3 replies to this topic

[QuantumEng]

Hello all,

 

I have a question regarding the valve/control valve inputs tab:

 

 

When specifying a valve on Aspen HYSYS, you are able to input a Valve Opening [%] and Cv value.

I currently have a metering needle valve, which has a MAX Cv (100% open) of 1.8. There is an existing chart which shows a linear relationship between valve opening and Cv values. 

This needle valve is meant to keep constant draining rate out of a separator vessel. 

When inputting the Cv value into HYSYS, should I put BOTH the MAX Cv of the valve under the Cv specification, and then the DESIRED valve % opening? Or should I put the DESIRED Cv value AT the DESIRED % opening according to the valve's characteristic curve? 

 

I am running into the issue that I specify the MAX Cv of the valve, then specify the desired % open to achieve the desired Cv (which is lower than the max Cv)  based on the characteristic curve, then my valve is open way too far.

 

In the other case, if I specify the DESIRED Cv, then let a PID determine the % opening based on the liquid level in the pre-filled vessel, it shows a % opening far below what is specified in the characteristic curve. 

Which setup will yield the closest results to the real life operation of this system? Please see attached image regarding this question. 

 

Many thanks in advance.

 

Edit: Attached jpg.

Attached Files

•  Cv Curve Inquiry.jpg   136.34KB   0 downloads

Edited by QuantumEng, 01 April 2024 - 09:54 AM.

[Pilesar]

Check to see whether the software allows you to put more than one point to characterize the valve. You may be able to input a table of values based on your known valve performance curve.

The program may be considering the pressure drop in the rest of the hydraulic system which will also change the pressure drop across the valve.

Edited by Pilesar, 01 April 2024 - 12:01 PM.

[QuantumEng]

Check to see whether the software allows you to put more than one point to characterize the valve. You may be able to input a table of values based on your known valve performance curve.

The program may be considering the pressure drop in the rest of the hydraulic system which will also change the pressure drop across the valve.

 

Pilesar,

 

You are correct.
HYSYS allows you to input the characteristic curve of the valve as a Cv % of max vs % Opening curve. Inputting the max Cv of the valve under the Cv specification (reference the original picture), the % Cv will adjust according to the % opening based on the curve the user has provided, if the "user table" option is selected. 
The calculated Cv may vary as time passes if the simulation is done dynamically with a limited water reservoir (lowering Q value in a Cv equation)... but at peak flow rate (or otherwise assuming an infinite water volume), the Cv should hold as predicted in steady state conditions.

Thanks for the guidance!

[Pilesar]

Just to be clear... the valve CV is only part of the flow calculation. Adjusting the valve Cv cannot predict the correct flow rate in the system without also considering the system pressure drop. It is the system pressure drop that will influence flow rate -- not just the valve pressure drop. In a typical hydraulic system, as flow rate increases, the downstream piping will have more pressure drop also. This will cause the pressure immediately downstream of the valve to be increased. So the valve must open further to compensate for the lower available valve pressure drop. You might already assume the piping pressure drop is inconsequential for your needs. That is okay as long as you have made this assumption deliberately with justification. Steady state modelling with the piping effects included will give your calcs higher fidelity.