Threaded connections are applied to pipes, as well as to connecting and branch fittings. But the simplicity and reliability of connecting any pipelines using threading still does not ensure its complete impermeability. Therefore, a mandatory addition is the sealing of pipes in all places of threaded joints. Only this combination allows you to create a reliable connection.

Threaded fittings for various purposes

Types and conditions of sealing

It is necessary to isolate joints when laying pipes for different types of pipelines. The only exception is welded joints, but this is also their disadvantage — they are one-piece. And the threaded joints make it easy to replace a damaged area or insert a new strapping.

Therefore, sealing is necessary for threaded pipe connections:

  • hot and cold water supply,
  • gas pipelines,
  • heating.

In practice, threads are used in small-diameter highways — in internal wiring at home and in an apartment, country cottages, and when laying networks to detached buildings with low water and gas consumption. Flanged connections and welding are used for large-diameter water supply and heating pipes.

Ventilation lines, as a rule, have a large cross section and are made of thin-walled sheet or corrugated pipes. Therefore, threaded connections are not used in them, but sealing of the joints is still necessary.

When choosing a method for sealing pipe joints, it is necessary to take into account their properties and the conditions in which they will function:

  • External temperature of the medium and internal carrier in the pipeline. Some types of sealants lose their properties at elevated temperatures or in the cold.
  • Line pressure. Liquid insulators can be squeezed out of the connection at high pressure.
  • Pipe diameter.
  • Final bond strength. This is the force required to disassemble the insulated joint in case of its repair or rewiring.
  • Environmental Safety. When laying pipelines for drinking water, substances that can release toxic compounds should not be used.

Most of the means and substances used for sealing, according to these parameters, fit into the range required for joining household pipelines. But especially careful choice is necessary when sealing the joints of gas pipes. Violation of tightness in water lines is noticeable from the very beginning by traces of leaks at the joints. Gas leaks are not visually evident and may remain undetected for a long time.

Methods for sealing threaded connections

According to the materials used, a distinction is made between filling the space between the threads with solid materials or paste-like sealants.

Standards recommend the use of pastes for conditional passages up to 65 mm. There are no strict restrictions for solid insulating materials.

Consider the main options for sealing threads on pipelines.

Plumbing flax

The winding of linen strands on threaded coils has been used for a long time, and is still considered a reliable method of sealing. But he is also the hardest to work with. Sequencing:

  • notches are applied to the thread so that the material does not slip when winding;
  • flax is separated from the bundle in small strands,
  • the first turn is wound crosswise to fix it on the thread;
  • after winding all the material, drying oil or waterproof paste is applied on top of it.

Without experience with this material, the winding often turns out to be sloppy, with protruding strands of flax. In addition, it is easy to stain parts and hands with applied lubricant. But the main problem is that it is necessary to put the right amount of flax into the coils. If it is not enough, the tightness will be broken over time. With a large layer, tightening the connection will be difficult, and screw-on fittings made of poor-quality material may burst.

Winding linen strands on thread


FUM Tape

This fluoroplastic film can be called a sealant for amateurs. Working with it does not involve any difficulties, since it easily fits into the threads. Only at the very beginning you have to keep the tape from slipping. The softness of PTFE allows you not to worry too much about the amount of wound tape. Even with excessive winding, the fittings are screwed onto the threads without much effort. And if everything is done according to the rules, then the connection can be made by hand, without the use of keys. But the downside of this ease is the likelihood of weakening the connection.


  • ease of handling
  • chemical resistance,
  • low price.


  • due to poor tightening quality, not recommended for threads over an inch (25 mm) in diameter,
  • application is not allowed under the condition of vibrations on the pipeline,
  • with large temperature differences, sealing failure occurs.

It is convenient for application on pipes of water — and heat supply in house conditions.

FUM tape in a reel and winding on a thread

Plumbing thread

In fact, it combines the advantages of linen and tape seals. It is made of synthetic materials impregnated with an additional sealing compound. It is also convenient when winding, like the FUM tape, but much stronger than it. If necessary, you can even slightly unscrew and tighten the connecting element again — the tightness will not be violated from this.

There are features of working with it:

  • winding is carried out not along the grooves of the thread, but a little obliquely;
  • to hold the thread in the desired position, notches on the thread are necessary,
  • the first two turns of the thread are left free, and on the last two threads are wound with an additional seal;
  • the number of turns for proper sealing depends on the diameter of the pipe and is indicated in the instructions.

Temperature resistance, reliability and tightness of the connection with the sealing thread allow it to seal not only heating pipes, but also gas joints.

Of the solid threaded seals, it is considered the most reliable. Of course, if you do not use cheap fakes. The most popular brand is Tangit Uni-Lock thread.

Winding with sealing thread


Liquid sealants

Such sealing compounds are filled into the threaded grooves of the connected pipes and fittings. After application, they do not harden, but remain in a paste-like state, providing impermeability of liquid or gas along the thread. Usually, the composition includes additives that protect the metal from rust.

The main disadvantage of such pastes is the impossibility of their use in high pressure lines. The pressure of liquid or gas will gradually squeeze out the soft substance from the threaded gaps.

The ease of connection on such a sealant turns into the fact that it is unscrewed without effort. Therefore, joints on liquid sealants must be controlled and tightened in a timely manner.

Applying liquid sealant to threads

Anaerobic sealants

Complex polymer compounds that retain fluid properties in air. Like non-hardening pastes, it is easy to apply on threaded grooves. After that, the connection is tightened without effort. Upon contact with the metal in the absence of air in the gaps, the substance is transformed, and it hardens, ensuring the sealing and reliability of the joint. The main requirement before application is that the metal of the joint must be free of rust and thoroughly degreased.

They differ in the degree of strength of the hardened composition:

  • Medium fixation (Medium). Usually produced in blue. After setting, it provides a reliable connection, but amenable to mechanical disassembly. It is unlikely that it will be possible to unscrew it with your hands, but completely with the keys.
  • Strong fixation (Strong). Has a red color. The connection formed by it is practically non-separable, since under normal conditions, even with the help of wrenches, it does not lend itself when unscrewing. Unlike welding, it can be disassembled after strong heating. Therefore, this type of sealant is not used in domestic mains.
Medium strength anaerobic thread sealant

It is not recommended to use a strong option for connecting metal pipes with polymer ones. If it is necessary to disassemble the joint, heating can deform the plastic section of the joint.

Sealing with gaskets


A gasket resting against the end of the pipe can also seal the joint. The clamping is carried out by a cap nut or a connecting fitting. In this case, the cutting itself can not be sealed, although sometimes it is also wrapped with a sealant for safety reasons. Rubber rings are gradually being replaced by modern gaskets made of PTFE and other polymeric materials.

Not all threaded connections can be sealed in this way. It is difficult to join two pipes together using gaskets. But connecting taps, faucets or flexible pipes through polymer rings is very convenient.

Threaded connections with gaskets

Sealing domestic air ducts

Ventilation in apartments and country houses is carried out by thin-walled elements. Often, air ducts are made of rectangular section or from corrugated pipes. Threading on these connections is not possible. Therefore, the sealing of ventilation joints is carried out in other ways:

  • through the flanges
  • bandaging,
  • socket method with the insertion of one air duct into another.

Flange connection is most often used for non-circular ducts. Sealing is carried out using gaskets made of a soft material, such as porous rubber. The shape of the gasket should follow the cross section of the duct.

With other methods, the voids formed after joining the air ducts are filled with mastics, according to their temperature characteristics corresponding to the operating conditions of the air line. Sometimes the joint of the air ducts is wrapped with self-adhesive tape.

Bandage clamps used for joining round ducts


Docking pipes made of metal and plastic

The popularity of polymer pipes in household mains often requires their connection to existing metal pipes. For this, special adapters are used. The most common of these are fittings called «American» or «nipple».

The American adapter consists of two parts. At one end of the metal part, an external or internal thread is applied, depending on the type of cutting of the pipe to be connected. At the other end is an external thread. The second part is made of plastic, ending with a union nut with an internal thread. Both parts are interconnected by screwing the nut onto the thread of the metal element. Joint sealing — through a sealing gasket inside the union nut.

American adapter connecting metal and plastic

The first part of the adapter is screwed into the metal pipe to be connected, and the outlet pipe is welded by polymer welding to the plastic pipeline.

A nipple fitting is a polymer sleeve, inside of which a threaded metal part is soldered. It provides a joint with a steel pipe, and the sleeve itself is welded to a plastic pipeline.


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