How to Minimize Fatigue Problems: A Fitness Scheme for Heat Exchangers

Plate-and-shell heat exchangers have become very popular thanks to their high thermal efficiency, compact size and ability to withstand high pressures. But they are also sensitive to metal fatigue. To avoid costly equipment failures and ensure long service life, design, construction and welding must be taken into account.

Plate-and-shell heat exchangers combine the heat transfer efficiency of plate heat exchangers with the ability to handle high pressures found in shell-and-tube types. This made them the preferred choice for a variety of applications in many industries, including high, fluctuating temperatures and/or pressures, which can cause material fatigue. The heat exchangers are often process critical, making it essential to avoid fatigue-induced failure. Unfortunately, most conventional units exchangers are designed and manufactured in a way that leads to unnecessary problems and equipment breakdown.

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Material fatigue occurs when a machine part is exposed to loads that vary cyclically over time, causing it to break at stress levels that would be safe under static conditions. First investigations into the nature of fatigue by August Wöhler in the 19th century led to the discovery of a logarithmic relationship between the load that a machine part can endure and the number of load fluctuations. The logarithmic nature of the relationship means that the lifetime of a component increases dramatically with even a minor decrease in stress. Design changes that reduce stress therefore have a significant effect.

Small cracks are the starting point for fatigue failures. They are caused by defects in the atomic structure called dislocations, atomic planes that end in the middle of a steel grain instead of traversing it. If dislocations gather at grain boundaries, they will eventually form microscopic cracks with stress concentrations at the tips. The crack will continue to grow with every load cycle, becoming so large that the remaining area cannot support the load.

Understanding Fatigue in Heat Exchanger Operations

As stress concentrations cause fatigue cracks, identifying them during the design phase is crucial in order to minimize fatigue. A second cause is that the welding process causes residual stresses due to the heat treatment of the material. The effect is strongest in the areas that have been heated the most, and adds to the external load. Many heat exchangers operate under conditions with cyclically changing temperatures and pressures, causing fatigue problems. The nature of the fluctuations has a big impact on the service lifetime, and there are many variables influencing the speed of the fatigue process:

• the amplitude of fluctuations
• the frequency of the fluctuations
• the base stress level
• variations of temperature, pressure or both
• design of the heat exchanger and magnitude of peak stress
• the quality of the welds.

The best way to reduce these problems is to examine the entire process from a fatigue point of view. Is it possible to reduce the amplitude of the cycles? Is it possible to avoid sudden starts and stops? A key question is the type of heat exchanger: Plate-and-shell heat exchangers have become a popular alternative to shell-and-tube models — but many engineers have learned the hard way that their fatigue resistance is poor.