Q1 2009 / Purity plus: How to prepare cylinders for today’s specialty gas operations

Virtually the first decision a distributor makes in preparing cylinders for specialty gas service is whether to use a new cylinder or an old cylinder. New cylinders typically have a more uniform and cleaner appearance. Those making significant investments in high-tech specialty gas equipment and instruments will be tempted to only use new cylinders in specialty gas service. A new cylinder can be put into the most demanding research grade service provided the inner wall is thoroughly dried. If a distributor does not have a cylinder drier, it might consider using cylinders that have been in service for a couple years since their last hydrostatic retest.

Inspect the cylinder’s internal surfaces

The cylinder’s internal surfaces should be carefully inspected prior to inserting the valve. The surface should be free of rust, dirt, debris, particulates, moisture, oil, and other contaminants. Some demanding electronics gas and reactive gas applications require additional surface preparation before valving such as polishing and nickel plating to reduce the porosity and reactivity with the inner surface.

Leak management

All cylinder preparation will fail if the valve leaks moisture and air into the cylinder. The cylinder should be valved by a skilled valve machine operator. Use a thread lubricant which is suitable for the 

intended gas service. Teflon tape works well for most applications. Use care to keep the teflon tape off the bottom thread as this will tend to deposit teflon particles in the cylinder. Teflon paste and other lubricants also help create a leak free seal, but they can be reactive with some gases such as EPA Protocol mixtures at low concentrations.

Moisture removal

Moisture is one of the hardest impurities to remove from a high pressure cylinder. The best way to remove moisture from a cylinder is with an automatic vacuum/purge bake-out system.

Some bake-out ovens rely simply on a hard vacuum cycle to remove moisture. A deep vacuum will indeed remove moisture from a cylinder. However, it is much faster and easier to dry a cylinder with heat, vacuums and purges. If you apply moderate radiant heat to a cylinder wall, the metal of the cylinder and pressure relief device will not be damaged. Using moderate heat, good vacuums and purges with the appropriate gas will typically dry a wet cylinder to under 0.5 ppm in a couple hours. High quality bake-out systems have built in logic that tests itself for vacuum leaks and high moisture levels.

Another advantage to a good bake-out system with automatic cycles is that vacuum pump lubricants will continuously purge with dry gases. This maintains the integrity of costly vacuum pump lubricants. Some have added low pressure nitrogen gas ballast systems to their vacuum pumps to continuously flush out moisture and other corrosives from lubricants.

Passivation

Some low ppm reactive gas mixtures require additional cylinder preparation beyond internal cleaning and drying the cylinder. Gases such as hydrogen sulfide, carbon monoxide and carbon dioxide at low concentrations can be absorbed or react with the inner wall of a steel or aluminum cylinder. However, the inner wall of the cylinder can be passivated to be less reactive to the low concentration reactive gases. Specialty gas companies use their own proprietary procedures for cylinder passivation. In some cases a cylinder is filled with a similar, higher concentration mixture. This higher concentration mixture conditions the inner wall of the cylinder so that subsequent low concentration mixtures remain stable. Low ppm mixtures of reactive gases can be stable for years if the cylinders are properly conditioned.

Effective cylinder evacuation

After the cylinders are connected to the filling manifold, care should be taken to assure that the cylinders remain dry. For example, the manifold should be checked for leaks before the cylinder valves are opened. Then the manifold should be vented and evacuated, the cylinder valves should be opened, and the cylinders should be vented and evacuated.

One theory of cylinder evacuation relies on deep vacuums to assure that all contaminants are removed. A faster and more effective vacuum strategy uses two moderate vacuums instead of a single deep vacuum. For example, if the cylinders are dry and the manifold is leak free, two 1,000 micron vacuums can be pulled on a manifold of cylinders in a fraction of the time that a single 50-micron vacuum can be pulled. Also, two 1,000 microns are much more effective than a singe 50-micron vacuum.

After cylinder drying, the vacuum strategy is perhaps the most important step in cylinder preparation.

External Cylinder Preparation

We typically consider the inside of the cylinder when we discuss cylinder preparation. However, customers will judge the value of the gas inside the cylinder by the external appearance of the cylinder. This includes the condition of the valve, shrink wrap, paint, and labels. If you are rolling mixture cylinders after filling, you should consider repainting the cylinder. Cylinder rollers often damage the paint or leave streaks in the paint. If you repaint a full cylinder, use extreme care. Many mislabeled cylinders have shipped because they were painted after filling. Consider a cylinder inverter instead of a cylinder roller to make your mixtures homogenous after mixing. This allows you to paint and label your cylinder before filling.

Post fill – cylinder preparation

Two additional measures can significantly reduce your cylinder preparation expense while improving the quality of your gases: Residual Pressure Valves and Ultrasonic Cylinder Requalification. Residual pressure valves reduce the likelihood of the contamination from the customer’s process or from leaving the valve open. Ultrasonic cylinder requalification reduces the exposure of the inner wall of the cylinder to moisture by testing the integrity of the cylinder walls without opening the valves.

Effective cylinder preparation can improve quality, reduce costs, and enhance customer satisfaction. SGR

Tom Badstubner

Tom Badstubner, President of AsteRisk, LLC, assists gas companies with their specialty gas and FDA technical support requirements. He has been in the specialty gas industry for over 30 years. He conducts regularly scheduled GC, gravimetric mixture and FDA compliance seminars at Weldcoa’s Precision University. AsteRisk was recently selected to represent GAWDA for FDA and specialty gas issues.

For more information contact Tom Badstubner, AsteRisk, LLC; email: tom.badstubner@AsteRiskLLC.com Phone: 508-883-0927.