14 replies to this topic

[sunil_chemical]

Dear all,

This is regarding question for wetted area calculation in Depressurization utility in HYSYS.

Normally HYSYS calculate Wetted area based on flat volume of vessel and intial liquid volume. But when I considered total system volume as per the Fire cirlce based on API-521 than I need to include all the equipments and piping which are coming under fire circle.

When I run the HYSYS Depressurization utility than calculated wetted area by HYSYS is different than I calculated manually.

So anybody knows how HYSYS calculate the peak blow down rate? or if i have to enter manually wetted area than I have to go in duty spread sheet and Put in D8 Cell. Is it right to enter manually wetted area in D8 cell for conservative design.

I am confusioning Because somebody told me that we have to enter manually wetted area in D8 cell because calculated wetted area by HSYSY is less than the manually calculated by me in duty spread sheet and I have doubt wheather this is right method.

and also one more thing that HYSYS calculate based on considering only one flat vessel and in my blowdown system 3 to 5 equipments and piping is
coming under one fire circle.

Thats why may be wetted area calcualted manually my me is more than HSYSY calculated. but question same as above it is worth to added wetted area manually!!!


If anybody knows please let me know

Thank you

Regards

Sunil

[aju_1807]

Sunil,

This is very known problem of HYSYS but somebody rightly mentioned we have to chngae the wetted surface area in backhand based on our wetted surface area. This also has it drawback also as it keeps the Q constant instead of decreasing it with respect to decrease in the liquid level. However, its a conservative approach to follow.

We have develop a spreasheet where we have to adjust the height of the vessel in such a manner that it matches the wetted surface area of our system. Although, it does not cloesly represnt the ideal situation but still better than entering it manually in backhand.

I personally feel that nothing wrong in adding the wetted surface area in back hand as its conservative approach. I hope this answers to your query.

Regards,

AJ

[sunil_chemical]

Thanks Ajay for the ans,

But if I want to decrease the peak load than how can I decrease because whatever system is existing one and I have to reduce the load. And only way is don;t put wetted area manually and let HYSYS calculate the wetted area so my blowdown rate is decreasing. But question is this is the right approach to reduce blowdown rate!!

Do you have any calculation how HYSYS calculate the blowdown rate? I am talking about manually. If you have any calculation let me know.

otherwise as you said only way to adjust the height to match the wetted area. that is the only way you to reduce the load. I have tried and peak load is also reduced somewhat.

Thanks once again.

If you get any calculation how to calculate the blowdown rate than tell me . I have tried to find but still I am not getting.

Regards

Sunil

Sunil,

This is very known problem of HYSYS but somebody rightly mentioned we have to chngae the wetted surface area in backhand based on our wetted surface area. This also has it drawback also as it keeps the Q constant instead of decreasing it with respect to decrease in the liquid level. However, its a conservative approach to follow.

We have develop a spreasheet where we have to adjust the height of the vessel in such a manner that it matches the wetted surface area of our system. Although, it does not cloesly represnt the ideal situation but still better than entering it manually in backhand.

I personally feel that nothing wrong in adding the wetted surface area in back hand as its conservative approach. I hope this answers to your query.

Regards,

AJ

[aju_1807]

Sunil,

You cant reduce the peak flow rate just by manipulating some factors in software. The system volume and initial liquid volume plays critical role in calculaitng peak flow rate for fire case so check that with respect to isometrics and final datasheets to ensure that it represnts correct system.

I dont have any manual calculation for depressurization. If your problem is not the sizing of tail pipe and flare capacity (due to simultaneous depressurization load) then you can work out an option of sequential depressurization.

If you tell me more details of your problem then I can share more information on this.

Cheers!!

AJ

[sunil_chemical]

Ajay,

Yaa that's right it is possiblt to do sequential depressurization and still we are not checked the flare model. Why we faced the problem because in existing calculation wetted area is not put by manual and HYSYS automatically calculated so blowdown rate is less than whatever present we calculated. Even I agreed about your ans.

Any other way to reduce blowdown rate. Like considering NLL instead of HHLL or HLL for volume calculation or change in Pressure and temp.

If you have any alternative than let me know.

Can u tell me where r u working presently?

Or if u don;t mine i add you in my list.

So that we can share some information.

Regards

Sunil

Sunil,

You cant reduce the peak flow rate just by manipulating some factors in software. The system volume and initial liquid volume plays critical role in calculaitng peak flow rate for fire case so check that with respect to isometrics and final datasheets to ensure that it represnts correct system.

I dont have any manual calculation for depressurization. If your problem is not the sizing of tail pipe and flare capacity (due to simultaneous depressurization load) then you can work out an option of sequential depressurization.

If you tell me more details of your problem then I can share more information on this.

Cheers!!

AJ

[ASOKA]

Hello all
can anyone clarify my doubt,
i have a Blowdown valve on a two phase line. the flow regime in the pipe is slug. now when i do depressurization in HYSYS , it calculates only the vapor flow. since the flow regime is slug, also high velocity during depressurization , certainly there will be liquid carry over to BDV/RO, i feel. can you help me how to calculate two phase line depressurization. PLEASE

Thanks
PR Ashokkumar[/font]

[pallavikhatri]

Hello all
can anyone help me in calculating the depressurization flowrate.
i have one column whose whose depressurization rate is to be calculated. Can anyone tell me how do we calculate the flowrate in case of emergency depressurization (not in fire case) like the spurious opening of the depressurization valve.

thank
pallavi

[fallah]

pallavikhatri,

Spurious depressurization flowrate would be calculated same as fire case with following differences:

1) No depressurization time specified
2) Final pressure should be 1 bara
3) Initial temperature should be normal operating temperature

Fallah

[pallavikhatri]

Thanks Fallah for the valuable input. Can u tell me how cane we calculate the flowrate in HYSYS?
But as per API 521 for emergency depressurization case dont we have to consider the final pressure as 7.1 kg/cm2g?
I understood emergency depressurization and spurious opening of the depressurization valve are one and the same thing.

Thanks in advance
Pallavi

Edited by pallavikhatri, 19 February 2012 - 09:35 AM.

[pallavikhatri]

Dear all
I have a distilation column separating ethylene and other heavier hydrocarbons (butenes, hexanes, octenes etc.) having an elevation of 10m. So as per API 521 fire case is not applicable in this column.
Can anyone suggest me that for this column whether depressurization assembly is required or not? Do we have to provide the depressurization line for maintenance and shutdown purposes?

Thanks in advance

Pallavi

[fallah]

Thanks Fallah for the valuable input. Can u tell me how cane we calculate the flowrate in HYSYS?
But as per API 521 for emergency depressurization case dont we have to consider the final pressure as 7.1 kg/cm2g?
I understood emergency depressurization and spurious opening of the depressurization valve are one and the same thing.

Thanks in advance
Pallavi

Pallavi,

In HYSYS you should select "Utilities" option from "Tools" and if you have relevant manual in your hand you can calculate the depressurizing flowrate.

As per API 521 (Fifth Ed.) for pool fire exposure, the final pressure should be 50% of vessel design pressure.

The differences between "spurious" and "pool fire exposure" depressurizings are mentioned in my previous post.

Fallah

[fallah]

Dear all
I have a distilation column separating ethylene and other heavier hydrocarbons (butenes, hexanes, octenes etc.) having an elevation of 10m. So as per API 521 fire case is not applicable in this column.
Can anyone suggest me that for this column whether depressurization assembly is required or not? Do we have to provide the depressurization line for maintenance and shutdown purposes?

Thanks in advance

Pallavi

Pallavi,

If the bottom of the column is elevated 10 m above the source of flame, heat input from the fire will only be considered if a retention structure appears.

Fallah

[pallavikhatri]

Thanks Fallah
Since there is no retention structure under the column so this implies that there is not need to consider a depressurization line for fire case.

Pallavi

[aju_1807]

<ul>
<li>Need to check that associated piping which always runs at lower elevation does not impose any risk to the system, as ultimately that Q is given to system. Moreover, more often we provide the depressurization system for high pressure application (as per API-521 higher than 17 bar) which allows you to bring down the pressure qucikly during anay emergency event (e.g. fire, vessel leak etc).</li>
</ul>

[vikramltv]

Dear sunil_chemical

Vessel area calculated by hysys is always less than the actual area. The reason behind this is, Hysys ignores the end/head of vessel and consider every vessel as cylindrical flat head. This works well for any header or pipeline but when depressurizing any spherial head vessel one must input the actual wetted area from vessel calculation sheet/ data sheet..