Hydroelectric power is the most widely used renewable energy resource in the world, comprising as much as 25 percent of total global electricity. Hydroelectricity is inexpensive to produce, adapts quickly to fluctuating demands for power and does not pollute the environment.
Despite its advantages, hydroelectric power presents unique challenges when it comes to monitoring the flow of water through a hydropower plant—an important step in ensuring safe and efficient plant operation.
Clamp-on ultrasonic flow measurement technology can handle the large pipe sizes and high flow velocities typically found in hydroelectricity applications.
Measuring Flow in a Large Pipe
As an example, a major municipal water company in northern California generates hydroelectric power and supplies water to local customers. Water is stored in a reservoir and released through a penstock, an enclosed pipe that travels downhill from the reservoir to a turbine. The water in the penstock often flows at high speeds, occasionally more than 40 feet (12.2 meters) per second. The force of this fast-moving water turns the turbine, which drives an electric generator to produce hydroelectricity.
The company needed to monitor the volume of water flowing through the 54-inch (1.4-meter) penstock and through an 8-inch (20.3-centimeter) bypass line. Turbine efficiency is directly related to water flow, and a reduced level of efficiency often indicates the need for system maintenance. As a result, accurate flow measurement is essential for maintaining precise control over the water’s movement and for receiving adequate warning of potential issues.
The company investigated a retrofit ultrasonic flow system that is welded into the pipe and sends ultrasonic signals between two transit-time sensors to compare the arrival time of sound waves traveling with flow and against it. The difference is directly proportional to the medium’s mean flow velocity.
This is a precise method of measuring flow, with an expected accuracy rate of 0.25 to 0.5 percent, but it tends to be costly because of the downtime required for retrofitting.
The company discovered a viable alternative—a clamp-on ultrasonic flow measurement device. This type of meter measures flow using the same transit-time method as its retrofitted counterpart, but the sensors are mounted externally and transmit signals through the pipe wall.
At about 0.5 to 1 percent flow, the accuracy rate is lower than that of a hot-tap solution but well within the accepted range for most water and wastewater applications. Clamp-on technology also allowed the company to eliminate pipe alteration and process downtime, which reduced installation costs by nearly half.
Because the sensors never touch the medium, they require minimal cleaning and maintenance.
Paths for Performance
A single-path clamp-on flow meter with one pair of sensors was sufficient to monitor the bypass line, but the penstock required a four-path system with four pairs of sensors. Pipes with larger diameters tend to exhibit a higher rate of eccentricity—the degree to which the pipe’s inner diameter is displaced from the outer diameter—and this can decrease measurement accuracy. Because it averages the measurements from multiple sensors configured in different locations around the pipe, the four-path system compensates for this imperfection.
To maintain flow meter performance despite the high velocity of the water, the company selected Lamb wave, or wide beam, sensors for the single-path and four-path systems. These sensors use the pipe wall as a waveguide to provide a wide area of vibration and to optimize the signal-to-noise ratio, which decreases sensor sensitivity to anomalies within the medium.
Today, supported by data from clamp-on ultrasonic flow meters, the company is producing hydroelectricity at maximum efficiency.