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Fire Emergency Depressurization Criteria For Thin Wall Vessels


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#1 SawsanAli311

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Posted 12 June 2020 - 10:02 AM

Dear All, 
 
I would like to see your views about the depressurization criteria under pool fire scenario for vessels with thickness less than 1 in. We all know that API 521 section 4.6 and Annexure A.3 explains that the require to reduce the pressure to 50% of design pressure is based on the API fire test and analytical validation to reduce the tensile stress of carbon steel grade 516 think un-wetted plate with 1 in thickness such that it doesn't experience a stress higher than it UTS of 138 MPa when the temperature exceeds 649 C. 
Now if we take 50% of that stress value we end up with 69 MPA which is 6.9 barg. Is this from where the criteria of:
- ''depressurizing to 50% of design pressure of 6.9 barg whichever is lower in  15 minutes  '' comes from. 
 
A- Do you also agree that for thin vessels and in absence of any stress based depressurization analysis, it is safer to dictate a final low pressure of 6.9 barg since this will conservatively lead to a sufficiently size restriction orifice in light of the fact that if we consider 50% of design pressure.. it would not be enough to mitigate the increase in the stress due to the low conduction heat transfer across the thin vessel. 
 
B- OR : you agree that for thin vessels.. we would still depressurize to 50% of design pressure but in a shorter time with a typical rule of thumb of 3 min reduction in the time for every 5 mm reduction in thickness. 
 
which criteria  do you think would be more influential in ensuring a safe design. 
 
thanks
 


#2 fallah

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Posted 12 June 2020 - 11:07 PM

 

I would like to see your views about the depressurization criteria under pool fire scenario for vessels with thickness less than 1 in. We all know that API 521 section 4.6 and Annexure A.3 explains that the require to reduce the pressure to 50% of design pressure is based on the API fire test and analytical validation to reduce the tensile stress of carbon steel grade 516 think un-wetted plate with 1 in thickness such that it doesn't experience a stress higher than it UTS of 138 MPa when the temperature exceeds 649 C. 
Now if we take 50% of that stress value we end up with 69 MPA which is 6.9 barg. Is this from where the criteria of:
- ''depressurizing to 50% of design pressure of 6.9 barg whichever is lower in  15 minutes  '' comes from. 
 
A- Do you also agree that for thin vessels and in absence of any stress based depressurization analysis, it is safer to dictate a final low pressure of 6.9 barg since this will conservatively lead to a sufficiently size restriction orifice in light of the fact that if we consider 50% of design pressure.. it would not be enough to mitigate the increase in the stress due to the low conduction heat transfer across the thin vessel. 
 
B- OR : you agree that for thin vessels.. we would still depressurize to 50% of design pressure but in a shorter time with a typical rule of thumb of 3 min reduction in the time for every 5 mm reduction in thickness. 
 
which criteria  do you think would be more influential in ensuring a safe design. 
 

 

 

Hi,

 

Even though appears the "A" option could be logical, but i think applying the "B" option would ensure a safer design especially due to be in the same track with API 521.



#3 SawsanAli311

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Posted 13 June 2020 - 02:43 AM

 

 

I would like to see your views about the depressurization criteria under pool fire scenario for vessels with thickness less than 1 in. We all know that API 521 section 4.6 and Annexure A.3 explains that the require to reduce the pressure to 50% of design pressure is based on the API fire test and analytical validation to reduce the tensile stress of carbon steel grade 516 think un-wetted plate with 1 in thickness such that it doesn't experience a stress higher than it UTS of 138 MPa when the temperature exceeds 649 C. 
Now if we take 50% of that stress value we end up with 69 MPA which is 6.9 barg. Is this from where the criteria of:
- ''depressurizing to 50% of design pressure of 6.9 barg whichever is lower in  15 minutes  '' comes from. 
 
A- Do you also agree that for thin vessels and in absence of any stress based depressurization analysis, it is safer to dictate a final low pressure of 6.9 barg since this will conservatively lead to a sufficiently size restriction orifice in light of the fact that if we consider 50% of design pressure.. it would not be enough to mitigate the increase in the stress due to the low conduction heat transfer across the thin vessel. 
 
B- OR : you agree that for thin vessels.. we would still depressurize to 50% of design pressure but in a shorter time with a typical rule of thumb of 3 min reduction in the time for every 5 mm reduction in thickness. 
 
which criteria  do you think would be more influential in ensuring a safe design. 
 

 

 

Hi,

 

Even though appears the "A" option could be logical, but i think applying the "B" option would ensure a safer design especially due to be in the same track with API 521.

 

Hi Fallah, 

Thanks a lot for your apply.. I was thinking as well that for thin walled vessels, it means its design pressure/design stress would not be in the high side and hence if we visualize the pressure profile during depressurization.. starting from a pressure which is relatively lower than the case of high pressure thick walled vessels.. the time required to reach 50% of design pressure can be more than the time to reach 7 barg.. again this might emphasize on the acceptance of 50 % of the design pressure. As an example, for a separator vessel design pressure of 10 barg, the design with a final pressure of 5 barg instead of 6.9 barg in 15 minutes would still be conservative. what do you think? In summary, I could demonstrate also that with high pressure going to 7 barg in 15 minutes is surely conservative.. the same might not be conservative in the flat region of the pressure profile where initial pressure is relatively lower... 

 

Also for the various grades of material discussed in API 521.. I could see in figures 1 & 2 that for Carbon steel Sa-516 grade 60 material, is design stress at 649 C is 138 MPa as I explained above.. and since fire gas temperature can reach 904 C.. 50% reduction in stress corresponds to 50% reduction to 6.9 barg final pressure in around 15 minutes (exact time is 12 minutes as per figure 1). 

On the other hand, if we look at figure  which addresses grade 70 material Sa-515, API 521 is saying that at the same 138 MPa .. the CS plate will fail in 6 minutes! Hence, do you think that in order to also cater for the differences in the material grade, we will still need to assess both the time criteria based on the thickness as well as the suitability of 50% of the design pressure.



#4 fallah

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Posted 15 June 2020 - 01:03 AM

Hi Fallah, 

 

 

 

Thanks a lot for your apply.. I was thinking as well that for thin walled vessels, it means its design pressure/design stress would not be in the high side and hence if we visualize the pressure profile during depressurization.. starting from a pressure which is relatively lower than the case of high pressure thick walled vessels.. the time required to reach 50% of design pressure can be more than the time to reach 7 barg.. again this might emphasize on the acceptance of 50 % of the design pressure. As an example, for a separator vessel design pressure of 10 barg, the design with a final pressure of 5 barg instead of 6.9 barg in 15 minutes would still be conservative. what do you think? In summary, I could demonstrate also that with high pressure going to 7 barg in 15 minutes is surely conservative.. the same might not be conservative in the flat region of the pressure profile where initial pressure is relatively lower... 

 

Also for the various grades of material discussed in API 521.. I could see in figures 1 & 2 that for Carbon steel Sa-516 grade 60 material, is design stress at 649 C is 138 MPa as I explained above.. and since fire gas temperature can reach 904 C.. 50% reduction in stress corresponds to 50% reduction to 6.9 barg final pressure in around 15 minutes (exact time is 12 minutes as per figure 1). 

On the other hand, if we look at figure  which addresses grade 70 material Sa-515, API 521 is saying that at the same 138 MPa .. the CS plate will fail in 6 minutes! Hence, do you think that in order to also cater for the differences in the material grade, we will still need to assess both the time criteria based on the thickness as well as the suitability of 50% of the design pressure.

 

 

Hi,

 

As per API 521, emergency depressuring for the fire scenario should be considered for large equipment operating at a gauge pressure of  1700 kPa or higher.
Depressuring to a gauge pressure of 690 kPa (100 psi) is commonly considered when the depressuring system is
designed to reduce the consequences from a vessel leak.

Edited by fallah, 15 June 2020 - 01:04 AM.


#5 breizh

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Posted 15 June 2020 - 02:36 AM

Hi,

Consider the document attached to support your work .

Good Luck

Breizh 






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