Ammonia inventory determination — maximum intended inventory, determined correctly
Your ammonia inventory determines whether you're subject to OSHA PSM (29 CFR 1910.119) and EPA RMP (40 CFR Part 68). We perform a complete maximum intended inventory determination — accounting for every component in your system at actual operating conditions.
The threshold question has a right answer. Most facilities don't have it.
If your ammonia refrigeration system holds 10,000 lb or more of ammonia refrigerant, you are subject to OSHA PSM and EPA RMP — two of the most demanding regulatory programs in industrial safety. If you're below the threshold, you're not.
That number — 10,000 lb — is determined by a maximum intended inventory determination: a component-level calculation that accounts for every vessel, heat exchanger, and piping segment in your system at actual operating conditions. Not nameplate data. Not estimates. Actual thermodynamic calculations at real operating pressure and temperature.
Most facilities have never had this calculation done properly. Nameplate charges and rough estimates frequently undercount by hundreds of pounds — putting facilities at risk of operating a covered process without required permits, programs, and protections in place.
If you're within 2,000 lb of the threshold
Inventory near the 10,000 lb mark is not static — it migrates seasonally as operating conditions change. A facility at 9,200 lb on paper may regularly exceed 10,000 lb during peak summer operations. We identify and quantify this risk.
Regulatory thresholds
OSHA penalty exposure
Operating a covered process without a required PSM program exposes your facility to significant willful violation penalties under OSHA and EPA — costs that far exceed the cost of compliance.
Why most inventory numbers are wrong
Nameplate charges
Equipment nameplates list design charge, not actual operating charge. They ignore operating conditions and piping.
Estimating by system size
Rules of thumb like "2 lb per ton of refrigeration" can be off by hundreds of pounds for real systems.
Ignoring piping
In large systems, liquid and suction piping can hold 500–1,500 lb of ammonia that never appears in vessel-only calculations.
Wrong operating conditions
A calculation performed in spring underestimates summer inventory, when condensing pressure is high and the HP receiver is fuller.
A proper maximum intended inventory determination addresses all of these. It accounts for vessel geometry, operating pressure, saturation temperature, liquid density, vapor density, and piping volume — at actual conditions, not design conditions.
Everything needed for OSHA and EPA submissions
Download the example inventory determination spreadsheet
See exactly how a proper ammonia inventory determination works. This spreadsheet walks through a complete component-level calculation for a real cold storage system — vessels, heat exchangers, and piping — showing exactly how maximum intended inventory is determined.
NH3Edge_Ammonia_Inventory_Example.xlsx
MII determination · Vessels, HX & piping · Example cold storage facility
Maximum intended inventory (MII)
EPA 40 CFR Part 68 requires covered facilities to report their maximum intended inventory — the largest quantity of a regulated substance you intend to have on-site at any one time. For ammonia refrigeration systems, this is not the same as your average operating inventory.
Maximum intended inventory must account for peak operating conditions — typically summer peak load at maximum condensing pressure — and any additional charge that could be added to the system for seasonal adjustments or makeup.
This number feeds directly into your worst-case and alternative release scenario calculations in your RMP submittal. Understating it reduces your apparent consequence distance — which can create compliance exposure if LEPC or EPA reviewers challenge your assumptions.
MII vs. operating inventory
Operating inventory
Actual charge at measured conditions
Maximum intended inventory
Largest quantity intended to be on-site
Worst-case scenario quantity
Used in RMP offsite consequence analysis
Our inventory determination reports include both your current operating inventory and a documented maximum intended inventory suitable for use in your RMP submittal.
Inventory determination questions
How is ammonia inventory determined for PSM and RMP compliance?
Ammonia inventory is determined with a component-level thermodynamic calculation, consistent with IIAR charge management guidance. Every vessel, evaporator, condenser, oil pot, and pipe run is calculated individually at actual operating conditions — pressure, temperature, and liquid level — and summed to a defensible total. Nameplate values and rules of thumb are not a determination.
Why does my ammonia inventory number matter so much?
The 10,000 lb threshold is the trigger for both OSHA PSM (29 CFR 1910.119) and EPA RMP (40 CFR Part 68). If your true inventory is at or above 10,000 lb, your facility is subject to the full 14-element PSM program and RMP submission requirements. If you claim to be below the threshold, that claim must survive an inspector's scrutiny — which means it needs a documented, component-level calculation behind it.
How close to the 10,000 lb threshold is too close?
If your facility is within roughly 2,000 lb of the threshold in either direction, you should have a formal determination. Ammonia migrates between vessels as conditions change — a system calculated at 8,800 lb on a mild day can hold meaningfully more charge on the high side during summer peak. A determination at documented worst-case operating conditions tells you whether you have real margin or only seasonal margin.
What is maximum intended inventory (MII)?
Maximum intended inventory is the largest quantity of ammonia you intend to have on-site at any one time, and EPA RMP requires it in your submission. It must reflect peak operating conditions plus any planned charge additions — not an average. Our reports document both current operating inventory and a defensible MII.
How long does an inventory determination take?
For a typical single-system facility, we gather system documentation (P&IDs, vessel data, pipe routing) up front, perform a site walkdown, and deliver the formal report within a few weeks of the visit. Well-documented systems can move faster; facilities with incomplete drawings may need additional field measurement time.
Ready to know your actual inventory?
We'll review your system, perform a complete maximum intended inventory determination, and deliver a formal report ready for OSHA and EPA submissions.