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Backpressure And Min Temperature In Depressurization

min temperature backpressure hysys depressurization

8 replies to this topic
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#1 raman123

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Posted 26 January 2018 - 06:14 PM

For depressurization, I am using HYSYS depressurization utility to calculate peak flow. This is based on 2 psig backpressure.

Using the peak flow, I enter it into flarenet which calculates higher backpressure

Is the backpressure from flarenet the correctly calculated backpressure?

Should I use that backpressure to recalculate the min temperature after valve? Or is the one from depressurization the correct one? I am going into LTCS based on HYSYS depressurization utility results..

#2 serra

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Posted 27 January 2018 - 02:29 AM

looks similar to a previous post about propane,

https://www.cheresou...ture-realistic/

is it the same problem ?

If your goal isto  estimate the outlet temperature consider the answers in previous thread (i.e. methods to verify results from simulation and alternative solutions to remove liquid),

(see also previous threads) you may try to estimate the outlet temperature (after valve) solving an adiabatic flash...

#3 Amit J

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Posted 01 February 2018 - 09:06 AM   Best Answer

Which tool is to be used depends on your objective. I think there are two scenarios & I am elaborating it because of lack of knowledge of your objective.

1) If you are going to do cold venting and intent of your study is to find out minimum design temperature of metal, then first use Hysys  depressurisation utility. In this utility, assumed backpressure of 2 psig will provide for maximum differential pressure across RO and thus it will give peak flow and corresponding minimum temperature. Further to this build model (as per isometrics) in flarenet and use peak flow rate (from Hysys depressurisation utility) to analyse vent header. In this case flarenet will calculate actual back pressure and corresponding minimum temperature. you can use this temperature to finalise your material.

Note that no need to go back to Hysys depressurisation utility with backpressure given in flarenet.

2) If you have flare and you are analysing flare network, then go for Flarenet. In this case first use Hysys depressurisation utility with Flare tip pressure drop (data to be taken from flare tip vendor, but for preliminary study you can assume same from literature) as backpressure and find out peak flow rate. Then build model (as per isometrics) in flarenet and use peak flow rate (from Hysys depressurisation utility) to analyse flare model. In this you case, you need to study various parameters like Mach no., rho V2 criteria, noise limits prior to freezing your flare network configuration. you can use minimum temperature from this study for metal selection.

Please note that if there are more sources releasing simultaneously into flare header, then consideration of conservative scenario with operational knowledge is important in this case.

On a practical note, if you don't have time or enough resources ( like isometrics, vendor data etc.) then use minimum temperature given by Hysys depressurisation utility (by assuming 2 psig backpressure) as minimum metal design temperature.

Hope this helps.

#4 raman123

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Posted 01 February 2018 - 08:27 PM

Which tool is to be used depends on your objective. I think there are two scenarios & I am elaborating it because of lack of knowledge of your objective.

1) If you are going to do cold venting and intent of your study is to find out minimum design temperature of metal, then first use Hysys  depressurisation utility. In this utility, assumed backpressure of 2 psig will provide for maximum differential pressure across RO and thus it will give peak flow and corresponding minimum temperature. Further to this build model (as per isometrics) in flarenet and use peak flow rate (from Hysys depressurisation utility) to analyse vent header. In this case flarenet will calculate actual back pressure and corresponding minimum temperature. you can use this temperature to finalise your material.

Note that no need to go back to Hysys depressurisation utility with backpressure given in flarenet.

2) If you have flare and you are analysing flare network, then go for Flarenet. In this case first use Hysys depressurisation utility with Flare tip pressure drop (data to be taken from flare tip vendor, but for preliminary study you can assume same from literature) as backpressure and find out peak flow rate. Then build model (as per isometrics) in flarenet and use peak flow rate (from Hysys depressurisation utility) to analyse flare model. In this you case, you need to study various parameters like Mach no., rho V2 criteria, noise limits prior to freezing your flare network configuration. you can use minimum temperature from this study for metal selection.

Please note that if there are more sources releasing simultaneously into flare header, then consideration of conservative scenario with operational knowledge is important in this case.

On a practical note, if you don't have time or enough resources ( like isometrics, vendor data etc.) then use minimum temperature given by Hysys depressurisation utility (by assuming 2 psig backpressure) as minimum metal design temperature.

Hope this helps.

Hi Amit,

Thanks for your reply. I am determining the low temperatures during maintenance depressurization of a vessel. As you said I followed the following steps

1. Used hysys depressurization utility with 2 psig backpressure and this shows the min temperature at valve outlet as -30F.

2. Used flarenet to get actual backpressure of 40 psig with peak flows

Now with 40 psig downstream of the valve, I will not get these low temperatures. And with -30F I will have LTCS. The flare line is not a complex network, it is just one vessel being depressurized at one time. So, should I use backpressure from flarenet or go with depressurization utility results? Or should the approach be to change some parameters such as depressurization starting pressure etc. to avoid cold temperatures?

Thanks for the guidance.

rs

#5 Amit J

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Posted 01 February 2018 - 11:13 PM

Hi Raman123

You can use backpressure from flarenet.

More over you are saying that it is maintenance flare. Then you need not bother about depressurisation time as well.

Take your client in confidence and you can play with depressurisation time being it maintenance flare.

#6 serra

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Posted 02 February 2018 - 02:14 AM

raman123,

before to put all your efforts on the software, couldn't it be more effective try to understand a bit better the process ?
In a similar thread (see my post #2) Pilesar has given some good suggestions...
also, as student, you may wish to start with Mollier or similar tools before to get numbers which you aren't able to verify...

process safety is critical and must always be verified by a engineer familiar with how these calculations are done...

#7 fallah

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Posted 02 February 2018 - 02:53 AM

For depressurization, I am using HYSYS depressurization utility to calculate peak flow. This is based on 2 psig backpressure.

Using the peak flow, I enter it into flarenet which calculates higher backpressure

Is the backpressure from flarenet the correctly calculated backpressure?

Should I use that backpressure to recalculate the min temperature after valve? Or is the one from depressurization the correct one? I am going into LTCS based on HYSYS depressurization utility results..

Raman,

To avoid cycling due to facing with various back pressures you can specify the minimum required depressurizing time, as per available maintenance procedure of the plant, instead supposing a back pressure to get a real peak flow rate from Hysys modeling. Then you can use this peak flow rate and other required info to model the system by Flarenet which will provide the actual back pressure and corresponding minimum temperature. you can use this temperature to finalize selecting the  material of construction.

#8 raman123

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Posted 02 February 2018 - 03:22 PM

Thank you fallah, amit, serra.

I will keep the points you have mentioned in mind, when I do this study.

#9 runsaep

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Posted 06 February 2019 - 07:24 AM

Something to be aware of with cold blowdown of a 1st Separator segment ?. I'll use Unisim.