HYSYS Dynamic Depressuring Utility

[jprocess]

Dear All,
I want to ask from those process engineers who have experience of working with HYSYS dynamic depressuring utility that how reliable the results including the peak flowrate or final temperature?
For example after modelling a relief valve,are the peak flowrate and relieving temperature accurate enough to use in orifice sizing equations of API 520?
Thanks in advance.

 

[Crof]

Blowdown these days is done via a ball valve and restriction orifice (RO). Control valves were used in the past and exist on some old plants but have been superceded primarily because they leaked and spring return fail open actuated ball valves are more reliable. Sizing relief valves and blowdown are two separate things, I'm just covering the blowdown here.

RW Miller will allow you to size an RO for a given flow, as would many packages inc. HYSYS and InstCalc. The hard bit is calculating the required peak initial flow, which is the RO sizing case, to give you the required drop in pressure in 15 mins. HYSYS purports to do it but two of the three options, the control valve ones (Masonellian and Fisher) are rubbish, so do it with the RO option (see above).

Cross check the HYSYS RO size by calculating the peak initial flow using Grote, compare it with the HYSYS peak initial, within about 10% is fine.

As a further check calc. the RO size (by whatever method).

Grote Equation;

Wi=Wt/[(Te/Pi)*{(Pi-Pe)/ln(Pi/Pe)}]

Where;

Wi= Peak initial flow kg/hr
Wt= total mass to be removed kg
Te= Blowdown time hrs
Pi= initial pressure bara
Pe= final pressure bara

And for your homework, remember integration!!!!;

The Grote equation is derived from the fact that pressure decay during blowdown is sonic, hence the curve is an exponential decay, with the equation;

Flow=xe^yT

Where

T=time and x and y are unknowns.

If you have any snotty wee graduates around this is the perfect task to set them, just to annoy them!!!!

 

[Crof]

Oh, sorry. Forgot to answer the bit on final temperature. If your vessel has liquid in it, just flash it to atmospheric in one stage and that is your vessel final temperature (ignore heat input).

The B/D outlet nozzle also gets cold due to the polytropic expansion of the gas and the vaporisation of gas from the liquid. Use 40% polytropic efficiency in HYSYS. This will give you the temperature of the gas upstream the blowdown orifice (again ignore heat input).

 

[MortenA]

Crof

I will only agree to your last statement as long as it dosnt lead to high extra cost due to exotic materials or other costs required by low temperature in combination with the normal operating conditions.

Even for pipe your assumption is a gross simplification (but as long as it does not carry any extra costs acceptable).

Oh and in most cases the fire blow down case is used for estimating RO size - and then heat input cannot be ignored.

Best regards

Morten

 

[jprocess]

Dear Crof,
You stated that:
"HYSYS purports to do it but two of the three options, the control valve ones (Masonellian and Fisher) are rubbish, so do it with the RO option."
Why do you believe that (Masonellian and Fisher) are rubbish?
What do you mean by "RO option"?
And about this staement:
"If your vessel has liquid in it, just flash it to atmospheric in one stage and that is your vessel final temperature (ignore heat input)."
You mean usung J-T valve will result minimum anticipated temperature?
Using the depressuring utility I found a final temperature lower than the value predicted by J-T valve in one of my case studies!

Cheers.

 

[MortenA]

JP this was discussed resently - you gotta be aware of up and downstream the RO.

During depress what goes on IN the vessel is (close) to adiabatic but what goes on across the RO/valve is close to isenthalpic (hope i got that right dont have my books to check on the wording). But as the pressure in the vessel approaches atm. then the dP across the RO/valve get low and the final gas temp is the equal to adiabatic depres.

But as i said before: If you can ignore heat input depends.. and for material temperature you should be extra carefull. If the vessel/pipe is uninsulated then it can be a good deal warmer than the gas.

Just so that you dont specify very exotic material just based on this.

Best regards

Morten