Peter Olschewski, Belzona Polymerics, Austria
Hydropower plant owners and operators are rehabilitating and upgrading their plants to increase the value of output, add capacity, improve reliability, reduce operating and maintenance expense, extend plant life, and comply with environmental and safety regulations or voluntarily imposed standards.
A hydropower station. located in Aschbach, Austria, had been experiencing problems because of a new larger turbine runner, which had recently been built to increase the turbine output. Before the installation of the larger turbine runner, it was necessary to manufacture and install a new turbine casing. Production and installation of the new turbine casing was carried out, including the welding.
Unfortunately, the heat generated during this process caused the turbine casing to become distorted and a circumferential gap of between 2.8 mm to 4.6 mm appeared between the two mating flanges. As a consequence, the correct assembly of the two parts became impossible without stressing the components and, potentially, additional leak sealing being required.
CALLING IN THE INSPECTORS
In response to this problem, the local Belzona authorized distributor was called in to inspect the problem. Following an on-site visit, it was proposed that a special repair method would be developed to solve the problem, based on Belzona’s experience in heavy industries, including steel manufacture and shipbuilding and repair. This repair could be carried out in-situ, and would provide a long-term solution for the hydropower station with additional cost-saving benefits because no new components would need to be fabricated.
It was important that the gap between the two parts was filled to ensure a leak-proof mating between the two flanges. Due to vibrations, very high compressive forces are applied to this flange and from the client’s evaluation there would be small movements despite the flanges being mated together. The decision was made to fill the inside part of the gap where the O-ring is normally located with an elastomer, Belzona 2131 (D+A Fluid Elastomer).
|Figure 1: The circled red area shows where the existing spiral casing (top section) had been bolted together with the new turbine casing (bottom section)|
The Belzona 2131 elastomer would act as the O-ring, which normally sits inside the flange. The outer part of the gap would then be filled by injecting Belzona 1321 (Ceramic S Metal). Belzona 1321 is a polymer repair system designed for surfacing metals attacked by erosion-corrosion, and also used in OEM situations to create irregular load bearing shims with high compressive strength. Further calculations showed that there would be a seating stress of up to 10.2 N/mm² on the flange. Belzona 1321 was specifically chosen for this application because it has been tested with compressive forces over ten times higher, and was therefore suited to permanently withstand the force.
|Figure 2: The Kaplan turbine: (A) stay vanes; (B) spiral casing; (C) circumferential gap; and (D) turbine runner output, 85 MW|
Following an analysis of the problem and the product specification, based of the knowledge of the acting force, a 1:1 model of the gap was manufactured.
It was on this model that the method which would later be applied to the turbine was tested. In the 1:1 model, as shown in Figure 3, a backer rod was placed to prevent the liquid Belzona 2131 elastomer from flowing into the outer parts, where later the Belzona 1321 was to be injected.
|Figure 3: This shows the 1:1 model of the gap and the backer rod, which was placed in the gap to prevent any liquid Belzona 2131 elastomer from flowing into the outer part of the gap and thus staying where the O-ring used to sit|
The flow of the liquid Belzona elastomer was monitored to show whether the entire cavity would be filled with the liquid elastomer. This step was essential in order to eliminate any potential problems that could later lead to problems during the main repair and the resulting downtime. Once the materials and static load had been tested, the repair of the turbine flange could be undertaken.
To begin the process, the surface in and around the gap was prepared by grit blasting; this was carried out to ensure a maximum adhesion of the Belzona 2131 elastomer.
|Figure 4: Blasting with the dust-free vacu-blast system, STF 50. During the blasting process there is negligible dust formation, which offers benefits in terms of the protection of the equipment, as well as a reduced risk to the system operator|
This was an essential part of the application because the Belzona 2131 elastomer would replace the O-ring, which would normally be assembled with a pre-tension, and would need to withstand the tensile stresses that are produced by the vibrations. The upper section of the flange is made from cast iron and the lower section is made from stainless steel, therefore a very angular abrasive had to be used to ensure a minimum profile depth of 90 microns.
Initially, the client did not want to blast because there were concerns regarding dust being generated, which could get into seals and subsequently affect the operation of the turbine. However, by using dust-free vacu-blast equipment, it was possible to blast practically without any formation of dust and the required minimum profile depth of 90 microns could be obtained.
|Figure 5: The profile depth of the chamber and the front section of the surface, which was 90-100 microns|
Testing showed the client that adhesion on the non-grit blasted steel was only 0.5 N/mm² to 0.7 N/mm², in comparison to Belzona who managed to achieve a three times higher adhesion of 1.8 N/mm² to 2.5 N/mm² on the grit blasted steel.
Results from both the dust-free blast equipment and adhesion testing convinced the client and allowed the blasting of the gap. This blasting with the dust-free equipment reduces the risk but more importantly it reduces the complexity because there is no need to build a tent around the surface, which would be necessary when blasting with usual equipment.
APPLICATION, APPLICATION, APPLICATION
As shown in Figure 3, the next step was to insert a foam backer rod into the gap to prevent the fluid elastomer from flowing into the outer section. In order to give the Belzona elastomer the best adhesion possible, the surfaces onto which the Belzona 2131 elastomer would be applied had to be pre-treated with a Belzona 2941 conditioner.
|Figure 6: Tape was used to prevent the elastomer from flowing out and to make sure the gap was fully injected with it|
An application onto grit-blasted steel without a conditioner, would lead to poor adhesion and to a possible failure of the seal. Therefore it is essential to apply the conditioner correctly and allow it to cure and dry before the application of the elastomer can commence.
The injection nozzle was elongated with a flexible straw to ensure that the elastomer could be injected into even the smallest area with only a gap of 2.8 mm. The injection method was carried out very slowly to prevent any air from becoming trapped in the product. After the product had cured, which in this case needed three-and-a-half days, the tape was removed and the elastomer was faired level to the metal surface.
|Figure 7: The completed repair with the elastomer on the inside section of the gap|
Once the inside section of the gap had been filled with elastomer, the outer part then needed to be prepared for the injection of Belzona 1321.
Figure 8 shows how the Belzona 1321 was injected into the 96 bolt holes. The edge was then sealed with Belzona 1111 (Super Metal) to ensure that the Belzona 1321 would not flow over the edge. As shown in Figure 8, an M3 bolt was inserted into the Belzona 1111 in the area of each bolt hole to control the flow and ensure that the cavity was completed filled with Belzona 1321. A specially prepared injecting bolt was inserted into the bolt hole. Through this bolt the Belzona 1321 was then injected. This bolt was treated with Belzona 9411 (Release Agent) which ensured, that the bolt could be removed easily after curing.
Figure 8: A diagram showing how the outer section of the gap was filled
Once the product was fully cured, the release agent treated bolts were replaced by the originals and the flanges could be mated together without any unnecessary stressing, but most importantly, the casing joint was now fully sealed.
Following this original application, two further power stations have benefitted from this technology, as increasingly more Kaplan turbines runners are upgraded to increase output.
The author would like to thank WG-Technik Werkstoffe und Technologie GmbH for their assistance in preparing this article.
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