Dear All
I have question regarding blow down calculation on HYSIS.
In HYSIS utility Depressurization Dynamics on valve parameter there are many equation I have confusion which one has to be used because when I am using General Equation its giving the peak load almost 6-7 times lower than if I am using Fisher.
The Fisher Equation calculate valve Cv and General Equation calculate the Area.
Please guide me

Attached is what I have referred.

Thanks in advance
Deepak

Deepak,

Please check your blowdown time. Are both cases having same blowdown time ? Fisher, Masoneilan and other equation are mainly to work out the peak blowdown rate.

JoeWong

Everything else are same no change at all .
Recently we have recived one feed doucment from Petrnas and they have used Genral Eqauation and i have some other cos feed document they used Fisher but i am not sure which is correct

Thanks

dgoyal,

I guess you are dealing with depressuring with non-controlled system (BDV/RO) and your depressuring system is in critical flow most the case, those i would say you should apply GENERAL or SUPERSONIC option. GENERAL option is preferred as it is well established and your fluid properties are known.

FISHER / MASONEILAN is more on controlled depressuring system.

Read this article for more explanation.

• How to apply valve equation in HYSYS Depressuring ?

JoeWong

Dear JoeWong,
Could you please explain more about controlled and non-controlled depressuring including their application,benefits and disadvantages?
I am only familiar with traditional method which comprises of BDV+RO.In this method can we do depressuring only using a full bore ball valve?
Thanks in advance.

Jprocess,

Non-controlled type depressuring is the most ordinary type of depressuring arrangement. Generally it consists of a Blowdown Valve (BDV) for isolation purpose with a correctly sized restriction orifice (RO) to limit peak depressuring rate. It can depressure the system from initial pressure (Pi) to final pressure (Pf) within required time frame (i.e. 15 minutes). As it is a fixed bore restriction orifice, the initial depressuring rate is high and gradually decrease to minimum depressuring rate at the final condition.

Controlled type depressuring is generally apply to those systems such as compressor and mole sieve bed, the depressuring rate needs to be controlled in order to avoid damage of compressor seal and mole sieve bed as well as limit to 15 minutes requirement . With this additional requirement, an ordinary non-controlled type depressuring (single BDV+RO) may not meet both requirements. A controlled type depressuring is required.

Read more on this post as well :

• Controlled and Non-controlled Type Depressuring

JoeWong

Hi JoeWong,

Here Kryz. It is very interesting discussion. And my work colleague asked me the question how to do controlled blowdown. I have many uncontrolled before. We want this time to select proper control valve. We want to ensure constant flow rate and by changing percent opening.

Do you know what to do in Hysys. When selected Fisher equation we must provide in Valve Parameters Tab both Cv (guess) and percent opening. Then in Operating Conditions Tab by selecting Vapour Outlet Solving Option - calculate Cv run the utility. It brings solution like certain Cv. However we wish to find answer like how change Cv graduately (i.e. open valve more and more) to ensure constant flow rate. The solution given to us is simply similar to uncontrolled blowdown, with difference that having Control valve as restrictor rather then orifice. And percentage of opening we need to give.

Generally I wonder if Hysys can do solution like vary Cv with time when pressure in vessel decreasing to ensure constant flow and report how we should change valve position. It is too difficult for Hysys.

Kind Regards,

Kryz.

Kryz,

Yes. You are right. HSYSY do not have the flexibility in manipulating valve opening with time. Secondly it also do not allow manipulation of C1 factor [Fisher module] which is different from valve to valve.

The only way to carry out depressuring by varying valve opening is to conduct depressuring by batches. This method is trial-n-error and really take time. Somehow, if you can program the repetitive works, it will save a lot of your valuable time.

By the way, if you choose flow control type (maintaining flowrate), you have bear in mind that failure of the controller loop will lead to excessive flow during starts of depressuring. Secondly the control valve inlet pressure is decreasing with time, the driving force across control change according. I think the valve and controller will experience difficulties in maintaining a good flow.
I don't really recommend my engineer to use this method.

Generally limitation is rate of pressure drop e.g. 20 bar / min. As long as the rate of pressure drop not exceeding the limit, it would sufficient. Generally i would advise to use multiple BDV + RO connected ESD system which is considered meeting the reliability & availability.

I will try to check out any better method. If you know any, please drop some note here.

JoeWong

It seem to be an error in HYSYS depressuring dynamics when using Fisher valve model. A response has come from them stating they will rectify the same in their next update..

see the response from aspentech below

""
Thank you for your mail.

I have seen your file with results and your observation is correct the peak flow is higher when you select Fisher valve under valve parameters in Dynamic depressuring Utility.

There is a problem with the Fisher equation calculation in the 2006 dynamic tool. A disproportionate scale term in the equation has been used such that it causes wrong Cv and peak flow calculation.

We have already reported this problem to our Aspentech development team in U.S. This bug is going to fixed in 2006.5 version which is going to be released in Jan 2008.

If you need any further information on this query, please let us know.

""

kandan,

Thanks for your information.
Do you mind to advise the incident number ?

JoeWong

Joe
i also got similer response from Aspentech and incident no was 881422 .

thanks
deepak

Hello, I'm a little confused on what the depressuring utility actually calculates. I essentially want to calculate the time it will take to blowdown an entire flowline, that is initially at some pressure Pi to atmospheric. I set the initial pressure, and other prop of gas as the stream inlet, and the back pressure being atmospheric; and the depressuring utility as a general valve. My final pressure should be atmospheric, and I want to calculate time required for blowdown. Can anyone help with this? Thanks in advance.

Youssef,
Welcome.

The response to your question may not 100% related to the topic of this post. It may NOT be easy for future search. Suggest you to open new post on your own.