From early 2005 to December 2007, the Sacramento Municipal Utility District (SMUD) experienced high vibration of an upper guide bearing in Unit 2 at its 150-MW Camino hydroelectric station. The highest vibration was more than double the specified bearing diametrical clearance. This high vibration on Unit 2 resulted in damages to the bearing insulation, labyrinth seal teeth, and upper guide bearing.
After multiple incidents and outages, in December 2007, the utility shut down the unit until it could determine the cause of the high vibration. The shutdown was a planned move, a decision engineers made as a result of previous shutdowns and the search for the root cause of high vibration. Investigators determined the high vibration was most likely due to a defect or flaw in the unit’s turbine runner. SMUD installed this runner – a spare – during a 2003 unit overhaul and experienced rough operation and high vibration in subsequent operation. Vibration testing identified a specific subsynchronous vibration that was unique to the spare runner.
During this overhaul, staff replaced the original runner with a spare runner (from the same foundry as the original). Each unit at Camino has a spare runner. When not in service, that runner is stored inside the powerhouse. There was no problem with the original runner other than worn-out seals. The seals were worn beyond acceptable seal ring clearances and, therefore, needed to be replaced. To ensure minimum downtime during the outage, before work began, SMUD outfitted the spare runner with new seal rings, making it ready for installation. As soon as the unit was taken off line, SMUD removed the original runner and replaced it with the spare fitted with new seal rings. The original runner then became the spare.
Four years later, when the utility shut down the unit to investigate the vibration, it discovered the problem was due to a flaw in the spare runner.
To solve the problem, the utility removed the spare runner and replaced it with the original runner. With the original runner installed, there was no onset of vibration as load was increased to 75 MW from 40 MW. Camino Unit 2 could now be operated at full load (75 MW) during sustained steady-state operation.
In addition, the utility invested in a $50,000 Bently Nevada vibration monitoring and machinery protection system. Prior to the use of this system, SMUD hired a consultant who brought in temporary equipment to measure vibrations. This method was time-consuming and did not provide immediately accessible data for operators to make course corrections in a timely fashion.
With the new vibration monitoring system, six radial vibration probes installed throughout the unit measure vibration in real time. Operators can view the results and make better informed decisions about how to best operate the unit. After two years, the unit continues to operate smoothly and with normal vibration levels at a load variance of 0 to 75 MW.
Situation at Camino
The Camino powerhouse is on the American River in El Dorado County, Calif. It is part of the Upper American River Project (UARP), under the Federal Energy Regulatory Commission license number 2101. In total, UARP includes 11 dams, eight powerhouses containing a total of 11 turbines, and six auxiliary dams and dikes.
Camino houses two vertical-axis, Francis type turbines. Unit 2, the subject of this article, became operational in mid-1968. Its hydraulic turbine is rated at 112,000 horsepower and 1,050 feet maximum static head. The associated direct-connected generator is rated at 75,000 kilovolt-ampere (kVa).
Between 2005 and 2007, Camino’s Unit 2 experienced high vibration. The highest vibration of the upper guide bearing was in excess of 50 peak-peak mils (pp mils), which is double the specified bearing diametrical clearance. Powerhouse operators observed this amount of vibration with the unit operating at full load (75 MW). Maximum vibration at the lower guide and turbine guide bearings was 29 pp mils and 12 pp mils, respectively.
Vibration caused damage to the upper labyrinth seal teeth, the upper guide and thrust bearing insulation, and the insulated bolts that fasten the upper guide bearing bracket to the upper bearing support bracket. It also caused contamination of the lubricating bearing oil in the reservoir.
Determining the cause of the problem
Finally, after two years of problems, in 2007, SMUD shut down Unit 2 to determine the cause of vibration. After a thorough investigation, the utility determined the most likely cause of vibration at loads above 40 MW was a flaw in the spare runner installed during the 2003 overhaul described above.
In fact, during the investigation, when this problem came to the surface, anecdotal evidence collected from operations and maintenance personnel pointed to the emergence of vibration problems soon after installing the spare runner.
Remedying the problem
To correct the vibration problem in Unit 2, SMUD pursued a two-pronged approach. First, the utility removed the runner in the unit (the spare) and replaced it with the original runner, which had become the spare after the 2003 overhaul. Utility staff permanently removed the flawed spare runner from service.
The staff recognized the importance of proper seals. Therefore, they installed new seal rings on the runner one year after returning it to service.
Second, SMUD installed a Bently Nevada vibration monitoring and machinery protection system on Camino Unit 2. The utility determined that continuously monitoring vibration, temperature, speed, and other condition indicators would provide numerous benefits, including protection from catastrophic failure.
Using the new system, SMUD is extracting vibration data for the manufacturer of the spare turbine runner to use to investigate how the flaw in that runner could cause the high vibration experienced at Camino Unit 2.
With the original runner re-installed, SMUD personnel brought the unit back on line and increased load from 40 to 75 MW. No high levels of vibration occurred. Radial vibration levels measured below 10.0 pp mil.
Consequently, SMUD returned the unit to normal operation at full-load capacity of 75 MW
– By Bill Collins, principal mechanical engineer for Sacramento Municipal Utility District (SMUD). In this capacity, he serves as the lead mechanical engineer for the Power Generation Department with responsibility for all mechanical systems of the utility’s power generation assets and related support facilities.
Collins, Bill, “Camino Unit 2 Material Failures and High Vibration,” Waterpower XVI Technical Papers CD, PennWell Corporation, Tulsa, Okla., 2009.