By Arthur Mendonça, technical services engineer, Belzona
As any other piece of equipment, shafts are subject to damage from corrosion and wear that can be accelerated by specific environmental conditions, insufficient lubrication of mechanical components or improper grounding for galvanic isolation. Worn and defective shafts can not only accelerate the wear of other components, but they can also potentially shut down the entire machine, halting production and resulting in revenue loss. Once damaged, they are conventionally repaired using hot processes, such as welding or metal spraying, and then machined down to the specific design parameters. If not carefully controlled, these conventional methods can cause residual damage to the shaft by generating potential thermal stresses, distortion and undesired metallurgical alterations. These repair methods also require the disassembly of the machine, which can be time consuming and expensive when considering the downtime of the equipment. In the wind turbine industry, replacement or disassembly generally involves contracting a crane service, lowering the components, transporting them to a fabricating shop and re-assembling of the machine. This process can take weeks to be completed.
The repair method
The problem and limitations of conventional methods can be avoided by using polymeric nonmetallic compounds in a cold applied in situ repair solution. The repair consists of injecting or forming the nonmetallic repair material using split formers. Such formers are pre-fabricated considering the design dimensions of the shaft being repaired. The repair procedure can be carried out in situ without the need for specialty equipment or hot work permits. Nonmetallic materials needed for the application are readily available and the standardized application procedure allows contracting or incumbent personnel to be effectively trained within hours.
Avoiding the disassembly of the system represents one of the biggest advantages of polymeric repairs over conventional methods because downtime and its consequential production losses are greatly minimized.
Different polymeric materials can be used to repair and rebuild shafts suffering from metal loss. However, some applications require products that can withstand harsher demanding service conditions than others without failure including elevated temperatures, corrosive marine environment or the combined action of erosion-corrosion. In addition, the repair system must be designed so that the operational limitations of the rebuilding compound are not exceeded within the desired lifetime of the shaft being repaired.
At least one manufacturer of nonmetallic polymer compounds recommends the use of 100% solids epoxy materials in environments prone to corrosion and wear. These 100% solids epoxy materials can be chemically designed with superior properties to withstand the environmental conditions to which they may be exposed.
Some of these properties include:
- Solvent-Free Materials – 100% solid epoxy rebuilding materials are designed so there are no volatile compounds leaving the adhesive material through evaporation at normal temperature and pressure. Thus, it is safer for the applicators to use, especially in confined spaces or small habitats.
- Quick Return to Service – 100% solid epoxies cure through exothermic reactions. The heat generated by the chemical reaction influences the drying times. Quick curing indeed provides a definite appeal for asset owners as enables quick project turnarounds.
- Excellent Resistance to Compression, Tension and Corrosion – Solvent-free epoxies have superior tension and compression strength when compared to other solvented rebuilding materials. In addition, they can perform well in highly corrosive environments.
At least one 100% solids epoxy manufacturer subjects its products to sensible testing protocols to ensure conformance with the most stringent standards set forth by the industry. These tests not only help understand the performance of the product and fitness for service, but they are also useful to fully characterize a material. Manufacturers decide which tests will best highlight their product properties; then, they employ a range of internal and external testing to verify the product’s performance. Some manufacturers benchmark their products to the requirements of internationally recognized industry standards such as ISO, ASME and ASTM. This allows asset owners to establish comparisons among different products and select those that best fit their needs. However, most asset owners are usually interested in performing further tests to simulate an environment that resembles their service conditions.
Case studies — The problem
During the maintenance schedule of a wind farm, the nacelle of the wind turbine was removed to inspect its internal components. Inspection revealed moisture ingress in the nacelle due to the humid environment and unproper weatherproofing of the external surface of the turbine. Excessive humidity inside the turbine and lack of lubrication caused the shaft to corrode and wear, respectively. As aforementioned, the metal loss on shafts can cause inefficient operation of the mechanisms inside the nacelle.
Repairs were required to prevent further corrosion and reinstate the operational efficiency of the components. Two conventional approaches were firstly considered: replacement and welding. Replacing the generator shaft for a new one would require the contracting services of a crane and proper installation and reassembly of the shaft into the nacelle. The cost of such procedures was estimated to be $300,000, not to mention the monetary impact of a two-week downtime and a non-operational turbine. The second option considered was to lower the equipment and transport the shaft to a fabricating shop for weld repairs to be performed. Although this option was $25,000 cheaper than the first option, doing so would also require a crane contractor and transportation logistics. The time estimation was also from two to three weeks.
To avoid excessive disruption of the operation of the turbine, a third option was proposed to the asset owner. This unconventional solution was a suitable in situ repair using nonmetallic 100% solids epoxy polymers. The repair could be completed in 24 hours for different diameters shafts operating in the nacelle.
Applying the solution
Before commencement of any maintenance procedure, all personnel involved in the repair gained full understanding of the repair extent, procedure and application logistics. Although both hand-applied forming and injection technique had demonstrable in-field testing evidence and success track records in different industries, the injection technique was chosen as the preferred method of application for this repair. After discussing different options, the following application method procedure was implemented:
- Design: The split former was constructed as per design (Figure 1) to be consistent with the length of repair area and diameter of the shaft being repaired (Figure 2). Holes were drilled and threaded to serve as injection and venting ports for the 100% solids epoxy fluid grade material (Figure 3).
- Surface Preparation: The damaged area of the shaft was mechanically prepared to the requirements of SSPC-SP11, Power Tool Cleaning to Bare Metal. A rough surface and minimum average profile depth of 1.6 mm around the circumference of the shaft were achieved to optimize the adhesion of the 100% solid epoxy to the substrate (Figure 4).
- A release agent was evenly applied onto the internal surfaces of the split former to facilitate removal of the former after full cure of epoxy material.
- The formers were then placed around the damaged area. The length of the former was confirmed to extend beyond the damage area to proper seal and mold to the dimensions needed. Once properly aligned, the former is clamped by using 4 M10 bolts (Figure 5).
- The fluid grade 100% epoxy was mixed in accordance with the manufacturer’s instructions and then poured into the injection cartridge. The cartridge was inserted into a pneumatic injection gun and injection procedure commenced. The material was slowly injected until the cavity was filled. This was confirmed by the product exiting through the ventilation ports. Injection ports were capped with a bolt as the product reached them.
- The fluid grade epoxy was subsequently allowed to cure in accordance with the manufacturer’s recommendations. The ventilation ports were removed, and the application was completed (Figure 7).
- After full cure of the fluid grade epoxy, the former was removed. The product was mechanically abraded by a fine sandpaper to smoothen any flashing and possible protuberances caused by the injection and venting ports (Figure 8). To remove the formers, extraction jacking bolts were used in each side of the mold.
Several conclusions can be drawn from this article:
- Using cold applied 100% solid epoxies is an effective and proven solution for shaft maintenance when conventional repairs such as welding and replacement are not feasible. Such polymeric materials allow for equipment or assets to be repaired in situ, with no heat, and in an easy and safe manner.
- This method offers the best compromise between cost and performance when compared to shaft replacement and welding repair procedures. Without the need for disassembly of the equipment and with faster turnaround times, the cost of the epoxy material solution is quickly offset by the savings in production and revenue loss caused by extensive downtime.
- When repairing wear and corrosion damage of shafts with a non-metallic material, the root cause of the problem is being solved. 100% solid epoxies will provide excellent corrosion and abrasion resistance to minimize future wear mechanisms.