Public Utility District of Grant County, US, had to assess the technical challenges posed by trunnion rehabilitation at the Priest Rapids dam in Washington state
The failure of a spillway gate in 1995 at Folsom dam near Sacramento, California, US, focused industry-wide attention on the potential for excessive coefficients of friction in trunnion assemblies. At Priest Rapids dam on the Columbia river in eastern Washington state, engineers inspected the dam’s spillway gates and found cracked thrust washers in the gates’ trunnion assemblies.
Owned and operated by Public Utility District 2 of Grant County, Priest Rapids dam has 22 spillway gates that provide flood control and fish passage. Gates at the dam have performed reliably since the late 1950s when Merritt, Chapman and Scott constructed the dam. The radial gates are 12.2m wide and 16m tall with a maximum rotating movement of 55?. Each gate has two trunnion assemblies.
An investigation of the gates’ trunnion assemblies pointed to a materials problem. Laboratory analysis indicated that stress corrosion cracking of the washers’ bronze alloy material was ammonia-induced. The bottom line was a corrosion problem likely precipitated by bird droppings.
As a result of the findings, district engineers and consultants made further investigation of the thrust washers and decided to remove and rehabilitate the trunnion assemblies from each of the spillway gates at the dam. The rehabilitation would involve replacing the bronze-alloy-based thrust washers with ones made of a corrosion-resistant composite material. Since the trunnion bearings were also bronze-alloy-based components, it was decided to use the opportunity to replace the bearings as well.
‘Our objective was to rebuild the assemblies with materials that offered optimum strength and durability so we can minimise any potential for failure,’ says George Thompson, the district’s project engineer.
The district is proceeding with the four and a half year rehabilitation project, which it expects to complete in 2005. Working with its contractors and consultants, the district devised an engineering and project approach that addresses many technical challenges and may be a model for other facilities with physical and operational similarities.
Several aspects of the project at Priest Rapids dam made trunnion rehabilitation a challenge.
‘The location of the trunnion assembly in the spillway gate makes the assembly difficult to access and remove,’ explains Thompson. ‘We had to develop a gate positioning procedure to isolate the forces in the spillway gate and restrain them to allow for removal of the trunnion assemblies.’
The process involves disconnecting the trunnion assembly from the gate arms, pushing the large gate – which weighs about 96tons – upstream about 5cm and then restraining the gate assembly in place until the trunnion was free. The trunnion is extracted by rotating it vertically and lifting it out with a crane. Later, to replace the trunnion, the gate is moved back in place using hydraulic jacks and a series of anchor pads, and the trunnion assembly is reconnected to the trunnion arms.
Another consideration for the project was the crane that would lift the trunnion assemblies from the spillway gates. The estimated weight of a trunnion assembly with rigging is 3175kg. The crane had to meet performance requirements, and its placement had to accommodate traffic that uses the roadway across the dam for access to the Wanapum Indian Village.
For the trunnion rehabilitation project, the district contracted WHECO, a Pasco, Washington-based company specialising in crane repairs, re-manufacturing, component upgrades and structural repairs with engineering support. WHECO addressed the crane issue by designing and manufacturing a 5ton capacity-rated, cantilever gantry crane tailored for the project at Priest Rapids dam. The four-legged, radio-controlled gantry crane travels with electric-over-hydraulic power, rolling up and down the spillway on a set of rails that can be repositioned at each gate. The crane’s footprint is fitted to the dam structure, and the reach of the boom (18.3m) extends from the top of the spillway to the gate’s trunnion.
The crane’s design and capacity makes it a workhorse for the trunnion lifting requirements and for installing needed fixtures such as work platforms and pulling fixtures. Because the gantry crane operates off the roadway, its use provides uninterrupted access to traffic along the dam’s roadway.
The limited construction season was another constraint on the project. From April to October, the district spills water for fish passage as required by the state’s Department of Fish and Wildlife, National Marine Fisheries Service, and Washington Department of Ecology. Consequently, there are only six months during the year – October through March – when project activities, including annual gate maintenance, can take place at the dam.
The project began with a prototype phase to refine procedures for removing and reinstalling the trunnion assemblies at the dam and for disassembling and changing components at the repair facility. WHECO completed the prototype project with a full rehabilitation of both trunnion assemblies at gate five.
‘We used the prototype project to test and refine the set of procedures developed by Stone & Webster,’ explains WHECO president Dave Wood. ‘The prototype provided learning and actual experience for developing efficiencies to meet the project’s tight timeline.
‘As a result, we were able to refine highly efficient procedures for work at the dam, and we found ways to streamline repair approaches in collaboration with Orkot, the bearing manufacturer,’ he says.
WHECO extracts the trunnion from the dam’s spillway gates according to contract requirements. The district requires that the contractor work on one gate at a time and that the contractor loosen or remove flange and web splice plate fasteners on only one trunnion assembly at a time.
Once extracted, the trunnion assemblies are transported to WHECO’s repair shop for disassembly and component replacement. For this task, the assembly is placed in a 200ton horizontal press, and the outer portion is warmed to 150°F. The 40.64cm diameter pin is pushed out of the assembly, and the assembly is removed from the press. The bronze-graphite bushing is cut from the trunnion housing, and the housing is then sand blasted and painted with a Wasser three-coat paint system. WHECO technicians machine, grind and polish the stainless steel-cladded pin to a millionth of 40.64cm or better finish. The precision polishing is essential for optimal fit, which allows the pin to rotate freely in the trunnion with minimal friction and wear.
The outside diameter (OD) of the pin is measured at eight locations, every 22.5?, at each end of the pin. The inside diameter (ID) of the casting, in which the pin and bearing fit, is cleaned and measured at eight inner locations, every 22.5?, at both ends. Those measurements are then transmitted to the bearing manufacturer, Orkot Composites, which manufactures and machines the trunnion bearings and thrust washers at its factory in Eugene, Oregon, US.
The district requires the trunnion bearings and thrust washers to be designed for continuous normal loads, intermittent immersion in water, and suitable clearances between trunnion pins and bearing surfaces in ambient conditions from -20°F to 120°F.
The required normal size specifications for the replacement trunnion bearings is 40.64cm ID, 46.4cm OD, 73.7cm length. The project requires the bearings to support a 4000psi normal load and a 12,000psi rated load. Specifications for the thrust washers are for a normal size of 61.9cm OD, 40.8cm ID, and 1.9cm thickness. The thrust washers require a 1600psi normal load and a 4800psi rated load. Material specified for the trunnion bearings and the thrust washers is a synthetic self-lubricating non-metallic Teflon polyester composite.
The corrosion-resistant composite bearing material selected for the project is the Orkot TXMM, which is a polyester fabric reinforced thermoset incorporating a Teflon bearing surface and solid lubricants for a self-lubricating low friction bearing. The Orkot TXMM material has a compressive strength of 43,500psi and a yield of 13,200psi, which exceeds contract requirements.
Orkot developed the material specifically for the hydro industry. The Orkot TXMM has excellent mechanical strength and wear resistance, performs with virtually no swell in water and offers dimensional stability, according to the company.
During prototype construction, it was discovered the trunnion pin and trunnion bore measurements were not consistent at every trunnion. Thus, each casting and pin is individually measured after cleaning, machining and polishing is complete. When Orkot receives the actual dimensions from WHECO, Orkot engineers run a calculation program to determine the correct size for bearing manufacture. Bearing interference, clearance, temperature and tolerances are taken into consideration.
Individual fitting of the bearing ensures clearances are uniform on all the gates and is considered a better approach than trying to make one size bearing fit all gates. However, the matched fitting takes time, and time is limited by the project’s schedule. The contract specifies that the bearings shall be delivered within seven calendar days after notification of the actual dimensions of the trunnion pin OD and the trunnion bore ID.
‘We conquered the challenging production schedule by manufacturing bearing blanks in advance of anticipated delivery dates,’ says Orkot engineering manager John Robinson. ‘When measurements come in, the blanks are ready for machining.’
When the newly manufactured components are delivered to WHECO, it reassembles the new trunnion bearing using a freeze fit technique. The bearing is submersed in a liquid nitrogen bath for approximately 45min to shrink the bearing through freezing. The frozen bearing is then placed in the casing and allowed to warm up; as it warms it expands for a precision press fit. Prior to inserting the pin, the ID of the bushing is lightly coated with a lithium-based grease to aid installation and the initial run. The completely reassembled trunnion with the new thrust washers and bearings is transported back to the dam and installed at the spillway gate.
With completion of the public works project, George Thompson anticipates no further problems with cracks in thrust washers at the dam. ‘We began assessments in February 2002 to quantify and measure improvements that result from the rehabilitation of the trunnion assemblies,’ says Thompson. ‘Overall, we expect the project will reduce trunnion friction, provide for optimum operation of the spillway gates and enhance reliability.’