What is Deflection?
Deflection in a piping system is an unavoidable reality, stemming from various factors like thermal expansion, equipment vibration, or structural movement. This article will explain the different types of deflection: axial, lateral, angular, torsional, combined, and cyclic, and how expansion joints are specifically engineered to handle them.
A solid grasp of these concepts is crucial for engineers to select the proper solution, extending system life and preventing costly failures.
Deflection, which is the bending or displacement of a pipe from its original position, can have significant impacts on a piping system. It introduces stress into the pipe and its components, particularly at connection points and bends.
If not properly managed, this stress can lead to fatigue, cracking, and eventual failure of the pipe or its fittings, resulting in costly leaks or system shutdowns. Expansion joints are specifically designed to absorb these movements and protect the system by preventing the transfer of damaging forces to the piping and its anchors.
Expansion Joint Function and Design
An expansion joint is more than just a simple pipe component; it’s a critical element that provides the necessary flexibility to a system. Its primary role is to absorb movement, which can be axial, lateral, or angular.
However, certain applications demand more than just flexibility. They require the expansion joint to have structural strength to support loads or control the bellows’ movement. In these cases, special features are added to limit specific types of deflection and resist forces like shear, tension, compression, and bending moments.
A successful piping system is the result of collaboration between piping specialists and expansion joint designers. While it’s easy to visualize deflections from thermal expansion or equipment vibrations, it’s important to remember that not all expansion joints can handle the same types of movement or resist external forces. Understanding the specific type, magnitude, and direction of these forces is essential for the safety and cost-effectiveness of your system.
Even with the aid of modern computer software to identify excessive stresses, the proper selection, placement, and optimization of expansion joints still require a designer’s experience. A designer needs to have a deep understanding of how expansion joints function and how the piping must be anchored and supported.
Often, multiple solutions exist for the same problem, but their costs can vary. Therefore, cost is always a critical factor in the final design.
This article will walk you through the different types of deflection expansion joints can accept and what forces they can and cannot resist.
Types of Deflection
Axial deflection is the change in the length of a component when a force is applied along its longitudinal axis. This deformation can be either a compression (shortening) or an elongation (stretching). This movement is directly related to the material’s elastic properties and the magnitude of the applied force.
Lateral deflection is the sideways displacement of one end of the expansion joint relative to the other, perpendicular to its longitudinal axis. This type of movement often occurs due to misalignment or thermal growth in different directions. Accommodating this movement is critical to prevent overstressing the bellows.
Angular deflection is the change in the angle of a structural component or joint due to an applied force or moment. It is often associated with the bending of beams, where one section rotates relative to another.
Torsional deflection refers to twisting along the expansion joint’s axis under torque. It is important to note that metallic bellows are not designed to accept torsional rotation, as it can significantly reduce their fatigue life. Designers must take care to avoid torsional loading through proper alignment and layout of the piping system.
Combined deflection occurs when a component experiences more than one type of deflection simultaneously, typically due to multiple applied loads. For example, a shaft might be subjected to both bending (angular deflection) and twisting (torsional deflection) at the same time.
Cyclic deflection is the repeated movement of the bellows caused by thermal cycles, vibration, or pressure fluctuations. The number of cycles a joint can withstand before fatigue failure is known as its cycle life. Understanding the expected movement frequency and magnitude is essential for the proper design of expansion joints.
Pressure Balanced / Hinged & Gimbal Overview
Pressure-balanced expansion joints use a balancing bellows to create a force that is equal in magnitude and opposite in direction to the pressure thrust force of the main pipeline bellows. This ingenious design internally neutralizes the pressure forces, so they are not transferred to the surrounding pipes and equipment. This eliminates the need for massive, costly anchors to absorb the pressure thrust, as the expansion joint handles it on its own.
Hinged expansion joints contain hinges that allow the bellows to bend in a single plane, while preventing axial (stretching or compressing) and lateral movement. They are used to absorb angular rotation in a specific direction. Gimbal expansion joints are an advanced version, using a floating gimbal box with two pairs of hinges to allow bending in two directions or planes. Both hinged and gimbal expansion joints are designed to absorb the pressure thrust of the piping system.
Deflection is inevitable in piping systems, but with the right expansion joint selection and proper anchoring, it can be safely managed. Recognizing the type, magnitude, and frequency of movement is critical to ensuring long service life and system reliability.