Working out whether the Control of Major Accident Hazards (COMAH) Regulations 2015 apply to your site sounds like a bookkeeping job. Add up what you hold, compare it with Schedule 1, read off the answer. Simple, right? COMAH tier calculations are one of the more commonly misjudged assessments in process safety, and the cost of getting them wrong can be significant. Under calculate it and you risk enforcement action, a late notification and a Safety Report nobody has budgeted for. Over calculate it and you have signed up for duties and regulatory scrutiny your inventory never actually triggered, which is an expensive way to be cautious.
Here is where sites most often come unstuck.
The Inventory Is Usually Where It Goes Wrong
Most flawed COMAH calculations have already gone astray before anyone opens Schedule 1. The regulations focus on on dangerous substances present, or likely to be present, at an establishment. That is not the figure on last month’s stock or inventory report. It means the maximum credible inventory, which often takes in material that never shows up in a stock system at all.
The gaps we see most often are tanks assessed at normal working level rather than full capacity, substances the process generates rather than ones you buy in, intermediates and waste streams, and road tankers or IBCs sitting on site during a delivery. CLP classifications copied across from a safety data sheet that is several revisions out of date are another regular culprit. Classification deserves as much attention as quantity, because a substance filed under the wrong hazard category ends up in the wrong sum, and everything downstream of it is wrong too.
The 2% Rule Comes With Conditions
Schedule 1 lets you ignore dangerous substances held in quantities of 2% or less of the relevant qualifying quantity. The condition attached is that their location within the establishment must be such that they cannot act as an initiator of a major accident elsewhere on site.
That condition does a lot of work and it is routinely glossed over. The rule tests where something is kept, not simply how much of it there is. A pallet of solvent parked next to your main tank farm will not pass, however modest the volume looks on paper. There is a second wrinkle as well. The relevant qualifying quantity differs between the lower and upper tier thresholds, so a quantity you can fairly leave out of one calculation may have to be counted in the other. Run the exclusion separately each time.
The Aggregation Rule: Three Sums, Not One
Two things reliably catch people out. The first is that a substance can appear in more than one sum, at its full quantity each time. A flammable solvent that is also toxic to aquatic life goes into the physical calculation and the environmental one. The second is that named substances in Part 2 of Schedule 1 sit alongside the generic categories in Part 1. And you have to choose where to include them (hint – they are named for a reason).
Where Sites Get Caught Out, and How OTECSA Can Help
The environmental sum is the one that catches operators off guard most often. A site that is comfortably clear of the flammables threshold can still find that a spread of aquatically toxic materials, none of them individually worth a second look, takes the environmental calculation past 1. Inventories creep up over the years. A new product line appears. A CLP update changes a classification. Any of those can shift a site across a tier boundary quietly.
None of the arithmetic is hard. What takes the time is the judgement wrapped around it: what counts as present, where you draw the establishment boundary, which category a substance genuinely belongs in, whether your 2% exclusion would stand up to challenge. That judgement is also what the regulator will expect to see written down and justified.
Help from the Experts
We have carried out COMAH inventory and tier calculations recently for distilleries, whisky maturation warehouses, anaerobic digestion plants and chemical manufacturers. Depending on what suits you, we can review calculations your team has already done, or take the whole assessment on and hand back a clear record of how the answer was reached, whether that puts you in lower tier, upper tier or safely outside COMAH.
If you are not sure which side of a threshold you sit on, or you would rather have your existing calculation checked before the regulator does it for you, get in touch with the OTECSA team.