6 replies to this topic

[prasad54]

Hello Everyone,

I am sizing a PSV on a vessel containing ethylene, the flowrate to be dischagred by this valve is 6500 kg/hr due to instrument failure. the design pressure of the vessel and the set pressure of the vessel is 7 barg and the operating and design temp. of the vessel is 40deg.C and 170deg.C

I am confused what will be discharge temperature........

Thanks in advance.......

[Andrei]

prasad,

You do not specify the discharge destination of your valve, the design case of your valve and also what type of PSV we are talking about.

I think you should determine first ethylene temperature inside your vessel at relief conditions. This will depend on your design case. Since ethylene is vapor at the conditions you indicated I am assuming that most probably your design case is not fire but some kind of blocked flow due to utility/equipment/instrumentation failure. So your PSV inlet relief temperature is normal operating temperature, 40 ºC in your case.

You should determine next what is your discharge back-pressure. For a conventional type of PSV your maximum back-pressure should be 10% of your set pressure, so max. 0.7 barg in your case. Using a simulator, or other way, you can do a depressurization calculation across the valve. You have pressure and temperature upstream and pressure downstream, you should be able to calculate the temperature downstream. I am calculating about 34 ºC for this back pressure and about 33.3 ºC when your discharge is at atmospheric conditions.

So, for a conventional type of PSV your discharge temperature should be between 33.3 and 34 ºC.

You should now see the actual configuration of your discharge line and calculate your actual pressure drop considering of course the actual discharge destination point pressure. You should get a back-pressure of max. 10% of the set pressure for a conventional valve, otherwise you have to change the discharge configuration or use a different type of PSV.

And of course you have to use the exact composition of your gas. I am assuming that you are dealing with 100% ethylene when I am giving those values.

[fallah]

Hello Everyone,

I am sizing a PSV on a vessel containing ethylene, the flowrate to be dischagred by this valve is 6500 kg/hr due to instrument failure. the design pressure of the vessel and the set pressure of the vessel is 7 barg and the operating and design temp. of the vessel is 40deg.C and 170deg.C

I am confused what will be discharge temperature........

Thanks in advance.......

As per API 521:
T1=(P1/Pn)*Tn
Where:
T1 Relief absolute Temperature®
P1 Relief Pressure or Design Pressure+Overpressure (absolute psi)
Pn Operating Pressure(absolute psi)
Tn Operating absolute Temperature®

[Andrei]

Hello Everyone,

I am sizing a PSV on a vessel containing ethylene, the flowrate to be dischagred by this valve is 6500 kg/hr due to instrument failure. the design pressure of the vessel and the set pressure of the vessel is 7 barg and the operating and design temp. of the vessel is 40deg.C and 170deg.C

I am confused what will be discharge temperature........

Thanks in advance.......

As per API 521:
T1=(P1/Pn)*Tn
Where:
T1 Relief absolute Temperature®
P1 Relief Pressure or Design Pressure+Overpressure (absolute psi)
Pn Operating Pressure(absolute psi)
Tn Operating absolute Temperature®

Fallah,

Here is API definition for T1: "The gas absolute temperature, at the upstream relieving pressure, determined from equation (11)", the one you are listing.
I thought the question was about the discharge temperature, so downstream of PSV.
And the entire paragraph refers specifically to fire cases; we don't know is that's the case.

[prasad54]

Hello Fallah and Andrei,

Many thanks for your answers and sujjestions..........

[fallah]

Fallah,

Here is API definition for T1: "The gas absolute temperature, at the upstream relieving pressure, determined from equation (11)", the one you are listing.
I thought the question was about the discharge temperature, so downstream of PSV.
And the entire paragraph refers specifically to fire cases; we don't know is that's the case.

Andrei,

Regarding the matter that the question was about the discharge temperature,you are right.But i think the discharge temperature would be obtained by modelling the gas through PSV as isentropic expantion with around 70% efficiency (due to isentalpic expantion between PSV nozzle and PSV outlet).

I think,because of fixed volume of vessel,the equation of API 521 that i mentioned would be applicable for the case described by Prasad54.Other che jedies are kindly requested to submit their viewpoint in this regard.

[JoeWong]

In my opinion,

- PSV upstream temperature may consider constant density method to derive the temperature.
- PSV downstream temperature may consider isenthalpic expansion (just sufficient).