Vessel Charge Calculator
Calculate ammonia mass in a vessel using ASHRAE/NIST thermodynamic property data. Results are for single-vessel assessment — total system inventory requires all vessels.
Vessel Parameters
Enter vessel parameters and click Calculate to see results.
How the calculation works
The vessel is treated as a horizontal cylinder. Liquid volume is the segment of the circle below the liquid level, multiplied by the shell length. The remaining volume is vapor. The operating pressure sets the saturation temperature, and the saturation temperature sets both the liquid and vapor densities from ASHRAE and NIST tables for R-717.
Charge = liquid volume × liquid density + vapor volume × vapor density. Head volume is not included, so results run slightly low for vessels with dished heads; add the head volume separately if you need it.
Reference only. A maximum intended inventory for 29 CFR 1910.119 or 40 CFR Part 68 should be prepared component by component by a qualified person and documented.
Vessel charge questions
How do you calculate the ammonia charge in a vessel?
Multiply the liquid volume by the saturated liquid density and the vapor volume by the saturated vapor density, then add the two. Liquid volume comes from the vessel geometry and liquid level; both densities come from the saturation temperature at the vessel's operating pressure. At 180 psia, saturated liquid ammonia is about 33.6 lb/ft³ and vapor about 0.61 lb/ft³, so the liquid dominates the total.
Why does operating pressure change the charge?
Liquid ammonia density falls as saturation temperature rises. A receiver at 95°F (about 195 psia) holds roughly 6 percent less mass per gallon than the same receiver at 60°F. Facilities that calculate inventory in winter at low condensing pressure and never revisit it in summer can misjudge their margin to the 10,000 lb threshold in either direction.
Does this calculator give total system inventory?
No. It sizes one vessel at a time. A maximum intended inventory determination for PSM and RMP adds every vessel, evaporator, condenser, and run of piping at its own operating condition. Use this tool to check individual vessels, then see the inventory determination service for the full method.
What liquid level should I use?
For a maximum intended inventory, use the highest level the vessel can reach in normal operation, typically the high-level alarm or the design operating level, not the level on a given day. For a routine operating check, use the level the gauge glass or level transmitter reads now.
Related tools
One vessel is a start. These cover the rest of the inventory question.
The saturation data behind this calculator, with densities and enthalpies.
OpenDoes OSHA PSM and EPA RMP apply to your facility?
OpenFull-system maximum intended inventory, documented for PSM and RMP.
OpenWhy a spreadsheet number and a live number disagree.
OpenOperating an ammonia refrigeration facility?
NH3Edge provides OSHA PSM and EPA RMP compliance consulting for ammonia refrigeration facilities — third-party compliance audits, ammonia inventory determination, and continuous real-time inventory monitoring. Based in Salt Lake City, serving facilities nationwide.