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Enhancement resistance to erosion and corrosion

2nd October 2014

Posted By Paul Boughton


Specialised polymeric coatings which can offer excellent resistance to erosion, corrosion and cavitation in hydroelectric equipment. Kyle Flanagan reports.

Currently accounting for over 16 per cent of global energy production and with expected growth rate of 3 per cent per annum for the next quarter century, hydroelectric power generation continues to grow as the frontrunner in renewable energy.

In recent years, maintenance of existing hydroelectric assets has become increasingly critical to ensure consistent supply of power. Low water levels due to factors such as higher local demand for water have resulted in decreased production in high profile hydroelectric stations such as the Hoover Dam. There, the problem has become so severe that the resulting drop in pressure difference has caused increased cavitation damage to turbine runners and a 20 per cent decrease in production levels. 

Ensuring turbine efficiency and up-time are at their maximum is key to achieving optimum production. However, as with any fluid flow equipment, the effects of erosion and corrosion will detract from this. If left unchecked, erosion and specifically cavitation damage rates increase exponentially to cause severe metal loss. Unbalancing and vibration of turbine runners can result requiring lengthy shutdowns for repair work to shafts and bearings. Loss of surface smoothness also results in increased turbulent flow and lower production rates.

The recommended procedure to determine the frequency of inspection and repair of hydroelectric runners (and turbines including stay vanes, wicket gates etc.) is to inspect the equipment at set intervals following installation to ascertain the rate of damage (including erosion, corrosion, cavitation, etc). Once the rate of damage is known, procedures are put in place to repair the damage once it reaches pre-determined levels of damage (commonly, depth of metal loss >Xmm).

Once a maintenance routine is put in place, repairs are carried out as per the recommended procedure. This procedure is often to replace the lost metal using conventional metal for metal replacement techniques. Large areas of pitting are repaired by welding plates or sheets of new metal in place as an erosion wear layer whereas areas of lighter damage are recommended to be repaired by weld overlay which is then ground back to the correct tolerance. The procedure is repeated at the next service interval as dictated by the rate of in-service deterioration.

This repair procedure is not without problems though. The most basic flaw is the replacement of the material which is being lost with more of the same material, a like for like repair.  Reintroducing the same base material simply allows the problems to reoccur and does not identify the root cause of the issue and work to limit its effects. Continued metal loss will result in continued shutdowns. As discussed above, metal loss will, in some cases, result in vibration due to imbalance and this can cause damage to bearings and shafts. 

In order to avoid distortion of finely honed parts, extensive rigging and supports are recommended.  Hot work is recommended to be carried out gradually, heating up the entire part first prior to application of the repair technique and lengthy cool down times between application of the repair 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. 

Modern polymeric repair systems offer an excellent alternative to traditional repair materials. These materials are supplied in either paste grade (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. Advanced polymeric coatings completely halt corrosion by isolating the metals and closing the corrosion cell. 

Polymeric coatings have been used for over 60 years in many different industries form hydroelectric generation, to offshore and onshore oil and gas production, pumps, sewage treatment, etc, and have proven themselves in these environments time and time again. Solvent free epoxy technology means that these products are very safe to use, even in enclosed spaces. 

Specialised filler materials such as ceramics and aluminium 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 huge advantages in terms of on-going maintenance. This is all down to the ease of application of the epoxy polymer systems. 

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

Polymeric repair and coating composites such as Belzona are supplied as two part products. These can be easily mixed in situ on site using spatulas and bowls or by paddle mixers for larger applications. This mixing initiates the chemical reaction which will see the product solidify to its final form. 

Generous application times allow the applicator to carefully apply the product to the area to be repaired. Application is commonly carried out using trowels for paste grade, rebuilding composites and by brush for coating grade epoxies. Many products can also be applied by airless spray, allowing for rapid repair times over large areas. The product is then simply allowed to cure for a period of time before the equipment can be returned to service. 

Because modern epoxy polymer materials are cold-curing, they immediately eliminate the requirement for hot work as necessitated when using traditional repair techniques described previously. This avoids problems such as:

* Risk of distortion of equipment;

* Requirement for specialist rigging and jigs;

* Lengthy repair times are required to allow cooling of welds;

* Grinding and finishing of weld overlay;

* Health and safety hazards associated with hot work;

* Need for specialist welding rods and expensive replacement metal; 

* Introduction of HAZ (heat affected zones) due to welding;

* Lengthy shutdown times.

Use of epoxy composites as a protective coating for the base metal also allows for much easier wear identification following service times. Different coloured layers of polymeric coatings will allow wear areas to be quickly identified. Repairing existing 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 while allowing application by airless spray, ideal for larger areas. 

Several polymer coatings have been specially developed by Belzona for applications in pumping and hydroelectric generation which aim to improve efficiency and reduce cavitation specifically.

As stated above, increasing the efficiency of existing equipment will allow asset owners to get the most from their equipment. One of the most effective methods to improve asset performance is by applying coatings which will reduce resistance to flow caused by friction with the substrate. Belzona 1341 (Supermetalglide) is an epoxy coating with a low electronic affinity with water molecules (a hydrophobic or, ‘water repelling’ material). Once applied, it forms an extremely smooth surface which reduces the boundary layer of the pumped fluid and reduces the internal turbulences in the flow, thus increasing hydraulic efficiency. 

When compared to polished stainless steel, it was found that Belzona 1341 (Supermetalglide) was 15 times smoother. Incorporation of ceramic fillers also allows Belzona 1341 (Supermetalglide) to resist erosion and protect the equipment for long service periods. 

Independent testing of Belzona 1341 (Supermetalglide) applied to a new pump gave a maximum 6% increase in the peak efficiency (NEL test report March 1986). Meanwhile, at this peak efficiency point, the power reduction has been measured at 5.1kW at duty point. Assuming a 5,000 hour operating cycle/annum, the power savings over this period would amount to 25,400kWh.

Similar efficiency gains can be expected in hydroelectric equipment. On existing, in-service equipment, the increase will commonly be even higher. Equipment which has suffered from heavy deterioration and loss of efficiency can be returned to better than original performance. On heavily deteriorated pumps, improvements of up to 17 per cent have been recorded (increase is from deteriorated condition not original performance).

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 vapour bubbles close to the metallic substrate results in powerful micro jets which impact and ‘chip’ the base material resulting in pocketed erosion.

Use of hard materials and specialist alloys is common practice in areas of cavitation but these measures are often very expensive and will eventually also fail under constant attack. In order to resist the effects of cavitation, Belzona specially developed Belzona 2141 ACR Elastomer. This is a two part elastomeric polymer applied using a brush as a coating specifically to areas subject to cavitation damage.

Belzona 2141ACR Elastomer followed a lengthy development and research process to determine the key conditions present in cavitation areas on fluid handling equipment. Exceptional bond strength, resistance to temperature and the ability to absorb the extreme impact pressures from the micro jetting were all requirements fulfilled by Belzona’s elastomeric polymer material. 

Extensive independent testing in accordance with ASTM standards yielded exceptional results. 

* Resistance without damage after 500 hours intensive cavitation testing;

* Resistance without damage after 20 hours at 130 knots intensive cavitation testing;

* ASTM G32 results significantly higher than 316L Stainless steel.

Over a decade of field applications which are still providing excellent performance are testament to the materials longevity including applications to turbines, wicket gates, stay vanes and turbine shafts.

Conclusion

Safer, more cost effective repairs which offer longer service intervals are continually sought by all industries. Polymeric coatings offer an excellent option for asset owners when maintaining hydroelectric equipment. They have been proven to reduce cavitation, halt corrosion and slow the effects of erosion on critical equipment for decades. Advanced application techniques and constantly evolving technology have allowed polymeric solutions to become a consistent option against traditional repair methods. 

Kyle Flanagan is Technical Services Engineer at Belzona Polymerics Ltd, Belzona Polymerics Limited, Harrogate, UK. 







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