Ammonia Refrigeration Charge Inventory and PSM Threshold

The anhydrous ammonia inventory of a refrigeration plant, summed vessel by vessel at each vessel's own liquid density, and the margin to the 10,000 lb threshold that triggers OSHA process safety management and an EPA risk management plan. Charge lives almost entirely in the liquid: a high-pressure receiver, a recirculator package, and the liquid and wet-return piping hold nearly all of it while the vapor side holds almost nothing. Liquid density is strongly temperature dependent -- around 37 lb per cubic foot in a warm receiver against about 42 at minus 20 degF -- so a per-vessel sum at each vessel's own operating temperature is the only honest way to do it, and a single bundled density understates a cold recirculator. The threshold is a cliff rather than a slope. Below it a plant has ordinary obligations; above it it has a written PSM program, process hazard analyses, mechanical integrity, management of change, and an offsite consequence analysis. That makes the useful question during a retrofit not what the charge is but how much headroom is left, which is why the margin and the additional volume that would cross it are reported next to the total. The piping term is the one that surprises people: a plant that counts only its vessels can believe itself exempt while the pipe runs carry more than the vessels do, so which of the two is larger is computed here rather than asserted. Volumes, fill fractions, and densities are ENTERED -- it does not read a P and ID, estimate pipe volume from a line list, or determine coverage. The plant's own charge calculation of record, 29 CFR 1910.119, 40 CFR 68, and IIAR 2 govern.

charge per vessel m = V x liquid fill fraction x liquid density at that vessel's own operating temperature, summed over the vessels and the pipe runs; the margin is the threshold quantity minus that total, and the volume that would cross it is that margin divided by the piping's liquid fraction times its density.

The IIAR per-vessel charge inventory by name, against the 10,000 lb threshold quantity for anhydrous ammonia in OSHA PSM (29 CFR 1910.119, Appendix A) and the EPA RMP rule (40 CFR 68.130). Liquid ammonia density is strongly temperature dependent -- roughly 37 lb/cu ft in a warm receiver against about 42 at minus 20 degF -- so densities are ENTERED per vessel. It does not read a P and ID, estimate pipe volume from a line list, or determine regulatory coverage.

A sum of volumes times densities; no standard text is reproduced.

Estimate. AHJ and licensed professional govern.

Field names used by the API: receiver_volume_gal, receiver_fill_fraction, receiver_density_lb_ft3, recirculator_volume_gal, recirculator_fill_fraction, recirculator_density_lb_ft3, piping_volume_ft3, piping_liquid_fraction, piping_density_lb_ft3, threshold_lb, receiver_lb, recirculator_lb, piping_lb, total_lb, margin_lb, pct_of_threshold

• Densities are entered per vessel liquid ammonia density varies strongly with temperaturethe plant's own property data
• Pipe volume is entered it is not estimated from a line list or a P and IDthe plant's charge calculation of record
• Coverage is not determined here the threshold is a quantity, not a compliance opinion29 CFR 1910.119 and 40 CFR 68