New pressure-relief valves installed at the historic 2,078-MW Hoover hydropower plant are allowing for quick shutdown of the generating units in the event of a trip, minimizing damage to the penstocks and extending reliable operation of the facility.
By Jeff Sage
Hoover Dam — one of the most impressive engineering feats of the 20th century – impounds water for a 2,078-MW hydroelectric powerhouse that generates electricity for millions of homes and businesses across the southwestern U.S. For the U.S. Department of Interior’s Bureau of Reclamation, it’s a constant challenge to keep the vital power source running smoothly. Recently, a team of engineering experts wrapped up a multi-year retrofitting and refurbishing project to make the facility safer and more operationally efficient.
The challenge was to overhaul and upgrade a series of 50-foot-tall pressure-relief valves (PRV) on the 17 turbines in the powerhouse and to convert them to a hydraulic control system. This was needed because the PRVs had aged and become corroded. Reclamation awarded the contract for the project to Precision Machine & Supply Inc. (a division of Andritz Hydro), which had been working at the Hoover Dam facility since the 1990s.
Subcontractors for the assignment included Parker Hannifin and Controlled Motion Solutions Inc. (Comoso). Parker Hannifin was tasked with designing and manufacturing a series of compressed-gas accumulators, and Comoso was responsible for providing engineering and sourcing the hydraulic components.
Upgrading an engineering marvel
Hoover Dam is often called one of the modern wonders of the world. Standing more than 700 feet tall and containing over 3,250,000 cubic yards of concrete, the magnificent structure spans the Colorado River between Nevada and Arizona, forming the 247-square-mile Lake Mead reservoir.
The plant generates more than 4 billion kWh of electricity each year by taking water from the lake, under extremely high pressure, and channeling it into the giant turbine-generator units at its base. The penstocks narrow from 30 feet to 13 feet as they descend to increase the pressure on the water. When the incoming water reaches this point, its pressure is 250 psi.
The water enters the turbine through large steel wicket gates, each over 6 feet tall and weighing 1,500 pounds. The gates open and close to control the volume of water going into the turbine.
Each of the 17 turbines weighs about 700 tons, with generator shafts rotating at 180 rpm. While a turbine is spinning, energy is being created and fed through power lines. However, if there is a sudden break or fault in the line – also called a load rejection – the turbine needs to stop as quickly as possible.
When a rejection takes down a primary line – which can be caused by a lightning strike or physical damage to a transmission wire — there’s no place for that newly generated electricity to go. If that happens, the spinning turbine tends to overspeed, which can cause serious damage to the mechanism. Therefore, the water driving it has to be immediately shut off at the gates and simultaneously diverted around the turbine. However, that necessity comes with problems of its own.
First, if the high-pressure water flow is stopped too abruptly, it results in a powerful “water hammer” effect when the backed-up pressure suddenly and violently slams into an obstruction. (Imagine trying to bring a fast-moving train to an immediate stop.) The water delivery system at Hoover Dam contains enough kinetic energy to reduce the life of the penstocks.
Diverting the flow and relieving pressure
To avert the dangers of those sudden load rejections, the original designers installed large PRVs that could quickly reroute incoming water to bypass the turbines, thereby taking the generators offline. The first PRVs used water head pressure to drive large pistons to close tulip valves.
In recent years, though, questions arose about the original PRVs’ functional consistency and ability to protect the aging waterlines. Installation of the turbines began in 1936, so the equipment and infrastructure inside the power plant were naturally affected by time and use.
“The turbines and all their plumbing are vintage – 80 years old in some cases – with a lot of wear and tear,” said Greg Paddock, hydraulic territory manager for Parker Hannifin. “So the Hoover people were very concerned about pressure spikes and the resulting negative impact they could have on the equipment.”
“Over the years, those PRVs became corroded,” agreed Dan Wenstrom, president of Precision. “Also, the original valves were mechanically actuated and water-operated, because that’s all the technology they had in the 1930s.”
Developing the solution
Aware of the critical need to optimize reliable performance of the older PRVs, the operations team at Hoover Dam launched a long-term project to upgrade them. The main objective was to make the PRVs more responsive and functionally efficient when a power line break necessitated a generator shutdown.
The initial plan called for overhauling the existing valves by restoring worn components to like-new condition. The scope of the challenge – plus the restriction of not being able to shut down multiple turbines at the same time – meant the work would require many years to complete. Hoover’s plant personnel and Precision Machine began the first remedial work on the valves in 1998 and 1999.
While that work was under way, Wenstrom came up with a unique design concept to standardize operation of the PRVs and make them digitally controlled. The original generating equipment was built by various manufacturers and installed over a long span of years, so it was far from consistent. There are five separate turbine designs in operation at Hoover. Even units built by the same manufacturer several years apart had differences.
“We showed them a design that would make the units fully compatible with their existing electronic control system that operated and controlled the generators,” Wenstrom said. “We proposed converting all PRVs to be operated the same way and all controlled by hydraulic cylinders.”
Each PRV at Hoover now uses one compressed gas piston accumulator with pressurized oil and two large nitrogen gas bottles, as shown here.
The decision was made to go with hydraulic-driven PRVs, which could provide precise control and extremely fast response. The system would also reduce the number of false PRV operations that often occurred with the old mechanically operated PRVs. Wenstrom brought in Comoso to engineer and supply the hydraulic power unit and manifold that mounted to the hydraulic cylinder.
Auxiliary power needed
The hydraulic controls also required accumulators for energy back-up. An accumulator enables a hydraulic system to respond quickly to a temporary demand, using a less powerful pump. Think of the accumulators as very large batteries with high levels of energy to operate the PRVs. The accumulator stores hydraulic energy until it’s needed for immediate use.
The Accumulator and Cooler division of Parker Hannifin was given the assignment to design the best units for Hoover’s unprecedented requirements. Working with Comoso and Precision, Parker was able to implement an ideal, cost-effective solution. An accumulator enables a hydraulic system to respond quickly to a temporary demand, using a less powerful pump.
Supplemental power from the accumulators is necessary because of how Hoover’s hydroelectric equipment is configured. Ironically, available electricity is very limited inside the huge power-generating facility. “Where the PRVs are located, there isn’t much access to electrical power,” said Steve Camp, Parker Hannifin regional manager. “The dam puts out 185,000 horsepower per turbine, but we only had the equivalent of ten horsepower in the area where we worked.”
The power that’s available comes from two smaller separate generators called “house units” in the powerhouse. The little units simply wouldn’t have the energy capacity to operate multiple high-pressure, high-horsepower hydraulic oil pumps to drive the cylinder when a PRV trips.
“The accumulators instantaneously allow 750 to 900 horsepower, so we have the energy we need at the drop of a hat to operate the valves,” Camp added. “It opens the bypass very quickly.”
Each PRV at Hoover now uses one compressed-gas piston accumulator with pressurized oil (180 gallons under 2,750 psi) and two large nitrogen-gas bottles. The accumulators have a 20-inch bore and an outside diameter of 23 5/8 inches. They’re 200 inches long and have a dry weight of 8,653 lbs. A total of 17 accumulators and 34 nitrogen gas bottles have been installed.
“As the upgraded PRVs are designed, we now get shaft movement typically within less than a tenth of a second after the signal is received that the generator is going into emergency shutdown,” said Paddock. “As quickly as the (water intake) gates are closing, the PRV has to open to bypass the same amount of water that was otherwise going through the turbine. That’s within ten seconds. And then the most critical aspect of it is once the PRV is fully open, it has to slowly reclose so that no water hammer is created.”
One of the new PRVs for Hoover is brought in via a large crane.
With the installation of each new PRV, a commissioning team – including representatives from Hoover, Precision Machine, Comoso and Parker Hannifin – conducts a detailed testing process.
“To do the commissioning, we bring the generator up to speed and then trip it to simulate an emergency shutdown,” Wenstrom explained. “Recording devices with transducers on the turbine side and the PRV side precisely measure the hydraulic pressures, the strokes, and the time it takes the cylinder to respond to the emergency closure signal. Then we measure the amount of time it takes the PRV to open and to reclose. The whole idea is that these PRVs have to open very quickly, as soon as the wicket gate starts to close, to avoid a water hammer.”
In addition to dramatically improving the functionality and reliability of the PRVs, the hydraulically driven system provides a solution to a new problem at Hoover Dam: Quagga mussels. By 2009, these invasive mussels started plugging up water passageways in the control valve system. They clog PRVs by clinging to the rods that open and close the valves, and in some cases even prevent them from opening.
“Dan’s design ensured the PRVs would operate regardless of any fouling factors such as the quagga mussels,” said Paddock. “The hydraulic-driven PRV could basically just plow through any obstruction in its path, by brute force. The impetus of the conversion to the hydraulic design wasn’t the mussels, but it turned out to be a great secondary benefit.”
The entire process of upgrading the PRVs and associated equipment has been an extraordinary team effort representing a lot of combined brainpower. Ultimately taking 20 years from start to finish, the scope and uniqueness of the project seem appropriate for such a magnificent historic facility.
“There are many long-term benefits to this whole project, from operational efficiency to safety and more,” said Paddock. “Parker Hannifin is grateful to have been part of it.”
Jeff Sage is product manager with Parker Hannifin’s Accumulator and Cooler division.