The final instalment in this series, Dr Roy Irani explores the compatibility of gases in specific cylinder packages and the plethora of associated safety concerns.
This is the last part in the current series dealing with gases in the industrial world. The first three parts dealt with three specific gases of particular relevance in the 21st Century.
This article attempts to cover general concepts surrounding the compatibility of gases when contained in particular types of cylinder packages.
Within a cylinder package one has to consider not only the gas cylinder but also the valving system whose internals would be ‘wetted’ by the contained gas.
Since valving systems consist of seals, and diaphragms, non-metallic materials need to be considered in addition to the metallic alloys used in our industry, for manufacturing the cylinders’ shells and valve bodies.
Additionally, some types of cylinders are manufactured also using non-metallics such as Type 4 composites, (see gasworld, August 2007) which have either a plastic liner with a fibre-reinforced plastic layer or those which are even linerless.
In the latter the gas is in direct contact with the composite material, though such cylinders are currently limited to designs with a low test pressure, capable of retaining hydrocarbon gases such as propane/butane.
Concerning metallic alloys, there is one primary source for the gases industry for confirming a specific gas (gas mixture)/cylinder package compatibility and that is in the standard EN ISO 11114-1.
Here you will find a large number of the commonly used gases whose compatibility is ranked against various alloys used for the manufacture of cylinders and valves. Steels (low carbon / C-Mn / Cr-Mo and stainless varieties) aluminium alloys and brasses are the listed alloys.
In virtually all cases gases are prone to attack alloys much more readily if the gases are “wet”. But when is a gas considered to be ‘wet’? Different standards and national regulations have slightly differing definitions of ‘wet’.
Until recently in the UK, a dew point of -46° for a gas at one atmosphere was considered to be dry. However, in EN ISO 11114-1 a gas is “dry” if there is no free moisture at one atmosphere. This is a somewhat subjective definition and a source of concern/error.
Furthermore, since a standard is written by a group of experts, each with their own views, in some subjective standards some of the conclusions are a ‘low common denominator’.
EN ISO 11114-1 is no exception in this respect.
So while it is chemically proven that halogen gases such as fluorine and chlorine, even with traces of moisture, are incompatible with aluminium alloy (AA) cylinders (a fact captured by EN ISO 11114-1), the latter does not tell you whether extremely dilute concentrations (say at the ppmv levels) of such gases may be filled in AA cylinders.
The need for such low concentrations of halogens is a requirement for some electronics applications and only AA cylinders with their smooth internals are capable of retaining such compositions in a homogeneous manner.
Hence considerable R & D needs to be undertaken to determine the reaction (if any) of a particular gas concentration (and its moisture content) with the cylinder shell. Such commercial information, once collated, is closely guarded by gas companies and not revealed in documents available to the public at large - and certainly is not included in EN ISO 11114-1.
Another major controversy has been around the filling of carbon monoxide (CO) and its mixtures, in cylinders. These are popular gas mixtures used for powering modern day lasers.
Filling these mixtures in AA cylinders is totally safe, even in the presence of moisture, due to the protection offered by the thin layer of alumina which forms a tenacious skin on the AA cylinder’s surface. But the same is not the case if CO is filled in steel cylinders.
If even a few ppmv (say 5ppmv) of moisture and CO2 are present in the CO, a dilute acid forms which then causes stress corrosion cracks (SCC) in the cylinder. This is a complex reaction and is a function not only of the gas contents but also the stress in the cylinder’s walls i.e. a function (steel strength/gas filling pressure/cylinder’s test pressure).
An excellent document (95/07) on this subject is published by the European Industrial Gases Association (EIGA), and must be consulted if catastrophic failures experienced in this area in the past are to be avoided.
However, if parameters are meticulously controlled, then trailers each with hundreds of high pressure Cr-Mo steel cylinders at 200 bar of CO are successfully operated on a daily basis. Here accurate pre-fill checks are the order of the day, with analysis of the gas(es) to be charged into the cylinder.
Finally positive pressure, non-return valves are fitted to the whole assembly, to ensure that the internals of the cylinders do not suffer ingress of moisture from the atmosphere or a customer’s feedback.
A final chestnut which keeps cropping up is the compatibility of oxygen in AA cylinders. It is known that if aluminium alloys are set on fire they will burn and the result is an exothermic reaction. However, the tenacious oxide film referred to above makes the actual burning phase difficult by delaying the ignition process.
So if the ignition process cannot occur then the flame propagation is totally irrelevant.
Testament to this understanding in the past decade has meant that the ‘oxygen debate’ voiced primarily in the US, during the 1975-1985 era, has been suitably resolved, by the presence of around 25 million AA cylinders in oxidising gas service, primarily medical oxygen.
This controversy did not occur in Europe, where AA cylinders were used in services such as medical oxygen for over 50 years.
A mention must also be made of valving materials which are invariably made using a brass.
This alloy also suffers from SCC and must be carefully machined to avoid any stress raisers in the valve’s design and with appropriate heat treatment, to avoid any unnecessary stresses in the component during its service. Particular substances, sometimes used for leak detection, must be carefully screened to avoid any ammonical contact with the brass.
Also, acid forming gases such as CO2 have in the past shown a tendency to exhibit SCC with brasses.
With all the caveats in mind from EN ISO 11114-1 mentioned above, the same apply to EN ISO 11114-2, which deals with non-metallic materials’ compatibility issues. Here of primary interest is the role of hydrocarbons/lubricants used in our industry.
To back up the objectivity of this standard, a third standard in the series dealing with test methods to determine a material’s oxygen compatibility, EN ISO 11114-3 has been published.
An important non-metallic which is in contact with the gas, is the sealing tape used on the valving system. Made from PTFE, the tape is contact with a lubricant during the drawing process to its final form.
Such tapes are prototype tested using EN ISO 11114-3 prior to issue for use. For many decades some companies preferred the use of a lead capsule as the thread lubricant rather than PTFE, to avoid all ignition related problems.
Similarly all ‘O’ ring seals, as found in parallel threaded valves, are also prototype tested using this standard.
Apart from oxygen compatibility, seals may also suffer from a swelling effect. Some seal/gas combinations, once pressurised, expand in their dimensions by the action of the gas dissolving within.
However, the seals are unable to contract when depressurisation takes place and have known to explode. Here too, EN ISO 11114-2 can be a useful guide.
Non-metallics, in some cases have released toxic fumes when alighted, which have asphyxiated patients. This hazard also must be excluded in the rare event when an ignition takes place.
There is no doubt that the obvious sources of incidents have been minimised in recent years by the use of EN ISO 11114 series. For example, no longer are ‘green clouds of chlorine’ seen to be escaping from AA cylinders nor steel cylinders with CO fragmenting in a catastrophic way.
But an AA cylinder containing a dilute ethyl chloride mixture did explode last year, due to the lack of total objectivity within EN ISO 11114-1 and the subsequent misinterpretation.
Readers are once again reminded to always conduct their compatibility research under closely controlled conditions for a particular gas or gas mixture and only sell such a product to the public at large, after extensive trials.
If the original cylinder shell material in contact with the gas proves to be incompatible, other inert shells may be used such as stainless steel, or the internals of the original shell coated with a suitable inert layer.
A look ahead to 2009!
Dr Roy Irani will return with more esteemed insights for gasworld readers in 2009, as he explores further cylinder-centric issues.
These topics will include Cylinder Pressures, Cylinder Identification, Valves, Cylinder Ancillaries, and much more.