Industrial expansion joints, by their very nature, are designed and fabricated to compensate for a combination of heat, pressure, movement, vibration and abrasion. An expansion joint, however, is not itself immune to these same stresses and inevitably will fail as it reaches the end of its design life.
The design engineer must create an industrial expansion joint that will perform reliably for a finite period of time under a specifically defined set of operating conditions. A plant engineer must successfully gauge these variables and accurately estimate lifespan tradeoffs of various expansion joint designs to ensure that process flows are cost effective, efficient and dependable over the long term.
What Is Cycle Life?
An expansion joint connecting two duct ends will shift position as changes in the gas flow temperature causes thermal expansion in the ductwork metal. This movement may involve compression (the endpoints move closer to each other), axial or lateral movement, or any combination of possible axis shifts. Regardless, the joint will start in an initial position, move to a new operating position that will add a certain degree of stress to the joint, and then resume an initial position once the system is no longer hot enough to cause ductwork thermal expansion. This initial position resumption typically happens when the plant system is taken offline for maintenance.
This set of position movements — from initial, to operating and back again — is a cycle. Each time the joint experiences a cycle, it becomes slightly weakened. The metal slowly changes shape and loses its original strength. The fabric begins to wear, thin and unravel. Eventually, after many cycles, this weakening will prevent the joint from operating reliably. The total number of predicted cycles that an expansion joint design can accommodate is called the joint’s cycle life. Depending on the type and design of the expansion joint, cycle life is usually measured starting at 1,000 operating cycles and can reach much higher.
Design Choices and Expansion Joint Cycle Life
To ensure optimal reliability, FlexCom engineers consider the following:
- Metal vs. fabric. A fabric expansion joint tends to have a much longer cycle life than a metal bellows. Failure is generally caused when heat deteriorates the outer fabric layers. The convolutions of a metal bellows, on the other hand, cause mechanical weakening with each cycle. Whether an industrial expansion joint is fabric or metal is typically determined by gas pressure, temperature and cost considerations, however, rather than cycle life of the expansion joint.
- Material types. Expansion joints are fabricated from a wide variety of metals and fabrics, all affecting the cycle life of the expansion joint as well as its performance. A titanium metal bellows, for example, will last for decades but comes with a high fabrication cost. Metal bellows are more commonly made of varying alloys of stainless steel, nickel, titanium, carbon steel and other metal types. Fabrics are typically laminates of fiberglass cloths and polytetrafluoroethylene (PTFE) of varying thicknesses, designed to compensate for different heat thresholds.
- Joint geometry. Different geometry decisions can have a significant impact on the performance and life cycle of an expansion joint. The number of metal plies and the number and size of convolutions are important variables in improving the flexibility, heat tolerance and cycle life of metal bellows. The size and shape of a fabric expansion joint, likewise, produce a specific balance of movement and heat tolerance capabilities that in turn affect how long it can remain in reliable service.
- Hardware options. Various additional design features are available for most expansion joint designs to prevent conditions that can prematurely weaken the joint. External tie and limit rods, for example, can help prevent overextension or excessive movement in the joint, leading to a more predictable estimated cycle life.