CNG gas tank requirements

Tally Ho · 2025-01-11T16:19:37-0800

Propane tanks have to be recertified after 12 years from manufacture…then every 5 years after that.

I would assume CNG tanks probably have similar requirements.

How to Tell If Your Propane Tank Needs to be Recertified or Requalified

Propane tanks are designed for long use, but they do need to be recertified or requalified. Ferrellgas offers tips to help you determine if you need propane tank recertification.

www.ferrellgas.com

Greg

Terry Cox · 2025-01-11T17:55:52-0800

Hi Alan. Our CNG tanks are round steel with a rounded bottom and will hold 2,500 pounds of CNG. If yours are the same, they must be re-certified every five years and will last until the re-certification company will no longer do so. As long as they are not rusted or heavily pitted from corrosion, they should last forever, according to my re-certification company. Ask your surveyor to provide any official documentation that states they must be destroyed or can no longer be used after twelve years.

thinwater · 2025-01-11T18:01:11-0800

Alan Backus said:
I'm selling my 1989 sailboat in the US. The surveyor claims that my CNG cylinders that fuel my stove exceed a 12 year end of life requirement. Is this requirement true? or applies to propane tanks? What is the standard that requires this? The cylinders have no date of mfg. but I'm sure are older than 12 years. They have been pressure tested and stamped within the last 5 years, which is required before refilling. Who still sells new CNG marine cylinders?
Also, the same surveyor claimed that a gas cylinder must have a OPD (overfill protection device), but I've learned that this only applies to LPG (propane) tanks, correct?
Thanks for any quick responses!

No date of manufacture? Is there any manner of code stamp, stating what standard they were built to? Without that information recertification is unlikely.

smokey73 · 2025-01-11T18:29:25-0800

The tank should have a DOT (Department of Transportation) certification stamped into the metal. That establishes to what standard the tank was manufactured. It will then need to be pressure tested every 5 years. Each time the tank is pressure tested the month and year of the test will be stamped into the metal somewhere on the tank. These tanks are very robust. I have been told not to attempt to remove surface rust or to paint over any surface rust as the place that tests them may refuse to recertify. As @Terry Cox said, there is no specific "end of service life" for a steel CNG tank. I understand that there may be some composite CNG tanks or "non-steel" tanks that may have a specific service life.

Richard19068 · 2025-01-11T18:30:18-0800

I think your surveyor is confusing CNG with propane. There should be a date of manufacture on the CNG cylinder but it doesent matter. As long as it passes the required five year hydrostatic pressure test they should be fine. Those cylinders are built to withstand many cycles of filling and use. Typical marine use cycles them relatively inoften. Mine are well over 20 years old and the guy at the company that hydro tests them said they ar like new.

thinwater · 2025-01-12T05:29:04-0800

Passing a hydro test is not the sole requirement. Excessive corrosion (over 0.011-inch metal loss) can also disqualify tank (require professional coating by the tech). Over 0.030 inch is grounds for the tank to be destroyed. Also dents, scratches, and damamge to the valves. The inspection list is long. These operate at much higher (15 times) pressure than propane tanks.

Because the corrosion allowance is so thin and because painting could hide it or the prep process could damamge the tank, it is better to leave this to the inspector. Obviously, it is in your interests to avoid damaging the paint and to keep the tank dry. This means careful handling, good clamping, and nothing else in the locker.

Correct, there is no specific retirement date for steel or aluminum tanks. Composite tanks do have a retirement date, typically 15 years, because UV and other oxidation and chemical exposure damamge cannot be gauged.

thinwater · 2025-01-12T05:51:47-0800

Alan Backus said:
Thanks for everyone's inputs which is mostly consistent with what I understood. Does anyone know the agency and specific standard to reference? The ABYC A-22 on CNG systems does not specify tank requirements and I can't find on AGA's website. I'll also reach out to my pressure test company. Thanks.

There are several standards. You really need to find the stamp. Without that, most inspectors will flat refuse, since they can't tell if it is even a proper CNG cylinder. There are additional standards as well, particularly for older tanks.

Good luck.

Before any Detailed Inspection can begin, the inspector must identify which of these standards
applies. There are currently two for CNG cylinders:
1. ANSI/CSA: NGV2 Compressed Natural Gas Vehicle Fuel Containers
2. DOT/NHTSA: FMVSS 304 Compressed Natural Gas Fuel Container Integrity
(NOTE: this is a US government standard referenced in 49 CFR 571.304 Code of Federal
Regulations)

smokey73 · 2025-01-12T06:08:24-0800

@thinwater Thanks for the specs. That explained why the place where I drop my tank off for recertification cautioned me not to paint the tank or try to "clean it up" by removing any surface rust. All of my tanks have a DOT stamp on the tank.