by FSA Member Lloyd Aanonsen
December 17, 2011

Today's pipe stress analysis software provides engineers with a tool for designing complete piping systems and performing structural analysis in compliance with ASME and other well-known piping codes. The typical PC-based software is easy to use with spreadsheet inputs and quick reporting. It has facilitated the design to analyze iteration cycle and enabled the engineers to quickly create better and more reliable piping systems.

Rubber expansion joints have long been recognized for reducing pipe and anchor stresses. Unfortunately, for equally as long, they have been viewed as being the weak link in any piping system. That an engineer would look to eliminate any inherent reliability risk is understandable, and with the advancements in pipe stress analysis, they have the tools to design the system without expansion joints.

Why are rubber expansion joints needed? The answer starts with disputing the theory that rubber expansion joints are a piping system's weak link. In fact, with finite element analysis (FEA) and strong engineering principals, it can be verified that the rubber expansion joints have an equal or greater safety factor than the adjacent piping or equipment.

The answer continues with the numerous design advantages that can only be obtained with the use of rubber expansion joints. These advantages can and should be quantified and added to pipe stress analysis data bases. This will enhance the tools available to engineers.

Expansion joint FEA
Figure 1. Expansion joint FEA

Performance and Reliability

Improved performance and reliability are directly related to the materials of construction and engineering design. Rubber is one of the most compliant and resilient materials available. This not only makes it an ideal material for car tires but for expansion joints as well. With a flexing cycle life in the tens of millions, fatigue is not a design concern.

Improved performance and engineered solutions are often accomplished by incorporating advanced materials and technologies into what would otherwise be considered a conventional product line. Advanced materials would include premium elastomeric compounds, tire cord and high tensile steel reinforcements. Using tire cord is a good example of optimizing a design with advanced materials.

Tire cord is engineered with a resorcinol-formaldehyde latex (RFL) coating for superior bonding to the rubber. The cords themselves can be aligned on an optimal-bias, ply angle, offering tailored strength in the directions needed. This often eliminates delamination and reduces the total number of plies required. The result is improved performance in the form of increased movement capability, decreased spring rates and improved reliability.

Today's rubber expansion joints are designed by licensed engineers using advanced software for simulation and FEA. Documented safety factors can equal or exceed that of the adjacent piping and equipment.

Critical design elements of piping systems are support and flexibility. An obvious consequence of inadequate support is catastrophic and potentially life-threatening system failure.

Less obvious, however, is that inadequate flexibility can produce the sa`me devastating consequence. System flexibility is required to absorb thermal movements, ground settlement, misalignment, vibration and shock.

The features and benefits of rubber expansion joints include absorbing these forces and displacements, and replacing them with the rubber expansion joint's low stiffness (spring rate). They absorb noise and vibration and provide easy access to piping and equipment.

Absorbs all Directional Movement 

Rubber expansion joints provide superior movement capability in axial compression, axial extension and lateral deflection, as well as in the angular and torsional direction. This performance cannot be obtained with metallic joints, grooved couplings or large pipe loops. The low stiffness and deflection forces can and should be incorporated into data bases accessed by pipe stress analysis programs.

Absorbs all directional movements and vibration
Figure 2. Absorbs all directional movements and vibration

 

Reduces Noise and Vibration

Rubber expansion joints dampen and insulate against the transmission of noise and vibration generated by mechanical equipment. This is important in HVAC applications where the disturbing frequency of pipe and fluid-conducting noise can resonate and amplify throughout a building. Metallic joints and grooved couplings do little to reduce noise and vibration.

High Resistance to Shock

The compliant and resilient characteristics of rubber expansion joints make them suited for earthquake and bomb blast isolation. They are also effective for pressure-surge and water hammer dampening.

Relieves Pipe and Anchor Stress

Thermal movements along with other external forces and displacements, including ground settlement, can exceed the allowable pipe and anchor stresses. Ground settlement is a good example of unanticipated system conditions that, without an expansion joint, could develop into serious problems. Rubber expansion joints absorb these stresses and replace them with their low stiffness (spring rate).

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