Selecting and Implementing the Best Gas Delivery System
By: John Henderson, Director - Product Development, Global Gas Equipment
Technical Consultant: Stuart Morgan, site services supervisor, Johnson Matthey Technology Center
For applications that require high purity gases, it is necessary to ensure the delivery system of that gas does not compromise its purity.
For research, development and industrial processing applications using high purity gas, the price of failure is high, so it’s imperative to carefully consider all equipment and components when building a gas delivery system – from the regulators and manifolds, through to the line, station and point-of-use regulation.
Applications that commonly use high purity gas and instrumentation (HP&I) for research and processing may include:
- Automotive applications, such as developing catalysts to control harmful exhaust emissions, fuel cell technology for clean energy, glass obscuration enamels and silver conductive pastes used in car defogging systems
- Pharmaceutical applications to develop active ingredients in medications and other treatments
- Medical applications to develop plastics and metals for surgical devices
- Petrochemical applications for oil and gas processing, production and refining
- Manufacturing of silicon wafers and LEDs
- Gas chromatography applications
The Price of Failure
Many gases can be expensive, sometimes running up to hundreds of dollars per cylinder. Laboratory and process procedures themselves can have a high cost, including labor, other ingredients used and an urgency to develop the desired result quickly. If high purity gases pick up contaminants during delivery, analytical results will vary, often requiring multiple tests to look for consistency. If a laboratory or process facility spends a lot of money on pure gases but doesn’t ensure the purity of the gas delivery is maintained, there is no guaranty of accurate results.
Accuracy and control are especially important in testing environments. A technician may need to stop and take time to identify the source of an impurity, which causes unnecessary downtime.
The ultimate goal is to perform testing as efficiently as possible and produce the optimum yield of the desired end product.
To select the right components when build a high purity gas delivery system, there are several questions to answer first:
- What gas is being used?
- Are there multiple gases involved?
- What is the gas purity? (see sidebar article on purity grading)
- What are the pressures of the cylinder and the process line?
- Are specialty cylinder connections required?
Once this information is understood, it will be easier to match the gas delivery components to the application requirements. Regulations and Standards from the Compressed Gas Association in the United States and Canada and the British Compressed Gas Association (BCGA) in the United Kingdom provide various codes and guidelines on how to distribute different gases and how gas delivery equipment should be used and maintained. These guidelines may indicate which components are recommended, optional or essential to each system or assembly.
The quality of the gas or reactivity depends on the material used in the construction of regulators. A Poor surface finish inside a regulator can cause particles to get caught in the gas stream, so high purity regulators must be composed of material with a very smooth surface finish. Barstock is metal that has been rolled or extruded into long continuous strips. The metal is stretched into finer grains, where impurities are minimized, and a consistent surface finish is achieved. This traps fewer contaminants, making barstock regulators the preferred choice for high purity applications.
In contrast, forging bodies cast in a mold create grain patterns that are not particularly smooth. In addition, forged bodies have a larger internal cavity that can trap contaminants, making forging a poor method for making high-purity regulators.
Diaphragm materials such as elastomer can act as a source of impurity by absorbing moisture and other contaminants, releasing these contaminants into the gas. Stainless steel diaphragms and PTFE seals do not absorb contaminants, making them ideal for high purity regulators. Elastomeric seals can corrode over time if the correct one is not selected, degrading the integrity of a regulators seal. Higher end regulators use stainless steel diaphragms and PTFE seals that do not degrade as quickly.
There are four main types of regulators ideal for process and high purity applications, each differing in their composition:
- Brass barstock regulators are intended for non-corrosive, technical grade gases up to grade 4.5 purity (99.995%). They feature a nylon seat, high-strength alloy bonnet, and neoprene diaphragm.
- Nickel plated brass barstock regulators are for non-corrosive, high-purity gases. They feature high-strength alloy bonnets, or nickel plated brass barstock bonnets, with stainless steel diaphragms and PTFE seals.
- For gas purity grades up to 5.0 (99.999%), nylon seats and high-strength alloy bonnets are used.
- For gas purity grade up to 6.0 (99.9999%), PCTFE seats and nickel plated brass barstock bonnets are used, offering a greater level of purity due to the higher compressive strength and gas tightness, reducing the both outboard and inboard leakage (backscatter).
- Stainless steel barstock regulators are ideal for corrosive, toxic and high purity gases up to grade 6.0 purity (99.9999%). They feature PCTFE seats, nickel plated brass bonnet, stainless steel diaphragm and PTFE seals.
Single stage, two stage, and line versions of each of these regulators are available. Single stage regulators are for applications where slight pressure variance is acceptable (as cylinder pressure decreases). Two-stage regulators are for applications where constant pressure is required.
When multiple gases are used, a manifold is required. Often this is the case when multiple gases are mixed or when banks of cylinders are set as duty and others set as reserve. Auto-change manifolds offer an uninterrupted gas supply with an alarm option to indicate when a reserve cylinder has automatically taken over. Manual change manifolds are for applications in which uninterrupted gas supply is not needed. Like regulators, manifolds are composed of different materials to accommodate different gas purity grades and types of gases (such as corrosive gases).
The Rest of the System
Tubing and hose: There are different tubing and hose options between the gas outlets. For gases at 5.0 to 6.0 purity, or for corrosive gases, coiled stainless steel hose or stainless steel tubing are required. For gases less than 5.0 purity, copper tubing is sufficient. Polymer hoses should not be used for high purity gas applications because it can easily pick up contaminants.
Fittings: Any gas at 5.0 purity or lower, copper fittings are industry standard. For gases higher than 5.0 purity or corrosive gases, stainless steel fittings should be used.
Other considerations: Vaporizers, heaters, flame arrestors, alarms and purging assemblies may be optional or essential components of a gas delivery solution. Every point in a gas delivery system needs to be carefully considered, especially in applications with corrosive gases. Each application should be evaluated to determine if system components are helpful or necessary.
Caustic and Corrosive Gases
Corrosive gases can wear down metals over time, which can quickly damage the integrity of a regulator. Stainless steel and PTFE resist corrosion, so regulators constructed with these materials should always be used with corrosive gases, regardless of purity. In addition, caustic and corrosive gases pose other challenges that often require additional considerations when building a gas delivery system to prevent hazardous conditions. For example:
- Hydrogen is highly flammable and under high pressure, so it requires special handling. Hydrogen cylinders manifolded together require flame arrestors to prevent flashback in case of fire. Pressure relief valves allows the safe discharge of gas in the event of regulator failure.
- Hydrogen chloride is very corrosive. If it is in contact with moisture, a buildup of hydrochloric acid can occur in the line. To prevent the introduction of moisture, the system should be purged with a high purity inert purge gas when changing cylinders and changing applications. Again, pressure relief valves ensure pressure is discharged to a safe environment in the event of regulator failure.
- Similar to hydrogen chloride, hydrogen sulfide also requires system purging during a change-out to prevent unwanted buildup of hydro-sulfuric acid.
- Ammonia is highly corrosive and requires robust gas delivery components to ensure the life of the equipment and the quality of gas in the stream.
It is important to follow regulations and establish policies for regular interval inspections of high purity gas delivery systems. Inspections validate that the equipment is operating properly and accurately. These inspections are important for all components in the system. For example the BCGA codes of practice recommend inspecting and/or replacing regulators and other critical components every five years. It's generally considered very difficult to inspect a regulator and guarantee it for another five years, so most companies just replace or refurbish their regulators. To manage such a labor-intensive task for companies that have large numbers of regulators and systems, a rolling maintenance program of switching out a certain number of regulators per month can keep such maintenance more manageable.
Get help with specification
Before building a new delivery system for applications using high purity and corrosive gases, it’s helpful to seek advice from well-established companies with a breadth of knowledge about gas delivery. Technicians should look for providers that understand requirements locally and globally, and are willing to translate a system’s needs into an effective and cost-efficient solution.
ESAB offers a complete line of specialty gas control products and can assist with your application needs. Explore our broad range of solutions.
Understanding Gas Purity Ratings
Purity grades are named with two numbers separated by a decimal point. The first number shows how many nines are in the percentage of gas’s purity. If there is a non-nine in the percentage, that number will appear after the decimal. For example, 99.995% pure gas is called 4.5 purity.
4.0 purity gas (99.99%) has one part in 10,000 of contaminants. 6.0 purity gas (99.9999%) has only one part per million of contaminants.
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The photos show a new grade 6.0 purity full gas distribution and management system, providing the user with a high purity gas aiding them to develop their next generation high tech product.
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