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Control Valves In Hysys


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#1 psuche03

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Posted 25 April 2015 - 08:04 PM

Currently, I'm trying to determine a PSV relief load using HYSYS.  My source is a H2 header with an operating pressure of 700 psig.  There are two control valves in series.  The first regulates the pressure that feeds the second valve, and the second valve regulates the flow to a stripping column.  I'm trying to simulate actual operating conditions so I know that my model is accurate.  Next, I need to simulate each of the valves wide open to determine my relief loads for the PSV.  I've entered the source information (pressure, temperature, composition) to my feed stream.  I've entered the valve characteristics (Cv, Km).  I was hoping that I could enter a destination pressure, and HYSYS would calculate the flow through the valves.

 

This is not the case.  HYSYS doesn't calculate the pressure drop through the valves, so the outlet of the first valve remains unconverged.  I then tried removing the destination pressure with the hopes of manually entering a flow until the outlet pressure of the second valve matches operating or relief conditions.  HYSYS seems to need a pressure drop across the valve which I do not have.

 

Does anyone have a suggestion on how to determine a flow through two valves without a pressure drop?  I'm using steady-state mode, so I'm not sure if dynamic mode would help.  Any suggestions are greatly appreciated.



#2 Bobby Strain

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Posted 25 April 2015 - 10:35 PM

Seems to me that calculating flow through 2 valves in series with a fixed inlet and fixed outlet with a bit of piping should be a simple chore. You only need to calculate flow with an estimated intermediate pressure until the flow through both valves is the same. You should not even need HYSYS. You should also examine your hypothesis that both valves fail open at the same time. Properly instrumented, this is not necessarily the case. If you have a week, you can use HYSYS dynamics. But I wouldn't.

 

Bobby



#3 Ajay S. Satpute

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Posted 25 April 2015 - 11:02 PM

Psuche03,

 

Hysys does Control Valve sizing just fine.

 

I would suggest, you first get the insight of the same trying with only one control valve. (Assuming you have checked the "Use sizing methods to calculate Delta P" check-box), If you have provided Cv, mass flow rate already, then Hysys shall calculate downstream pressure, similarly if you provide downstream pressure, then mass flow rate shall be calculated for given valve output.

 

Also you need to use the realistic values of Cv and mass flow rate. Try the "ADJUST" application to let Hysys find out mass flow rate for the given downstream pressure.

 

You may also try calculating the flow through individual control valves with maximum pressure drop available and compare with the valves  in series. You may realize smaller capacity control valve decides the flow rate value through PSV.

 

If it doesn't work, you might post Hysys file here for us to get a closer look.

 

 

Regards.

 

Ajay S. Satpute



#4 psuche03

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Posted 27 April 2015 - 02:39 PM

I had some good learning experiences with this.  I first tried simulating one control valve, and I kept getting a message for the valve of, "Failed to Find Pressure Drop."  After changing a few thing, I eventually lowered the flow and had the valve converge.  It seems that the calculations are determining choked flow.  I added the second valve, and I had to lower the flow even more to get below choked flow.

 

At this point, I question the accuracy of the simulation.  The valves are designed for a flow of 1-2 MMSCFD.  Typical operation is 0.1 MMSCFD.  My flow ends up choking near 0.02 in the simulation with the current % open for each valve.  Suggestions are greatly appreciated.



#5 psuche03

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Posted 27 April 2015 - 02:41 PM

Seems to me that calculating flow through 2 valves in series with a fixed inlet and fixed outlet with a bit of piping should be a simple chore. You only need to calculate flow with an estimated intermediate pressure until the flow through both valves is the same. You should not even need HYSYS. You should also examine your hypothesis that both valves fail open at the same time. Properly instrumented, this is not necessarily the case. If you have a week, you can use HYSYS dynamics. But I wouldn't.

 

Bobby

Perhaps I need to clarify.  I'm not looking at the case where both valves fail open simultaneously.  I'm looking at two cases: (1) - first valve fail open and second hold position  (2) - second valve is fully open and first valve holds position



#6 Ajay S. Satpute

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Posted 27 April 2015 - 10:04 PM

Psuche03,

If you can, please post the hysys file here to learn more about this problem.


Regards.

Ajay S. Satpute




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