May-2011

Safety, quality and integrity of gas supply depends on equipment competency

Protecting the integrity of gas throughout the supply chain, from manufacture to end user, is an ongoing challenge, particularly as gases and gas mixtures become increasingly sophisticated to meet needs across the full spectrum of contemporary global industry.

Stephen Harrison, Linde Gas

Article Summary

Gas molecules are continually in motion, filling every corner of every space and being capable of flowing through minute gaps. Gases stored within a container or in a gas supply system are constantly seeking ways to escape, while at the same time other gases present in the atmosphere have the ability to migrate into the container or system. Unintended gas mixtures and contamination could adversely affect production processes and, at the extreme end of the scale, potentially damage the environment, cause safety risks and be harmful to human health.

Speciality gases applications are of particular concern. These are extremely sensitive, high purity gases, often intentionally containing very low levels of trace components — in some cases as low as parts per trillion — which are used in equipment calibration and gas analysis. These trace components are easily compromised by a reaction with contaminant gases.

In response, leading gas manufacturers have developed increasingly high-tech cylinders, valves, regulators and supply systems, developed from new-generation materials, to ensure that gas products remain pure and uncontaminated throughout their lifecycle. This mitigates threats to the environment and human life, avoiding the exposure of all parties to the high cost of integrity failure.

Moving gas in a contained way
When the intention is to deliver gas out of a cylinder, regulators are required to control pressure and valves to control flow rate. These are separate tasks and generally need multiple devices to achieve this successfully. Moving a gas or a gas mixture from a cylinder to the place where it is to be used requires a range of valves, regulators, piping and sometimes the inclusion of other gases and procedures, such as purge techniques, to achieve the correct result.

“We need to move gas in a contained way from supply point to point of use and be able to guarantee that there are no leak points that could allow the product to move out into the environment or allow atmospheric gases to get in,” says Arron Varne, Director of Equipment Manufacturing, Linde Electronics and Specialty Gases, North America. “We also need to control how quickly gas flows from the storage container to the point of use. The device used to achieve this is a valve.

“Coupled with this, regulators are required to control the pressure of the gas. We store our gas products at high pressure to optimally fill their containers. However, our customers generally use the product at much lower pressures and therefore regulators are required to reduce the pressure to the application.

Gas supply pyramid
Head of Specialty Gases and Specialty Equipment at Linde Gases, Stephen Harrison, says at the most basic tier of the gas supply pyramid are the industrial and technical gases used for applications such as welding and cutting in heavy industrial settings. The associated equipment is robust, high flow and tough because of the often physically demanding environment in which the equipment is used and also the high volumes of gas passing through it. In these applications, ball valves are often used for tight shut off and needle valves used for flow control.

Quality and product sophistication increases in the next tier, which is occupied by medical gases such as oxygen and nitrous oxide. This is a safety-sensitive application where the principal concerns are the materials of construction and the cleanliness of the equipment supplying it. In this environment, errors can be life-threatening, therefore valves and regulators are specially designed to eliminate the possibility of connect- ing the wrong gas cylinder to the supply regulator. Through using a combination of diameter, thread type and thread direction, CGA fittings used in North American make it virtually impossible to make the wrong connection. Pin index connectors used in Europe and other parts of the world produce the same result through using an identifying code consisting of a specific combination of holes in the face of the valve into which connecting pins for a particular type of gas must fit in perfect alignment.

Food and pharmaceutical applications fall into the next tier — clean environments where gases and associated equipment are of a high quality and batch traceability is required. Valves and regulators are manufactured from materials that have no potential to harm the person consuming the end product. In the US, Generally Recognised as Safe (GRAS) is the American Food and Drug Administration (FDA) designation often used to specify equipment requirements for food and pharmaceutical applications.

Legislation impacting equipment delivering gases to food applications have also recently come into play in Europe, which has had far reaching implications for the food processing industry. These are stringent regulations governing materials used in food processing. An example is Regulation No 1935/2004, which is primarily intended to regulate the use of primary packaging of foods and to provide a legal frame work to ensure that only safe materials are used as wrapping and other packaging materials that come directly in contact with food. Current European food legislation defines food gases as food. Therefore, storage tanks, cylinders and valves, as well as gas supply systems and food processing equipment such as freezers, would fall under the requirements of this regulation.

The next tier in the gas supply pyramid is the area of speciality gases — extremely high-purity gases that can also be toxic or corrosive. Storing, handling and using these gases correctly is critical, so there is a huge focus on the leak integrity of supply systems. In these applications, diaphragm valves are considered best in class as process flow regulation devices, because they employ a flexible diaphragm to effectively control a process flow line either partially or completely as needed in order to regulate the transport of the gas stream and eliminate the possibility of contamination. Leak-free connections between these valves and other piping components are often achieved using national pipe tapered (NPT) thread.