By Mark Bitter, Randy Furbush, Jacob Hostler, Adam Lyman and Loren Nauss
Aging hydro facilities present challenges, and the hydro plant discussed in this article provides an illustration of how the life of such an aging unit can be extended. The facility is located on a river in a northeastern U.S. state that was home to a number of paper mills constructed during the early 20th century, many of which were converted to generate hydroelectric power.
This mill was built in the early 1900s before being converted to electric power production in 1914, when three vertical Francis turbine-generator units were installed. The plant owner still operates three units with an installed capacity of 8 MW operating at 45 feet of head.
The plant’s Unit 2 had survived the river’s rugged waters since it was assembled in the early 1910s, but by 2015 it had degraded to the point where it was in need of restoration and rehabilitation. The other units are identical to Unit 2, but both have had refurbishment work done in the recent past, along with at least one unit having the generator rewound. Unit 2 was next in line for a proper makeover. The hydro project owner hired Hydro Consulting & Maintenance Services to provide its expertise on the process.
Initial inspection of Unit 2, performed in July of 2015, showed the refurbishment project would involve work on several items, including all 20 wicket gates that would need to be replaced or refurbished (regardless of whether the runner was replaced or refurbished). Parts of the generator, thrust bearing and bottom ring were also seen as needing rehabilitation. In addition, the HCMS team found signs of cavitation, contact wear on both the runner crown and band, a damaged balance weight, and general overall degradation – all common in a unit with this age and environment. To return Unit 2 back to service, the plant owner only had a couple of viable options.
The first and most expensive option would consist of replacing the runner with a new one, requiring custom fabrication from scratch to fit the hydraulic constraints and design of Unit 2. In addition to the hefty price tag – which ranged anywhere from about $125k to $200k for a “like” runner or even more for an upgrade – constructing a new runner would take about a year. The old runner could continue operating in the interim but would continue to be working at a reduced efficiency with the possibility of furthering damage not only to the runner, but the possibility of something detrimental like breaking the shaft of the unit.
|The original runners at the plant had experienced significant wear since their installation in the early 1910s.|
The second option consisted of refurbishing the unit through mechanical and polymer composite methods. These consist of applying a scratch coat as a base, layering that with a metal rebuild material, and then following with a protective coating of a product with chemical resistance.
After reviewing the runner’s surface for weldability, the owner selected the second option because it was more cost efficient for the situation and directed HCMS to move forward with the refurbishment.
The plant’s powerhouse is unique in that it is located inside a paper mill that was built in the mid-1800s. The history of the powerhouse complicated the disassembly process in that many of its areas did not accommodate the use of a crane or other forms of mechanical removal once parts were detached from the unit. Because of the rare features and the initial structure of the site, each part’s disassembly and removal process became that much more important. In certain instances, walls and other configurations of the plant had to be altered or removed to fit items in and out of the plant.
To ship parts and equipment in and out of the facility, they would have to be placed on a rail cart before traveling through the mill’s starch room to get to the powerhouse’s entry. This was a difficult task in itself, especially when following all necessary codes and standards associated with the construction to be completed. Following the guidelines of the American National Standards Institute (ANSI), American Society of Mechanical Engineers (ASME), Occupational Safety and Health Administration (OSHA) and International Organization for Standardization (ISO), it was important to handle the process not only efficiently, but as safely as possible.
|A Hydro Consulting & Maintenance Services worker grinds one of a number of trouble spots identified as needing repair.|
Likewise, environmental laws were almost nonexistent at the time of Unit 2’s installation, which led to a high level of pollution in the area. The mills discharged an unbelievable amount of toxic pollution, specifically dioxin – a chemical byproduct of chlorine compounds used to bleach paper pulp.
While the river has made a remarkable comeback thanks to various federal environmental laws and is nearing a point at which it can support aquatic life once again, early contact with this pollution – along with the cases of pulpwood, rocks and debris in the river – led to considerable wear and sometimes damage to the runner, including contact wear to the crown and band, complicating disassembly even more.
Once the runner and major components were finagled out of the powerhouse and shipped to the HCMS runner facility in York, Pa., the needed maintenance was very visible. This piece of history was in need of upkeep due to the long life it had endured and the tough exposure the river presents.
Cavitation repair and rehabilitation
The beginning of the rehabilitation process involved media blasting the 10-foot-diameter runner to remove as much of the rust and corrosion as possible, allow workers to inspect the substrate’s integrity and provide a clean surface for repair.
The two types of inspection are as follows: magnetic particle inspection (MPI) – a method to locate surface and near subsurface discontinuities in ferromagnetic materials such as iron, nickel and some of their alloys – and liquid penetrant inspection (LPI), a widely applied and low-cost inspection method used to locate surface-breaking defects in all non-porous materials. HCMS used it to detect the welding surface for defects such as cracks, surface porosity and leaks. Per the MPI and LPI specifications, there were more than 50 relevant gross defects and indications greater than 2 inches that needed to be addressed.
|HCMS employees apply a protective Henkel Loctite epoxy coating to the Unit 2 runner following the completion of cavitation repairs.|
The blades of the Francis runner were to be ground and shaved down on the locations with the most significant damage – a process that is tedious, yet necessary to provide the unit with the best chance of working as efficiently as possible. Once ground down, the metal overlay progression commences. This involves rebuilding the parts of the runner with the most cavitation damage, where the unit needs to be at its best when in action at site. Between the two types of repair, roughly 300 man-hours were spent bringing the runner back to proper form.
With the turbine now looking like something that could be a more productive asset for the plant owner, it had to be media blasted two more times before coating the unit with Henkel Loctite protective polymer composite.
Loctite application process
As with any coating job, surface preparation is critical in ensuring the best performance of the product being applied. Per Henkel’s scope of work documentation, HCMS cleaned, media blasted and post-cleaned the rebuilt runner within several hours of beginning to apply coatings.
To wet out the surface and obtain the highest epoxy contact area, a thin-mil brushable ceramic epoxy called a “scratch coat” was worked into the blasted profile of the runner. Once the coat had cured to a tacky but transfer-free consistency, a metal rebuild epoxy paste was applied to the entire runner surface.
|Coatings manufacturer Henkel said applications at similar projects have seen performance improvements of as much as 10%.|
Within the life of the secondary top coat, the team inspected the entire surface, coating over any pinholes or inadequately covered areas. The runner was then tented, using heaters to accelerate and enhance the cure.
From start to finish, the coating process took about 24 hours to complete. Material included a 4 kg kit of EA 3478 Superior Metal, two 6 lb. kits of PC 7333 Brushable Ceramic, and a 12 lb. kit of PC 7319 chemical-resistant coating.
From the beginning of cavitation repair on both the runner and all 20 wicket gates, to the epoxy coating process done with Loctite, these were completed inside of the York runner facility.
The rehabilitation work taken to get this turbine into its current condition was considered a success. The methods used could potentially add more than a decade on to the life of the runner’s exterior. It was restored to the best possible condition and the key customer savings will show years down the road, as maintenance and repair costs for the runner will have substantially diminished.
|The plant’s No. 2 runner is pictured here awaiting a protective epoxy coating after having received cavitation repairs.|
The battle against degradation is continuous in the hydro world. These long-term numbers are hard to quantify, but we can quantify various other savings up front. The difference rehabilitating the existing equipment compared to fabricating a new unit are $100,000 or more, while the use of Loctite products, which produced a frictionless top coat, allow for efficiency improvements in the runner.
Similar applications have resulted in pump efficiency improvements up to 10% from what the unit was producing before the application, compared to conventional refurbishment techniques, according to cases documented by Henkel. This translates into improved performance and increased lifetimes for the associated components.
Looking back on the process, HCMS and Henkel were pleased with the outcome of the Unit 2 rehabilitation. However, several logistical improvements will be taken into account for the next job, based on lessons learned here.
Media blasting prior to coating is a time-critical preparatory step that should be scheduled immediately before applying polymer composites. It reduces the chances of and provides the smallest window for any corrosion to form prior to coating, a major factor in jeopardizing coating adhesion and performance. Scheduling the final media blasting in advance would have potentially saved several days during refurbishment.
When calculating the necessary quantities of epoxy coatings for large jobs, porosity, pitting and erosion play a significant role in the variability of the product’s coverage. Aside from calculating surface area, an additional 25% to 50% should be incorporated into the order quantities of product required, depending of the severity of the substrate’s wear.
Henkel also offers protective coating products in sprayable forms, which greatly reduces man-hours when coating large and geometrically complex parts. Moving forward, it is highly advised that a sprayable ceramic epoxy coating be used for application efficiency.
Mark Bitter is a project engineer, Jacob Hostler is marketing coordinator, and Randy Furbush is a senior technical consultant for Hydro Consulting and Maintenance Services. Adam Lyman is a market application engineer and Loren Nauss is business development manager for Henkel.