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Everything You Need to Know About Slip Joints

What is a Slip Joint?

Slip joints are mechanical joints that enable the contact surfaces of two pipes to slip away from one another. Slip joints can be utilized to overcome expansion stress concerns when there is significant thermal movement in the pipe and an expansion loop cannot be provided because of other constraints. The slip joint’s robust design makes it suited for use in extreme circumstances such as inside a ditch, underwater, or underground. However, because the sliding surfaces may not provide perfect sealing, leakage along the surface is possible.

This issue limits the usage of slip joints in dangerous materials. Significant force must be maintained on the gasket or packing to ensure slip joint tightness. As a result, there is a significant internal friction force resisting the slipping action. In rare circumstances, the internal friction force is so great that the joint loses its ability to bend. Slip joints are often referred to as slip-type expansion joints.

What are the different types of Slip Joints?

Slip joints are classified into two groups based on the modes of sliding motion:

Axial slip joint – An axial slip joint allows the pipe to slip into it axially while simultaneously allowing the pipe to rotate axially. Axial slip joints are available in a variety of types. The compression sleeve is used for low pressure and temperature lines. Similar clamp-on couplings are widely used when some significant thermal expansion must be absorbed. The compression sleeve, also known as the Dresser Coupling, is widely used in water distribution systems when temperature fluctuations are mostly caused by climate change. An internally guided structure with a secure packing gland is used for large movements at higher temperatures and pressures. To resist the pressure thrust force, all axial slip joints, like bellow expansion joints, require main anchors.

The internal friction force of an axial slip joint is significant and should be considered while designing and analyzing pipes and supports. Because an axial joint is typically placed to handle the expansion of a straight piping run, the anchor design load can be correctly determined by adding the pressure thrust force, joint internal friction force, and support external friction forces. A computerized analysis is usually required for pipelines that are not completely straight. The spring force of the axial flexible joint can be used to simulate the internal friction force. The internal friction force changes with the movement since it is the product of movement and spring constant. The analysis may require several attempts to match the spring force with the internal friction force reported by the joint’s manufacturer.

The axial slip joint may also support the pipe’s axial rotation. Occasionally, a joint is specifically designed to allow only axial rotation of the pipe. Internal lugs can be positioned to resist the pressure thrust force because no axial movement is permitted. As a result, no anchor is required for this sort of axial rotation joint. The resulting piping system, similar to a multi-hinged system, may support very significant movements by strategically positioning a couple of these sorts of rotational joints.

Rotational Slip Joint – A ball joint, or a ball-and-socket joint, is another name for a rotational slip joint. The most common type of ball joint is made up of three major parts. It comprises a two-piece dome-shaped housing with an inner ball-shaped adaptor. The gasket is installed at the junction of the two housing parts. The joint can rock within a certain range, as permitted by the opening in the outer housing. It also can rotate 360 degrees axially. It is assumed to be capable of rotating in any direction.

The rotational slip joint, also known as the ball joint, is a mainly robust component that is appropriate for use in harsh situations such as offshore and loading dock applications. It can withstand a lot of pressure from piping and equipment operations. Again, adequate force must be provided and maintained at the gasket or packing to keep the joint tight. This produces a significant friction force, resulting in a rather significant resistant moment against joint rotation. The break-off moment is the moment required to rotate the joint, and its magnitude is accessible from the joint’s manufacturer. This break-off moment can have a major impact on the flexibility of the piping and must be considered in the design and analysis of the piping system.

Uses of Slip Joints

In the wastewater plumbing industry, slip joints are commonly utilized. They are also widely used in the piping sector for non-hazardous pipe networks. Other uses for slip joints include:

  • Slip joints are utilized in large structures to allow massive components to move independently while still being connected. Slip joints on bridges and overpasses allow the deck to move relative to the piers or abutments. The joints are built using elastomeric pads to facilitate movement, or they can be built with rollers on flat surfaces to allow the ends to move smoothly.
  • Slip joints are used in the automobile sector to alter the length of the propeller shaft when rear axle movements are required.

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Everything You Need to Know About Slip Joints

by Piping Mart time to read: 3 min