Rank: New forum user 
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I am trying to get my head round how these calculations work, using the "Understanding COMAH: A guide for new entrants" document.
As I understand it, you do the calculation for three broad categories, toxic, flammable and environment. For each toxic material, you determine the fraction of the allowed amount for the tier.
Presumably then some substances count in more than one category. Potassium permanganate should be counted in the flammable category as it is oxidising, with a top tier threshold of 50 tonnes, and also in the environmental category, with a threshold of 200 tonnes.
Some chemicals are named (schedule 1 part 2), for example methanol. This has a higher tier limit of 500 tonnes - I guess that is for both toxic and flammable?
Methanol is both flammable and toxic.
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Rank: Forum user 
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F2Andy,
If I understand what you are doing then this is what you do. Refer to L111 for finer details.
If a substance is named then you should use this rather than generic names such as toxic, etc. So for example if you have, as you say, potassium permanganate, which is classed as oxidising then there are limits of 50 tonnes lower tier and 200 tonnes upper tier. If you exceed 50 tonnes you are lower tier and if you hold more than 200 tones you are upper tier. The only thing you have to concern yourself with then is whether there are other COMAH materials on site and dependent on the amount you have the hazard might be different e.g. Highly flammable.
The problem you may be facing is that associated with 'aggregation' (Para 403, L111). This is when you have amounts less than the COMAH limits but added together could bring you into COMAH e.g. You have 30 tonnes flammable liquid, 25 tonnes oxidising. Lower tier limits are 50 tonnes so you calculate as follows 30/50 = 0.6; 25/50 = 0.5 - add together = 1.1 as you are greater than 1 then you are a lower tier COMAH site. Likewise to do an upper tier calculation you do the same calculations but based on upper tier limits.
this is a bit of ramble, PM for more information or just contact you local HSE person, they can help you do the calculation as well.
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Rank: Super forum user
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drop me a pm with your email and I will send you a copy of my s/sheet... :)
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