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Safety Valve For Mounded Bullets For Propylene

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#1 vishadenav


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Posted 05 October 2021 - 05:13 AM

I have a mounded storage which houses the following:
Propylene 2 x 1823 mt
Propane. 1 x 1200 mt
Each bullet has two fire case safety valves. The capacity is 30% of a similar above-ground bullet.
Do we need to add all the 3 safety valves and determine the flare load?
Mounded storage eliminates external fire case, hence, why all 3 should be added?
OISD 150 also allows for vent on mounded bullets at a height of 3m above the mound.
Adding the 3 safety valves is leading to a grossly over sized flare... For an unlikely event.
Has anyone seen such installation or designed... Would appreciate some help on this.

#2 Pilesar


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Posted 05 October 2021 - 09:52 AM

There is a minimum recommended flare size based on engineering evaluation of the system and combination of relief scenarios. This does not mean that flare always has to be sized for simultaneous maximum loads of individual relief valves. Guessing at a flare size requirement is not an engineering best practice. 

#3 Chemitofreak


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Posted 08 October 2021 - 05:39 AM

This project is in which country??


If it is in India, then OISD is applicable, if OISD is applicable then the fire relief valves can be relieved to atmosphere at a safe location (as per your statement, I have no clue what is specified in OISD). If the fire relief valve is reliving to atmp. then why are you asking for adding the load in flare system??


Note that, the non fire RV can relief to flare during fire case as it has a lower set.


Revert in case of any query. 

#4 vishadenav


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Posted 13 October 2021 - 02:38 AM

Yes.... It's in India. OISD art 6.3 allows for open vent as mentioned. We have a case where flare was specified by Engg company, adding the loads.

#5 latexman


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Posted 13 October 2021 - 06:46 AM

Did the Engineering Company over-specify, or was it in your company's scope?  Didn't Engineering Company determine the basis for the flare, and the contributing load from the mounded tanks?  A fire relief rate of a mounded tank being 30% of a similar above ground tank sounds too high, or was a different scenario driving the PSV size besides fire?


My company has mounded tanks containing flammables.  We've had them over 30 years.  The fire case was deemed not credible.  We do have a normal vent to an incinerator and an emergency vent with higher set pressure to atmosphere on each tank.  The basis for both vents is API 2000 venting rates.  No other credible scenarios.

#6 sonu19921103


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Posted 13 October 2021 - 07:14 AM

As per OSID -150 


- Each vessel shall be provided with at-least two safety relief valves whose capacity shall be minimum 30% of the capacity required for an equivalent size of above ground vessel of above ground vessel.

- All vents and drains are routed to closed Flare system where Flare is available. Otherwise vented vertically upwards to atmosphere ensuring effective dispersion of hydrocarbons

The best option is to have atmospheric vent at safe height its tough to design a flare header to accommodate relief load of mounded bullets.


Best Regards

Sonu Singh

Edited by sonu19921103, 13 October 2021 - 07:16 AM.

#7 Bobby Strain

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Posted 13 October 2021 - 09:26 AM

It's widely accepted that the article you quote is generally ignored. As well as the whole code. And companies in India follow API or ASME. You should check if the rules are enforceable.



#8 Chemitofreak


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Posted 15 October 2021 - 05:11 AM

Yes.... It's in India. OISD art 6.3 allows for open vent as mentioned. We have a case where flare was specified by Engg company, adding the loads.


1) You need to follow the law of the land


2) If the relief is routed to the flare, it is technically correct, but that leads to higher flare load.


3) Considering fire case for mounded bullets is also conservative, but if OISD says that, then you have to do it, or else you don't get the permit to operate.




a) You need to ask the Engg. Contractor the basis for their design


b ) If you relief to atmosphere, then dispersion calculations are a must.

Edited by Chemitofreak, 15 October 2021 - 05:11 AM.

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