Thousands of man hours have been spent at various forums trying to agree how gas cylinders and their contents may be identified.

Even when a national consensus has been reached, extending this to a regional or international level then presents additional obstacles.

The main reason for all this conflict, is that this characteristic of gas cylinders is taken out of the hands of technical experts and ‘Joe Public’ has then been able to contribute to the debate. So what are these identification features?

1. Identification methods
These are three primary aides to identify gas cylinders and their contents:–

1.1 Stamp Marking
1.2 Labelling
1.3 Colour Coding

Whilst 1.1 and 1.2 are mandatory requirements, 1.3 remains either voluntary or even non-existent in some countries.

However, the external surface of cylinders is used in varying ways by different gas companies to promote the product, or to inform the public of other useful information such as the next due test date (often a plastic ring trapped between the valve and the cylinder neck), the cylinder’s tare weight (especially for LPG cylinders which is stencilled on to the shoulder area), or a label advising customers to avoid getting oil near the cylinder (in the case of oxidising gases).

1.1 Stamp Marking
This is essentially the way by which the empty shell of the gas cylinder is identified. The majority of the information is hard stamped, or engraved, onto a gas cylinder by the cylinder’s manufacturer.

Aspects such as the cylinder’s serial number, the standard to which it is constructed, it’s internal volume/empty weight/pressure ratings, the inspection body, date of manufacture, the manufacturer and the latter’s country, and valve thread are some of the important data which uniquely identify a gas cylinder, are present.

A further feature of the stamp marking procedure which needs a mention, is a special mark ‘H’ required for steel cylinders. When present it signifies that the cylinder’s mechanical strength is compatible for it being used in hydrogen (embrittling gas) service. In other words, the actual tensile strength of the steel is less than 950 MPa.

A full list of the mandatory and optional stamp markings, which are usually found on the thickened shoulder area of seamless cylinders, can be seen in an international standard,
EN ISO 13769 (1). A typical example is shown
in Fig.1.

In the latter standard, not only are the required marks stipulated but so also are their required locations, so that it is an easy matter to decipher the information. This is in contrast to the former status quo whereby each manufacturer had his own style of stamping the information onto the cylinder.

However, after the cylinder has been put into service, it is the owner’s responsibility to periodically inspect and test a gas cylinder and stamp it in a given area on the shoulder with the date of the test (month/year) and the mark of the retest station - provided the test is satisfactory.

1.2. Labelling
This is the only reliable method and often a legal requirement, whereby the contents of the cylinders are recognised.

Sometimes known as ‘Precautionary Labels’, International transport agreements such as ADR require suppliers of gases to fix a label to the shoulder of the cylinder.

Often banana shaped, the label contains vital safety information such as the shipping name of the gas together with its UN number, the pressure within at a reference temperature, the owner’s name and a telephone number by which they can be contacted in case of an emergency, and nature of the hazard the gas may present in the form of safety risk phrases.

It is the duty of the filler to ensure that the appropriate label is present at the time of filling. Full details here can be found in international standard ISO 7225 (2).

One feature of labelling which needs to be carefully monitored is the nature of the adhesive used. Since gas cylinders labelling is a mandatory aspect of many nations, companies attempt to ensure that the labels do not detach from the cylinder’s surface by using a very strong adhesive.

However, some adhesives chemically attack the cylinder’s surface. This is true for halogen containing adhesives when applied to aluminium alloy cylinders, where on occasions a ‘step’ been found at the label’s interface with the cylinder, signifying that some corrosion had taken place!!

1.3 Colour Coding
This form of identification of a cylinder’s content, on surface, would appear to be the most direct, but it is also the most controversial.

For example, during the early 21st century a group of experts spent several years attempting to develop an international standard for colour coding.

Although the final draft gained the support of the majority of the committee (ISO TC 58 which deals with gas cylinders), and it was intended to be only an informative standard with no legal standing, the standard was blocked by some nations on the grounds that such a standard is not of ‘global relevance’.

The truth of the matter is that the basis of such an action relies on the fact that in some industrial countries, individual companies use different colour codes as a means of asset control and not for the cylinder’s contents.

But for medical gases there is far less controversy and an international standard ISO 32 has been respected by many countries for several decades.

In Europe, a voluntary standard EN 1089-3 (3) has been widely used for almost a decade. The key here is to identify by means of a colour code, one of four major hazards which the contents of the cylinder presents.

These hazards are whether the contents are oxidising, inert, flammable or toxic, which are represented by white, green, red or yellow colours respectively painted on the shoulder section of the cylinder. The exact shade of the colour is specified to prevent any confusion using the RAL system (4).

When a gas or mixture has two hazard properties, then the cylinder shoulder shall be coloured in accordance with the primary hazard. The colour of the secondary hazard can also be applied to the cylinder shoulder.

Additionally, whilst it is possible to cover most of the commonly used gases with a further unique colour to the body of the cylinder, it is impossible to have a unique combination for the many thousands of gases and their mixtures which are in current use. For this latter reason alone, colour coding must remain as a guide and only the precautionary/hazard label must be sued to establish the contents.

3. Concluding observations
When cylinders are found in derelict areas, often after many years of without being used, great care is taken to ensure that the contents are correctly identified.

Often the label has disappeared and there may not be any paint left on the externals. In such cases the operative uses a variety of other methods such as the type of cylinder present (seamless? welded? composite?); the valve outlet’s specification, since often gases are grouped by their physical and chemical properties and a valve outlet is assigned.

If there is still doubt then the cylinder is safely transported in a ‘coffin arrangement’ to a suitable site for disposal.

It cannot be stressed enough that it is vital to read the cylinder’s label to be able to make a positive identification of its content – cylinder colour acts merely as a guide and should never be used in isolation to determine cylinder content.

As an overall rule, a cylinder without a label should not be used but returned to the supplier.

Some interesting implications
Since labels are unidirectional and not easily legible at a distance, in some cylinder applications colour coding, either on its own or coupled with stencilling of the contents, is still a most useful indicator used by the gases industry.

For example, in Europe the emergency services such as the fire brigades assume that a maroon cylinder will contain acetylene (see Fig. 2), whilst the off-shore drilling industry relies on a brown cylinder with ‘Heliox’ stencilling for it’s divers.

Reasons why colour coding, even in countries who have embraced EN 1089-3, should not be relied upon other than as a guide include:

* Some paints fade with the effect of ultraviolet light and colours change.

* Some owners paint over previous surfaces in order to avoid excessive shotblasting and if the new paint does not adhere correctly, then the old paint shows through, causing confusion. This is particularly for ageing aluminium alloy cylinders which cannot be routinely shotblasted and require rather sophisticated paint removal techniques.

* Some paints peel off with the passage of time which also makes identification ambiguous.

* Over the past couple of decades or so there has been an amazing growth of the use of fully-wrapped composite cylinders. The latter cannot be hard stamped but they will always bear a manufacturer’s label which is trapped on the body of the cylinder just below the surface of the composite overwrap. The user related label is then stuck onto the composite in the usual way.

For acetylene cylinders it is becoming increasingly important to identify the contents of the porous substance which may
contain asbestos. Here again, the hard stamp marking is invaluable.