Im preparing to do a DSEAR assessment at one of our sites and was wondering if anyone could help me out? I've read the Regs, ACoP and BS and have no problem determining the zoning etc, what I do have a problem with is calculating the extent of the zones (vertical and horizontal) from the source. Can anyone provide any clarity on how to do this, and is it actually necessary to do so to complete a suitable assessment? One source I spoke to said that provided I justify the decision on what zoning to assign to the souce, the actual extent isnt necessary. Thanks in advance.

Bleve, I hear you and Im still open to the possibility of handing this out to a third party. As I say, I have no problem determining everything else required such as the chemical properties, LEL/UEL etc, degree of ventilation, grading of release etc etc. Its just solely the issue of calculating the extent of the zones which is an issue. There just seems to be so many variables to make it a straight forward (or reasonably straight forward) calculation.

I think thats what Im struggling with. Looking at the formulas in BS EN 60079-10 it looks relatively straight forward (assuming all the information necessary is available) but when it comes to estimating the release rate, how do you know the cross section size of, for example a failure in a flange? Surely it would depend on the extent of the failure (i.e. small to total failure)?

All the above is correct. However, provided you are not undertaking anything unusaul you can work within the 'standard' diagrams within the BS and HSE guidance. However, you must justify that it is appropriate to do this. E.g. if you have a flammable gas at low pressure in a pipeline a 1m radius around the flange will be accepted by the HSE (as per the BS for LPG in a pipeline). Don't forget combustable mists. If you have pressurised combustable materials these can give rise to zoned areas (e.g. diesel). Also, note that the 2009 BS standard has a new section on mists that should be referred to. Hope it helps Tony.

You could even have the bonus that not only does the pro know what he/she knows what they are doing but also has Phast and knows how to use it and save even more money. Imagine that. ;)

I agree but finding a decent consultant isnt that easy. The cost of Phast means that this isnt an option given that the use I'd have is limited to the immediate operations.

Thanks Iter, The first one is relatively straightforward. We're planning on installing 2 x 12,000L external vertical storage tanks (integrally bunded) which will contain solvent (LEL 3.3%, UEL 19%, Flashpoint 13oC, Density 0.791, Gas relative density 0.791, Ignition Temp 365oC, Group&Temp 2A T2) with natural ventilation. The others are little more difficult as they are printing presses and I dont have all the info at hand right now for those. If you have anything that you think might be useful to me please feel free to send it over. The spreadsheets sound particularly useful if you didnt mind sharing but obviously I respect the fact its your work and intellectual property etc and fully understand if you wanted to hang on to them.

Sorry head space of tank should state Zone 0 Vent Zone 1.

Jason I assume you/your company is the client? If the tanks are yet to be installed on your behalf and hence designed by someone else - then the designers should undertake the various hazardous area calculations, so the vents can be sized properly, equipment specified etc. They should pass the information on to you and/or you should get the information from the layout engineering drawings of the tank and system etc. The figures quoted by others/Zone classifications will be ok. From the information you now appear to have given, I'm not so sure you need to get involved with the calcs. Just take the design calculations and make site personnel aware etc for future reference e.g. when undertaking maintenance, allowing heat sources into the area, electrical equipment etc. bob - HAZOPS/HAZANs studies will simply tell you to that you need to calculate/estimate the extent of the hazardous areas to comply with DSEAR. And yes I am a safety case engineer 'bod'

Tank fabricator will only size vents and pressure relief PSV based on breathing losses for atmsopheric storage tanks and fire case for pressure vessels. This will not have an input into the HAC. The only calculation necessary will be that of jet/spray release from tanker unloading hose, comparison of extent of unloading area bund. In addition, consider any flanges in transfer lines from bulk storage tanks into process building and of course emission points at the printing equipment. Also consider that your explosive limits will be far in excess of your IH limits.

60079 does not take into account liquid jet release, formation of pools, pool evaporation rate, mixtures of liquids or gases, effect on extent of zone due to obstructions etc. The BS also clearly states that this exercise should be carried out by an expert. As previously stated the extent of zoning associated with the storage tanks can be determined by exemplar (the extent I have already described). The extent of other zones particularly in relation to the machinery you describe will need to be determined through study and calculation.

Point taken. I guess I just wanted to be able to work it out for myself. I dont like not being able to do something and was hoping that with enough reading and research I'd be able to figure it out.

The IP 15 tables and standard examples contained in the guide should suffice for what you need. Hazardous area calcs are not an exact science - IP 15 is a pragmatic guide based on conservative engineering judgement. You could also read a book by Frank Lees -'Loss Prevention in the Process Industeries'.... but it is about 900pages long! ok maybe not. He has also written another book specifically about hazardous area classification - the exact title slips my mind right now - but if you Google Frank Lees, I'm sure you should be able to track it down quite easily. As for the other printing machines etc you mention - if the pipes containing flammable are small, then its probably unlikely that the haz zones will be greater than 1-2m. As regards spherical zones - correct, thats the advantage of using PHAST you can factor in weather conditions, wind direction and strength etc to get a pretty picture showing the calculated extent of the cloud. Given the basis for the ventilation in 60079-10 is a best guess fudge factor, if you use a factor that is more conservative i.e. worse ventilation, then in practice, for external tanks/pipes the haz zone will in practice be smaller. Remember 60079-10 assumes ventialtion wind speeds outside are rarely less than 0.5m/s. Disagree that the tank fabricator will only size atmospheric vents - I have worked for 2-3 design consultant companies in the oil/gas sector - we always did the full haz zone calcs using IP15 and/or 60079-10 and passed on the results to the client. Just insist that you get them supplied by the designers!

But the zone willbe dependant on the distance and size of any pool formed. If ventilation decreases in actual fact the hazardous area (extent of zone) increases.

Jason, go back to your source and aske him/her,if you do not know the extent of a zone how can you ensure that the electrical and or mechanical equipment is suitable for use within the extent of the zone?what about zone transition?

I actually understand what you are saying, from one safety case bod to another I hope you are not as cryptic in any safety case document though.