Lately we’ve been seeing quite a few misunderstandings of the inventory calculations required by a PSM program so I thought I would try and explain it as simply as I can. Both the EPA’s RMP and OSHA’s PSM require you to declare a “Maximum Intended Inventory.” What does that actually mean? Well, it’s often confused with the inventory calculation – or what’s more commonly called the charge calculation.
First, let’s first look at the guidance from OSHA and the EPA:
OSHA: Process technology information will be a part of the process safety information package and it is expected that it will include… employer established criteria for maximum inventory levels for process chemicals; limits beyond which would be considered upset condition…
EPA: You must document the maximum intended inventory of any vessel in which you store or process a regulated substance above its threshold quantity. You may want to check with any trade association or standards group that develops standards for your industry to determine if there are any limitations on inventories. For example, in some cases the maximum capacity of a tank may be 10,000 gallons, but an industry standard may recommend that the tank never be filled to more than 85 percent capacity. If you follow the standard, your maximum inventory would be 8,500 gallons.
We’re going to want to declare three things by the time we are done with this inventory issue:
1) The ACTUAL charge of the system – The ACTUAL amount of Ammonia in the system. You can calculate this directly with NH3 properties charts and the PSI information on your piping and vessel volume. This is the number that decides if you are required to have a PSM/RMP program.
2) The MAXIMUM inventory of each process vessel. Ammonia moves around the system and therefore vessels will often have more or less than the amount listed in the actual inventory calculation. We have to set a limit that we will allow in each vessel. MANY people decide to set this as 80% based on old RETA books. I would think that’s a maximum fill level to avoid hydraulic trapping but you shouldn’t rely on a rule of thumb – you will need to have an engineering reason for this number. This number is decided upon based on your PSI information and the Process Hazard Analysis you do.
3) The MAXIMUM intended inventory of the system – the MAXIMUM amount of Ammonia you would ever want to put in the system under any circumstances. You will want to include some sort of procedure or policy that ensures this level is not exceeded – make sure this procedure or policy is audited as well. You could figure out this number by adding up all the figures you calculated in step #2.
It’s a good idea to document all the above in a letter to file explaining the quality control SOPs and administrative controls you have in place in regards to inventory level. Make sure you also describe the auditing component to these inventory controls so you can ensure your administrative solution is working. It wouldn’t hurt to write this letter to file in plain language so it’s easily understood. I love these “Letters to File” is that when an inspector asks me a question about something, such as my inventory in this case, I can just hand her a document.
The difficult part – the part you will want to do with your PHA team is the second item: the maximum levels for each vessel. Some choose to set this level at their limit of their normal operating range. The problem with setting maximum intended inventory levels at your normal operating levels is that when you exceed them (and you will from time to time) you’ve invalidated the document and any reasoning you made from it.
So let’s take a simple recirculator package that normally runs at 30% full. You might set your normal operating levels between 20-40% and your maximum intended inventory at 80%. Of course, if you have issues with inadequate liquid separation above 70% or so you might set that maximum intended level lower. The number you choose here can’t be arbitrary – you need to have some sort of engineering reason to explain this choice. Furthermore you must DOCUMENT that reasoning to answer the most often failed OSHA question:
Why have the implementation decisions and priorities been made as recorded in the PSM documentation?
If you really want to understand this question and why it’s even asked, check out the CSB video on the texas city disaster which resulted from not understanding what the maximum intended level actually was.