In order to deliver gases from a cylinder to the point of use, a gas delivery system is used. This can be as simple as a single cylinder regulator or a complex system involving a manifold, pipelines, line regulators, outlet points, or a long list of many other components.
However your system is designed, there will be many gas-tight connections. In this article, we’ll focus on one type of fitting: tapered thread fittings.
Tapered thread fittings are commonly used to connect gas fittings in many different industries and applications. Depending on where you are in the world, you may find different tapered thread standards being used. For most of the world, the commonly used standard for specialty gas applications is National Pipe Thread, or “NPT.” You will find NPT threads on most specialty gas oriented gas control equipment, including pressure regulators, manifolds, outlet points, line regulators, and point of use panels.
National Pipe Thread standards include specifications for many different sizes between 1/8” and 24”. Most commonly ¼” NPT is used but occasionally 1/8” NPT is used for smaller fittings, such as for lecture bottles or miniature regulators.
NPT threads are called ‘male’ or ‘female’, sometimes abbreviated NPTM for male (external) threads and NPTF for female (internal) threads.
Tapered threads are sealed when the threads of both the male and female fittings deform into each other, as the fitting is tightened. The seal occurs on the first few winds of thread, counting from the end of the male fitting. For this reason, tapered threads can only be re-used a finite number of times. This differs from other types of gas-tight seals where the thread is used only to apply force to a separate sealing method, like a compression fitting or flat face seal.
Thread Sealant
It is important to use thread sealant when connecting tapered fittings for two reasons. Firstly, the thread tape fills any micro-gaps between the threads which may not be filled when the threads deform, preventing leaks. Secondly, it provides lubrication between the threads to prevent galling (thread damage resulting in seizure). Thread sealant is most commonly ‘thread tape,’ sometimes called ‘PTFE tape’ or ‘Teflon tape.’
It is important to ensure the tape you use is suitable for your application. The tape you choose needs to be compatible with the gases you will be using, otherwise leaks may form. Generally speaking, “PTFE” tape is recommended. Other types of thread sealants are liquids, which have their own advantages & disadvantages. For most speciality gas applications, thread tape is preferred due to the inert nature of the material and there is no need for ‘drying time’ as there is with liquid sealants.
Thread sealant is for use on tapered fittings only. Thread sealant should not be used on any parallel threads, which includes cylinder valve connections. This is because the threads on cylinder connections are not used to make a seal. Taping threads which are not intended to form the seal can cause the tape to break apart and enter the gas stream, which can lead to damage of control equipment and instruments downstream.
To connect a tapered fitting, follow these steps:
- Ensure both male and female threads are totally clear of all debris, including residual thread tape from previous connections.
- Apply 2-3 winds of PTFE (thread) tape to the male fitting, starting from the first thread. (see video: https://youtu.be/S6-akMHP5Rc )
- Thread the male fitting into the female fitting until finger tight.
- Use a wrench to tighten until “wrench tight.” There is no specific torque rating for tapered fittings. Wrench tight is normally 2-3 full turns from finger tight. Do not apply excessive force as the thread will become damaged and the fitting will leak.
- Use a leak detector (like Swagelok Snoop or some soapy water) to check the connection for leaks.
