An ASME propane tank is a stationary pressure vessel built to Section VIII of the ASME Boiler and Pressure Vessel Code and refilled where it sits, while a DOT cylinder is built to Department of Transportation specifications so it can be moved, and tops out at a nominal 420 lb of propane. DOT cylinders must be requalified within 12 years of manufacture and periodically after that; ASME tanks are not on that federal schedule.
The short answer
An ASME propane tank is a stationary pressure vessel built to Section VIII of the ASME Boiler and Pressure Vessel Code. It is set in one place, on a pad or underground, and refilled where it sits by a delivery truck. A DOT cylinder is built to U.S. Department of Transportation specifications in 49 CFR so it can be moved, filled and handled as a portable container. Every grill cylinder is a DOT cylinder, and so are many of the 100 lb and 420 lb cylinders set beside cabins and workshops. The practical differences come down to size, how the container is filled, whether it can be buried, and requalification: DOT cylinders run on a federal clock that starts at manufacture, and ASME tanks do not.
Two rulebooks for one fuel
NFPA 58, the national LP-Gas Code, requires every propane container to be designed, built, tested and marked under one of a few recognized rulebooks. For consumers, those are the DOT regulations and the ASME Code.
- ASME tanks are built to the ASME Code's rules for unfired pressure vessels. Manchester Tank, for example, x-rays the welds and pressure tests its tanks to those rules. Residential ASME tanks run from small vertical tanks to 1,000 gallons and up, and larger commercial tanks follow the same code.
- DOT cylinders are built to DOT specifications and sized by weight. Under NFPA 58 a cylinder tops out at 1,000 lb water capacity, about 119 gallons, which is a nominal 420 lb of propane. Anything larger for a home or business is an ASME tank.
ASME vs DOT at a glance
| Feature | ASME tank | DOT cylinder |
|---|---|---|
| Built to | ASME Boiler and Pressure Vessel Code, Section VIII | DOT specifications, 49 CFR |
| Sized by | Water capacity in gallons (or pounds) | Pounds of propane; water capacity in pounds |
| Largest size | 2,000 gallons in this store's range; far larger for bulk storage | 1,000 lb water capacity, nominal 420 lb propane |
| How it is filled | In place, by a bulk delivery truck | At a filling station or exchange, or in place by a truck when set as a stationary supply |
| Can it be buried? | Yes, if built and marked for underground service | No, above ground only |
| Identification | Stainless steel nameplate that stays visible after installation | Markings stamped into the collar, including the manufacture date; no attached nameplate |
| Federal requalification | Not on the DOT requalification schedule | Within 12 years of manufacture, then at intervals set by the test method, most often 5 years |
DOT cylinder requalification, explained
Requalification (often called recertification) is the periodic check that lets a DOT cylinder keep being filled. Here is how it works:
- The clock starts at manufacture. The manufacture date is stamped into the cylinder's collar. A new cylinder may be filled for 12 years from that date.
- After that it must be requalified. Most cylinders are requalified by an external visual inspection, which is good for five years at a time. A cylinder requalified by volumetric expansion (hydrostatic) testing can go 12 years instead of five.
- Only an authorized person can do it. Inspections must be done by someone holding a current DOT requalification identification number (RIN), who records the result and marks the new date on the cylinder.
- An out-of-date cylinder will not be refilled. Fillers must confirm a container meets the code's requalification provisions before filling it. Requalification usually costs something, and not every filling location can do it.
- It applies to stationary cylinders too. A 420 lb cylinder set beside a house and filled by truck is still a DOT cylinder, and it still needs requalifying on schedule.
DOT forklift cylinders follow the same rules. Texas regulators list forklift cylinders in service past their requalification date among the most commonly cited violations. Our forklift propane tanks guide covers fleet use, and cages for storing cylinders are in forklift and BBQ cylinder cages.
Do ASME tanks need recertification?
ASME tanks are not on the DOT 12-year requalification schedule, which is written for DOT specification cylinders. That does not mean an ASME tank is never checked. Before filling any container, the filler must determine that it meets the code's design, fabrication, inspection, marking and requalification provisions. Manchester Tank tells owners to ask their authority having jurisdiction about in-service inspection and maintenance. NFPA 58 requires containers with excessive denting, bulging, gouging or corrosion to be removed from service, and repairs to ASME tanks must follow the National Board Inspection Code. A buried ASME tank also has cathodic protection testing on its own schedule; see underground vs above ground propane tanks.
If you are buying a used ASME tank, the nameplate is the first thing to check. Texas regulators found tanks reconditioned out of state whose nameplates did not meet state requirements, and those tanks were removed from service. Ask for clear photos of the nameplate before you buy, and have your supplier confirm they will fill the tank.
How to tell which one you have
- Look for a nameplate. An ASME tank carries a stainless steel nameplate, placed so it stays visible once the tank is installed. It shows, among other things, the service (aboveground, underground or both) and the water capacity. Older tanks may carry the ASME "clover leaf" stamp; new tanks do not necessarily have it.
- Look at the collar. A DOT cylinder has no attached nameplate. Its manufacture date is stamped into the collar, and it is marked with its water capacity and tare weight in pounds, along with any requalification dates.
- Check the size. For a home or business tank, anything over about 119 gallons of water capacity is an ASME tank.
Same size, two formats
At 420 lb (120 gallons) both types are made, and this store lists both:
- DOT cylinder: 420 lbs (120 Gallon) Propane D.O.T. Tank, $1,025.00
- ASME tank: 420 lbs (120 Gallon) ASME Propane Tank, $1,095.00
In the 200 lb size the store lists the ASME version: 200 lbs (45.6 Gallon) ASME Propane Tank, $749.99.
Choose the DOT version if the cylinder will ever travel to be filled or exchanged. Choose the ASME version if it will stay put and be filled in place, and you would rather not track a requalification date. Either way, check that your propane supplier fills the type you pick.
Which one you need
- Grill, patio heater, camping or a portable generator: a DOT cylinder you can carry to be filled or swapped.
- One or two appliances on a fixed supply: a 100 to 420 lb cylinder filled by truck, or a small ASME tank such as the 420 lbs (120 Gallon) ASME Propane Tank or the 120 Gallon Above Ground ASME Steel Propane Tank, both $915.00.
- Standby generator: a fixed supply sized to the run time you need; see propane tank for a generator.
- Whole-home heating: an ASME tank, since whole-home sizes are far above the DOT ceiling, such as the 500 Gallon Above Ground Propane Tank - ASME at $1,650.00. The size chart helps you narrow it down.
- A tank you want out of sight: an ASME tank built for underground service.
- RV onboard tank: manufacturers build ASME tanks for mounting on recreational vehicles, such as the Manchester 6828 - 14″ x 40″ ASME Propane Tank at $700.00, made as a motorhome's onboard tank. Match any replacement to your vehicle's specifications.
- Forklifts: forklift cylinders; DOT forklift cylinders must be kept within their requalification dates.
Whether to own the tank or lease it from a supplier is a separate choice with real advantages on both sides; see buy vs lease a propane tank and propane tank cost.
Filling, setup and safety
Neither type is filled to the brim. A "full" container is about 80% liquid, which leaves room for the liquid to expand as it warms. Tanks from this store ship empty by freight (shipping and delivery). Setting a stationary tank or cylinder, connecting the regulator and gas line, leak testing and the first fill are jobs for a licensed installer and your licensed propane supplier, not for the homeowner. Our installation page explains what that work involves.
Common questions
Do ASME propane tanks need to be recertified?
No, not on the DOT schedule. The federal 12-year requalification rule applies to DOT specification cylinders, not ASME tanks. ASME tanks are still checked by the supplier during setup and filling, must be taken out of service if badly dented, bulged, gouged or corroded, and may have in-service inspection rules set by your state or local authority. Buried ASME tanks also need cathodic protection tests on a schedule.
How do I know if my propane tank is DOT or ASME?
Look for a nameplate. An ASME tank has a stainless steel nameplate that stays visible after installation, listing its service and water capacity. A DOT cylinder has no attached nameplate; its manufacture date is stamped into the collar and it is marked with water capacity and tare weight. Any tank larger than about 119 gallons of water capacity is an ASME tank, since that is the ceiling for a DOT cylinder.
How long is a DOT propane cylinder good for?
A new DOT cylinder can be filled for 12 years from the manufacture date stamped on its collar. After that it must be requalified by someone holding a DOT requalification identification number. A visual inspection extends it five years at a time; a volumetric expansion test extends it 12 years. A cylinder can keep being requalified as long as the inspector judges it safe, and an overdue cylinder will not be refilled.
Can a DOT cylinder be filled in place by a delivery truck?
Yes. Larger DOT cylinders, such as 100 lb and 420 lb cylinders, are often set beside a building as a stationary supply and filled on site from a bulk truck. They remain DOT cylinders, so they still need requalification on schedule and must stay above ground on a firm foundation. If you want a fill-in-place container with no requalification date to track, choose an ASME tank of similar size.
Is a 420 lb propane tank DOT or ASME?
It can be either. A 420 lb cylinder, about 120 gallons, is the largest DOT cylinder, because NFPA 58 caps cylinders at 1,000 lb of water capacity, a nominal 420 lb of propane. Manufacturers also build ASME versions of similar size for fixed installations. Check the nameplate or collar markings, or the product listing, to see which one you are buying.
Can I bury a DOT propane cylinder?
No. NFPA 58 says cylinders may be installed only above ground, on a firm foundation or otherwise firmly secured, and must not be in contact with the soil. Only an ASME tank built and marked for underground service, with a burial coating and cathodic protection, can be buried. A licensed installer should set either type.