4 replies to this topic

[Cue]

I need to check the relief load for a existing relief valve installed on a Test Separator. The only info that i have is the RV size is DN80 x 600# at the inlet and DN100 x 150# at the outlet. This RV is pilot operated. I do not know what type of orifice has been used.

Set pressure of the RV is 1000 psig which is less than the design pressure of the vessel at 1415 psig. What is the impact on setting the RV set pressure less than the design pressure of the vessel?

I have try to simulate this case in hysys where i performed the constant density method to get the relieving temperature at relieving pressure (21% overpressure - fire case (wetted surface area)). At this condition, hysys give me a weird result - the fluid phase in the relieving condition is liquid phase but the vapour fraction is 1. No mass heat of vaporisation value is provided which i cannot proceed with the normal sizing method.

I suspect this is a supercritical fluid since the relieving pressure is extremely high compare to the operating pressure at about 5 barg. However, when i checked the critical properties in the relieving stream from hysys, the relieving pressure exceeds the critical pressure but the relieving temperature is still lower than the critical temperature. In this case, this fluid can still be consider as supercritical fluid?

Anyone can advise me how to proceed to size this valve?

Thanks.

Regards,
Penny

[djack77494]

What is the impact on setting the RV set pressure less than the design pressure of the vessel?

This is perfectly permissible. The only impact is that the PSV will pop at a lower pressure.

At this condition, hysys give me a weird result - the fluid phase in the relieving condition is liquid phase but the vapour fraction is 1. No mass heat of vaporisation value is provided which i cannot proceed with the normal sizing method.

Why do you say that the fluid phase is liquid? Please let us know if this is a pure fluid or a mixture; if practical give us a bit more info on the critical P & T, etc. I can say that I've worked with a pure substance above the critical pressure and I remember that Hysys sometimes "refered" to the fluid as a liquid and sometimes as a vapor. I'm not sure of why, but it should be easy enough to investigate further.

Anyone can advise me how to proceed to size this valve?

I thought this is an existing valve? Do you mean to confirm that the size is adequate? Please be specific. Also, check for a tag on the valve and look for an orifice size. Even if you do not see one, get the manufactureer's name and see what orifice sizes they install in an DN80x100 PSV. Usually only one or two orifices are available for any given body size.

[fallah]

What is the impact on setting the RV set pressure less than the design pressure of the vessel?

The size of RV to be increased for the same relief load.

[Cue]

Thanks for the quick response from all here. Let me clarify a bit further.

Why do you say that the fluid phase is liquid? Please let us know if this is a pure fluid or a mixture; if practical give us a bit more info on the critical P & T, etc.

This is a mixture of hydrocarbon from the well. I say the fluid phase is liquid as it is defined by Hysys in the properties worksheet for the relieving stream. I have try to simulate a case for pure propane at its boiling point -44 F, Hysys give me the same results where the total vapour fraction is 0 but the liquid phase vapor fraction is 1. When i increase the pressure slightly by 0.1 F, the indicated stream change instantly to vapour phase with vapor fraction = 1.

For the relieving stream, the true and pseudo critical pressure is about 47.5 barg. True critical temperature is 510.4 C and pseudo critical temperature is 411.2 C. Relieving pressure for my case at 83.4 barg exceeds the critical pressure but the relieving temperature at 311.8 C is less than the critical temperature. Will this still be consider as supercritical fluid?

I thought this is an existing valve? Do you mean to confirm that the size is adequate? Please be specific.

Currently i need to study the blowdown and relief conditions during SIMOPS operation on the existing platform where the relief load will be routed to the rig vent. I need the relief load of the RV to input to FLARENET to analyze whether the new vent header is adequate to handle the load if fire happens at the platform.

Also, check for a tag on the valve and look for an orifice size. Even if you do not see one, get the manufactureer's name and see what orifice sizes they install in an DN80x100 PSV. Usually only one or two orifices are available for any given body size.

I plan to check at site on the relief valve but i doubt the tag at site will still be in proper condition since the platform is more than 20 years in operation. However, thanks for the suggestion.

[Fletch]

Anyone can advise me how to proceed to size this valve?

One method for dealing with supercritical fluids, is to use a thermal expansion method treating the fluid as a liquid.
Using the constant density approach as you have done up to the relief pressure to determine the relief temperature, copy this stream and add 1 degree to the temperature. Take the two densities and there respective temperatures and you can determine the cubic expansion coefficient for the fluid and size the relief valve for thermal expansion as in API.