A seamless gas cylinder usually has a lifetime extending several decades. If only that were also true for the ‘bit at the top’; an integral component of a gas cylinder package is its valving mechanism.
Even though it has been around for over 130 years, the reliability of a valve is still under constant scrutiny, since rarely does a valve last more than the periodic inspection test interval assigned to a cylinder. Until recently the primary reason for this poor performance has been the lack of attention paid to this valuable widget within the Gases Industry.
In recent years however, due to pressure from the Gases Industry and its customers, much effort has been devoted to increase the safety aspects and to develop specialist valves for demanding applications such as automotive fuel tanks, the electronics industry and medical applications. Bringing down costs will never be off the Valve Industry’s radar.
Where we were yesterday
In the late 19th Century, gas cylinder valves were very unsophisticated in their construction, consisting of a few parts (none of which were moving), often using waxed leather or felt for its gland seals, and had metal-to-metal seats.
But they did enjoy the benefits of a brass technology that had evolved during that period. Also, in those days, safety considerations were not a primary driver as they are today within the Gases Industry, so the industry tolerated leakages from the valve.
With working pressures in cylinders at around 124 bar (1800 psi) at the turn of the 20th Century, once again there was a lower risk factor than current operations. Though such simple valves are still manufactured today, where cost and ease of maintenance are the main considerations, a minor revolution has occurred within this industry ensuring current needs are met.
Where we are today
In the last couple of decades, valve manufacturers have been forced to improve their products for a number of reasons.
** In many parts of the world 200 bar (even 300 bar in some countries) is now a common working pressure of the cylinders.
** Productivity at filling sites is much higher today, with cylinders being filled in a few minutes, rather than the slow fills extending hours, as was the case in the past.
** Ergonomic (Repetitive Strain Injury, RSI) considerations, such as low valve closing and opening torques, are now relevant.
** Customers are striving for longevity of products, rather than treating valves as a consumable.
** High gas quality is essential for some specialist applications.
** Demanding Authorities expect greater safety and reliability
** Not forgetting LITIGATON!
Many of the above requirements have been possible by the ease of availability of non-metallic materials, particularly elastomers, which form the backbone of many current day gland seals. Also, the single key operated spindle of the past has been replaced by a two-part spindle arrangement fitted with a hand-wheel and a valve seat made using an elastomer.
The provision of a hand-wheel and a soft seat means that a low torque is required to shut off the valve. Hence valves can be operated more quickly with the added benefit that the valve seat is not destroyed and survives longer because of this low torque.
Furthermore, some valves have built-in pressure relief devices (mandatory for some applications e.g. liquefied gases (CO2) and compressed natural gas (CNG), while others have a residual pressure device.
The latter, when fitted is known as a Residual Pressure Valve (RPV), and is particularly valuable when it is necessary to ensure that the internals of cylinders are kept dry, since the device operates by shutting off the cylinder from the external environment when the pressure within the cylinder drops below a predetermined pressure (usually about 4 bar).
An RPV provides the Industrial Gases Industry with many benefits, but one that can be readily realised, is that since the internals of the cylinder are kept dry, the periodic interval for testing a cylinder may be extended. Currently, over 90% of all cylinders tested are rejected for their internal corrosion, and 30% of all cylinder incidents reported are also due to internal corrosion.
This ensures significant savings as a result of an increased cylinder retest frequency. Cost savings apart, reliable RPVs will ensure that modern, efficient (lightweight) cylinders, made from high strength steels, which are susceptible to a high rate of corrosion, will also enjoy longevity without risk of any incidents.
One, albeit essential, way to ensure that the RPV is functioning, is to depress the close-off cassette with a special adaptor, each time the cylinder comes in for a refilling. Presence of gas indicates a satisfactorily functioning valve. Posts filling the usual checks are made on the valve’s connections using a suitable leak detection fluid.
Care must be taken to ensure that such fluids do not contain any ammonia, since even parts per million of ammonia in contact with the valve’s brass, will initiate stress corrosion cracking in the brass body.
Indeed, the reliability of valves generally has increased over the past couple of decades, aided by the formulation, publication and subsequent adoption into Law, of stringent International Standards. The latter, coupled with advanced engineering practices and the use of robotics, which helps to engineer out human error, are all key factors in increasing the image of this technology.
Today, 100% of the valves have to be tested via a variety of procedures during their production cycle, while a demanding prototype testing regime precedes their mass production. In some parts of the world the safety of valves is taken very seriously and they have to be hard stamped with an identifying mark (e.g. in the EU with a ‘π’ mark) to confirm that they meet all essential safety requirements.
These prototype tests not only ensure that the design and manufacturing of the valve are robust, but also seek to establish that the valve’s anticipated operating conditions over its lifetime are also met.
The advent of new/different gas applications has meant that at all times, all materials coming into contact with the gas filled in the cylinder must be both chemically and physically compatible. Stainless steel and even more sophisticated alloy are now used for some applications.
To help the valve manufacturer a number of International Standards exist for this aspect. Notable is the ISO 11114 series, where Part 1 concentrates on metallic materials, while Part 2 is for non-metallics.
An important topic related to the stability of materials in a high-pressure oxidising gas stream has resulted in ISO 11114-3. Oxidising gas compatibility apart, it is essential that in a medical breathing application, IF, despite all the testing an ignition does occur, products from the combustion must not be toxic.
Where will we be in the future?
Modern valves have many moving parts and with this there is a greater risk of unreliability, which must be eliminated. With gas cylinder customers expecting a greater ease of use of the package, cylinder valves with devices such as an integral pressure regulator (VIPR) and a contents gauge will become the industry’s norm.
Coupled with all these built-in features will be the hunt for miniaturisation. The electronics industry will continue to strive for near zero particle emission, which will be met via meticulous internal cleaning, such as by employing electropolishing techniques.
Pressure considerations apart (850 bar working pressure is not uncommon now in the automotive fuel application Industry), containment of toxic, pyrophoric, corrosive and flammable gases is very much a safety related issue, and a serious escape of gas from the cylinder package can prove expensive.
Hence, future focus will be to manufacture cylinder valves with utmost safety and reliability, so that their life matches at least the retest interval of the cylinder to which they are fitted. Such is the optimism in this area that already International Standards exist for the refurbishment of valves.
One thing’s for sure, whatever happens, manufacturers will always be expected to produce cylinder valves at ever decreasing prices.