Thread Sealant Types - Selection for Instrumentation Applications
What is Thread Sealant?
Thread sealing compounds are products used to eliminate leaks in threaded joints, and make them pressure tight. Thread sealing compounds are sometimes refered to as, "thread sealants", "pipe dope", or just simply "dope". In the world of instrumentaion thread sealant is used on the threaded connections on instrument tubing in process impulse lines, pneumatic lines, and analyser fast loop circuits.
Types of Thread Sealant
There are two distinct types of sealant, namely: thread tape, and thread paste. And within each of these broad categories of thread compound there are tapes and pastes designed for particular applications, e.g. oxygen use, connections in cryogenic service, high temperature service etc.
Selecting the correct thread sealant for your particular application is essentail to obtain a pressure tight joint – more on this below.
How do Thread Sealing Compounds Work?
Machining and finishing variations during manufacture of threaded connections means that in practice it is unlikely that 100% contact between the male and female thread forms will be obtained, in other words, you will have an imperfect seal.
Thread sealing compounds do two things:
First, they lubricate the threads, which allows them to be more easily screwed together providing a deeper seated joint. The lubricating effect may also allow the mating threads to deform, providing a solid seal.
Second, the compound fills any gaps that may be present between the mating threads.
Thread Sealant Selection
When selecting a thread sealant consideration should be given to:
- Compatibility with the thread materials,
- Compatability with the process fluids,
- Pressure and temperature of the process media,
- Curing time (this is often forgotten),
- Ease of application, and
- Experience of instrument technicians with the product selected.
Sealant Application Problems
Overuse or misapplication of thread sealants can cause problems.
Using too much sealant, either tape or paste, can prevent mating threads from fully engaging, or a deep seated connection being made. This will prevent a leak tight seal.
Tape that extends beyond the thread into the process make break free and form a foreign body which could constrict down stream equipment e.g. ports of needle valves, some types of pressure gauge accessories etc. Likewise, excessive paste may be forced into the process during assebly of the connection giving rise to the same problem.
Thread Seal Tape
PTFE tape is one of the most popular sealants used on instrument fittings, for many reasons:
- It is chemically inert, making it suitable for a wide range of process fluids including acids, and solvents.
- It is suitable for use over a wide temperature range, from sub zero to around 200° C, depending on manufacturer.
- It is suitable for high pressure applications.
- It is easy to carry and store, and has an indefinite shelf life.
- No special tools are required when applying the tape.
- It is easy to apply. The tape is sold cut to specific widths and wound on a bobbin, making it easy to wind around pipe threads. Typically the instrument tech will wrap the tape for 2 or 3 revolutions in the direction of the thread spiral of the male pipe thread beginning with the first thread. The male end connection is then screwed into the mating female end connection until finger-tight. The connection can then be completed as per the srew fitting manufacturer’s instructions.
- Tapes are available for different applications e.g. pure PTFE tape that is lubricant free can be used in oxygen service. Most manufacturers colour code their grades of tape to help prevent installation errors, for example green is frequently used for oxygen use in the United States.
There are two different types of thread paste: solvent based compounds, and anaerobic resin compounds.
Solvent based compounds are the origianal pipe dope, having been around for decades. These compounds rely on a solvent carrier and harden when the solvent evaporates. The resulting seal adheres to all plastic and metal pipes and effectively blocks leak paths. Over time this type of sealant has a tendency to shrink leading to leaks. This type of paste is rarely used for instrument fittings.
Photo courtesy of Swagelok
Anaerobic resin compounds cure when the sealant is confined within the threads of the metal pipe connection and air is excluded. This type of sealant does not shrink over time. They do generally require a cure time of up to 24 hours before the joint can be pressurised. There may be compatability issues with anaerobic resin compounds and some plastics, so this should be checked before use on plastic threads – not usually an issue in instrumentation hook-ups.
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