HYSYS - depressurizing

[hskk]

Is depressurizing of a vessel designed for fire case only?
Is it correct to think of fire when designing the depressurizing of a system?

As per API RP 521, 3 factors need to be considered during depressurizing.

1) temp increases due to heat input to the vessel causing liquid to vaporize.

2) density change of the gas in the vessel due to the decreased of pressure during depressurizing.

3) liquid flash due to the decreased of the pressure in the vessel.

However, I can find only the heat input to the wetted surface (liquid) in HYSYS -depressurizing- under 'heat flux' under 'API521' (by default-"adiabatic". How about the heat input to the unwetted surface (how to specify in HYSYS)?

Notice that there is another option under 'heat flux', which is 'fire'
it shows the equation as below (anyone know about this equation? from which reference?)

Q = C1 + C2*Time + C3*(C4-Vessel Temp)+ C5*Liqvol(time=t)/liqvol(time=0)

 

[joerd]

Yes, you'll need to take fire heat input into account when doing a depressurizing case. Just think about when you would be emergency depressurizing - in a fire event, right?
The heat input to the wetted surface is much higher than the unwetted surface - see API RP521.
You can account for it, and do other customizations, by using the general equation (heat flux - fire). You can specify C1 thru C5 to come up with your own heat flux as a function of Time, Vessel temperature, and Liquid volume.
If you like to compute the (initial) heat flux yourself, for example, set C1 to your value, and set C2 - C5 to 0 (zero). We have done this many times.
Hysys will then do multiple flashes to determine the temperature, pressure and compositional variations over time (as suggested by your items 1/2/3)
For a good reference, read the material on support.aspentech.com There is also a training module on dynamic depressuring there (as part of the Advanced Modeling course). Also, I recommend that you check the Known Issues section for depressuring.

Cheers,
Joerd

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