The Importance of Properly Sealing Threaded Connections for Hazardous Gases and Liquids


When storing and transporting cylinders, especially those containing flammable or corrosive liquids and gases, leak-tight seals are of critical importance. As any distributor of packaged gases can attest, a number of problems arise when a cylinder is found to leak. It can be dangerous to both the customer and those nearby to have a cylinder with a leaking valve. If the leaking gas or liquid is flammable or caustic, it can ignite or explode. In such instances, there is a real potential for injury or even death in addition to equipment and property damage at the customer’s location. Leaking valves are also costly, not to mention frustrating, to the distributor who supplied them. When it is discovered that a cylinder leaks after delivery to the customer, the distributor has two options. If the cylinder can be easily moved, a truck can be sent to the location to pick up the cylinder and deliver a replacement. In other cases, however, replacement is not an option. A distributor needs to send someone to the customer’s location to repair a connection that cannot easily be returned to the distributor. In this case, a leaking cylinder is repaired in situ. Both options incur additional expenses for the distributor.

Many distributors are involved with the selling and/or assembling of many different types of gas handling systems. The threaded connections and valves that connect flowmeters, delivery systems, generators, and regulators all have the potential for leaks. As with cylinder valves, if the threaded connections are not properly sealed they may leak in time, creating a possible hazard for the customer’s employees and equipment.

To eliminate this problem, many packaged gas distributors use PTFE tape, a thread sealant, or a combination of both on threaded connections prior to distributing their cylinders. When using these products, it is important to select the proper tape or thread sealant for the application. For example, during their manufacture many PTFE tapes acquire a residue of hydrocarbon oil, which is deposited on them during the extruding process. Hydrocarbon oils are flammable and can potentially create a fire or explosion when used to seal cylinder valves containing oxidizers such as oxygen. Oxygen compatible PTFE tape or thread sealant is preferred. These products are tested for autoigniton temperature to meet nonflammability requirements. PTFE tape is relatively thick and compacts into threads, thus providing a good initial seal. With time, however, the tape can become distorted or cold flow, and leaks can develop sometimes days, weeks, or even months later. Using an oxygen compatible thread sealant in place of or in conjunction with the tape is an accepted and popular practice that gives the user a very high rate of leak-tight seals. Oxygen compatible thread sealants eliminate fragments of tape in the flow path of orifices and valves in regulators and metering systems, as well as ensure a leak-tight seal. Many gases distributors have had success in sealing their threaded connections with OC Five Paste, an oxygen compatible thread sealant manufactured by MPT Industries of Dover, New Jersey.

All of the OC Products are oxygen compatible and chemical resistant. There are some differences, however, among the four types. OC Three is a water soluble, economical light thread paste designed for low pressure cylinders, and applications that do not involve moisture or water. OC Five is a waterproof, thick thread paste designed for high pressure cylinders, most aggressive chemicals, and applications involving water or moisture. OC Seven is a thinner, less viscous version of OC Five that is often used as a grease, but some customers use it as a thread sealant because they find it easier to apply. OC Nine is an even thinner version of OC Seven that is used as a grease for moving parts and O-rings in systems involving oxygen or aggressive chemicals. Some distibutors will use an oxygen compatible thread sealant in conjunction with Teflon tape in all of the assemblies. In the early days, thread sealant was primarily used on stainless steel components to not only ensure leak-tight connections, but to also help prevent galling of the stainless components during assembly. It was quiickly realized that by using the thread sealant on all compatible materials, there was a reduction in the small margin of leak rejection to virtually zero. So if you are connecting threaded components to cylinders or other gas handling apparatus, thread sealants provide an added measure of safety that can give gases distributors and their customers the assurance that they will not encounter any leak problems due to a poor seal in a threaded connection. It’s sort of the “belt and suspenders” approach to leak-tight connections. And it is well worth it!

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