flash twice since it is equilibrium with vapor in the separator. (assumed hydrostatic is negligible) 

 

Many thanks,
 

[eliealtawil]

Assuming a flow rate of about 40 gpm, RO = 0.250", use thick plate..

Attached Files

•  LCV, RO Sizing.png   90.5KB   28 downloads

[Pilesar]

If the restriction orifice is there to limit the flow of high pressure gas during a level control failure, then the RO should be sized assuming the gas conditions inside the separator and not the normal liquid conditions. Calculate the system pressure drop on a gas basis to determine the RO size needed for the worst case scenario with all valves open. Without the RO in the line, the level control failure case may become the controlling case for sizing the relief valve on the downstream equipment. Once the RO size is set by the gas flow case, for the normal oil flow case it becomes just another restriction in the line that must be considered when calculating the control valve performance.

Edited by Pilesar, 29 January 2014 - 01:30 PM.

[fallah]

Hi,

 

Where the transfer line isn't capable on which a RO being installed to limit the flow of high pressure gas during LCV wide open due to relevant control loop failure, another option would be considering mechanical stop facility on the LCV to prevent LCV being opened more than a prespecified value during such failure...then no need to RO and LCV maximum opening would dictate the maximum flow rate of the gas breaks through...

[Trunk]

If the restriction orifice is there to limit the flow of high pressure gas during a level control failure, then the RO should be sized assuming the gas conditions inside the separator and not the normal liquid conditions. Calculate the system pressure drop on a gas basis to determine the RO size needed for the worst case scenario with all valves open. Without the RO in the line, the level control failure case may become the controlling case for sizing the relief valve on the downstream equipment. Once the RO size is set by the gas flow case, for the normal oil flow case it becomes just another restriction in the line that must be considered when calculating the control valve performance.

Can i use relief valve in HYSYS for simulating RO?

[PaoloPemi]

if RO is on a liquid line and Pout < vapor pressure (true vapor pressure or bubble pressure for a mixture)

then you should consider vapor at outlet conditions, too,

in choked flow vapor expands to reach critical condition,

there are models as HEM, HNE or Constant Energy which allow to model that,

I use the flash operations available in PRODE PROPERTIES for solving such problems

but you probably can adopt a different thermo if available,

 

If RO is on a gas line there could be (or not) liquid condensate at outlet

and, depending from inlet, outlet  conditions, you can have (or not) a critical condition,

vapor flow is easier to model than two phase flow and there are many

correlations which you can adopt,

as alternative the procedures for two phase flow can be adopted (rigorous energy solution)

[eliealtawil]

Use thick RO for dP = 1000 psig

Attached Files

•  Untitled.png   32.05KB   16 downloads

[S.AHMAD]

Hi averyone

1. My analysis shows that the main reason for the LCV & RO configuration is to ELIMINATE cavitation issue due to too high pressure drop

2. By studying the diagram, I can simply conclude that the control valve is LCV and the pressure 85 psig must be controlled by downstream pressure control valve.

3. If my assumption is correct,let me offer the solution, as below

 

Step 1: Size the LCV based on the downstream pressure (let say P₂) of higher that the vapor pressure (VP) of the liquid at the operating temperature or at the most probable highest operating temperature.

 

Step 2: Size the first RO based on this P₂ and downstream pressure (let say P₃) such that P3 > VP. Use some safety factor such as P₃ = 1.25VP (this is an example of 25% safety factor). This safety factor is to ensure that the pressure at the VENA contracta is higher than the VP. That is to say if P2 = VP, the pressure at the vena contracta is definitely less than VP. leads to cavitation. Use pressure recovery factor as a guide to determine the safety factor.

 

Step 3: If P₃ > 85 psig then continue adding one more RO until the downstream pressure is equal 85 psig.

 

For protection of downstream equipment from overpressure, size the PSV based on the flowrate when LCV ii in  fully open position. Overpressure is resulted from inadvertent closing of any valve downstream of the last RO, not as a result of 100% LCV opening.

Edited by S.AHMAD, 31 January 2014 - 12:45 AM.

[S.AHMAD]

Hi

Post #22 is for VP<85 psig. There is great possibility that VP> 85 psig. If this is the case than ensure that the last RO and the downstream piping is using the right material to prevent erosion/damage due to cavitation. If this case is true then there shall be another flash drum downstream and the pressure 85 psig is controlled by the pressure control on this flash drum.

Edited by S.AHMAD, 31 January 2014 - 01:54 AM.

[Trunk]

Liquid outlet in separator will flash with small pressure drop unless the separator has high elevation.