I am into process engineering and part of my job involves specifying
thermophysical data and flows for various safety valves.Currently I am working on a process which contains C1-C4 hydrocarbons.I have this process fluid which is vapor at normal condition but when I flash it adiabatically to a higher pressure i.e. at its relieving pressure under a blocked outlet condition I get a two phase fluid.Now when I am reporting data for the the relief valve should I take into account DIERS criteria or simply report data for two phase flow

ab84 - I suggest you re-state your inquiry. It's not clear.

I'm guessing that you're trying to describe a supercritical fluid in the protected vessel, which transitions to a 2-phase stream on the outlet side of the PSV. But, that's not what you wrote, so I'm not sure I'm interpreting it correctly.

I wanted to know whether DIERS analysis should be carried out for any scenario involving two phase flow in sub-critical region

I intend to say two phase flow at PSV inlet at relieving pressure

Yes, Diers methodology should be used whenever there is 2-phase flow entering the PSV.  

What pressures and fluids are you talking about? It looks like you have a supercritical relief (reported as two phase by simulators?)

Well I was of the view that DIERS analysis only applies when we are concerned about fire scenario....but does it applies to all other relief scenario's as well

It also applies to exothermal reactions, decompositions, anything that may makes your liquid foam and have a biphasic relief stream at the relief device inlet.

DIERS is simply a methodology to evaluate relief valve capacity for two phase relief.

The PSV doesn't care how/why it has to handle two phase flow.

I was of the view that DIERS is applicable when you are not certain about the mass fraction of the phases at the PSV inlet.For a blocked outlet case when you know the mass fraction of the phases at PSV inlet,I am certain that DIERS does not apply

@ab84 - DIERS methodology provides a way to calculate the orifice size for a relief device that has a 2-phase stream entering the device. That calculation methodology doesn't depend on the cause of the 2-phase relief. There's a wide range of possible causes, and fire exposure is just one of them. In almost every 2-phase case the phase composition is a transient - the phase split varies from the time the relief begins until the time that it ends. In many cases, the 2-phase stream will transition into an all-vapor relief. In others, such as those in which the fluid is foamy, the 2-phase release may continue until the vessel is empty.

So, be cautious about says you know the phase fraction, and regardless of that split, the DIERS method is the best available technology for safely sizing the relief device.