Jump to content



Featured Articles

Check out the latest featured articles.

File Library

Check out the latest downloads available in the File Library.

New Article

Product Viscosity vs. Shear

Featured File

Vertical Tank Selection

New Blog Entry

Gas Turbine Re-Ratings- posted in Ankur's blog

2

Fire Depressurization Criteria For Jet Fire


5 replies to this topic
Share this topic:
| More

#1 SawsanAli311

SawsanAli311

    Veteran Member

  • Members
  • 34 posts

Posted 20 November 2018 - 01:15 PM

Dear All, 

 

Having been discussing the emergency fire depressuring criteria explained in API 521, we came across that for jet fire causing potetnial vessel failure and leakage, the recommendation in API 521 section 4.6.6 and section A. 3.5.4.2 is to depressurize to 7 barg while taking into account the impact of system's design pressure (and hence thickness) when dictating the required depressurization duration (Yet min 15 minutes). Jet fire heat input would definitely be calculated by API 521 analytical method equation A.1. 

 

The above would be comparatively conservative than the pool fire emergency depressuring criteria mentioned for 1 inch thick plates of reducing the pressure to 50% of the design pressure in 15 minutes (the time to have the plate temperature reaching the limit of 593 C). 

 

I was discussing with some consultant regarding the jet fire requirement of depressurization, they recommended not to enforce the criteria of 7 barg in minimum 15 minutes as this would lead to large flare system capacity whilst jet fire may be remotely applicable in some cases depending on the safety emergency response plan of the plant. 

 

However, in my opinion, design shall cover jet fire scenarios and accordingly the appropriate criteria shall apply. 

 

I need your views on the criteria for jet fire fire depressuring and whether specific deviations would be accepted?



#2 Sharma Varun

Sharma Varun

    Gold Member

  • Members
  • 81 posts

Posted 21 November 2018 - 05:05 AM

its a vast area to discuss about designing of system for jet fire or not. Its always optimization between the capex & probability of jet fires. Normally if the system has high pressures licencors themselves specify depressurization, its more on gas filled vessels exposed to fire (unwetted).

 

Regarding your queries on time & flare capacity, please refer Figure A.4 (Fire Depressurization Work Flow Diagram) & Table A.6 (Typical Starting Points for Step 1 in Figure A.4 when Designing Depressurization System for Unwetted Walls Exposed to Jet Fires) for some more clarity, API 521 6th edition.



#3 SawsanAli311

SawsanAli311

    Veteran Member

  • Members
  • 34 posts

Posted 26 November 2018 - 10:49 AM

Thank you Mr.Sharma, 

 

Actually I was discussing with the same consultant the requirements of table A.6 which actually show that designing for jet fire may mandate higher depressuring rate (flare system capacity) due to shorter depressurization time than 15 minutes depending on the vessel's design pressure. However, yet the consultant stated that these are starting points for the design of jet fire. 

I do believe that providing PFP is highly expensive and providing insulation may not be practically because it can be eroded by the jet fire. 

Additionally, the empirical equation of API 521 concerning pool fire would not be applicable for jet fire and the analytical method shall be used instead to estimate the local and surface average heat fluxes and which are also a strong function of the jet geometry and its spread. 

 

From your experience and for gas filled systems (high pressure vessels unwetted), in order to cover the jet fire in the design, would using table A.6 be sufficient coupled with the analytical method to calculate the heat load?? or a safety fire analysis needs to be developed taking into account emergency response plans/evacuation. I also do believe that we should not be giving credit for active fire fighting systems even if they are installed. 

 

Appreciate your advice. 


Edited by SawsanAli311, 26 November 2018 - 10:55 AM.


#4 Sharma Varun

Sharma Varun

    Gold Member

  • Members
  • 81 posts

Posted 03 December 2018 - 01:32 AM

I guess by PFP you mean Passive Fire Protection.

 

Actually this topic needs to be discussed with specific details & general discussion has limitations. 

 

In my opinion you must get RRA done & check RRA report w.r.t. jet fire. 

 

For Unwetted vessels we normally specify sprinklers if the relieving temperatures are expected to be higher, irrespective of any study.

Credit to fire fighting depends on client policies.

 

Just for additional information as per one of the standards (probably by BP) jet fire failure can occur in time as small as five minutes. 



#5 SawsanAli311

SawsanAli311

    Veteran Member

  • Members
  • 34 posts

Posted 04 December 2018 - 01:25 PM

Thank you Mr.Sharma, 

 

Yes, I meant passive fire protection which API 521 also states that they can be eroded by the localized impingement of jet fire. Additionally, section 4.4.13.4 also states that the fire fighting system can actually be impacted by the jet fire. 

 

I believe you meant by RRA asrisk reduction assessment which can resemble an emergency response plan or a detailed fire analysis, kindly please confirm.

 

Additionally, in case you use the analytical method in Annex A for the jet fire heat flux calculation, can you clarify what area you have considered for converting from flux to heat load (KW) for the system being impacted by jet fire since it is not entirely engulfed in fire. Also see section A.3.5.4.2.2 describing the flame size and the dynamics of jet fire. 

 

Thanks



#6 Sharma Varun

Sharma Varun

    Gold Member

  • Members
  • 81 posts

Posted 05 December 2018 - 12:22 AM

Thank you Mr.Sharma, 

 

Yes, I meant passive fire protection which API 521 also states that they can be eroded by the localized impingement of jet fire. Additionally, section 4.4.13.4 also states that the fire fighting system can actually be impacted by the jet fire. 

 

I believe you meant by RRA asrisk reduction assessment which can resemble an emergency response plan or a detailed fire analysis, kindly please confirm.

 

RRA means Rapid Risk Analysis- This study identifies the hazards associated with the facility, analyses the consequences, draws suitable conclusions and provides necessary recommendations to mitigate the hazard/ risk.

 

In past we have provided PFP based on RRA recommendations. 

 

Additionally, in case you use the analytical method in Annex A for the jet fire heat flux calculation, can you clarify what area you have considered for converting from flux to heat load (KW) for the system being impacted by jet fire since it is not entirely engulfed in fire. Also see section A.3.5.4.2.2 describing the flame size and the dynamics of jet fire. 

 

Unfortunately I personally have never carried out above calculation. The RRA itself provides us intensity (heat flux) radii for jet fire. 

 

Thanks






Similar Topics