Parts 1 and 2 of this series examined the cavitation process and a novel approach to preventing it with the use of an innovative, high-performance coating. The new technology can best be described as a polyurethane or as an elastomer specifically designed for coating extremely localized high-pressure areas where abrasion, cavitation, erosion and corrosion resistance are required. We will now look at case studies to examine this new protection technology in action.
Case Study 1: Combating Turbine Runner Cavitation
A hydroelectric plant in Ontario, Canada was faced with a cavitation problem. Twin 36'' (914mm) cast iron Francis turbine runners, constructed in 1945 and operating with a 42 feet (13m) head, had suffered erosion leading to the loss of performance.
Both semi-rigid and flexible epoxy coatings had been applied in 1998 to combat cavitation, but these coatings had eroded after three years in this aggressive environment.
Figures 1, 2 and 3 serve as good training material, as they clearly show the damage occurring in areas of pressure change, where low pressure generated cavitation bubbles, reaching the higher pressure areas and imploding. The problem was consistent on each channel.
Figure 1: Francis turbine showing existing cavitation damage in LP area.
Figure 2: Semi-rigid epoxy at 36 months, erosion damage.
Figure 3: Flexible epoxy 36 months in service showing delamination.
Figure 4: Elastomer coating after 34 months in service without damage.
The fluid elastomeric coating was installed in 2002 and later inspected that same year with no damage noted. Subsequent inspection after an additional 34 months in service (Fig. 4) showed no sign of damage to the coating.
Case Study 2: Repairing an In-service Shaft since 2006
The main 6.29'' (160 mm) diameter shaft, running at 1000 rpm at a hydropower station in France, was damaged by cavitation and erosion (Fig. 5). A system comprising a two-part hydrophobic epoxy and an elastomeric top coating was applied in 2006 to repair the defective area and prevent further damage (Fig. 6).
Figure 5: Cavitation damage to the shaft.
Figure 6: Shaft repaired and protected.
Figure 7: Inspection after 9 months in service.
Inspection of the shaft in 2007 (Fig. 7) and in 2010 showed the system still in perfect condition.
Case Study 3: Repairing an Unusual Cavitation Problem on HSS Ferry
The port bow thruster tunnel of a Fast Ferry built in 1997 suffered extensive cavitation damage.
Figure 8: HSS ferry.
Previous attempts with both glass flake and semi-rigid epoxy coatings lasted less than 12 months before repair was necessary (Fig. 9 and 10). The problem was particularly unusual as it only affected only one of two identical bow thruster tunnels on the port side (whereon the vessel ties to the quay), despite the fact that both tunnels were used equally, and were of identical design.
Figure 9: Damage to glass flake coating.
Figure 10: Damage to semi-rigid epoxy coating.
In 2002, the fluid elastomeric coating was installed. To repair heavy pitting and provide a smooth surface, an epoxy-based composite material was utilized.
Initial inspection after one year showed localized wear in the top of the tunnel only, where a coating application error resulted in insufficient thickness. Otherwise, the elastomer provided a significant improvement over all previous attempts to protect this tunnel. Application of a further coat of elastomer resulted in two additional years of service without the need for repair, allowing the owner to extend periodic dock maintenance from annual to every two years.
This case also provided a learning experience. It was believed that the impressed current cathodic protection systems of both the catamaran and the quay may have been interacting with each other, together with the cavitation, to cause conventional coating/lining systems to degrade prematurely.
In this instance, the two-part epoxy base coat provides excellent corrosion and cathodic disbondment protection, while the elastomeric coating offers cavitation resistance. Applied together as a system, both coatings ensure excellent in-service performance.
Figure 11: An elastomer coating system was applied in 2002.
Figures 12, 13 and 14 demonstrate the elastomeric coating’s performance in service, with the latter two photos showing inspection after seven years in service.
Figure 12: Inspection after 5 years in service.
Figure 13: Inspection after 7 years in service shows deterioration of anti-fouling paint.
Figure 14: Inspection after 7 years in service illustrates the need for periodic inspections.
Figure 13 particularly demonstrates the beginning of deterioration of the conventional marine anti-fouling paint. In this case, the breakdown of the paint had led to undermining the main system, the solution being to extend the elastomer system.
The latter example demonstrates the need to maintain the inspection of coatings. Coatings are a tool for the designer/operator to provide an optimum blend of performance and cost efficiency. However, coatings can be damaged through misapplication, changes in operating conditions or mechanical forces. Periodic inspections afford the operator the opportunity to take corrective action.
Coating Design Challenges
High-intensity cavitation damage involves four main considerations for the polymer coating designer:
- Suction forces (adhesion needed)
- Extremely high instantaneous pressures (resilience needed)
- Instantaneous temperatures in excess of 9032°F / 5000°C (heat resistance needed)
- Permeation in immersion (diffusion resistance needed)
When we add the potential corrosion forces present when metals are used, we realize that the polymer choices for protection are very limited.
Research and development has shown that there is a consideration of the use of epoxy and elastomer technology, which can address these issues and offer the asset owner reassurance with respect to the operation of fluid flow equipment.
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