How to Check a Propane Tank for Generator Runtime and Vaporization

Why this matters

A propane standby can fail two different ways on the same tank, and customers only ever ask about one of them. "How long will it run?" is a gallons question, and it has a clean answer. "Will the tank keep up?" is a vaporization question: whether liquid propane can boil off fast enough, on the coldest night, at a low fill, to feed the generator and the furnace together. A tank with plenty of gallons in it can still starve the engine. This procedure answers both, writes them down, and puts the second one in front of the people who own it.

Who owns what

The propane supplier sizes, sets and fills the tank, and owns its regulators. The generator tech does not adjust a first-stage or second-stage regulator, open the tank's service valve beyond normal use, or move a tank; tank setting, filling and regulator selection fall under NFPA 58 as adopted by your jurisdiction, and that code binds the supplier. The tech's job is the arithmetic and the question: here is the load in gallons per hour and BTU per hour, here is the coldest weather this site sees, can your tank and regulators deliver it?

If anyone smells gas at the tank or the unit, everyone moves away, no switches, no engine starts, no phone used at the tank, and the call goes to the supplier from a distance. Nothing on this procedure continues until the supplier clears it.

Reading the tank's data plate and float gauge is the only hands-on part, and it involves nothing that opens the system.

Step 1: Read the tank's data plate, not its size on the invoice

Tanks are rated in water capacity, the gallons of water the vessel holds full. Find it on the data plate on the tank, along with whether it is above or below ground. A "500-gallon tank" holds 500 gallons of water; nobody fills it to 500 gallons of propane.

Record also the tank's orientation and setting. A buried tank sits in ground that holds a steadier temperature than winter air, which matters for vaporization later.

Step 2: Apply the 80 percent fill and a working floor

Maximum fill is about 80 percent of water capacity, set by the supplier's fill practice, because liquid propane expands as it warms and needs vapor space. So a 500-gallon tank holds about 400 gallons when full.

The bottom of the tank is not usable either. As the level drops, the wetted wall area that conducts heat into the liquid shrinks, vaporization falls off and pressure sags under load, so most suppliers schedule a refill well before empty. This procedure uses 20 percent of water capacity as the working floor. Ask your supplier what floor they plan deliveries to and use theirs if it differs.

Usable gallons = (80 percent minus floor) times water capacity.

Step 3: Get the generator's full-load consumption

From the generator spec sheet, take the propane consumption at full load, in gallons per hour. Use the full-load figure. The brochure often leads with half load, and a tank sized on half load runs out halfway through the outage it was sold for. See related: Reading a Generator Spec Sheet and Derating.

The fuel-content numbers used to convert BTU and gallons are owned by a sibling card; this one uses about 91,500 BTU per gallon of propane. See related: Natural Gas and Propane Heat Content and Generator Consumption.

Step 4: Add every other propane load on the tank

An outage is exactly when the furnace runs hardest. List every appliance that draws from this tank: furnace, water heater, range, fireplace, pool heater. Convert each rating plate's input in BTU per hour to gallons per hour by dividing by 91,500.

For runtime, decide what you are modeling and say it on the record. The honest worst case is the generator at full load and the furnace at full fire; that is what the artifact below does.

The runtime calculation

 TANK                 500 gal water capacity, above ground
 max fill 80%         0.80 x 500        = 400 gal
 working floor 20%    0.20 x 500        = 100 gal
 USABLE FROM FULL                         300 gal

 LOAD, worst case (full load + furnace at full fire)
 generator full load  spec sheet         3.60 gal/h
 furnace 80,000 BTU/h 80,000 / 91,500  = 0.874 gal/h
 TOTAL DRAW                              4.474 gal/h

 RUNTIME FROM FULL    300 / 4.474      = 67 h
 check: 67 h x 4.474 gal/h             = 300 gal

 RUNTIME TODAY        gauge reads 45%
 usable now           (0.45 - 0.20) x 500 = 125 gal
                      125 / 4.474        = 28 h

 VAPORIZATION DEMAND  4.474 x 91,500     = 409,000 BTU/h

The 3.60 gallons per hour is illustrative; read the sheet for the unit you are working on. As a cross-check, 3.60 gallons per hour times 91,500 is about 329,000 BTU per hour, roughly 330 cubic feet per hour of natural gas at 1,000 BTU per cubic foot, which is in the range a residential air-cooled standby of that class draws on natural gas. If the sheet's propane and natural gas figures land far apart once both are converted to BTU per hour, re-read the sheet; you may have a half-load figure in one column.

What the two runtimes tell the customer. From full, about 67 hours, a little under three days of worst case. On the day of this visit, at 45 percent, about 28 hours. That second number is the one that strands people: the tank's size did not change, the gauge did. This is why maintenance plan customers get the gauge checked and a fill scheduled before storm season. See related: The Storm Season Readiness SOP for Maintenance Plan Customers.

What would shorten or stretch it. A real outage rarely holds the generator at full load all night; at half load the generator draws well under its full-load figure (the spec sheet gives the half-load number) and the runtime stretches past 67 hours. A second propane appliance running hard (a pool heater someone forgot to shut off) shortens it. The full-load figure is the one to quote, because it is the one the customer can rely on.

Step 5: Put the vaporization question to the supplier

Gallons say how long; vaporization says whether. Liquid propane turns to vapor by absorbing heat through the tank wall wherever liquid touches it. Three things set how fast it can do that:

• Wetted wall area. A fuller tank has more liquid against the steel. Vaporization falls as the level falls, so the worst moment is late in a long outage, at a low fill.
• Temperature around the tank. Heat flows in from the air or ground. The colder it is outside, the less flows in and the lower the tank pressure: propane's vapor pressure is roughly 110 psig at 70 F and drops into the mid-20s psig around 0 F. Propane boils at about minus 44 F, where the tank has essentially no pressure left to push.
• Draw rate. Pulling vapor off faster than the wall can supply heat chills the liquid, which drops its pressure further. The tank "freezes down": a frost band forms on the outside at the liquid line, pressure falls below what the regulators need, and the generator leans out and quits.

Give the supplier four numbers in writing: the tank's water capacity and setting, the total vapor demand (409,000 BTU per hour in the example), the coldest temperature this site sees, and the lowest fill the customer is likely to reach before a delivery (20 percent here). Ask whether the tank, at that temperature and fill, can deliver that demand continuously, and whether both regulator stages are sized for it. Write down the answer and who gave it.

The supplier owns the vaporization tables for their tanks and the regulator selection. A tech quoting a vaporization rate from memory is quoting a number nobody can stand behind.

When the answer is no

A small tank. Vertical cylinders and small horizontal tanks that are fine for a range and a water heater are the classic freeze-down case under generator load. The fix is a larger tank or two tanks manifolded, which the supplier sets.

A large load in a cold climate. Where even a large vapor tank cannot keep up at the design temperature, the supplier may move to liquid draw with a vaporizer. See related: Propane Vapor vs Liquid Draw.

A buried tank. Ground temperature holds steadier than air, so a buried tank often vaporizes better in a cold snap than the same tank above ground. That is the supplier's figure to give, not an assumption to make.

How it fails in the field

The freeze-down failure looks like a generator fault. The unit starts and carries the house for hours, then hunts, droops and shuts down on low frequency or underspeed, usually in the early morning when it is coldest and the furnace has been running all night. A tech who arrives at noon finds a warmer tank, normal pressure and a unit that starts fine, and may leave having changed nothing.

The tells are a frost band on the tank wall at the liquid line during the event, a gauge reading low, and a timeline that tracks the overnight low. Check the tank first on any propane unit that quit late in a cold outage, before opening the fuel train. See related: The Propane Tank That Starved the Generator in a Cold Snap.

How to verify you got this right

• Water capacity read off the data plate, not the invoice.
• Usable gallons computed from the 80 percent fill and the supplier's floor, with the floor written down.
• Full-load consumption from the spec sheet for this exact unit and fuel.
• Every other propane appliance on the tank listed with its gallons per hour.
• Runtime from full and runtime at today's gauge, both on the record.
• The supplier's written answer on vaporization at the coldest temperature and the lowest fill, with a name and a date.

References

• NFPA 58, Liquefied Petroleum Gas Code, in the edition your jurisdiction has adopted, which binds the supplier's tank setting and fill practice
• The generator spec sheet for full-load propane consumption; the propane supplier's tank and regulator data
• See related: Natural Gas and Propane Heat Content and Generator Consumption; Propane Vapor vs Liquid Draw; The Storm Season Readiness SOP for Maintenance Plan Customers; The Propane Tank That Starved the Generator in a Cold Snap