Prediction of Relieving Temperature in PSV

[Processoil]

Dear sir,
Please explain how do we predict the relieving temperature of process fluid in a vessel at fire case for designing the PSV, while we know the set pressure of PSV.
How do we predict the relieving temperature.Please explain.
Thanks.

 

[Latexman]

This FAQ may help: faq1203-1293

Good luck,
Latexman

 

[SeanB]

You find the bubble point temperature of your fluid at relieving pressure. If you have HYSYS or some other simulator create your fluid stream, set your pressure at relieving condition and set your vapor fraction to zero. The resulting temperature - the bubble point temperature, will be your relieving temperature.

 

[JoeWong88]

Processoil,

INITIAL Condition
First establish your INITIAL condition prior to fire inception.

RELIEVING Condition
Second establish the RELIEVING condition using constant VOLUME & MASS method by considering the inventory is trapped in confined system.

At RELIEVING condition, the fluid in vessel could be two phase (Vapor-Liquid) or single phase (Liquid Only or Vapor only). For Vapor-Liquid case, the bubble point (hence temperature) could varies from Initial Boiling Point (IBP) to Final Boiling Point (FBP) if you have Multi-component system. Temperature variation could be small or big subject to Volatility of lightest and heaviest component. As the vapor is in equilibrium with liquid at a boiling point in many boiling points, the relieving temperature will be the boiling temperature at the particular boiling point. The relieving temperature will vary from IBP to FBP till last drop of liquid vaporised.

At RELIEVING condition, if the fluid is in Vapor form when first relieve occurred, then the temperature will continue to rise until the vessel failed.

Hence, if your system are in two phase at INITIAL condition, the RELIEVING temperature would varies from IBP to FBP and continue upto temperature where vessel failed.

Which temperature to use in PSV sizing ?
Choose the RELIEVING condition which give you the maximum required relief area. Keep in mind, maximum relief load does not mean maximum relief area.

JoeWong
Chemical & Process Technology

 

[Processoil]

Thank you.

 

[don1980]

If the protected vessel contains liquid and that liquid boils at a reasonable temperature, then the relieving temperature during a fire case is no different from that caused by loss of control over the heater or reboiler.

If it's a gas filled system then the temperature will usually contine to climb until the vessel fails. Yes, the gas temperature will increase and the pressure will rise, but even if the gas heats up enough to lift the PSV, that will usually not stop the vessel temperature from rising until it eventually fails.

In the case of a high-boiling liquid (mineral oil, Dowtherm, etc) the results will be similar to that of a gas filled vessel.

In those last two cases, the PSV provides compliance with code, but it can't effectively protect the vessel from fire exposure. Based on the specific risks and consequences, you should consider whether or not other protective measures are warranted. Those include water spray, additional insulation, or automatic depressurization.

 

[roker]

dear all,

where this temperature is measured, before or after the expansion at the outlet of the orifice, what is the temperature at the outlet along the header?

regards,
roker

 

[don1980]

Roker,

The relieving temperature is the temperature in the vessel when the safety valve is open. The temperature in the tailpipe may be the same or different, depending on fluid and the pressure in the tailpipe.

Regards