Reasons for Heat Exchange Tube Failure

Why do heat exchange tubes fail?

1. Pipeline oxidation

For shell and tube heat exchangers using carbon steel tubes, the biggest threat is oxidation (corrosion) of the heat transfer surfaces of the tubes.

The reaction between oxygen (O 2 ) and iron (Fe 2 , Fe 3 ) is the most common form of corrosion. This reaction forms a layer of iron oxide (Fe 2 O 3 ) on the carbon steel pipe, resulting in reduced thermal permeability and ultimately pipe damage. This problem is difficult to resolve and is usually only discovered when the pipes are severely corroded, thermal performance levels are reduced, fluid flow is significantly reduced, or the pipes are perforated and leaking.

2. Pipe corrosion

Pipe corrosion is the physical wear and tear of fluid on metal. Fluids containing high amounts of total dissolved solids (such as silica, silt, or seawater containing salts, sand, and marine life) can accelerate corrosion inside pipes and at the leading edges of inlet pipes.

While all pipes will erode over time, the weakest points in a pipe are typically the U-bend (if there is one) and the leading edge of the inlet pipe.

U-shaped bend erosion

Pipe-side fluid velocities in excess of manufacturer's recommendations can cause erosion damage to the inner surface of the U-bend return outer bend. The change in flow direction at this point creates resistance to its flow, causing the force of the fluid and any particles in it to concentrate on the far wall of the tube, where it continues to erode the tube.

Inlet pipe end erosion

Severe corrosion of the pipe also occurred at the front end of the inlet pipe, which is connected through the tube sheet and bears the full force of the incoming fluid. At this point, the fluid splits from a single stream into many smaller streams, resulting in turbulence and extremely high local velocities.

3. Steam or water hammer

Steam or water hammer is a powerful force that can cause a heat exchanger casing or tubes to rupture or collapse. Steam or water hammer usually occurs in the presence of a surge in pressure, usually caused by a sudden interruption of cooling water flow, rapid evaporation of stagnant water, or pump failure. This phenomenon can be observed in feedwater heaters, as high steam pressure increases the likelihood of steam strike.

Hammering sounds are often heard, but damage to the casing is rare. The tube is more fragile than the casing and more susceptible to hammer blows, but damage to the tube will only be discovered during internal inspection or if a leak occurs.

4. Thermal fatigue

Heat exchanger tubing is prone to tearing and cracking due to accumulated stress caused by continued thermal cycling or high temperature differences. Thermal fatigue occurs when extreme temperature differences between the casing and the pipe cause the pipe to buckle. Thermal fatigue can cause pipes to buckle, creating stress loads that exceed the tensile strength of the material and ultimately cause the pipe to rupture.

Another result of high temperature differences is physical thermal expansion and contraction of the pipe along its length, which may ultimately compromise the integrity of the pipe-to-tubesheet connection, resulting in leaks.

It is nearly impossible to diagnose the threat of thermal fatigue before it fails.

5. Vibration/Resonance

Vibration and resonance, regardless of the source, whether caused externally or internally, exert powerful forces on heat exchanger tubes, and once vibration or resonance begins, its intensity increases until the tube ruptures, fails, or becomes incompatible with the tubesheet The seal is lost and leaks occur.

Baffles provide important support for the tubes in a shell-and-tube heat exchanger and direct the flow of shell-side fluid to assist in the exchange of heat energy. Heat exchanger tubes are often welded or tightly rolled into the tube sheets to ensure the joints do not leak. The locations where the pipes come into contact with the baffles and tubesheets are weak points.

Excessive fluid velocity on the tube side may cause high-frequency vibration or resonance of the tube, causing wear between the tube and the edge of the baffle. This may result in tube rupture or failure of the tube-to-tubesheet bond. Equipment or machinery equipped with a shell and tube heat exchanger may also transmit its external vibrations to the heat exchanger tubes and cause damage or failure.

6. Pitting corrosion of pipes

Chemical corrosion can cause pitting and even pinhole formation in heat exchanger tubing, leading to tube failure and leaks.

Pitting corrosion is caused by the electrochemical potential difference between the inside and outside of what is commonly known as a concentration cell. The oxygen-rich environment in a concentration cell acts as the anode and the metal surface acts as the cathode, causing pitting corrosion to occur slowly on the metal surface due to chemical reactions.

Concentrated electrochemical gradients of oxygen (O2) and carbon dioxide (CO2) are often the cause of pipe wall pitting, and excessive amounts of chemical compounds such as chloride and sulfate are often present in inadequately treated cooling water.

7. Manufacturing and installation defects

The heat exchanger may have quality problems during the manufacturing process, such as poor welding, improper material selection, etc. Improper installation may also cause heat exchange tube failure. If the operating temperature and pressure exceed the design range, or the fluid flow rate is improperly controlled, it may cause the heat exchange tube to be overloaded and accelerate failure.

Failure prevention measures

To prevent heat exchange tube failure, it is recommended to take the following measures:
Clean pipe bundles regularly to remove dirt and corrosion products.
Choose corrosion-resistant materials or increase pipe wall thickness.
Control proper flow rate to reduce corrosion and scaling.
Strengthen the operation monitoring and maintenance of heat exchangers to ensure that operating conditions meet design requirements.
For fluids that are prone to scaling, clean them regularly to remove scaling.

Optimize structural design to reduce the impact of fluid-induced vibration.

Summary

Heat exchange tubes are generally stainless steel pipes, including welded pipes and seamless pipes. Through the introduction of this article, you can understand the heat exchange tube, why the heat exchange tube fails and the preventive measures.