Performance testing is a key activity when procuring new hydro turbines and when rehabilitating units. Also, testing is central to maintaining and improving plant performance. Advances in testing methods and technology have made high-quality testing more economical and effective, and less labor intensive.
By David O. Hulse
Performance testing of hydropower facilities is a vital, yet challenging activity. It is important because, without testing, assessment of how well a facility is performing – vis-à-vis how well it possibly could perform – is, at best, a difficult task.
Of course, water is the “fuel” for a hydroelectric plant – and, to evaluate performance, it’s necessary to know how much fuel is used. What if, in your car, you wanted to determine gas mileage performance (miles per gallon or kilometers per liter) but could not measure the amount of gasoline put into the fuel tank?
Unlike measuring the fuel that you put in your car, determining how much water through a hydro plant (i.e., the flow rate) is not an easy thing to do. Nonetheless, succeeding generations of technologists have taken on this challenge – and by applying creativity, innovation, and increasingly sophisticated methods – have continually improved the state-of-the-art of performance measurement.
At the U.S. Department of the Interior’s Bureau of Reclamation, we have a long, more than 70-year history of measuring performance. In fact, Reclamation’s experience could reasonably serve as a chronology of the development of hydro turbine testing techniques.
At Reclamation, the performance testing of pumps is also important, and the methods used are in many cases very similar to what’s used for hydro turbines. While Reclamation operates 58 hydro plants having 196 turbine-generators (ranging in size from 350 kW to 805 MW), we also operate hundreds of large pumps in carrying out our roles of serving many millions of users of both water and electricity.
Why do performance testing?
Performance testing is an essential practice to ensure that plant performance has not degraded to unacceptable levels. Moreover, testing is needed as a part of programs to improve the efficiency, output, and economic performance of single hydro plants and of multiple plants that operate in a coordinated manner. Optimization has been an important focus in recent years. Optimization typically requires performance testing in order to verify the performance of units and facilities and to establish benchmarks.
Another important reason for performance testing has to do with the procurement of new turbines or runners and new pumps or impellers, or in contracting for the rehabilitation of such equipment. Specifically, performance testing is often necessary for verifying contractual guarantees.
We routinely use a test code such as the American Society of Mechanical Engineers’ Performance Test Code 18 for Hydraulic Turbines and Pump Turbines (ASME PTC-18). This code, originally published in 1923, was last revised in 2002 by experienced test engineers who represent turbine and equipment manufacturers, owners, and consultants. Because few of Reclamation’s hydro plants have excess water that would justify capacity increases, the majority of Reclamation’s turbine runner replacement contracts focus on efficiency improvements aimed at reducing the amount of water for each megawatt-hour generated.
The way the procurement process works is that, in a request for proposals (RFP), we ask turbine manufacturers to guarantee the efficiency of a new or rehabilitated turbine. Suppliers are provided with incentive to push the limits of hydraulic efficiency they will provide and guarantee, which will earn them a lower evaluated proposal price. For such a contracting method to work, it is essential to have a performance testing method that has the confidence and acceptance of both the purchaser (in this case, Reclamation) and the supplier – and, owing to the involvement of both owners and manufacturers on the code committee, the ASME PTC-18 standard has earned this acceptance.
Performance testing through the years
During its 70-year testing history, Reclamation has employed a variety of methods. Some key practices are described in the following sections.
Gibson (pressure-time) method
Reclamation’s records of performance tests date back to the mid-1930s. These reports show water flow rate and head measurements being performed by the Norman R. Gibson Company of Niagara Falls, N.Y., using the traditional Gibson method. The Gibson method of flow measurement is a “transient” method. It uses the pressure rise associated with a decelerating column of water due to rapid closure of the turbine wicket gates as the basis for calculating velocity.
The Gibson method, now called the pressure-time method, originally relied on recording the movements of a column of mercury – used to indicate the pressure wave – by having the mercury column obstruct a light source. A rotating piece of photographic paper captured the movements of the mercury. The method has been modernized using pressure transducers, data acquisition systems, and computers.
Salt velocity method
Because the Gibson method of flow measurement did not work for pumps, a method was required to verify pump performance. Charles M. Allen of Alden Research Laboratory developed such a method, known as the salt velocity method. This is a tracer method using the fact that a brine solution is more electrically conductive than water. With the method, brine is rapidly injected into the penstock or discharge pipe through an array of pop-valves. To ensure adequate mixing, a turbulator – an array fabricated from I-beams to create turbulence – is installed downstream. The salt cloud is detected by an array of electrodes both immediately downstream of the pop-valves and at a known distance downstream. Based on measurements from a chart recorder used to record the current passing between the electrodes versus time, the velocity is calculated.
Reclamation used the salt velocity method for pumps and later for turbines up until the late 1980s. Reclamation had all the equipment to perform salt velocity flow measurement and made extensive use of it for both pump and turbine efficiency testing, including participating in an EPRI comparative flow measurement testing research project at BC Hydro’s 580-MW Kootenai Canal generating station in 1983 and Reclamation’s 6,809-MW Grand Coulee plant in 1984. This testing provided comparison data with code-accepted flow measurement methods, which was partially responsible for the acoustic flow measurement method acceptance into the 1992 revision of ASME PTC-18. Owing to the acceptance of the acoustic flow measurement system into PTC 18, Reclamation disposed of its salt velocity test equipment in 2003.
Acoustic flow measurement
For the past 20 years, Reclamation has used the acoustic flow measurement method for the vast majority of its pump and turbine performance tests. From Reclamation’s viewpoint, it has been a major contributor to shorter and less expensive performance testing.
This testing method is an especially useful and relatively inexpensive way to conduct performance tests of older units to determine current unit efficiency, in order to analyze the economics of a runner replacement.
The acoustic flow measurement method calculates water velocity based on the differential travel times for ultrasonic pulses as they travel between pairs of transducers that are mounted in diagonal planes on the periphery of a penstock. Sixteen transducers typically are used, arranged to trace eight paths. For acceptable accuracy, the method requires 20 penstock diameters of straight penstock upstream of the measurement planes, and five penstock diameters of straight penstock downstream.
Because most of Reclamation’s turbines are of the vertical shaft Francis type and have reasonable lengths of penstock upstream, acoustic flow measurement works quite well. The method has the advantage of being permanently installed. In addition, measurements can be taken continuously, and while unattended. The design of transducer installation is site specific. In some cases, we use through-the-wall transducers, in others internally mounted transducers and, in some cases, a combination.
Since 2000, Reclamation has used an Accusonic Technologies model 7510 portable flowmeter for field testing pumps and turbines. This unit has eight-path capability and is used for temporary installations, or in plants where one processor is computing flow for many penstocks. This portable flowmeter has proven to very useful in a variety of flow measurement situations.
Most significant advances
In the past, performance and efficiency testing was expensive and labor intensive, especially when the Gibson or salt velocity methods were used. However, in the past 20 years, the conduct of testing has changed significantly. Advances in test equipment and personal computers have decreased in the number of people required and the quantity of equipment that must be shipped to the testing site. Thirty years ago, a testing crew of 20 people could be required for conducting a field performance test. Measurements were made and recorded by hand. Synchronizing of measurements was accomplished by stringing indicating lights throughout a plant to measurement stations – these white, yellow, and green lights would indicate to staff what was going on. Today, a scanning voltmeter and various transducers take the place of most of yesteryear’s test crew. Using permanently installed acoustic flow transducers, two people can perform the field work for a turbine test in three to four days.
An important component, pressure transducers have evolved to be quite reliable and accurate. Of course, they need to be selected with appropriate characteristics, such as high accuracy and quick response. Until the mid-1970s, Reclamation used mercury manometers. That was when reliable pressure transducers became available. Moreover, by then the dangers of mercury were starting to be better understood, and this raised a case against the use of this type of device.
There have been other improvements that have made field testing easier. For example, until recently, a dead weight tester was required to be shipped to testing locations to calibrate pressure transducers. This 150-pound device was occasionally damaged in shipment. Today, the same task is accomplished with a lightweight, easy-to-use, portable calibration device.
Benefits of performance testing
In recent years, performance testing has been used to improve performance curves that are used as the basis for operating Reclamation’s hydro units. Overall efficiency gains of about 1 percent were obtained as a result.
While 1 percent may not seem like a great amount, consider this: at Reclamation’s 6,809-MW Grand Coulee plant (the U.S.’s largest hydro plant), a 1 percent improvement in efficiency equates to producing additional electricity each year that’s worth more than $10 million. Owing to the great value of efficiency improvements at this one plant, Reclamation and the Bonneville Power Administration, which markets Grand Coulee’s power, are working together in a program to optimize unit dispatching at the facility.
Going forward, information resulting from Reclamation’s continuing program of performance testing will be used to make economic- and engineering-related decisions about unit rehabilitation. In addition, performance testing results will be used to further improve unit operations and scheduling.
At Reclamation, improved performance testing technologies and practices have both decreased testing costs and improved the quality of results. More than ever, performance testing is an important contributor to maintaining – and further improving – high levels of performance.
Mr. Hulse may be contacted at the U.S. Department of the Interior, Bureau of Reclamation, P.O. Box 25007, Denver, CO 80225-0007; (1) 303-445-2881; E-mail: [email protected]
David Hulse, P.E., is manager of the mechanical equipment group and lead turbine performance test engineer for the U.S. Department of the Interior’s Bureau of Reclamation.