dear sir,


how do we desire the Reliving temperature in PSV sizing for Hydro carbon service
at operating condition of 80 deg C. 3.5 Barg is operating pressure.

please explain any body

thanks in advance.,
regards,
dhn

You must develop relief scenarios or situations that could develop that would require activation of your PSV. Fortunately there are many available standards to assist you in this regard, and they are easily located and well described in various professional forums.

Let's take a simple situation to provide an example. Say you have a pure chemical compound in a pressure vessel. If it is exposed to a fire (the fire case scenario) then the temperature of the contents will rise due to heat absorption. As the temperature rises, so does the pressure, which is assumed to be approximately the same as the vapor pressure of the contents. For a pure compound you typically know the relationship between temperature and vapor pressure. So, when the vapor pressure reaches your PSV setpoint, the temperature will be that which corresponds to this vapor pressure. That is your relieving temperature.

I must advise your that your question is very basic and indicates a lack of knowledge about this aspect of engineering design. That's fine - we all need to start out at that point. However, because PSV are critical safety devices, have your work checked/confirmed by knowledgable co-workers or outside consultants.