On day one, when a newly purchased gas cylinder comes into the filling branch, it is a thing of beauty.
With a good coat of paint, no external damage/corrosion and a bright shiny valve, the owner is indeed pleased with his purchase. But gas cylinders have a high longevity.
During my spell at an industrial gas company, one of our drivers retrieved a cylinder from the garage of a deceased customer, which had been manufactured in 1898! So this one was over 100 years old!!
Although an exception, cylinders with a useful life of greater than 50 years abound in their hundreds of thousands all around the world.
To keep them functioning both efficiently and safely, cylinders need a maintenance programme throughout their lives.
In most countries there are statutory requirements whereby a competent person must examine a gas cylinder at regular intervals, with the outcome recorded and the cylinder permanently marked accordingly.
This is known as the ‘Periodic Inspection and Testing’ (PI&T) of a gas cylinder and reputable standards, such as ISO 6406 and ISO 10461 for seamless steel and aluminium alloy cylinders respectively, are used worldwide.
However, due to the detailed requirements of such standards, the PI&T is performed infrequently. Hence, typically cylinders containing inert compressed gases such as argon and nitrogen are subjected to a PI&T at intervals of about ten years in most countries.
For other gases, for example HCL where there is a risk of corrosion, shorter intervals are used.
The intervals of five and ten years have been arrived at taking into account past safe experience, operational difficulties of despatching the cylinder from service to the test location (and hence also losing the cylinder from the fleet for a period of time) and the cost of the test.
Some decades ago, it was realised that such a time interval was far too long for safe operational control, and hence a series of standards was developed for ‘Inspection at Time of Filling’ (ITF).
A good example of the latter is ISO 24431, which covers virtually all cylinders on the market for volumes up to 150 litres, except those used for storing acetylene.
This article focuses on the ITF procedures needed to keep cylinders in safe operation, for the >50 year lifetime most operators wish to enjoy from their new cylinders.
Inspection at Time of Filling (ITF)
Typically most industrial gas cylinders are filled several times a year. Therefore in between the PI&T period, of say ten years, a cylinder would be filled 30-40 times.
Here lies the power of the ITF operation, as the owner can withdraw a cylinder from service and rectify it (or even scrap it, if it is unsuitable for further service), if a defect is observed in between the PI&T intervals.
Specific aspects have to be checked according to the standard prior to the cylinder having been filled, during the filling process and immediately after the filling. At all three steps vital information can be gathered on the condition of the total gas package, which includes not only the cylinder, but also the valve and all permanently fixed attachments.
But the pre-fill checks remain the most critical, as described below.
Before filling operations
The most important point to check prior to filling is to ensure that the cylinder is still within test – that it does not require a PI&T.
Various companies have different ways of establishing the due date for the PI&T, but most use a plastic ring which is trapped between the valve stem and the cylinder neck.
The ring is both colour-coded and shaped as per the guidelines given in Annex
G of ISO 6406, thus making the identification of the due date easy from a distance.
If the cylinder is still in test, the filler confirms that the working pressure limit of the cylinder is compatible with the filling rig and that the materials of construction of the shell/valve are compatible with the intended gas to be filled.
The latter is particularly important when filling embrittling gases such as hydrogen into steel cylinders and a number of gases, which are corrosive, when filled into aluminium alloy cylinders. Full details are in ISO 11114-1, which is currently undergoing a revision.
Next comes a very critical and difficult operation, whereby the filler has to decide on the internal condition of the cylinder.
‘Critical’ since, when most cylinders are responsible for an incident, it is because they have been internally corroded and ‘difficult’ since this part of the inspection needs to be performed without removing the valve!!
Here the filler needs to use all his knowledge and experience to establish the internal condition. Each company has its own set of guidelines/procedures to maintain the internals of cylinders.
These include residual pressure checks; check weighing; and moisture analysis for example. If any of these are breached then the cylinder must be isolated from the filling area.
The external condition must also be satisfactory before filling and here, use is made of the list of damage criteria including corrosion, heat damage and abuse as listed in ISO 6406/10461, as appropriate.
An example is where a previously heat damaged aluminium alloy cylinder was sufficiently softened, and when it was undetected by the pre-fill checks, it subsequently exploded on a filling line.
Different companies have developed various heat detection techniques such as the use of plastic tags, which soften when exposed to elevated temperatures.
Difficulties of external examination encountered with the interior cylinders in a pallet are overcome by ensuring that the filling company’s representatives load only serviceable cylinders into a pallet.
In particular, unauthorised repairs by customers, unwilling to pay for damaging a cylinder, need to be detected by competent staff.
It is at this stage of loading single cylinders into pallets, that all permanent attachments are also checked for further safe service. Hence, shrouds/foot rings on welded cylinders and neck rings on seamless cylinders must all be fit for service.
For liquefied cylinders, validity of the stamped tare weight must be verified with the actual weight of the cylinder.
Finally, and just as crucially, the integrity of the valve and its suitability for the intended gas service need to be assessed.
The valve needs to be free of contaminants (especially all traces of hydrocarbons if oxidising gas service is involved) and must be easy to operate.
The body of the valve must be undamaged, with the outlet threads of full form such that the filling adaptor fits satisfactorily. Any pressure relief device, if present, must also be undamaged.
Now we can fill
Once all the pre-fill checks have been satisfactorily completed, the cylinder may be filled.
As the filling commences, due to adiabatic compression, the cylinder gets warmer during the operation.
In the case of a liquefied cylinder a gradual weight increase signifies satisfactory filling. It is vital that all pressure gauges used during the fill cycle are accurate and correctly calibrated.
It is common practice for the gauges on the filling lines to be checked every week against a Master gauge, which must be calibrated against national standards at a yearly frequency.
Similarly, weighing scales must always be in calibration. A common source of incidents with liquefied gases, resulting in burst cylinders, is that they have been overfilled - this is particularly pertinent in warm climates, where a slight overfilling can easily result in a ruptured cylinder.
Ready for the customer
Once filled, the filler must ensure that the valve and the pressure relief device on the cylinder do not leak, with the valve in the closed position and disconnected from the filling connection.
For permanent gases the filler must also make sure that the filling pressure is consistent with the intended working pressure. Whilst for cylinders filled by weight, the weight must be checked immediately after disconnection from the filling line.
Finally, the cylinder must be checked to ensure that it is correctly identified and labelled. The above procedure is not an easy one to follow, but it must be respected at all levels of management within a gas filling operation.
It requires a high level of competence, an in-depth knowledge of a cylinder’s characteristics and sometimes, a piece of good fortune too.
Lack of attention to even a seemingly minor aspect could result in a serious accident, causing damage not only to the filling plant and its personnel, but also to the reputation of the gas company.