The main challenge is to reduce the reactivity of certain gases with alloys in the metal container, reactions to the metal interior surfaces in a pressurised environment, and to reduce the possibility of moisture retention.
Gas stability within a cylinder means that the composition of a gas or gas mixture will remain within a defined and acceptable limit for a designated period. There are many factors that can affect the gas stability in a cylinder, these include reactions occurring within the gases themselves, pressure, the length of storage time, temperature variations and moisture.
Recently maintaining stability has become even more difficult with the introduction of more complex, multi-component gas mixtures which could be certified to purity levels of 99.999% or higher. It is not uncommon for highly specialised gas mixtures to contain 60 or more components.
Aluminium alloy cylinders in specialty gas services
Aluminium alloy cylinders were first developed in Germany in the 1930s. In France in 1941, Société Métallurgique de Gerzat (SMG, now Luxfer France) used a hot extrusion method to produce the world’s first commercially available seamless high-pressure aluminium alloy cylinder, and Luxfer Gas Cylinders in England made the first commercially available, cold extruded seamless higher-pressure aluminium alloy cylinders in 1958.
As aluminium alloy cylinders are typically much lighter in weight than conventional steel cylinders, they became more popular for portable gas containment. It was also found that they provided better gas stability than steel cylinders for a variety of reactive gases. In 1973, Airco worked with Luxfer Gas Cylinders at its Riverside facility in California to produce the first cylinders that would be subjected to Airco’s proprietary Spectra-Seal process that the company described as an anodising process on the cylinder interior.
This and another process reportedly eliminated pinhole coating porosity and shielded the wall from reactive gases. BOC acquired Airco in 1978, and Linde then acquired BOC in 2006. Linde still markets the Spectra-Seal process.
Source: Luxfer Gas Cylinders
In the late 1970s another US-based company, Scott Speciality Gases (acquired by Air Liquide in 2007), introduced its own propriety aluminium treatment called Aculife which was also applied to cylinders manufactured by Luxfer. “Aculife is a chemical vapour deposition process that eliminates gas to cylinder surface interactions to provide guaranteed stability,” explains the Air Liquide website. There are many industrial applications that use specialty gas applications utilising aluminium alloy cylinders. These include electronics manufacturing, including semiconductor production that requires a high purity nitrogen and arsine, which is a highly toxic and flammable gas.
Aluminium alloy cylinders are also used in medical markets. For example, these specialty gases can be used for biotechnology and biopharmacology, including research and laboratory gases as well as high purity therapeutic gases. Other applications include chemical and refining processes industries; and environmental monitoring using calibration gases which are used to calibrate sensors and instruments to warn personnel of dangerous levels of gases which may not be detected without these monitoring devices.
Commonly used calibration gases include hydrogen sulfide, hydrogen chloride, hydrogen cyanide, sulfur dioxide, chlorine, nitric oxide, nitrogen dioxide, phosgene, ammonia and carbonyl sulfide. To meet strict environmental protocols, the user is required to calibrate sensors on a regular basis which requires the device to be exposed to a controlled concentration of a target gas to verify that the device identified the gas correctly. To work correctly, the test gases must have properties that are known and stable. If the test gas becomes unstable in the cylinder, then the results could give false readings leading to dangerous conditions.
“To enhance gas stability and shelf life, gas companies have developed techniques to minimise the potential potency of local active sites for catalytic surface in aluminium cylinders before filling with specialty gases”
Increasing specialty gas stability
To enhance gas stability and shelf life, gas companies have developed various techniques to minimise the potential potency of any local active sites for catalytic surface in aluminium cylinders before filling them with speciality gases. Some common techniques are outlined in EIGA 161/16 paragraph 11. It advises:
“The cylinder shall be dried by suitable heating and by evacuation or other equivalent methods. The moisture in the cylinder shall be minimised and the concentration established by analysis or other means.”
This can be achieved by:
- Introducing an inert dry gas into the cylinder at approximately 10 barg
- Leaving the gas to equilibrate with the cylinder over several hours
- Measuring the concentration of moisture present, and
- Repeating the process until the required level of dryness is achieved.
The final moisture concentration shall be compatible with the mixture to be produced and shall be equal or less than 10 ppm (parts-per-million). A residual pressure valve (RPV) – non-return valve (NRV) cylinder valve should be fitted in order to avoid back flow and exposure of the internal cylinder walls to air during the use of the mixture.
All cylinder preparations mentioned above involve treating the interior of a cylinder post manufacture. Using more than 40 years of collaboration with gas companies, Luxfer Gas Cylinders has concentrated on improving the cylinder manufacturing process itself to create a superior cylinder internal surface that can increase the efficiency and effectiveness of post manufacturing techniques to improve gas stability.
At Luxfer’s factory in Nottingham, England, aluminium alloy gas cylinders for speciality gas applications use L6X, its own version of AA6061 formulated by Luxfer metallurgists to provide optimal durability, fracture toughness and improved tearing resistance.
Luxfer Nottingham uses a proprietary manufacturing process that is able to generate a superior surface finish specifically intended for demanding specialty gas service. This manufacturing route includes specialised procedures and more rigorous controls. The result is a cylinder with Luxfer’s SGS™ (Superior Gas Stability), a high performance internal surface.
Any gas cylinder is pretty much unusable if it cannot be transported. The idea of a pressurised gas container to compress a large volume of gas into a relatively small package is an accepted norm with potentially millions of cylinders in service at customer premises worldwide. Transport compliance is therefore critical to the usability of a gas cylinder.
Different economic regions adopt varying compliance rules for the transport of compressed gas cylinders, considering not only classification groups (UN numbers) for the gas content, but also requirements for the cylinders themselves.
For UN Member States, the guiding principle is found within the UN Model Regulations which attempts to normalise requirements for the classification and transport of dangerous goods. However, different regions of the world will adopt different versions of the UN Model Regulations as their guide and hence local Dangerous Goods Transport regulations can vary region to region. In Europe, it is enshrined with ADR/RID/ADN. In the US, DOT rules apply and Australia has its own ADG rules. Luxfer Gas Cylinders aim to bridge these regional variations and offer a worldwide solution.
Phil Edwards of Vonix Limited, previously Scientific and Technical Gases (StG) operations director, said, “The development of the superior internal finish for small calibration gas cylinders enabled improvement to processes that provide excellent gas stability of reactive gas mixtures.”
“Having a cylinder that removes 75% of the variables known to contribute to stability failures assured a class-leading calibration gas was available to market.”