A good question, and one that comes up fairly often.
I am a Safety Director in the compressed gas industry and I have a cylinder retest facility as one of the branches in my territory, so I can field this one with confidence.
DOT requires that compressed gas cylinders transported in commerce be retested by Hydrostatic pressure testing (Hydrotest) or Ultrasonic Emission testing (UE) on a periodic basis. The retest time frame depends on a number of factors - the service (type of gas contained), the cylinder composition, DOT Special Permits, etc.
The retest date is composed of a 2 digit month, followed by a 4 digit Retester Identification Number (RIN) which is read from the top left clockwise, followed by a two digit retest year. This can be additionally followed by the letters UE (for ultrasonic testing), a Plus symbol (+) which certifies that the cylinder qualifies for a 10% overfill, and/or a Star symbol (*) which signifies that the cylinder is qualified for a 10 year retest. *Most* cylinders are either on a 5-year or 10-year retest cycle. So the retest markings from this month for a DOT 3A 2015 steel cylinder could look like this for RIN# A123:
02 32 14 UE + * This cylinder could be filled to 2216 psig and would require retesting in February of 2024.
A cylinder can be filled at any point from the manufacture date - or prior retest date - up until the "expiration". After that point it cannot be filled for transportation in commerce without a passing a new retest. I have personally seen cylinders from the 1890s and early 1900s still in service, still in good condition, and still passing retest, still being safely filled and used.
Case-in-point: A cylinder of industrial nitrogen with a retest date of March 2014 could be filled at any point beforehand - and WILL be filled so that it can be sold. The cylinder is safe for filling - a process that stresses the cylinder by rapidly heating and expanding the metal. The cylinder does not experience that kind of stress at any point in normal transportation or usage. The cylinder and the product it contains are perfectly usable until that cylinder is "empty". After that expiration date, it cannot be refilled until it passes retest.
Please allow me to touch on one last subject -
While I have seen a lot of old cylinders in good condition and used for many decades, I have also seen a very large number of cylinders that have been misused, damaged, abused, shot full of holes, drilled, cut, welded, etc. by customers. During pre-filling inspection, these cylinders are routinely rejected and sent for retest; very many fail retest and are scrapped. You would be amazed how often customers avoid taking responsibility for damage they've done by trying to hide the damage - using body putty, paint, etc, and going to extreme lengths.- just so they don't have to pay the cost of replacing the cylinder they
Consider the cost to the industry of each scrapped cylinder - a hundred years or more of rent and product fees. It is significant.
However, there are MUCH GREATER COSTS to our friends and co-workers being hurt, maimed, or killed by cylinders which fail during the stress of the fill process by cylinders whose damage has been successfully hidden. To me this is criminal. And yes, these cylinders, as rare as they are, pose a danger to everyone. Our fillers realize the risk and are extremely careful, but nobody is 100% perfect.
Please make sure your people handle cylinders properly using cylinder carts, use regulators in good condition, and report any damage that may have happened while in their care. It could be the difference between a mother or father coming home to their family at night or never coming home....
Thanks for listening. Please feel free to contact me if you have any questions.
Todd Perkins BSc, MBA
Mid America Region
Personal phone: 314.803.2318
From: Benjamin G Owens <bowens**At_Symbol_Here**UNR.EDU>
Sent: Tuesday, February 4, 2014 7:54 PM
Subject: [DCHAS-L] Gas Cylinder Hydrostatic Testing
I understand that compressed gas cylinders must have a current hydrostatic test to be filled. I have read a vendor site that indicates that a cylinder may be transported after the hydrostatic test expiration date if it was filled prior to the expiration date. If cylinders that have exceeded the hydrostatic test date are considered safe to transport are they considered safe to continue using? In other words, if a compressed gas cylinder is not empty but has exceeded the hydrostatic test expiration date is it required by regulation or prudent practice to be returned for testing or can it continue to be used?
Assistant Director, Laboratory Safety
University of Nevada, Reno
EH&S Dept., MS 328
Reno, NV 89557