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By Jeffrey S. Close
When processes change and manufacturing plants need to install any of several types of ultra-high-purity process gases, itís the plant engineer who will be responsible for specifying an appropriate piping system. A more critical aspect of ultra-high-purity gas piping systems is the way gases are delivered to the process tools.
No two specialty gas suppliers use the same language to identify gas purity or specifications. Common ground is found in reference to the term ultra-high-purity , or UHP, gases. Ultra-high-purity gases have a minimum purity of 99.999% or better. This is often referred to as ďfive ninesĒ pure. This purity level is determined by subtracting the sum of the concentration of possible trace contaminants from 100%. Therefore, a minimum purity level of 99.999% is equivalent to a maximum contaminant level of 10 parts per million by volume.
Compressed gases typically found in a manufacturing facility are classified as oxidizers, inert or flammable. Oxidizers arenít inherently flammable, but will contribute to combustion as an oxidant. Examples of oxidizers include air, chlorine, fluorine, nitric oxide and oxygen. Inert gases donít take part in combustion processes nor do they react with other materials. An inert gas introduced into a room or confined space reduces the amount of oxygen and limits a combustion process such as a fire. Inert gases are commonly used in extinguishing systems when itís important to avoid water damage. Examples of inert gases are argon, carbon dioxide, helium, nitrogen and xenon.
Flammable gases, together with air or oxygen in the right concentration, burn or explode if ignited. Examples of flammable gases include ammonia, ethylene, hydrogen, methane and silane.
The three most common ways to supply ultra-high-purity gases to a facility are onsite gas production plants, bulk delivery from onsite storage tanks, and individual gas cylinders. Select the most reliable and cost-effective gas supply system by evaluating plant-specific factors such as onsite production capability, size and number of components in the design, number of connections to the equipment, and how much product needs to be stored for backup supply.
Onsite gas production plants typically are installed if you need large quantities of nitrogen, hydrogen or oxygen. The technologies these on-site plants use include membrane, cryogenic systems and pressure-swing adsorption systems. Each can provide an ultra-high-purity product at a much lower cost than other available delivery methods. Advantages to onsite gas production are the convenience of not having to place orders or wait for deliveries, and having no containers, cylinders or liquid gas to handle. Factors to consider when selecting an onsite gas production plant are the real estate required to install gas generating equipment and the additional piping required to distribute gas to the facility.
Bulk delivery to onsite storage tanks is the preferred source of gas for many facilities that require large quantities of liquid argon, carbon dioxide, hydrogen, nitrogen, oxygen and several semiconductor process gases. Tanker trucks transport these high-purity gases from the gas companyís production plants to bulk storage tanks located at the manufacturing facility site. A variety of storage tank sizes are available to suit individual customerís needs. These storage tanks can be purchased outright or rented through a gas distribution company. An advantage to this gas delivery method is the ability to have large quantities of the gases used in high volume. Items to consider when selecting a bulk gas storage system include flow rate, pressure, usage pattern and site location.
Individual gas cylinders commonly are used when gas volume requirements are small. If the consumption rate increases, multiple cylinders can be manifolded together in banks to provide a greater source of supply and to reduce the amount of cylinder handling. These cylinders must be constructed to withstand high pressure and they must conform to specific U.S. Department of Transportation regulations. An advantage to this gas delivery method is the ability to have an unlimited quantity and variety of gases on site for different process requirements. Storage is a major factor to consider when multiple gas cylinders are onsite. Depending on the types and quantities of gases being stored, several separate gas rooms may be required.
Producing and purifying a gas to advanced specifications is of no value unless it can be delivered to the point of use without introducing contaminants. Your system design must avoid sources of direct contamination and minimize sources of back-contamination and cross-contamination. System installation must follow rigorous specifications and procedures, including onsite inspection and additional cleaning.
The common piping material of choice within the industry is AISI 316L stainless steel tubing for the majority of gas delivery applications. Lines either can be single-wall or double-wall, depending on the hazard level of the gas involved. Gas delivery piping and manufacturing equipment is machined to a very smooth finish, electropolished and welded. The supplier carefully cleans components and tubing before assembling them into systems and equipment to remove contaminants from surfaces that are expected to come into contact with the process gas.
The supplier delivers piping connections and accessories in sealed bags to prevent contaminating them after cleaning. The fittings in systems and equipment are assembled either with metal-to-metal seals or by orbital welding. Metal-to-metal seals provide a leak-tight seal for services from vacuum to positive pressure. Orbital welding is performed with a power supply, orbital welding heads and fixture blocks. Together these components comprise a welding system that can join of pieces of stainless steel piping together precisely to eliminate mechanical seals or joints.
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