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

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.

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

Originally Posted by: chris42

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 again Stevedm, the link is very interesting and for the clarification on the book references. Again, thanks to everyone that has responded, all very helpful.

Peters question has been in my head all week about why I’m looking into this. Indeed, the thought of why more people aren’t looking into this has also crossed my mind. As far as I can see that if you have anything that can form an explosive atmosphere however small and however unlikely should have an assessment. So, this is everything from a propane powered fork truck and you have just one bottle on the fork truck, or a oxygen /acetylene or propane welding / burning set, up to a full petrochemical site. So, you may have a zone 0, 1 or 2, but could have a zone of negligible extent, either way to get to these you need to do some sort of assessment. Without which you can’t define a zone volume or if it is cubic, spherical or more likely not a geometric shape but some sort of fluffy cloud shape, altered by everything it touches / flows past.

For me I’m thinking more about the lower end of the scale and not necessarily to do the assessments myself, but have a much better understanding of the process, so I can have better discussions with others. I find the best way of learning something like this is to take myself through the whole process, with a working example. I’m not a consultant but employed and its difficult to explain properly how my job just touches on this subject without giving away the industry I work in on the open forum. I have my own reasons for this. I know some of the responders I have communicated with in the past and so know where I work (thank you for not divulging it), and for those helpful responders that don’t I would be happy to tell via PM if they want.

It is a very big subject in its own right, and as we have discussed above it is all based on a good guess. The likes of Steve and Ian etc their good guess is an informed and experienced educated guess opposed to my just guess (like to think it will be a reasonably educated guess). Guesses aside if I’m talking to an expert, I can ask their logic for their guess. As Antbruces001 notes the hole size will make a massive difference and smaller than I was thinking of. If I’m talking to a non-expert who have just made a decision, then I hope to be able to challenge it if required.

The referenced documents are not particularly light reading I think the BS is 240 pages as is the Purple book (thanks for the link). So will take me some time to go through ( I may / will have more questions). The formula does look interesting, but it is a case of getting the units correct and it is just maths after that.

From what I have read so far there is a good bit of info gathering first not just on the substance, but environmental conditions at the location ie Temperatures and temp gradient, ventilation and obstructions etc which makes sense.

This should keep me out of trouble for a while anyway and hopefully some CPD (points / hours or whatever it is now).

Chris

Yes, Chris, LOTS of CPD however it might be counted and no need for a course in sight (at least not for now)!

Yes as with all thing H&S or anything else for that matter the person needs to be competent. As I said for me, I want to understand the process so I can better discuss with people primarily. I was also interested if there were different levels of assessment. By that I mean would you go completely through the same process for a Oxygen/ Acetylene set as for a chemical plant, and it seems you would. However, where you would readily bring in the experts for the Chemical plant, would people bring in the same experts for example the weld / heating set? We get lots of questions about how far away something should “A” be away from something “B” which really would be dictated by the DSEAR assessment, if they had one. Do people turn a blind eye at the lower end of the scale?

The other interesting thing I have noted that the way the documents seem to be written although applies to everything, things like failure rates etc apply to fixed tanks and pipework, joints and valves etc. But what about the potentially non fixed things, say an LPG tanker. I’m sure if they were refilling something on your site you need the assessment, but what about if say just coming for an MOT or service inspection. Nothing is going to be touched just inspected, but you have brought them into a workplace, so require assessment. All just examples I could think of, and I guess there will be many more examples.

It is all quite interesting, as I said not looking to branch out or change job.

Chris

Chris

HSE Fred https://www.hse.gov.uk/landuseplanning/failure-rates.pdf

Will give you some basic faiure rates to go along with there are more detailed lists in OREDA and it is even better if you can get actual plant history failure data...but mobile equipment and transport is covered by both the flammable areas extent in 60079/Ei15 calcs and from on the road risks and other parts which fall under QRA again not for the faint hearted - reference would be the purple book for that one...

As regards QRA estimates, when I first got involved with COMAH and often QRA I was remineded on a variation of the old phrase about statistics.

There are lies, damned lies and then there are QRA statistics.