Propane Tank Filling Limit and Outage

ScopeA 500 gallon tank delivers 400 gallons at most and a 'full' tank reads 80% on the float gauge; neither is a shortfall, and this computes both so the conversation can be had with numbers. THE FILLING LIMIT IS ABOUT THERMAL EXPANSION rather than a safety margin in the abstract. Liquid propane expands roughly 1.5% per 10 degF, so a tank filled solid on a cold morning and warmed by the sun becomes hydraulically full and then lifts its relief valve -- the relief valve working exactly as intended, and still a large release of flammable gas over whatever is beneath it. The outage is the room that expansion needs, and the temperature rise it buys is reported here, because an overfill does not read as dangerous until that figure is put next to a sunny afternoon. THE FIXED LIQUID LEVEL GAUGE IS THE PHYSICAL ENFORCEMENT and it is why filling needs attention rather than a meter to watch: the bleeder valve is cracked open during the fill and sprays white when liquid reaches the dip tube, and that spray is the stop signal regardless of what the float gauge or the meter reads. The float gauge is an indication; the dip tube is the measurement. HOW LONG THE GAS LASTS IS DELIBERATELY NOT COMPUTED HERE. The tank's energy content is reported, but dividing it by a connected load gives a continuous-firing floor rather than an estimate, and the propane run time calculation carries the duty cycle, the delivery trigger and the degree-day history that make the answer usable. The usable figure is lower again, because the last of the liquid cannot maintain vapour pressure against the load in cold weather -- that limit is wetted surface rather than volume, and the propane vaporization capacity calculation is where it is read. This computes fill, outage and energy content from an entered filling limit. It does not determine the applicable filling limit, which is temperature-corrected and differs between aboveground and underground containers and with the filling method, verify or calibrate a float gauge, address the fixed liquid level gauge's setting or the filling procedure, evaluate relief valve sizing or discharge location, or determine what any standard requires. NFPA 58, the adopted fuel gas code, the container manufacturer, and the AHJ govern.

Formula or tablemaximum fill = water capacity x the filling limit; outage = the remainder; the expansion headroom is the free fraction divided by liquid propane's roughly 0.0015 per degF.

Source and editionThe filling limit is ENTERED because it is temperature-corrected and differs between aboveground and underground containers and with the filling method.

Free accessTwo products and one division.

What governsEstimate. AHJ and licensed professional govern.

API field nameswater_capacity_gal, fill_limit_pct, current_gauge_pct, btu_per_gal, liquid_temperature_f, max_fill_gal, outage_gal, current_gal, deliverable_gal, full_tank_mmbtu, current_mmbtu, limit_headroom_f, hydraulically_full_at_f

• The filling limit is entered it is temperature-corrected and differs by container typeNFPA 58 as adopted
• Expansion at 0.0015 per degF the coefficient varies with the propane-butane mixpublished liquid property data
• The float gauge is an indication the dip tube is the measurementLP-gas filling practice