Rehabilitation and upgrades of the Miller Creek Hydroelectric Facility have allowed new owner Innergex Renewable Energy Inc. to maximize its investment.
By Derek McCoy and Richard Blanchet
Derek McCoy, M.Eng., P.Eng., is engineering manager and Richard Blanchet, P.Eng., M.Sc., is senior vice president – western region with Innergex Renewable Energy Inc.
After acquiring the Miller Creek Hydroelectric Facility in 2012, Innergex Renewable Energy Inc. identified a number of components needing repair or rehabilitation, including the intake, penstock, turbine inlet valve, deflectors and runner, and plant control system. The work has increased the plant’s output capacity by about 17%, restoring it to nearly its original design capacity.
Identifying areas for improvement
Innergex Renewable Energy completed the acquisition of the Miller Creek Hydroelectric Facility in October 2012. Innergex acquired the facility along with the 7-MW Brown Lake Hydroelectric Facility in northern British Columbia for a combined total of US$63 million (C$68.6 million). The Miller Creek facility was a strategic acquisition for Innergex due to its close proximity to existing operating assets, including the 49.9-MW Rutherford Creek Hydroelectric Facility and the Fitzsimmons Creek Project, and to the Upper Lillooet River and Boulder Creek projects that are currently under construction.
|Oxidation caused by a bacterium inside Miller Creek’s penstock created “turbercles”, which protruded up to three-quarters of an inch from the interior pipe wall.|
Miller Creek, located about 7 km northwest of Pemberton, British Columbia, Canada, is a high-head (762 m), low-flow (5.5 cubic m per second) run-of-river facility rated for a total generation capacity of 33 MW. The facility consists of a high-elevation intake (the normal headpond operation level is at 1,120 m above sea level [masl]), a 4,250-m-long steel penstock (with a diameter ranging from 36 inches to 48 inches and a thickness ranging from 9.5 mm at the intake to 25.4 mm at the powerhouse), a powerhouse with two units (an Alstom-manufactured 30-MW vertical Pelton turbine and Powerformer generator and a 3-MW horizontal Pelton turbine) at an elevation of 358 masl, and a small switchyard. The main characteristic of the Powerformer is the technology of the generator, which is rated at 25 kV and, therefore, does not require a step-up transformer. Electricity is sold to BC Hydro under a 20-year electricity purchase agreement, which commenced in 2003, with the utility holding two consecutive five-year renewal options.
Although the Miller Creek facility is relatively new — having been commissioned in 2003 — several components of the project were identified as in need of repair during the acquisition process. A capital expenditure program to upgrade the penstock and water intake was announced at the time of the acquisition of the facility, with funding for the program provided by the proceeds of a private placement (non-public offering of shares in a public company) concluded in July 2012. The program was considered in establishing the purchase price of the Miller Creek facility and was completed on budget at a cost of about US$6.44 million (C$ million).
The main issues to be addressed were the excessive sand and sediment intrusion and poor hydraulic performance of the intake, the extensive friction losses of the penstock, malfunctioning of the turbine inlet valve and runner for the large unit, excess vibration of the large turbine and Powerformer, and modernization of the plant control system. To complete this work, a minimum two-month shutdown was scheduled from August to October 2013. Timing of the shutdown was selected to correspond to the tail end of the freshet period, when creek flows are historically at their lowest levels of the year. With the intake at such a high elevation, the onset of winter conditions is highly unpredictable. Therefore, it was critical that the rehabilitation work be completed by the end of October.
The penstock head losses were the item that required the most immediate attention. The source of the head losses was identified to be corrosion caused primarily by the bacterium Gallionella ferruginea, which is prevalent in the high alpine environment of coastal British Columbia and is known to oxidize the iron in steel. This oxidation results in the development of so-called “tubercles,” which at the Miller Creek facility had protruded up to 0.75 inch from the interior pipe wall.
The consequence of this corrosion was twofold. First, Miller Creek’s total generation capacity had peaked at about 28 MW due to the excess head losses caused by the increased wall roughness and decreased effective diameter. Second, and more importantly, when the tubercles were removed from the pipe wall, they left behind pits that can compromise the integrity of the pipe wall to withstand pressure. Fortunately, the unlined penstock was designed an extra 4 mm thick to allow for some corrosion.
Innergex had previously encountered this same corrosion issue at its Rutherford Creek Hydroelectric Facility and, therefore, understood the scope of work required to repair the Miller Creek penstock. The process ultimately selected consisted of surface preparation, followed by two coats of epoxy paint that included a high-adhesion prime coat of Devoe Bar Rust 33H an abrasion-resistant topcoat of Devoe Devgrip 238.
Surface preparation work was completed by Mac & Mac Hydrodemolition Services Inc. and consisted of removing the tubercles from the pipe wall and cleaning the pipe down to bare steel. Instead of using a more expensive and time-consuming dual process of sand-blasting followed by a water rinse, Innergex opted for the ultra-high-pressure (UHP) hydro-blast technique developed by Mac & Mac, which only required one pass down the penstock. To complete the UHP hydroblast process, Mac & Mac had to first fabricate a manually operated robot capable of delivering a 40,000-psi water jet circumferentially around the pipe.
Once inside the penstock, the robot was capable of cutting through the tubercles and cleaning the pipe at an average rate of 150 linear meters per day. Delays were initially encountered due to extensive compacted glacial silt built up along the upper flat sections of the penstock, which impeded the ability of the robot to progress down the penstock. As a result, actual production was closer to 130 m per day, resulting in a total of 32 days to complete the entire surface preparation program. As expected, extensive tubercle development and consequent pitting were observed within the pipe. Tubercles as thick as 0.75 inch were removed from the pipe wall. Pitting in the pipe wall was generally measured to be 0.02 to 0.05 inch; the deepest pit measured was 0.08 inch.
The pits were not considered to be deep enough to compromise the integrity of the penstock given that the pipe was designed with a 4-mm (0.16-inch) “corrosion allowance.” This was confirmed by non-destructive testing (NDT), which included spot-checks of penstock wall thickness using an ultrasonic measurement device.
The surface preparation work was followed by the coating program, which was completed by Corrcoat Services Inc. It was essential that the coating application commence as soon as possible after the cleaning to avoid excessive flash rust on the pipe wall. The Devoe coating mix specified for this work was deemed to be more tolerant to the humid environment and provide a good balance between adhesion strength and abrasion resistance. Coating was applied using a machine similar to that used for the hydroblast process. However, welds, manholes and other transitions and irregularities needed to be manually coated to ensure adequate paint coverage.
Machine application of the prime coat progressed at an average of 105 m/day, versus the top coat at 90 m/day. Excessive wear to the coating application pump was the main factor for the reduced production rate of the top coat; this wear was due to the aluminum oxide content of the abrasion resistant paint. Care was taken to ensure that the coating thickness was sufficient to fill in the pits left by the tubercles, and additional coating was applied as required. Quality assurance services for the surface preparation and coating application was completed by Norske Corrosion and Inspection Services Ltd.
As a result of these improvements, the head losses along the penstock were reduced from 150 m to 60 m and the facility’s long-term average annual production is expected to increase from 97,900 MWh to 102,795 MWh.
In addition to the penstock rehabilitation work, Innergex identified improvements to the intake structure that were necessary both to protect the investment being made in the penstock and to improve the hydraulic performance of the intake. The existing intake structure was undersized for the design flow rate and resulted in excessive head loss and debris accumulation on the trashrack. Further, there was no safe method of cleaning the trashrack. Instead, the facility had to be shut down and the intake fully dewatered to remove accumulated debris, a process that required two to three hours of plant shutdown several times per week during critical times of the year.
Unfortunately, the opening of the original intake was situated flush to the floor of the sluiceway. This provided a direct path for accumulated sediments to enter the penstock and rendered the sluiceway irrelevant. In addition, water had to travel through two openings prior to reaching the regulation chamber, which increased turbulence and head loss.
The goal of the intake rebuild was to maximize the hydraulic performance while limiting the amount of concrete work required, as this would both reduce overall costs and mitigate the potential for work to creep into the winter months. With this in mind, the intake was widened to accommodate an overflow-type intake, which reduced approach velocities from about 1.3 m/sec to less than 0.8 m/sec and also allowed the sluiceway to function as intended. The redesigned intake was also evaluated for flow distribution and vortex generation using Flow Science’s FLOW-3D computational fluid dynamics modelling software. Hydraulic design of the new intake was completed in-house by Innergex personnel. Structural design and geotechnical design were completed by Allnorth Consultants Ltd. and Golder Associates Ltd., respectively.
|An area treated with the ultra-high-pressure hydro-blast process is visible in the foreground, while tubercles still cover the penstock in an untreated area farther down the pipe.|
The intake works consisted of installation of a small cofferdam, shoring up of the excavation area using injection boring micropiles and soil nails, concrete cutting and removal, new concrete poured and tied into the existing structure, and miscellaneous metals such as walking platforms and safety rails. The intake works were completed by prime contractor North Construction Ltd. and concrete works sub-contractor Duro Construction Ltd.
Innergex decided to use the hybrid micro-pile and soil nail wall to limit the volume of excavation into the earthfilled abutment but, more importantly, to prevent undermining of the existing intake control building, which was adjacent to the excavation area. This innovative solution of combining the use of groutinjected micro-piles with 6-m-long soil nails and shotcrete facing allowed for a vertical excavation face that was as deep as 7.5 m.
Concrete work included cutting/demolition of portions of the original structure and pouring a new base slab and walls. To tie new reinforcement into the existing concrete structure, the contractor used ground-penetrating radar to locate and mark existing rebar to avoid encountering it while drilling new dowel holes. All concrete work was completed in five weeks, well ahead of the onset of winter conditions at the intake.
Due to delays in steel delivery, however, the miscellaneous metal work was forced to be completed later in temperatures below freezing. This included the installation of the trashrack and support beams, new decking and safety rails.
During the due diligence inspection prior to purchasing the facility, several improvements within the powerhouse were identified. These improvements included replacement of the turbine inlet valve (TIV), the runner and the deflectors (all for the 30-MW Powerformer unit), as well as a clean-up and improvement of the PLC logic.
Inspection of the existing TIV identified severe scoring, the root cause of which was the fact that the TIV did not have the adequate mechanical strength to withstand the static pressure, thereby causing deformation as the TIV ball came into contact with its housing. This scoring had resulted in seating problems such that the valve would not close properly. A new hydraulically-seated TIV was supplied by D2FC Energy Valves SAS. The runner, too, had been excessively eroded by the high sediment concentrations. A spare runner that was available in the powerhouse was installed to replace the existing one. The new TIV and runner were installed by Lethbridge Millwright & Welding Ltd.
In addition to the TIV and runner, a major component of the powerhouse improvements was the replacement of the deflectors. The geometry of the existing deflectors was found to induce excessive vibration of the entire Powerformer assembly, which caused the generator to trip into emergency shut-down mode. This, in turn, caused the turbine needles to close too fast, resulting in elevated potential for so-called “ramping events,” where the downstream creek levels drop faster than allowable rates. This vibration issue was mitigated by changing the geometry of the deflectors from a flat design to a slightly concave-face. The new deflector design was provided by ABMS Consultants Inc.
Lastly, the programmable logic controller (PLC) required some reconfiguration to accommodate the new TIV. Since this work was required, Innergex took advantage of the two-month plant shutdown to modernize the entire PLC logic. This brought the Miller Creek control system to a level on par with Innergex’s other facilities, which allows the operations team to work seamlessly between facilities.
The all repair work were completed for about C million (US$6.4 million), within the initial budgetary estimate. The work has resulted in an estimated decrease in head loss from 150 m to 60 m and a resultant increase in generation capacity of about 17%, from 28 MW nearly to the design capacity of 33 MW. More importantly, by mitigating further corrosion, the penstock has now been deemed safe and should satisfy the life expectancy of the facility.
The following contractors were instrumental in delivering the rehab project on schedule and within budget. Penstock rehabilitation work included Mac & Mac Hydrodemolition Services Inc. (prime contractor), Corrcoat Services Inc. (coating subcontractor) and Norske Corrosion and Inspection Services Ltd. (quality assurance services). Intake reconstruction work included North Construction Ltd. (prime contractor), Duro Construction Ltd. (concrete works), Atlas Polar Company Ltd. (trashrack and rake), Allnorth Consultants Ltd. (structural engineering) and Golder Associates Ltd. (geotechnical engineering). Powerhouse improvement work included Lethbridge Millwrights and Welding Ltd. (TIV and turbine runner installation), D2FC Energy Valves SAS (TIV supply), ABMS Consultants Inc. (deflector supply), Coast Automation Inc. and Hydro ECI Inc. (PLC modernization).More HR Current Issue Articles
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