Hello
I am hoping someone may have some advice on an issue I'm having with rating a control valve in supercritical gas Service. The gas is High pressure, low temperature hydrocarbon gas at 105.5 bar(g) and -15 degC. Molecular weight is in the range 20 - 22 kg/kmol. The valve is an emergency depressuring valve with downstream pressure at maximum flow of ~ 0 barg.
I am using ANSI/ISA 75.01.01 to do the rating calculations (company requirement). I note that the premise of ISA 75.01 is to use it with gases that do not deviate substantially from the ideal gas law & this gas does deviate substantially with a z factor of ~ 0.4 and Real Gas Cp/Cv ~ 2.4 (phase envelope places it very close to the critical point to complicate things further). However, I also note that in the Masoneilan control valve sizing manual (same equations as ISA 75.01) it notes that compressible fluid equations can be used for supercritical fluids when using the real gas heat capacity ratio.
The problem I am facing is that when determining the Xsizing value one would select the lower of the X or Xchoked value...with a real gas Cp/Cv of ~ 2.4, Xchoked is very large and therefore following the logic of ISA methodology it would suggest that the gas does not go sonic. I know from a practical sense that the gas will definitely choke across the valve which is a depressuring valve ejecting fluids into the Vent& flare system.
Therefore my questions are as follows:
Is it acceptable to over-ride the logic and select xchoked for x sizing even though x = 0.99 and Xchoked = 1.2 because I know the gas will definitely choke?
Should we derive the actual choked pressure from first principles and use that instead? Currently xchoked is calculated as per the standard using the recommended XT values for the valve style.
Is there a better method available for sizing/rating valves in supercritical service?
Thanks for any help/comments! Happy to provide any further information.
I am hoping someone may have some advice on an issue I'm having with rating a control valve in supercritical gas Service. The gas is High pressure, low temperature hydrocarbon gas at 105.5 bar(g) and -15 degC. Molecular weight is in the range 20 - 22 kg/kmol. The valve is an emergency depressuring valve with downstream pressure at maximum flow of ~ 0 barg.
I am using ANSI/ISA 75.01.01 to do the rating calculations (company requirement). I note that the premise of ISA 75.01 is to use it with gases that do not deviate substantially from the ideal gas law & this gas does deviate substantially with a z factor of ~ 0.4 and Real Gas Cp/Cv ~ 2.4 (phase envelope places it very close to the critical point to complicate things further). However, I also note that in the Masoneilan control valve sizing manual (same equations as ISA 75.01) it notes that compressible fluid equations can be used for supercritical fluids when using the real gas heat capacity ratio.
The problem I am facing is that when determining the Xsizing value one would select the lower of the X or Xchoked value...with a real gas Cp/Cv of ~ 2.4, Xchoked is very large and therefore following the logic of ISA methodology it would suggest that the gas does not go sonic. I know from a practical sense that the gas will definitely choke across the valve which is a depressuring valve ejecting fluids into the Vent& flare system.
Therefore my questions are as follows:
Is it acceptable to over-ride the logic and select xchoked for x sizing even though x = 0.99 and Xchoked = 1.2 because I know the gas will definitely choke?
Should we derive the actual choked pressure from first principles and use that instead? Currently xchoked is calculated as per the standard using the recommended XT values for the valve style.
Is there a better method available for sizing/rating valves in supercritical service?
Thanks for any help/comments! Happy to provide any further information.
