1. Junior Member

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Jun 2008
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I am using Depressuring utility from HYSYS to check time taken by blowdown valve to depressuring the inventories. There is RO before the blowdown valve and I can size a relief valve using RO size say 25 mm , 50 mm Orifice dia.

My query .. if there is no RO in the piping and only blowdown valve .. shall I take full bore dia.. as RO in case I have 4 "pipe and 4 " blowdoen valve..

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3. Member

Join Date
Feb 2009
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My friend, I dont understand the reference you made to a "relief valve", but regarding your question I suggest you to proceed as follows: if you dont have a RO downstream your blowdown valve (BDV), then the only restriction that limits the flow rate is such BDV. BDVs are normally ball valves, either regular (reduced) port or full port.
To calculate the peak flow you need to know the Cv of your BDV, which can be obtained from vendors literature (be carefull: full port valves have a higher cv than regular port valves).
Once you have the Cv of your BDV you may calculate the peak flow using the following equation (for critical flow conditions P2 < 0,53 x P1):
q = 660 x Cv x P1 x sqrt (1/(SG x T1)), where q [=] SCFH, P1 [=] psia, T1 [=] &#186;R.
After obtaining the peak flow rate through your BDV, go to Hysys and modify the orifice area in so as to obtain the same peak flow rate.
In this way, the depressuring time calculated by Hysys will be reasonably realiable.
Hope this helps.
Good luck!
Mart&#237;n

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5. Senior Member

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## Re: Adiabatic Depressuring using HYSYS

Thanks

6. Junior Member

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Oct 2008
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## Re: Adiabatic Depressuring using HYSYS

Also, in Hysys, you may also input valve CV and let it runs.

7. Member

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Apr 2011
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## Re: Adiabatic Depressuring using HYSYS

I am sorry Martinmas I don't understand.
For Cv of the valve I must know the peak flow, so how may obtain the Cv without know peak flow rate?

8. Member

Join Date
Apr 2011
Posts
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## Re: Adiabatic Depressuring using HYSYS

I am sorry Martinmas I don't understand.
For Cv of the valve I must know the peak flow, so how may obtain the Cv without know peak flow rate?