Cv for manual piping globe valves

[Sawsan311]

Dear All,

As part of performing depressurization using HYSYS universal gas sizing equation, Cv is required for the manual globe valve installed on a vent line.. can we obtain initial assumptions of this Cv from control valve's Cv vendor data or you recommend to use the ''size valve'' feature in HYSYS for achieving the required pressure drop as an initial assumption.

Regards,

 

[marcsant]

to simulate a plant depressurization the procedure estimates the flow through valves, Ro and piping (from vessels to venting points) at each time step (numerical integration),
if you have globe valves you should enter the (operating) value of Flow Coefficient (Cv) which depends from position of flow controlling element in the valve (or you can estimate Cv from flow and dP),
for vapor phase the procedure included in Prode Properties can estimate the (critical and subcritical) flow through valves with ISA correlations from a Cv value, as alternative there are HEM, HNE-DS etc. models but these require additional parameters,
if Hysys requires a value for Cv enter that value as discussed.

 

[Sawsan311]

Thank you Marcsant for your response.. I was wondering about the value of Cv which is to be entered for a globe valve which is manual and is not actuated as a control valve.. would using equations of Cv applicable for control valve and tables given by control valve vendor for linear/ equal percentage control globe valves of various sizes like 2'' or 3'' be a correct approach.. I am suspecting these manual globe valves which are procured as piping items have a standard Cv which is not to be confused with tabulated vendor control valve Cvs.. what do you think?

 

[marcsant]

the definition for Cv (number of U. S. gallons per minute of water that will pass through a given flow restriction with a pressure drop of one psi) is the same for control valves and (manually operated) globe valves,
you can calculate Cv (from flow and dP) or estimate the value with a graph (for example Cv vs. Opening degree)
note that ISA procedure includes a few additional parameters to estimate critical flow conditions but I suppose the software will include default values.

 

[mk3223]

Normally, you may contact the Vendor or Manufacturer to provide the Cv of the manual globe valve, instead of using the Cv of the control valve.

 

[Sawsan311]

Thanks all for your inputs..

I have a follow up question to the above: if I want to restrict the Cv trim of a globe valve in order to limits its opening such that the capacity of my downstream drain or flare system is not exceeded.. and if the valve body size is 2''.. would you think that restricting the trim would accordingly cause an solids or slits to be accumulating inside the valve, specially if there is icing.. I know that the same doesnt happen with restriction orifices since they are normally installed on vent lines connected to the vapor space of the pipe. However, in my globe valve case it is connected to a liquid drain connection where a multiphase flashing would also occur across the valve.

Appreciate your views.

Regards,

 

[georgeverghese]

While RO d/s globe valve in gas/vapor depressuring service is accepted by Operations, the same is not generally accepted when used in liquid draining service at vessel bottoms. Here, solids can accumulate in these drain lines, erosion is much worse at the RO, and often the RO impedes draining when the line is partially blocked. Hence, the safe approach used for both vapor and liquids draining operations are for these tasks to be carried out under supervision and with work permit.

 

[MortenA]

For me its a “it depends”. If you design you May want to use the hysys option. If you a verifying then alway the right cv and trim type