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Repair and modernization of heat exchangers.
CTI technology for the repair and modernization of shell-and-tube heat exchangers using metal tube inserts.
Typically, most damage to heat exchanger tubes used in industrial plants occurs in the first (150 mm) section of the tube bundle. Tube damage manifests itself in the form of sinking of the tube, formation of grooves, pitting, cracks, which can eventually lead to leakage and failure of the tube bundle (Figure 1).
Fig. 1. Corrosion damage to heat exchanger tubes
a – on the outer surface;
b – at the end sections;
c – crevice corrosion
In the past, this rather localized problem was solved by replacing the entire tube bundle, even though more than 95% of the tube bundle length remained intact. In some cases, the bundle could be “saved” by shortening the heat exchanger, but this reduced the heat transfer surface area.
Any of these radical solutions are expensive and time-consuming.
In the 1970s, a new technology for repairing heat exchanger tubes was developed, which uses flared metal tube shields. This universal method of in-situ repair has also been successfully applied in the case of tube failure along its entire length by installing inserts (shields) along the entire length of the damaged tubes.
Any of these radical solutions are expensive and time-consuming.
In the 1970s, a new technology for repairing heat exchanger tubes was developed, which uses flared metal tube shields. This universal method of in-situ repair has also been successfully applied in the case of tube failure along its entire length by installing inserts (shields) along the entire length of the damaged tubes.
Common failure mechanisms of heat exchanger tubes
Inlet pipe end erosion is common in carbon steel and copper alloy coolers, process heat exchangers, and condenser tubes and is caused by the kinetic force of the fluid (or gas), especially if it contains abrasive impurities.
At the inlets of the heat exchange tubes, changes in the direction of the medium flow and the air bubbles that form create strong turbulence, leading to damage to the protective passive films on the inner surfaces of the tubes. After 150 mm of tube length, the turbulent flow becomes laminar, and the aggressive effect of the medium decreases sharply. Other factors contributing to strong turbulence are the irregular shape of the distribution chamber (channel) and the supply pipe.
If the medium contains corrosive components, the negative consequences for the heat exchange tubes can be much greater. This is the so-called erosion-corrosion phenomenon. The combined effect of erosion and chemical influence of the medium is much higher than their separate effects. The cause of erosion-corrosion is most often corrosive process fluids. However, this phenomenon can also be caused by water, for example, when the heat exchange tubes are exposed to cooling water with an admixture of sulfides and/or ammonia.
Stress corrosion cracking (SCC) is another common type of failure in heat exchanger tubes. It is caused by the combined action of tensile stress and corrosion. SCC often occurs at the flared area of the tubes directly behind the tube webs, especially when the flare is excessive. The most common forms are ammonia corrosion cracking of copper-based alloys, chloride SCC of austenitic corrosion-resistant steels, and alkaline SCC of carbon and alloy steels.
Other types of corrosion that occur at tube ends and tube-to-tube grid joints are pitting and crevice corrosion. Crevice corrosion is a dangerous form of deep local penetration that most commonly occurs in austenitic (Cr-Ni) corrosion-resistant steels exposed to chlorides.
The rate of both pitting and crevice corrosion increases significantly with increasing temperature.
The cause of failure in the areas of the inlet grille and the ends of the heat exchange tubes can also be mechanical factors (for example, improper flaring of the tube) and poorly executed welds between the tube and the tube grille.
Fig. 2. CTI Shield/Seals
Thin-walled metal inserts for repairing tube ends (shields)
Thin-walled metal inserts (shields) were first used in 1976. This method of repair protects, repairs and seals damaged pipe ends. The shields (Fig. 2) are manufactured to specific dimensions and retain the ductility necessary for flaring.
As with the installation of tubes in a heat exchanger, it is important to choose the right alloy for the manufacture of shields depending on the tube material, heat exchanger functions, and failure mechanism.
Inserts are made of copper alloys (Cu-Ni and brass), conventional corrosion-resistant steels (austenitic, ferritic, martensitic, dual-phase), hyperaustenitic corrosion-resistant steels (six molybdenum alloys) and 4276-based alloys. This allows the selection of the required alloy for protection against specific failure mechanisms, such as chloride pitting, CRC, ammonia pitting, etc.
The installation process is performed “in-place,” starting with cleaning the inside diameter of the pipe with a wire brush to create a tight seal. After blowing the pipes with compressed air, the inside diameter of the pipe is measured to determine the flaring requirements. The shields are then inserted into each end of the pipe. The outer end of the shield is flared with a torque limiter using a conventional pipe flaring tool, while the inner section of the shield is flared using a mechanical stop, thus reducing the possibility of over-flaring. The final step is to flare the end of the shield to match the profile of the pipe grid.
The shields were installed in a high pressure (30 MPa) and high temperature (300°C) installation. In some cases, the shields could connect completely disconnected pipes. Pipe restoration using flared metal shields is an economically viable repair method.
Due to the thin-walled design and the possibility of hydraulic expansion of the shields, the cross-sectional area of the heat exchange tubes is slightly reduced.
Thin-walled metal inserts (shields) were first used in 1976. This method of repair protects, repairs and seals damaged pipe ends. The shields (Fig. 2) are manufactured to specific dimensions and retain the ductility necessary for flaring.
As with the installation of tubes in a heat exchanger, it is important to choose the right alloy for the manufacture of shields depending on the tube material, heat exchanger functions, and failure mechanism.
Inserts are made of copper alloys (Cu-Ni and brass), conventional corrosion-resistant steels (austenitic, ferritic, martensitic, dual-phase), hyperaustenitic corrosion-resistant steels (six molybdenum alloys) and 4276-based alloys. This allows the selection of the required alloy for protection against specific failure mechanisms, such as chloride pitting, CRC, ammonia pitting, etc.
The installation process is performed “in-place,” starting with cleaning the inside diameter of the pipe with a wire brush to create a tight seal. After blowing the pipes with compressed air, the inside diameter of the pipe is measured to determine the flaring requirements. The shields are then inserted into each end of the pipe. The outer end of the shield is flared with a torque limiter using a conventional pipe flaring tool, while the inner section of the shield is flared using a mechanical stop, thus reducing the possibility of over-flaring. The final step is to flare the end of the shield to match the profile of the pipe grid.
The shields were installed in a high pressure (30 MPa) and high temperature (300°C) installation. In some cases, the shields could connect completely disconnected pipes. Pipe restoration using flared metal shields is an economically viable repair method.
Due to the thin-walled design and the possibility of hydraulic expansion of the shields, the cross-sectional area of the heat exchange tubes is slightly reduced.
Fig. 3. CTI Liners
Pipe inserts (doublers) for the full length of pipes
The successful use of shields to repair pipe end damage has led to the development of a similar repair technology that allows for the repair of pipes with damage along their entire length. Such repair involves the installation of an insert (liner) equal to the length of the entire pipe and its subsequent hydraulic expansion to ensure metal-to-metal contact.
Leaks occur due to pitting on the inner surface and pitting on the outer surface, a reduction in wall thickness along the entire length of the heat exchange tubes, and damage due to impact corrosion in the tube bundle. In such cases, plugs are usually installed on the damaged tubes. In this case, the heat exchange tubes are taken out of service.
As the heat exchanger wears out over time and the number of plugged tubes increases, the efficiency of the installation begins to decrease and fluid consumption increases. If more than 10% of the tubes are plugged, the complete tube bundle of the heat exchanger unit must be replaced.
In such cases, installing full-length inserts on the tubes is an attractive alternative. Regularly reconditioning of plugged tubes can provide additional years of service life for the heat exchanger.
The successful use of shields to repair pipe end damage has led to the development of a similar repair technology that allows for the repair of pipes with damage along their entire length. Such repair involves the installation of an insert (liner) equal to the length of the entire pipe and its subsequent hydraulic expansion to ensure metal-to-metal contact.
Leaks occur due to pitting on the inner surface and pitting on the outer surface, a reduction in wall thickness along the entire length of the heat exchange tubes, and damage due to impact corrosion in the tube bundle. In such cases, plugs are usually installed on the damaged tubes. In this case, the heat exchange tubes are taken out of service.
As the heat exchanger wears out over time and the number of plugged tubes increases, the efficiency of the installation begins to decrease and fluid consumption increases. If more than 10% of the tubes are plugged, the complete tube bundle of the heat exchanger unit must be replaced.
In such cases, installing full-length inserts on the tubes is an attractive alternative. Regularly reconditioning of plugged tubes can provide additional years of service life for the heat exchanger.
Fig. 4. Installing inserts in a finned air cooling unit
tubes
tubes
The process of installing tube inserts (liners) begins with removing plugs and thoroughly cleaning the tubes (hydraulic, hydromechanical, using various brushes and scrapers made of metal and nylon). Next, liners are installed in the standard heat exchange tubes (Fig. 3, 4) with some allowance in length, which allows the use of special nozzles for water supply and air bleeding.
Fig. 5. Hydraulic expansion of inserts.
The next stage is hydraulic expansion of the liners (Fig. 5). When water is supplied to the liners under high pressure (15 ... 75 MPa), their size increases (in diameter along the entire length) until they collide with the standard tubes.
Hydraulic expansion stops only after the liner walls are in full contact with the walls of the standard tubes with the necessary tension. Despite the high pressure created inside the liners, the hydraulic expansion technology practically eliminates the risk of damage to the standard tubes. Then the liners are cut, milled and flared in accordance with the technical specifications.
Previously blocked pipes that have been deemed unusable are returned to service.
Thin-walled metal inserts have been used effectively for over 35 years to restore failed tubes and return heat exchanger tubes to service worldwide.
Due to the wide range of alloys suitable for manufacturing tube inserts, this inexpensive method can extend the service life of heat exchangers in corrosive environments and under high temperature and pressure conditions.
Currently, the design of new heat exchangers often also includes the installation of full-length shields and inserts.
Hydraulic expansion stops only after the liner walls are in full contact with the walls of the standard tubes with the necessary tension. Despite the high pressure created inside the liners, the hydraulic expansion technology practically eliminates the risk of damage to the standard tubes. Then the liners are cut, milled and flared in accordance with the technical specifications.
Previously blocked pipes that have been deemed unusable are returned to service.
Thin-walled metal inserts have been used effectively for over 35 years to restore failed tubes and return heat exchanger tubes to service worldwide.
Due to the wide range of alloys suitable for manufacturing tube inserts, this inexpensive method can extend the service life of heat exchangers in corrosive environments and under high temperature and pressure conditions.
Currently, the design of new heat exchangers often also includes the installation of full-length shields and inserts.