In order to safely deliver gas from a cylinder to the point of use, a gas delivery system is required. In some applications this may be as simple as a single cylinder regulator connected directly to an instrument. In others, it may involve manifolds, pipelines, line regulators, outlet points and multiple control components.
Regardless of how simple or complex the system is, one thing remains the same — every connection in the system must remain completely gas tight.
In this article we’ll look at one of the most common connection methods used throughout the gas industry: tapered thread fittings.
Tapered thread fittings are widely used across specialty gas, industrial gas, laboratory and analytical applications. Depending on where you are in the world, you may encounter different thread standards, but for most specialty gas equipment the most common standard is National Pipe Thread, or NPT.
You will find NPT connections on many gas control products including pressure regulators, manifolds, line regulators, outlet points, point-of-use panels and gas distribution systems.
NPT standards cover a wide range of sizes from 1/8″ through to larger process pipe sizes. In specialty gas applications, 1/4″ NPT is by far the most common size, although 1/8″ NPT is frequently used on smaller equipment such as miniature regulators, sampling systems and compact gas assemblies.
NPT threads are available as either male or female connections. Male threads are located on the outside of the fitting, while female threads are machined into the body of the component receiving the connection.
Unlike compression fittings or gasketed connections, tapered threads create the seal directly through the threads themselves. As the fitting is tightened, the male and female threads progressively deform into each other to create a pressure-tight seal.
The seal is normally formed within the first few threads nearest the end of the fitting.
Because the sealing surface is part of the thread itself, tapered fittings can only be assembled and disassembled a limited number of times before wear begins to affect sealing performance.
This differs from compression fittings or flat face seals where the threads are only used to apply force to a separate sealing surface.
Thread Sealant
Using the correct thread sealant is an important part of assembling tapered thread fittings properly.
Firstly, the sealant fills any microscopic gaps that may remain between the threads after tightening, helping prevent leaks.
Secondly, it acts as a lubricant during assembly, reducing the risk of thread galling or damage which can occur as metal surfaces tighten against one another under load.
The most common sealant used in specialty gas applications is PTFE thread tape, often referred to as thread seal tape or PTFE sealing tape.
It is important to ensure the sealant selected is suitable for both the gas being used and the operating conditions of the system. If the material is not compatible with the gas, leaks or premature seal failure can occur.
Liquid thread sealants are also available and are widely used in some industries, however PTFE tape is generally preferred for specialty gas systems due to its chemical resistance, inert properties and the fact that it can be placed into service immediately without any curing or drying time.
Thread sealant should only ever be used on tapered thread connections.
It should never be applied to parallel thread cylinder valve connections or any fitting that seals using washers, gaskets or metal-to-metal sealing faces.
This is because the threads on these connections are not designed to create the seal. Applying thread tape to these fittings can cause fragments of tape to break away and enter the gas stream, potentially damaging regulators, valves, analysers and instrumentation further downstream.
Connecting a Tapered Thread Fitting
To achieve a safe and reliable connection, the following procedure is recommended:
- Ensure both the male and female threads are clean and completely free from dirt, debris and any remnants of old thread tape.
- Apply approximately two to three wraps of PTFE thread tape to the male fitting, beginning at the first thread and working backwards in the direction of the thread.
- Thread the fitting into the mating connection by hand until finger tight.
- Using the correct spanner or wrench, continue tightening until the fitting is securely seated.
There is no universal torque specification for tapered thread fittings. In most applications this will require approximately two to three additional turns beyond finger tight.
Avoid excessive tightening as over-tightening can damage the threads and actually increase the risk of leaks.
- Once assembled, always perform a leak test using an approved leak detection solution before placing the system into service.
Spending a few extra minutes checking threaded connections during installation can prevent leaks, improve reliability and significantly increase the safety of your gas delivery system.