Pipe joints are integral to any piping system as the length of the pipe is limited. So, to have a continuous piping network pipe joints must be used. Also, these pipe joints are usually the weak sections of a piping network. So, depending on the durability of the technique used to form the joint, the actual working of the piping or plumbing system varies. A variety of joints or joining methods are used in a piping assembly as per the requirement. So, pipe joints are an important component in any piping or plumbing system. Usually, the pipe joints are able to sustain the piping pressure.
The design and selection of a proper type of pipe joints can have a major impact on the initial installed cost. Also, the long-term operating and maintenance cost and the performance of the piping system are dependent on piping joints. There are various factors that must be considered in the pipe joint selection like
- material cost,
- installation labor cost,
- degree of leakage integrity required,
- periodic maintenance requirements, and
- specific performance requirements.
Additionally, the pipe joint selection should meet the applicable code requirements.
Types of Pipe Joints
Depending on application requirements various types of pipe joints are used. They can be grouped as follows:
- Threaded joint
- Welded joint (butt welded, socket welded)
- Brazed and Soldered joint
- Grooved joint
- Compression joint
- Flanged Joint
A threaded joint is piping joining where pipes are joined by screwing in threads provided in the pipe. One of the pipes is provided with internal threads whereas the other pipe has external threads. Threaded piping joints are ideal for non-critical, low-pressure-temperature, and low-cost applications like domestic water, fire protection, and industrial cooling water systems, etc. The productivity of such pipe joints is moderate and installation is quite easy. Screwed piping joints should not be used for vibrating lines as vibration can lead to leakage due to differential thermal expansion.
Threaded pipe joints are widely used for PVC, CI pipes, copper pipes, GI pipes, and brass or bronze, carbon, and alloy steel pipes. Even though threaded joints are available from 6 mm diameter to 300 mm diameter pipes, economic considerations normally limit industrial applications to 80 mm. Threaded construction is commonly used with galvanized pipe and fittings for domestic water and drainage applications. A screwed piping system is a good choice where maintenance or the process requires frequent disassembly and reassembly. Note that, threaded pipe joints should always be used within the limits of the governing code.
Welded joints (Butt-welded joints)
Butt-welded pipe joints are the most common for pipes in large commercial, institutional and industrial systems. Skilled pipe welders and fitters are required for butt welding. Butt-welded pipe joints provide extremely good strength and leakage integrity. This pipe joining results in a low-pressure drop as the inside surface of a butt-welded piping system is smooth and continuous.
Butt-welded pipe joints are the preferred joints for high-temperature-pressure large-bore pipe applications. Most butt-welded piping installations are used for size NPS 2 (DN 50) or larger. They produce a smooth external pipe surface.
Welded joints (Socket-welded joints)
Socket-welded pipe joints are used for systems with high leakage possibility. In socket-welded pipe joints, one pipe is put inside the other and welded around the joint. The construction cost of this type of pipe joint is usually lower than butt-welded joints. However, the internal crevice is prone to corrosion. The mechanical strength of socket-welded pipe joints is better than other types of pipe joints.
Brazed and Soldered Joints
Brazing and soldering joints are widely used to join copper and copper-alloy piping systems. The brazing of steel and aluminum pipe and tubing is also possible. Both the brazing and soldering joining methods involve the addition of molten filler metal which is drawn into the joint by capillary action. This metal then solidifies to fuse the parts together. The melting point of the pipe material has to be more than the chosen filler material to avoid the melting of the parent pipe.
Brazed and soldered joints provide very good leakage integrity and installation productivity. For medical gases and high-purity pneumatic control installations, brazed and soldered joints are popular. However, the mechanical strength of these systems is low.
The main difference between brazed and soldered joints is that brazing is done with molten filler material at above 840°C while soldering is performed at much lower temperatures. They are preferred for moderate pressure and temperature services.
Grooved joints are widely used for fire protection systems, ambient temperature service water, and low-pressure drainage applications. They are quite easy to install and uninstall; Hence, labor cost is lower. In these piping joints, two pipes are assembled together by making grooves at the end of pipes with the help of sockets or couplings. They usually provide good leakage integrity and can accommodate some axial misalignment. The grooved construction prevents the joint from separating under pressure.
The use of elastomer seals limits the use of grooved joints in high-temperature applications. Also, they have poor resistance to torsional loading.
Compression joints are common for joining plain piping ends without any end preparations. Such pipe joints have the ability to absorb a limited amount of thermal expansion and angular misalignment. Using compression joints, pipes of dissimilar materials can be joined. The installation cost is low.
In compression joints, the pipe ends are joined with threaded fittings or couplings. To avoid leakage problems, the joints need to be properly placed to check the flow pressure.
In instrument and control tubing installations and applications where high seal integrity and easy assembly and disassembly are desirable, Compression joints are used.
In all modern piping and pipeline systems, flanged connections are extensively used due to their ease of assembly and disassembly. But they are costly due to the high cost of the flanges themselves and the labor costs of attaching the flanges to the pipe using bolting. Flanged joints are used for all high-pressure and temperature applications. But they have a tendency to leak. Dissimilar pipe materials can be easily joined using flanged pipe joints. A gasket is inserted between two flanges to prevent leakage. To learn more about flanged joints kindly refer to the following articles: