Normal use shouldn't be of any concern. Filling, on the other hand, is, because of the stress that it places on the cylinder during the fill process. Normal use doesn't place that kind of stress on the cylinder, and cylinders are routinely in use for many years past their retest date without any safety concerns.
Obviously everyone here must set the standards for his/her own organization, and you can set those standards as high as you feel appropriate, but please don't confuse product expiration with packaging (cylinder) retest, because its not a true equivalency.
A couple other notes for anyone interested:
The DOT stamp on a cylinder is a packaging specification. For example, DOT 3AL 2015 is specific to aluminum cylinders with a working pressure of 2015psig, while a DOT 4BA 260 is a low working pressure (260psig), thin walled cylinder used for propylene and other compressed liquids with low vapor pressures, and a DOT 4L 200 is a cryogenic liquid cylinder with both an outer shell and an inner container, and a service pressure of 200 psig. A lot can be learned from these packaging specifications - see 49CFR178 for more info.
A cylinder that is removed from transportation in commerce to become part of a stationary cylinder bank (aka ground tubes, et al.) is not *required* to be retested because it does not fall under DOT's jurisdiction, regardless of how many times it is filled. Similarly a cylinder that was filled in test can be perfectly safe in circulation (normal use) a long long time before returning for a retest and refill.
Cylinders that have been sitting for multiple years at a customer location, which contain highly oxidizing "acid gases" (HBr, HCl, HF, and others) should be handled with extreme care as over time the cations can attack the iron, leaving Hydrogen. The original gases are usually a compressed liquid with a moderately low vapor pressure, but the hydrogen gas created can eventually over-pressurize the cylinder causing the safety or the cylinder to fail.
A similar danger is posed by cylinders of HCl, HBr, HF, Cl2, and others, if water is introduced back into the cylinder. Two different things can happen:
1) the heat of reaction can cause an over pressurization - and/or
2) the steel inside can be etched, leaving the cylinder wall thinned and weakened. This will also happen with Carbon Dioxide, as it forms Carbonic Acid which can etch the steel over time to the point of catastrophic failure. This is unfortunately common with beverage grade CO2 cylinders.
A typical hydrotest pressurizes a cylinder to 5/3 its normal working pressure, while a UE test looks at the thickness and microcrystalline structure of the steel to find any weak spots or invisible damage that could cause the cylinder to fail. Both have their advantages, but UE will fail cylinders that can pass a hydrotest, for reasons a hydrotest cannot detect.
When in doubt, talk to a professional in the compressed gas industry. Airgas has Safety Directors all across the US, as well as a nationwide network of highly trained Airgas Emergency Response Organization (AERO) team members to assist customers experiencing compressed gas cylinder incidents.
Todd Perkins BSc, MBA
Mid America Region
Personal phone: 314.803.2318
From: "JAKSAFETY**At_Symbol_Here**AOL.COM" <JAKSAFETY**At_Symbol_Here**AOL.COM>
Sent: Wednesday, February 5, 2014 2:30 PM
Subject: Re: [DCHAS-L] Hydrostatic Test Date
I would be prudent to not use a compressed gas cylinder after its hydrostatic test date has expired.
Laboratory Safety Institute
In a message dated 2/5/2014 12:00:35 A.M. Eastern Standard Time, LISTSERV**At_Symbol_Here**listserv.med.cornell.edu writes:
Date: Wed, 5 Feb 2014 01:54:50 +0000
From: Benjamin G Owens <bowens**At_Symbol_Here**UNR.EDU>
Subject: 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
Assistant Director, Laboratory Safety
University of Nevada, Reno
EH&S Dept., MS 328
Reno, NV 89557