RV Propane Tank Won't Fill: The Four Causes, and Which One Is the Tank

A refusal at the pump is a conversation, and the attendant is usually right. What they are applying is a pre-fill inspection the regulation requires, and knowing what is on that list is what tells you whether the problem is your tank or a valve on it.

The four causes below are in the order to check them: three of them cost a valve, and one can cost the tank.

A stuck overfill device

The overfill device is a float inside the service valve. As liquid rises past roughly 80 percent of the tank's volume the float lifts and closes a check, and the filler stops taking gas. When the float sticks, one of two things happens at the pump: the flow cuts off early, or the attendant cannot get a seal at the outlet at all.

PHMSA describes what the device is for: "An OPD is a backup safety shutoff device incorporated into the filling valve of a propane cylinder. It is designed to prevent cylinder overfilling in excess of the cylinder's maximum permitted filling limit."

A stuck float is a valve replacement rather than a tank replacement, and it is the most common of the four.

A requalification date that has run out

Every DOT propane cylinder carries a manufacture date on its collar and, after its first requalification, a requalification date beside it. The regulation is not a recommendation: "A cylinder may be repaired and requalified only as prescribed in subpart C of part 180 of this subchapter." A cylinder past its date cannot lawfully be filled, and an attendant who fills it is the one exposed.

The date is the one cause on this list that is about the tank rather than a part on it, and the page below covers how the intervals work.

A valve with no overfill device in it

The older valve type is a POL, a left-hand internal thread that needs a wrench. It has no overfill device inside it, and the handwheel is round rather than the three-lobed shape that identifies an OPD valve.

The requirement is not federal. PHMSA states the position plainly: "The Hazardous Materials Regulations (HMR; 49 CFR Parts 171 - 180) do not contain specific design requirements for cylinder valves. We have not adopted the OPD requirements into the HMR." It comes from the LP-Gas Code instead, and PHMSA gives the terms: "NFPA 58 requires an OPD on all cylinders with 4 to 40-pound propane capacity that are: Fabricated (manufactured) after September 30, 1998; Requalified after September 30, 1998; or Refilled on or after April 1, 2002."

Most states have adopted the code, which is why a POL valve is refused on sight rather than argued about.

A fill valve seal the filler must reject

The 2020 edition of the LP-Gas Code added an inspection that catches people out, because the part it checks cannot be replaced on its own. A state agriculture department describing the requirement puts it this way: "The code directs those who fill cylinders to inspect this seal prior to filling every time a cylinder with these valves is presented for filling. This seal is not replaceable, so the cylinder must not be filled until the valve is replaced."

It applies to the connections used on nearly every cylinder from 4 to 40 pounds of propane capacity. And the same document covers what happens when the filling station cannot do the work: "If this is the case, filling the cylinder must be denied and the customer must be given written notification of the reason for this denial."

That written notice is worth asking for. It names the reason, which is the difference between driving to another station and driving to a shop.

The pre-fill inspection is written into the regulation, and it is short enough to read in full. It is also the list an attendant is working from, which makes it the list to check your own tank against before you leave.

The pre-fill inspection, in the regulation's own words

49 CFR 173.301, in the paragraph that governs filling a cylinder: "Before each filling of a cylinder, the person filling the cylinder must visually inspect the outside of the cylinder. A cylinder that has a crack or leak, is bulged, has a defective valve or a leaking or defective pressure relief device, or bears evidence of physical abuse, fire or heat damage, or detrimental rusting or corrosion, may not be filled and offered for transportation."

Read that list against your own tank and the refusal usually explains itself. Rust on the bottom is named. A dent, a bulge or a valve that has been knocked are named. A relief valve that weeps is named.

One more line sits beside it and matters after a fill: "Pressure relief devices must be tested for leaks before a filled cylinder is shipped from the cylinder filling plant."

What a refusal means, and what it does not

A refusal is not a judgement about your coach and it is not a diagnosis. It is one of two things: a condition on that list, or a station that cannot do the work the code requires it to do.

The distinction is worth making out loud at the counter. A station that refuses because the seal is damaged is applying the code correctly, and the same tank may be accepted at a station equipped to change the valve. A station that refuses because the collar date has run out is applying the regulation correctly, and no other station will take it either until the cylinder is requalified.

The service valve is not one device. It is three, and each does a different job. Cavagna describes the assembly on its Type 1 valve: "This valve has a vapor service outlet, relief valve, captive fixed liquid level gauge, and an overfill prevention device (OPD)."

Owners tend to treat the whole valve as one thing called the OPD, which is why a fault in any of the three reads as "my OPD is broken".

It stops flow in, at about 80 percent, during filling only

The overfill device's job is the one its name gives it. It closes the fill at roughly 80 percent of the tank's volume, and the reason for the 20 percent that is left empty is expansion: liquid propane grows as it warms, and a tank filled to the top has nowhere for that to go.

It is a backup rather than the primary check. The primary check is the fixed liquid level gauge, which is the small bleed the attendant opens while filling.

It is not a level gauge

Nothing in the valve tells you how full the tank is. The bleed tells the filler when to stop, and the gauge on the wall inside the coach reads a sender rather than the float.

RegO is explicit about the bleed's role: "The fixed liquid level gauge on a cylinder valve is, when present, essential to prevent overfilling the cylinder." The same document adds the check that keeps it honest: the gauge valve "must operate freely, venting vapor when loosened, and sealing gas-tight easily when tightened with the fingers", and the diptube inside it "must be of the proper length, and be in proper position", with a periodic weighing of the cylinder as the way to confirm it.

A bleed that will not seal by finger pressure is a valve fault, and one worth fixing before the next fill rather than at it.

The pressure relief valve is the device that keeps a tank from becoming a bomb in a fire, and it is the one part of the assembly with no service procedure at all.

RegO states it twice, in two registers. In its service guidance: "The pressure relief valve is of critical importance: Its proper operation is vital in avoiding excessive pressures during emergencies, such as overfilling or exposure to excessive heat. No repair of this device is allowable." And in the warning printed for the cylinder itself: "This valve cannot be repaired. If it is obstructed, the entire cylinder valve must be replaced."

What it does when it works is alarming, and it is not a fault: "The Pressure Relief Valve can expel a large jet of LP-Gas into the air if the cylinder is (1) exposed to high temperatures - over 120°F (49°C), or (2) overfilled and exposed to a temperature higher than the temperature at the time it was filled."

A tank that vents on a hot day has done one of two things. It has been overfilled, or it has been sitting in more heat than it was filled in. Either way it is a tank to stop using and have looked at, not a tank to top up.

The same document gives the routine check that catches a leaking one before it matters: "Each time the cylinder is filled, the entire Cylinder Valve must be checked for leaks (with a leak detection solution... leaks cause bubbles to grow)."

Every DOT cylinder carries its dates stamped into the collar, and they are the first thing an attendant looks at. Reading them yourself takes a moment and settles the question before you drive anywhere.

Five, ten or twelve years, and what decides which

The interval is not one number. The regulation's table gives DOT 3A and 3AA cylinders a period of 5, 10 or 12 years depending on the conditions in the section, and the long interval has its own paragraph: a cylinder made to DOT specifications 4B, 4BA, 4BW or 4E, "protected externally by a suitable corrosion-resistant coating and used exclusively for non-corrosive gas that is commercially free from corroding components may be requalified by volumetric expansion testing every 12 years instead of every 5 years."

The alternative test carries a different clock: "the cylinder may be subjected to a proof pressure test at least two times the marked service pressure, but this latter type of test must be repeated every 10 years after expiration of the initial 12-year period."

What that means in practice is that the number on the collar is the answer, and the way it was tested is what decides the next one.

The requalifier's mark, and the letters that matter

Requalification is done by a shop holding a requalifier identification number, and the mark it stamps carries a letter for the method. An E is the external visual method and it buys five years. An S is the proof pressure method and it buys ten.

Only a current requalifier can do the work, and the cylinder has to be presented to one rather than filled in the hope that nobody looks. A cylinder that has not been used exclusively for the listed gases can still take the ten-year interval if it meets the other provisions and is first requalified and examined as the section prescribes.

A propane generator draws from the same tank as everything else, which is the whole argument for it: the fuel is already on board, and there is no can of gasoline in the tow vehicle. What it costs is consumption, and the makers publish the figures.

What the makers publish for consumption

Cummins Onan's operator manual for its 7000-watt LPG model gives consumption at three loads, in the units the fuel is actually sold in:

• No load: 0.5 kg/h, which is 1.1 lbs per hour.
• Half load: 1.2 kg/h, which is 2.5 lbs per hour.
• Full load: 1.8 kg/h, which is 3.8 lbs per hour.

Those are pounds rather than gallons because propane is sold by weight at the fill and by volume in the tank, and the conversion is where most of the guesswork about runtime comes from. The same manual gives the gasoline figures beside them, so the two can be compared directly rather than estimated.

The fuel grade and the supply pressure

The generator asks for a specific grade, and it is not the same as whatever is in the tank. Cummins Onan's instruction is to "Use clean, fresh HD-5 grade liquefied petroleum gas (LPG) or equivalent product consisting of at least 90 percent propane."

The reason is butane, and it matters in cold weather: "Commercial LPG fuels may contain more than 2.5 percent butane which can result in poor fuel vaporization and poor engine starting in low ambient temperatures (below 0 oC (32 oF))." A generator that starts hard on a cold morning may be running on a winter blend that is fine for the furnace and marginal for the engine.

The supply pressure is the other figure, and it is a range rather than a setting: the LPG vapor connection is specified at 228 to 330 mm, which is 9 to 13 inches of water column. That is the same low pressure the appliances run on, which is why the generator can tee into the coach's system at all.

The cost of this fault is decided by which of the four causes it was, and three of the four are a valve.

At the bottom is the diagnosis itself, and the cheapest outcome is a tank that was fine: a bleed that needed finger-tightening, a station that could not do the seal inspection, or a date read correctly at the counter before anyone filled anything.

In the middle is the service valve, which covers a stuck overfill device, an obsolete valve and a damaged seal. It is a modest part and a short job, and it is the outcome for three of the four causes.

At the top is the tank itself, and there are only two ways to arrive there: a cylinder that has failed the pre-fill inspection, or one whose requalification has run out with no requalifier able to take it. Neither is repairable at a filling station, and both are the reason the page leads with the other three.

The gas system the tank feeds, including the regulator and the pressure it should hold, is in the propane furnace guide. The alarm that watches for a leak is in the propane detector guide, and the requalification interval sits on the maintenance schedule as one of its annual checks. When the generator is the thing that has stopped rather than the fuel, the generator not charging guide takes it from there.