Heat exchangers: A review of common heat exchanger failures
This review provides an overview of the most common modes and causes of heat exchanger tube failure. Failure investigations of heat exchangers reported in the literature have been studied using failure mechanisms. Fatigue, creep, corrosion, oxidation, and hydrogen attack cause the vast majority of heat exchanger components to fail. Fouling, scaling, salt deposition, weld defects, and vibration are the most common causes of failure. This post discusses a few important failure modes.
Two important definitions
Fatigue and creep
Creep is the tendency of materials to deform when subjected to long-term stress, especially heat. Fatigue occurs when a material is subjected to cyclic loading, causing damage that can lead to failure.
The fatigue and creep properties of the material are the most important for heat exchanger durability at the material level. Load spectra and temperatures differ significantly between heat exchanger types.
Creep–fatigue is expected to be the primary damage mode for the very high-temperature heat exchanger. Transients during start-up and shut down produce cyclic loadings that is fatigue. While the stresses relax during steady operation induces creep damage. Tubing, particularly in the U-bend area, can fail due to fatigue caused by accumulated stresses from repeated thermal cycling. As the temperature difference across the length of the U-bend tube increases, so does the problem.
Creep-fatigue diagram of typical alloys
Y-axis is creep and X-axis is fatigue. You can see a far superior creep-fatigue interaction with 9Cr-1Mo-V steel. 9Cr-1Mo-V steels (Grade 91), shows an excellent performance at high temperature in mechanical properties and hydrogen resistance, has been used for tubing and piping materials in power industries, and can be a candidate material for high-temperature processes.
Three important mechanical failures
-Metal erosion,
-Steam or water hammer, and
-Thermal expansion and cycling
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Metal erosion – Why?
Excessive fluid velocity on either the shell or tube side of the heat exchanger can cause metal tubing erosion. This can accelerate any existing corrosion because erosion has the potential to remove the protective film on the tube material, exposing the fresh metal to further attack. The U-bend of U-type heat exchangers and the tube entrances are the most prone to erosion. Material loss can occur in tube entrance areas when excessively high-velocity fluid from a nozzle splits into much smaller streams as it enters the heat exchanger. Excessive velocity occurring at the entrance area of tubes typically produces a horseshoe-shaped erosion pattern.
The solution:
The solution is to keep the flow in the tubes and entrance nozzle at the maximum recommended velocity. This value is affected by a number of factors, including the material of the tube, the fluid handled, and the temperature. Carbon steel, copper-nickel, and stainless steel can withstand higher tube velocities than copper. Copper — 8 ft/s; carbon steel — 9 ft/s; 90/10 copper-nickel — 11 ft/s; and stainless steel — 11 ft/s are typical tube velocity limits.
Impingement of wet high-velocity gases, such as steam, can cause erosion on the outside of tubes. Wet gas impingement can be reduced by oversizing inlet nozzles or installing impingement baffles in the inlet nozzle.
Impingement of wet high-velocity gases, such as steam, can cause erosion on the outside of tubes. Wet gas impingement can be reduced by oversizing inlet nozzles or installing impingement baffles in the inlet nozzle.
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To prevent impingement erosion on the outside of tubes, use the following equation to calculate typical shell-side nozzle velocity limits: rho x V2 = 1,500, where density is in lb-m/ft3 and velocity is in ft/s.
Water hammer or steam hammer- Why?
Pressure surges or shock waves caused by a liquid's sudden and rapid acceleration or deceleration can result in steam or water hammer. The resulting pressure surges can reach 20,000 psi, which is high enough to rupture or collapse heat exchanger tubing.
In a water/steam heating application, damaging pressure surges can cause the flow of cooling water to be interrupted. The stagnant cooling water is heated above its boiling point to produce steam; the resumption of flow causes a sudden condensing of the steam, resulting in a damaging pressure surge or water hammer.
The solution:
The solution is to always start cooling water flow before heating the exchanger. Use modulating control valves instead of fast-acting shut-off valves, which open and close abruptly, causing water hammer. Vacuum breaker vents can help prevent steam hammer damage caused by condensate accumulation if you work with condensable fluids in the shell or tubes.
By preventing condensation from accumulating in the shell, properly sized steam traps with return lines can help prevent steam hammer. Additionally, make certain that the lines are pitched to a condensate receiver or condensate return pump.
Thermal expansion and cycling – why?
Stresses accumulated as a result of repeated thermal cycling or expansion can lead to tube failure. Because the bundle can expand and contract within the shell, exchangers with U-tube construction handle thermal expansion and cycling the best. The tubing in a straight-tube fixed-tube-sheet design cannot expand or contract.
As the temperature difference across the length of the tube increases, the problem worsens. The temperature difference causes tube flexing, which generates stress that acts additively until it exceeds the material's tensile strength, at which point it cracks. The crack typically runs radially around the tube and frequently results in a complete break. In other cases, the crack runs longitudinally through the tube halfway through. Failures caused by fluid thermal expansion are most common in steam-heated exchangers.
The solution
To avoid this type of failure, install relief valves in the heated fluid system. It's also a good idea to have some way to absorb the fluid expansion. Installing a tank in the heated fluid system, for example, prevents the periodic discharge of relief valves, which results in system fluid loss and places an undue burden on the valve. Place these devices between the heat exchanger and any control or shut-off valves.
Fouling from scale, mud, and algae – why?
A film or coating on the surfaces of heat transfer tubes can be left by various marine organisms or deposits. The film acts as an insulator, limiting heat flow while also protecting the corrosive components. Tube wall temperatures rise as a result of this insulating effect, as does corrosion.
Scale is formed as a result of dissolved minerals precipitating out of heat transfer fluids. The solubility of these minerals is affected by forces within the heat exchanger, such as temperature changes or chemical reactions. When heated, calcium bicarbonate, a common constituent of many waters, emits carbon dioxide. The material is reduced to calcium carbonate, a relatively insoluble compound that precipitates and coats heat transfer surfaces.
The Solution:
Experience shows that increasing the fluid velocity decreases the rate of precipitation. Of course, fluid velocity must be matched to the tube material's ability to withstand the erosive effects of velocity.
Suspended solids are typically found in one or both heat transfer fluids as sand, iron, silt, or other visible particles. If velocities aren't high enough to keep particles suspended, they settle out, causing the same problems as a scale from dissolved solids. Furthermore, many suspended solids are extremely abrasive to tubing and other heat exchanger components. When working with abrasive suspended solids in a heat exchanger, fluid velocity must be kept low enough to prevent erosion. Algae and other marine organisms pose a significant threat.
Credit: Google