SUMMARY (130 words): Below post is quite long, so here's a quick summary. Pakistan imports CNG cylinders manufactured according to NZS 5454:1989. NZS 5454:1989 does NOT require the manufacturer to put an expiry date on the label. There are cylinders found in other countries which are manufactured according to other contemporary standards such as ANSI/NGV 2, ECE R110 & ISO 11439 which do have an expiry date on the label clearly marked as 'DO NOT USE AFTER MM/YYYY' and not in coded nomenclature (e.g D06, D011) which would require deciphering.
The CNG cylinders which come with an expiry date still need to be checked at periodic intervals as required by the law of the respective country. It is just that after the expiry date it cannot be used even if passes the mandatory test.
I have found that the above claim is not 100% correct. Also, it is not put forward in the best possible, self explanatory and uncertainty-reducing manner.
#1: "No one in the world tests cng cylinders anymore". How come? Have all the firms/departments providing CNG cylinder testing service closed their doors and hung a notice, "We do not test CNG cylinders anymore?" Or has everybody said, "I will not get my car's CNG cylinder tested anymore?"
This looks like a false claim. This is because there are many many CNG cylinder testing facilities in the world and none of them has gone out of business, gone bankrupt or just closed down because nobody was coming in to avail their service. Instead, the business of CNG cylinder testing & validation is booming as witnessed (in Pakistan) by the recent induction of many private sector testing firms into the CNG infrastructure. This means governments and businesses (both) also see the potential of the market, and not only the existing facilities are unable to fulfill the demand, new entrants are expecting to recover their initial capital investment and go on to being actually profitable. I guess the poster wanted to say, "there is no more a need to get CNG cylinders tested because now they come with an expiry date".
There is no evidence of any public service announcements anywhere in the world like, "Dear citizens, if you have bought your CNG cylinder after dd/mm/yyyy date, there is no need of periodic testing, just keep using it and discard it after the expiry date written on it has passed". Even if this were to happen someday, what about the existing CNG cylinders which were sold without any expiry date in the first place? Surely those people who sit on the helm of infrastructure administration would not like to leave any open ends in their policy?
In addition, all the circulars and notices only mention use of "untested" cylinders, none mention the use of "expired" cylinders.
Even if people all over the world stopped getting their CNG cylinders tested, (at least in Pakistan) it is a legal requirement to get it checked every 5 years. Just because people have somehow fallen out of habit, or just because the manufacturers have changed their method does not waive anybody from their legal responsibility.
#2: Now let's look at the issue of expiry date as well. In the past there have been threads on Pakwheels telling people about expiry date of CNG cylinders. Most did not have any convincing photograph, manufacturer's data or any other which would help people find out the expiry date of their cylinders. Also, nobody I've met in Pakistan told me, "Oh! I have an extra expense at hand. The CNG cylinder of my car has expired and I have to get a new one". Neither on news after a CNG cylinder blast accident, "the accident occurred due to use of expired cylinders", though you usually hear, "Oxygen cylinder was being used instead of proper CNG cylinder".
The lack of evidence compels me to believe that posts like +++ CNG Cylinder Life +++ and CNG Cylinders ..Expiry Date (Very Important ) are baseless fear-mongering, albeit the intention of the poster may not be bad, and they merely wanted their fellow human beings to come to no harm. Also, with the lack of evidence related to the expiry date format, threads such as CNG expirey[sic] date?? true/false? also appeared, came to no conclusion and were abandoned. Some people tried finding expiry date in this format: D06, D011 etc. and were shocked and felt that they had bought poor quality/fake CNG cylinder, doubted the honesty of shopkeeper where they bought their CNG cylinder from, and suffered emotionally.
You will seldom find expiry date on a CNG cylinder sold/being used in Pakistan. Why? Because (as previously noted in post #9 of this thread) the government of Pakistan has adopted the NZS 5454:1989 Standard requirements for lightweight steel automotive compressed natural gas cylinders for use in New Zealand which does NOT require CNG cylinder manufacturers to include an expiry date on the label of the CNG cylinder. However North America have their own standard for CNG cylinder manufacturing, and the CNG cylinders sold there need to conform to that. This standard is called ANSI/NGV 2-2007: American National Standard for Natural Gas Vehicle Containers.Unless the CNG cylinder was made to conform to other standards also in addition to NZS 5454:1989 (some times companies make their products for more than one market, so they adopt an inclusive approach), there is limited chance you'd find an expiry date on your CNG cylinder in Pakistan.
Note: ANSI standards are industry guidelines and not necessarily a legal requirement. Usually industry standards meet or exceed government regulations but not necessary. Now what I gather is that ANSI/NGV 2 only calls for putting expiry date on the cylinder. It does not enforce decommissioning of the cylinder once the expiry date has passed. It is the U.S.DOT NHTSA FMVSS 304 and NFPA 52 which require that CNG cylinders should be removed from service after the expiry date has passed.
The NZS 5454:1989 stays 'Current' and not marked as obsolete. (EDIT: A few years after this post was published, NZS5454:1989 has been changed to "withdrawn" due to various reasons speculated in post #80, page #4. Still, it has not been declared "obsolete" or "superseded").
A drawback of NZS 5454:1989 is that it only covers all-metal (all-steel or all-aluminium) CNG cylinders in detail (though composite cylinders are also mentioned, but not in detail), which are also called Type-1 by most standards, such as ISO 11439 and the ANSI/NGV 2. [If you go to this page, you'll find more info about different types of CNG cylinders]. This should not be a problem in Pakistan because I do not know of any CNG cylinder here which is not all-steel. Also, all-steel CNG cylinder though heaviest is the cheapest of all options. Considering the status of purchasing power here (and also the ban on CNG equipment imports since Feb 2012) it is less likely we will ever see CNG cylinder of any other type of material/construction in Pakistan.
There are other standards for CNG cylinder manufacturing as well, which include ISO 11439:2000 Gas cylinders -- High pressure cylinders for the on-board storage of natural gas as a fuel for automotive vehicles. (In an year or two, it will be superseded by ISO/DIS 11439.2, but right now it is current) . ISO 11439 also requires an expiry date to be written on the cylinder. Another is ECE R110, which is given by UNO, and it also requires expiry date to be written (see page 95 on that link). A comparison between different types of CNG cylinder testing standards can be found here.
The standard format of expiry date on CNG cylinders manufactured to the ANSI/NGV 2:2007 and ECE Regulation 110 standards is 'DO NOT USE AFTER MM/YYYY' in clear words (example: 1, 2, 3, 4 and for all-metal CNG cylinders: 5 and 6). It is NOT some obscure code etched on the wall of the CNG cylinder. I have reason to believe that the expiry date format on ISO 11439 CNG cylinders is also the same.
Is the Pakistani consumer somehow disadvantaged due to the absence of CNG expiry date? CNG cylinders is a relatively new product and used only ~30 years (In stoves, LPG is used and even where CNG is used it is not at the extreme pressure that vehicular use requires). Therefore not a lot of manufacturer data or field data is available. CNG cylinders are usually designed and manufactured to have a design life of 30 years and a service life of 20 years. This is a rough value encompassing all types of CNG cylinders. As you may know, products can wildly fall short of or exceed their design life. With little time-data available, CNG cylinders initially marketed in USA were suggested to have expiry dates of 15 years. But given the good safety track record, revised standards are allowing for longer periods of use and manufacturers are now (in USA since 2007) allowed to label cylinders having up to 25 years before expiry. More on this here. (If the manufacturers want to profiteer they could still sell long-life cylinders with short expiry periods). This means it is good that in Pakistan, CNG cylinders do not come with an expiry date. Since the manufacturers themselves (or anybody else for the matter) do not know of the actual service life of the CNG cylinders, it is only good to adopt the wait-and-see and periodic testing approach. In this way the administration could also establish the data about the expected reliable-operation life of CNG cylinders (Pakistan has a lot of CNG cylinders which are 0-15 years old, and nearly all pass the 5-year, 10-year and 15-year test. Let's look to the future for knowing whether 99.999% of them also keep passing 20-, 25-, 30-, 35-, 40- ,45-year milestone and at what age a lot of cylinders keep start failing the test). Otherwise CNG equipment suppliers could become a mafia and people would be forced to ditch CNG cylinders with a great amount of accident-free life ahead of them just because of an expiry date even the manufacturer does not know for sure where on the bath-tub curve it lies.
One more thing: Even in countries where CNG cylinders come with an expiry date, there are laws that required them to be tested. Look on slide #7 here. This means you have to get the cylinder tested at regular intervals, but cannot use it anymore if the expiry date has come whatever the test performance may be.