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chris42  
#1 Posted : 05 August 2022 16:23:50(UTC)
Rank: Super forum user
chris42

Hi all you DSEAR experts that sometime visit this site.

My job occasionally touches on DSEAR related issues, and I have been trying to get a slightly better understanding of the subject. Now the concept that an explosive atmosphere may exist for varying periods of time will give rise to zone 0,1 or 2 for liquids and gasses, is fine. Then say a zone 2 (present occasionally), extent can be calculated using the tables/ formula/ info in BS60079-10-1 and Energy institute document EI15, if you have enough money to purchase them (not me). However, I accept it can be calculated.

The thing that I don’t get is the release determination. Ie say you had a fixed tank or even a tanker vehicle inside a building just sitting there, you could look up information in the above mentioned tables for say a release from a 1mm diameter hole or 2mm etc or even a pool 2m diameter on the floor. The bit I don’t get is how you know what potential size hole or pool to consider. You obviously don’t plan on a pin hole in your tank, I could accept spilling a small container of substance, but struggle with the leak scenario. Not only the size of the hole to be considered but the frequency it may happen, I just don’t get what sort of logic you would use to get to this point. Are there more tables inside these documents that give some rational / likelihood scenarios/ expected failure rates.  

Is anyone willing to shed some light on this particular aspect.

Chris

stevedm  
#2 Posted : 05 August 2022 16:54:27(UTC)
Rank: Super forum user
stevedm

the calculations are contained within IEC 60079-10 there are also other documents known as the yellow and purple books for the determination of physical effects...all of that is support with various software tools again all very expensive to buy which provide dispterion modelling...all of which is the domain of us Process Safety Engineers/Chemical Engineers...so not for the faint hearted...but at the end of the day it is just maths and physics...thise of us who have been doing it for a wee while have our own spreadsheets etc that support the cals in 60079...

Ian Bell2  
#3 Posted : 05 August 2022 17:18:09(UTC)
Rank: Super forum user
Ian Bell2

A key point to remeber is that hazardous area classification doesn't apply to gross/catastrohpic failures. Only credible expected leaks/emissions.

As Steveedm says you can use the Dutch yellow and purple books for estimatig leaks. Also BS60079-10-a has a table of typical leak hole sizes. There is also HSE guidance on the likely leak frequency of pipes, flanges and vessels etc. Just search on the HSE website for 'failure rate data', which should give you a document Failure Rate and Event Data for use within Risk Assessments (06/11/17)

Its all a best guess really.

peter gotch  
#4 Posted : 05 August 2022 20:16:50(UTC)
Rank: Super forum user
peter gotch

Hi Chris

Is your question academic or related to the place or places you actually work?

Academic - then all the advice already given.

Real life - most of the time, you can probably rely on the guidance that is available from HSE and others to provide a "conservative" or "precautionary policy" viewpoint as to what would or would not be reasonably practicable.

That is going to come down to considering what materials in what quantities and what potential interactions.

You probably only need to look in detail when you are on the margins and arguing about whether e.g. exclusion zones should be 5m, 10m or whatever

So, as example, there is a huge difference between having one tank holding say LPG or some flammable liquied and being on a petrochem site where there are multiple tanks of X, Y, Z and much more besides.

If in the complex scenario SOMEONE (other than YOU!!) should be making the expensive (but not expensive to an organisation) reading material available and you should have process safety people at hand.

Edited by user 05 August 2022 20:17:54(UTC)  | Reason: Omittted two words

chris42  
#5 Posted : 07 August 2022 14:09:40(UTC)
Rank: Super forum user
chris42

Thanks all for the feedback.

Peter the answer to your question is yes, both academic and I have previously stated my job touches / comes into contact with DSEAR related issues. (have tried very hard to avoid). However, I felt it time to get more of an idea to maybe do simple assessments / know enough to be able to tell if decisions made by other are indeed seemingly correct. The academic side is getting my head around the process as much as doing an assessment.

With that thought in mind regarding the process, from my current investigations I could not understand how you would tell what may possibly happen and so work out what size hole or puddle you would get. What logic was employed to get to this point, before any maths calculations. A zone 2 by its name is a zone is a 3-dimensional volume of explosive atmosphere. Now its not going to be a nice cube, but you would probably represent it as such on a zone plan. Now:

Quote from Ian “Its all a best guess really.” This is where I got to in my considerations of the subject, but no surely not! I thought. I mean you guess a potential hole size / puddle size, then apply complicated maths to probably many decimal places to work out a volume around such puddle / hole. Madness, I thought, or is it? Obviously, a general risk assessment does exactly that, it considers the worst, most likely thing to happen (not just the worst or everything would be death – papercut ->infection ->gangrene -> death eventually).

Quote Ian again “only credible expected leaks/emissions” So why would you expect a 2mm hole in a tank opposed to a 1mm hole. I could understand say the danger of spearing a tank with a fork truck, but then you would have a barrier to prevent, so back to not possible.

So it is just down to experience and any newcomer would have to mentor an experienced person for 10 year or more to also be able to do assessments. Seems a little old fashioned approach in this day and age and felt there had to be something else out there.

I started working life off as a mechanical engineer and I admit that when I left school I thought it was all about making things, which it is but only after a lot of maths. So back in the day lessons on thermodynamics and fluid dynamics, and pretty much all the other lessons come to think about it was a lot of maths and applying formulae, so a bit rusty but sure I could get my head around whatever is in the BS or EI15. Do I want to compete with Ian or Steve and do it full time for a living -God no, but I would like to be able to work through some basic assessments? Does my employer want me to -probably not, does he need me to be able to do it? Uncertain.

Yellow book and Purple book not come across these references Steve so thank you. By yellow book I assume you mean “TNO Yellow Book CPR 14E from the Dutch Ministry of Social Affairs and the Environment” and the Purple book about process safety produced by the Scotch Whisky Association. Can’t help but feel if I had a tank /tanker of whiskey at work the least of the problems would be DSEAR (I think I may insist on taking it home with me for safe keeping and quality sampling purposes).

So the answer to my actual question is a good guess followed by detailed maths on the good guess. You just got to love H&S

Chris

Kate  
#6 Posted : 07 August 2022 19:01:05(UTC)
Rank: Super forum user
Kate

I would go for a sphere every time, not a cube.

thanks 1 user thanked Kate for this useful post.
chris42 on 08/08/2022(UTC)
antbruce001  
#7 Posted : 08 August 2022 06:58:56(UTC)
Rank: Forum user
antbruce001

Originally Posted by: chris42 Go to Quoted Post

The bit I don’t get is how you know what potential size hole or pool to consider. You obviously don’t plan on a pin hole in your tank, I could accept spilling a small container of substance, but struggle with the leak scenario.

Chris, going back to your specific question relating to the leak size. 

This has always been one of the hardest aspects of applying any calculation method. For info the the leak sizes used are normally far smaller than 1mm diameter. They have a massive effect on the result, and until 2015 there was no guidance provided in BS 60079 as to what size to use. One of the best improvements to the standard in 2015 was the inclusion of a table that allows for a justification for the hole size used (Table B.1). I have known of cases where an assessor has spent a lot of time trying to justify a 'smaller hole size' to reduce the Zone size, as applying the calculated Zone causes serious problems in terms of potential igntion sources. Clearly, this goes against acceptable professional behaviour - but when we had no guidance to support the selection of hole size, it was common. It was also common to just use a 'standard' smallest hole size in all cases. 

It is commonly thought by the unimformed that calculations give a definite answers, but they don't. No only is the validity of the calculation method questionable (in all cases), but some of the data used for Zone 2 calculations has to be estimated (most signficantly the potential hole size), therefore removing all the 'precision' in undertaking the calculation.

Hope this answer the orignal question.

Tony.

thanks 1 user thanked antbruce001 for this useful post.
chris42 on 08/08/2022(UTC)
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