Polymeric Materials Present Alternatives to Traditional Turbine Blade Repair

Developing polymer technology has led to specialized polymeric coatings that offer hydroelectric turbine runners excellent resistance to erosion, corrosion and cavitation. In applying the coatings, testing indicates the technology has positively affected flow efficiency.

By Kyle Flanagan

Supplying 16.4% of global electricity annually and 85% of global renewable electricity, hydropower has experienced an upsurge in development, with an International Energy Agency report stating emerging economies have the potential to double hydroelectric production by 2050. Total installed capacity of hydro has grown by 27% since 2004, with an average growth rate of 3% per year. During the past decade, hydropower has reached 1,000 GW of total capacity, with 40 GW installed in 2013 alone, according to a 2015 report by the World Energy Council.

To maintain hydroelectric power supplies, maintaining hydroelectric assets is increasingly critical. At the same time, flow levels at many hydro projects have been reduced because of higher local demand for water. For example, the U.S. Department of Interior’s Bureau of Reclamation owns and operates the Hoover hydroelectric facility on the Colorado River in the state of Colorado, USA. According to the U.S. Geological Survey, from 2000 to 2015, Hoover’s reservoir water volume dropped 37 m. As a result, total capacity of the facility, which was 2,074 MW when it was commissioned in 1936, was dropped to 1,592 MW in June 2014 and is projected to continue to decline. The drop in water volume in the reservoir has decreased head available to the turbines, resulting in increased risk of cavitation damage.

As with any fluid flow equipment, the effects of erosion and corrosion will decrease production efficiency. If left unchecked, erosion from cavitation damage increases exponentially and causes severe metal loss. If this causes turbine runners to become unbalanced and vibrate beyond tolerances, the result would be lengthy shutdowns to repair a unit’s turbine shaft and its bearings, in addition to the turbine blades. Deterioration in surface smoothness also produces increased turbulent flow, which can be an additional factor that contributes to lower power production rates.

Traditional repair techniques

Following its installation, inspecting equipment at set intervals is the recommended and effective means to ascertain the rate at which runner damage occurs. Once the rate of damage is known, procedures are put in place to repair damage once it reaches pre-determined levels (i.e., depth of metal loss >3 mm).

Polymeric coatings can offer options to asset owners in maintaining equipment.

Traditional repair procedures often use conventional metal-for-metal replacement techniques. Welding plates are used to repair large pitted areas or new metal sheets are put in place to act as an erosion wear layer. Weld overlays are used to repair areas that have light damage and the welds are then ground to correct tolerances.

Traditional techniques

Reintroducing the same base material to a damaged runner is a flaw in traditional repair techniques because metal-for-metal replacement does not mitigate the root causes of runner deterioration.

Increased damage to runner blades is one of the major drawbacks of using hot work, a long-standing metal repair method. In addition, complex rigging and support must be set up to avoid distorting finely honed parts. Hot work is a gradual process during which personnel must heat the entire part before applying the specific repair technique. Conversely, between applications of the hot work repair, the component needs lengthy cool-down times to avoid excessive heat distortion. Care is also required when selecting the repair metal (plates or welding rods), as different materials can introduce local galvanic corrosion, initiating even more repair requirements.

Reclamation has similar experiences, saying, “Extensive weld repairs can result in runner blade distortion, acceleration of further cavitation damage and possible reduction of turbine efficiency. Also, extensive repair can cause residual stressing in the runner resulting in structural cracking at areas of high stress.”

Polymeric coatings

Modern polymeric repair systems offer an alternative to traditional repair materials. These materials are supplied in paste (filler-type repair composites used to infill damaged areas and restore profiles) or coating grade products used to provide long-term protection to equipment against specific damage. Polymeric coatings completely halt corrosion by isolating the metals and closing the corrosion cell.

Polymeric coatings have been used for more than 60 years in many different industries, from hydroelectric generation to offshore and onshore oil and gas production, pumps and sewage treatment. They have proven themselves in these environments. For instance, in 2003 an inspection revealed that a Francis turbine runner at a hydro plant in Czech Republic had suffered from erosion, corrosion and mechanical damage. After the repair and coating of both the runner and wicket gates, a 10% to 15% increase in output was recorded and the turbines were able to run at a greater range of heads than previously possible.

Epoxy polymer materials (in blue) are cold-curing, eliminating the requirement for hot work.

After a year of operation, the runner was inspected and found to have only suffered minor damage due to impact with pieces of wood. A further inspection in 2006 showed the runner to be in excellent condition, and in 2010 the client expressed satisfaction with the application.

Specialized filler materials, such as ceramics and aluminum oxide, allow epoxy coatings to achieve incredible wear resistances where required. Exceptional bond strengths mean these epoxy coatings combine with the metallic substrate to provide a composite component with minimal requirements for ongoing maintenance.

Application

First, thorough surface preparation is required in the area to be repaired. This is commonly achieved by grit blasting to clean and roughen the metal, allowing the polymer to form an intimate bond with the base metal.

Polymeric repair and coating composites are supplied as two-part products. These can be easily mixed in situ using spatulas and bowls or by paddle mixers for larger applications. This mixing initiates the chemical reaction, which results in the product solidifying to its final form.

Usable life of mixed epoxies is designed with application in mind. Depending on application size, fast acting (usable life of 9 minutes at 20 degrees Celsius) or long working life (usable life of 40 minutes at 20 C) epoxies are specified, allowing faster back to service or more time to apply the material. Application is commonly carried out using trowels or brushes to lay up rebuilding composites and by brush for coating grade epoxies. Many products can also be applied by airless spray, allowing for rapid application over large areas. This can again drastically reduce downtime as Belzona has recorded application rates as fast as 100 m2/hour. The product is then allowed to cure in accordance with the manufacturer’s recommendations. Cure times are temperature and reaction specific but can be three to four hours up to several days. Once fully cured, the equipment can be returned to normal service. Overall start to finish for an epoxy repair can be as little as 24 hours from preparation and in many cases will not involve extensive dismantling of equipment.

Modern epoxy polymer materials are cold-curing, so they eliminate the requirement for hot work needed when using traditional repair techniques. This avoids problems such as:

  • Risking equipment distortion;
  • Requiring specialty rigging and jigs;
  • Lengthy repair times required for welds to cool;
  • Grinding and finishing overlay welds;
  • Health and safety hazards associated with hot work;
  • Need for specialist welding rods and expensive replacement metal;
  • Introducing heat-affected zones due to welding; and
  • Lengthy shutdown times

Using epoxy composites as a protective coating for the base metal also allows for easier wear identification after service. Different-colored layers of polymeric coatings would allow wear areas to be quickly identified.

Repairing existing polymeric coatings is simple as they can be locally prepared using powered hand tools or similar and patch repaired as opposed to continually repairing damaged areas by welding during each shutdown, as required using conventional repair techniques.

Advanced application methods, such as airless spray equipment, have allowed even faster repair times.

Turbine casings, draft tubes and outlets are all subject to the same damage as the turbine runner and can be repaired using the same epoxy polymer products.

The newest generation of epoxy coatings now incorporate advanced polymer fillers, providing even more improved erosion resistance. Traditional erosion resistant coatings have relied on “ceramic” or “alumina” type fillers to provide a hard wearing surface. These have had limitations in impact resistance as they form a hard but brittle surface once cured and applicability using high pressure spray equipment as the hard filler particles tend to quickly wear application equipment. Using “thermoplastic ductile alloy” fillers, which have excellent resistance to sliding particles and fluids as well as a ductile capacity capable of withstanding moderate impacts, resolves these issues.

Belzona has developed several polymer coatings for pumping and hydroelectric generation designed specifically to improve runner efficiency and reduce cavitation.

Efficiency enhancement

Increasing existing equipment efficiency would allow asset owners to get the most from their facilities. One of the most effective methods to improve asset performance is by applying coatings to the substrate, which will reduce flow resistance from friction. Belzona offers a 1341 Supermetalglide product that is a hydrophobic epoxy coating with a low electronic affinity with water molecules, making it a water-repelling material. Testing carried out in 1994 by a European pump supply company has proven that when compared to polished stainless steel, the Belzona product is 15 times smoother. Once applied, it forms a smooth surface that reduces the turbulent boundary layer of the pumped fluid, thus increasing hydraulic efficiency.

Turbine runners can be repaired using epoxy polymer products.

In 1986, the U.S. National Renewable Energy Laboratory conducted an independent test on a new pump that had not been in service previously and on which the Belzona hydrophobic epoxy coating was applied. Test results indicated the coating gave a maximum 6% increase in peak efficiency. Meanwhile, at this peak efficiency point, the power reduction was measured at 5.1 kW at the pump duty point – the point on the curve where the flow and head match the application’s requirement. Assuming a 5,000-hour operating cycle/year, the power savings over this period could amount to 25,400 kWh.

When the same coating is applied on existing equipment, the increase could be even higher. Equipment that has suffered from heavy deterioration and loss of efficiency may be returned to better than original performance. According to a 1997 report from the city of Fayetteville, in the U.S. state of Arkansas, in comparative condition-to-condition tests on heavily deteriorated pumps, equipment using polymeric coating Belzona 1341 showed improvements of up to 17%.

Cavitation

Occurring in areas of pressure change across fluid flow equipment, cavitation is possibly one of the most damaging and difficult forms of erosion encountered in hydroelectric equipment. Rapid implosion of vapor bubbles close to the metallic substrate results in powerful micro jets that impact and “chip” the base material, resulting in pocketed erosion.

Using hard materials and specialty alloys is common practice in cavitation mitigation, but these measures are often expensive, involving expansive outlay on materials, specialist installers, rework following repairs, heat treatment and finishing. Reinstating the original material that has been lost will also simply reintroduce the same issue in the future and the same outlay can again be expected once the issue returns. To resist the effects of cavitation, Belzona developed its 2141 ACR Elastomer, a two-part elastomeric polymer applied specifically to areas subject to cavitation damage. When used in conjunction with Belzona rebuilding epoxies to reinstate lost material, these materials significantly reduce repair costs as well as withstand the cavitation mechanism.

Elastomeric coatings focus on resisting the key forces present in areas where cavitation commonly occurs on fluid handling equipment. Exceptional bond strength, resistance to temperature and the ability to absorb the extreme impact pressures from the micro jetting are all requirements fulfilled by the elastomeric polymer materials in use today.

For instance, the owner of a hydroelectric plant in Ontario, Canada, was faced with a cavitation problem. Twin 914 mm cast iron Francis turbine runners, constructed in 1945 and operating with a 13 m 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.

The fluid elastomeric coating was installed in 2002 and later inspected that same year with no damage noted. Inspection after an additional 34 months in service showed no sign of damage to the coating.

In another example, the port bow thruster tunnel of a Fast Ferry, built in 1997, suffered extensive cavitation damage. Previous attempts with glass-flake and semi-rigid epoxy coatings lasted fewer than 12 months before repair was necessary. In 2002, an epoxy-based composite material was utilized to repair heavy pitting and provide a smooth surface.

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.

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.

Kyle Flanagan is the former technical services manager at Belzona Polymerics Ltd.

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