The “Do’s and Don’ts” of CGA Cylinder Connections

by Frank Kandl, Director of Business Development – Airgas Specialty Gas Equipment |  Winter 2017 Issue

You may be familiar with CGA (Compressed Gas Association) connections, but what do the different types of connectors tell you?

First, they tell you if there is a hazard with the gas even before you look at the labels on the cylinder. The CGA Connections Committee designs and assigns a unique fitting for each gas, along with that gas cylinders’ maximum pressure, and does this for a reason. For example, the connection for a cylinder containing toxic, poisonous or flammable gases (pure gas or mixtures) will be LH (left-hand). A left-hand nut has a cut mark on the hex of the nut. This contrast from the typical RH (right-hand) configuration warns the user that the gas is hazardous and must be handled properly.

Want to ensure product safety and integrity with all of your CGA connection systems? Uncover the “do’s and don’ts” for successful CGA connections below that’ll help eliminate hazards and assist you in optimizing your operation to achieve the high purity valves.

Do: Different CGA Connections for Different Fill Pressures
There will be different CGA connections for the same gas but at different fill pressures. There are also either RH (right-hand) or LH (left-hand) threads. LH threads are backwards or left instead of the normal right threads and this is odd, because it is backwards of how you loosen or tighten a connection. As noted above, all toxic, poisonous, corrosive and flammable gases will have a left-hand thread. This will carry over to mixtures as well. When the mixture qualifies for one of these designations based on percentages of the minor components it will become a LH connection.

Don’t: Change CGAs on a Regulator
Why is it not recommended to change CGAs on a regulator? If someone removed a CGA 580 off a regulator and replaced it with a CGA 680, the gas service would be the same but the pressure is more than the CGA 580 and regulator are designed for. If the regulator was put into service on a cylinder with the CGA 680, it would over-pressurize the regulator. It could fail and someone could get hurt. In this case, if the regulator were to fail at the inlet gauge on the regulator, it would probably be the first component to become faulty as it would not be rated for the higher pressure. This failure would result in gas venting, and also there is a good probability of material failure that could result in someone being injured. The pressure could cause the regulator body and other components to fail, as well, and this would be catastrophic.

Don’t: Use a Fitting that Allows 2 Different CGA Connections to Be Connected
Never use a fitting that allows two different CGA connections to be interconnected—these are sometimes called “Wild Card Fittings,” where one side of the fitting would be a CGA 320 and the other a CGA 580. These fittings should never be used and present both a danger to the operator and also a risk for cross-contamination. As discussed, different fittings are for different pressures and you are subverting the design factors of the fitting that CGA has set forth by using these. Cross-contamination is another issue you are risking.

Do: Be Aware of Cross-Contamination
Cross-contamination is another reason for not changing CGAs or using wild card fittings. There are two issues this creates. For analytical applications, it can introduce contaminants into the system and create inconsistencies and other performance issues. These contaminates can cause damage to components such as purifiers, columns and detectors, and also cause performance issues.

The other area of concern is in the case of oxygen. The previous service may have introduced contaminants like halocarbons or hydrocarbons that are not compatible with oxygen and ignition can occur in the regulator as a result. Typically, whenever there is ignition in a regulator, it results in material fragmentation which results in both personal injury and possible fire. Most fires occur in oxygen regulators as a result of modifications to a regulator that has previously been in service without incident. The modification of changing components is typically determined to be the cause of ignition inside the regulator and subsequent event.

Do: Know Your CGA Connections
CGA connections are of two types: a metal-to-metal seal—called bullet-nose connections, as on a CGA 580—or a seal that incorporates a gasket, as with a CGA 320. The connection has two or three components: the nut, the nipple and some have a gasket. The gasket can be made from different materials. The choice is based on the gasket material being compatible with the gas.

Sometimes the nipple and valve may be different materials. Care must be taken not to overtighten these connections as the harder of the two materials may damage the softer material and cause issues when a new connection is made.


Don’t: Use Teflon Tape to Create a Seal
Another common mistake people make is applying Teflon tape to the connection to get to the seal. Actually the tape will cause a leak and typically not prevent a leak. When tape is applied to the CGA nut, it prevents the nose of the bullet-nose CGA to fully contact the mating surface inside the cylinder valve. A gap is created and the only thing stopping gas from escaping is the tape. Small molecules like helium and hydrogen will easily escape through this taped connection. The same is true for connections with gaskets—there the tape will prevent the gasket from fully compressing.

Most people think huge amounts of force are required to get a positive seal, but look at the chart below for the CGA recommendations for torque by connection number.

Not only do the cylinder labels tell us about the gas, but also CGA numbers and whether it is a LH or RH connection. Remember not to change a CGA on a regulator, nor use “Wild-Card Fittings.” Left-hand connections are backwards and they are specifically so we know there is a hazard associated with the gas, so care must be taken.

The charts shown in this article are available to you. To receive them, contact your local Airgas representative or location. Always take care whenever replacing a spent cylinder. If you have not had proper training contact your gas supplier to receive this training. Airgas provides safety training to all of our customers as part of our responsible care responsibility.

I hope you find this information useful. If you have any questions on this article, I can be reached at