The power industry has a critical need to measure the bin level inside fly ash hoppers under electrostatic precipitators (ESPs). The inherent problem associated with these hoppers is that they are usually ganged together per generating unit in arrays from four to 32, depending on unit capacity. It is important that a properly maintained level monitoring system be implemented to prevent material from backing up in these hoppers and damaging the precipitator, reducing ESP efficiency, or both.
Fly Ash Properties
Several properties of fly ash present difficulties in applying process instrumentation correctly. Repose angle, temperature, dielectric constant, material buildup, and space limitations are the most prominent.
Not all fly ash is the same. Fly ash collected from the slag in a cement plant is different than that from a coal-fired power plant. Both processes might burn similar coal, but the temperatures the coal was burned at and also the process of burning itself has an impact on the properties of the fly ash collected.
Naturally, fly ash collected from heavy-oil or waste-burning installations will have different properties as well. These main properties are the weight of the fly ash, size of the particles, dielectric constant, temperature, and last but not least, how adherent it is.
Typically the power industry has focused on using radiometric (gamma) or capacitance-type devices to monitor hopper levels (Figure 1). However, the low dielectric constant of fly ash and the temperature extremes make reliable setting of a traditional capacitance probe very difficult. The sensitivity needed for low dielectric product detection often leaves the probes in a state where false triggers can be caused due to changes of temperature.
Radiometric devices provide a noncontact method of measurement where the radiation passes through the sidewall of the hopper to a detector on the opposite side. This eliminates material properties from affecting the level measurement and gives a relatively simple installation. However, the drawback to radiometric devices centers on the cost of ownership. Documentation, periodic testing, training and maintaining a site radiation safety officer, which is normally required, can be expensive. Furthermore, disposal costs can sometimes exceed the original purchase price, adding insult to injury.
Traditional capacitance devices have provided a lower cost alternative, with limited success. Whether designed with an insertion-style probe or a plate type, the sensor must come in contact with the ash to detect the absence or presence of material. When the weight touched the level in the silo, the electronics detected the slack in the cable or band, reversed the motor and started counting the length of the tape that indicated the amount of free space in the silo. However, the properties of the fly ash, such as heat, abrasion, and buildup, may degrade sensor performance over time. Uneven loading can cause damage to the precipitator even if the sensor is working correctly, because it is limited to sensing at the precise point where it is inserted in the bin wall, rather than scanning across a span of the bin.
This is a simple way of measuring in fly ash silos, but it also can be costly. It requires relatively high maintenance as the moving, mechanical parts wear out and need replacement, especially in a dusty environment. With the current focus of modernization and emphasis being placed on reducing maintenance and ownership costs, this technology is being abandoned.
Optical devices have not been reliably proven in this application due to the dust and film coating present.
Today’s Technological Solutions
The latest technology, which addresses several material-handling concerns for the power industry, utilizes a microwave-based switch system. This uses radio frequency (10.525 GHz) to propagate a wave of energy from a sensor to a detector. It is similar in concept to the gamma switch, where an alarm relay is tripped if material interrupts the signal path. However, the radio frequency (RF) energy solution has several significant advantages over other technologies, including:
■ It is a span-measuring device, similar to the radiometric switch, and it detects material in the path of the energy beam. The dielectric constant of fly ash is in the ideal range for RF energy.
■ It is a noncontacting device, eliminating the possibility of process conditions fouling the sensor.
■ It uses a low-energy source, compliant with Federal Communications Commission Code of Federal Regulations, Title 47, Part 15 rules for radiation emission, and it does not require warning sign posting for use. Output is fewer than 10 microwatts per square centimeter, which is less than typical commercial microwave oven leakage.
■ It is a high-powered device, with a maximum range more than 300 feet.
■ The oscillator is solid state; therefore, the signal intensity will not degrade over time.
Fly Ash Precipitator Application
The fly ash precipitator application presents some interesting challenges for proper operation. The first obstacle with installation is that there must be a nonconductive path for the RF signal to propagate through, otherwise it will not penetrate steel bin walls like the gamma source. This, coupled with elevated temperatures of up to 398C (750F), requires the use of ceramic lenses to allow the signal to pass through the sidewalls.
The next hurdle is dealing with air temperatures outside of the bins. The oscillator used in the microwave sensor is limited to a maximum continuous temperature of 71C (160F). The oscillator is located in the sensing head between the mounting flange and the window face. The siamese bottoms of the fly ash precipitators can act as a heat trap, and ambient temperatures at the highest point can reach 93C (200F) outside the flashing. To overcome the problem, an aluminum conduit specifically designed for the RF band of these switches can be used to relocate the oscillator to a more suitable ambient temperature.
The final step is making it easy for field personnel to install, operate, and maintain. The systems for fly ash precipitators are provided as an application-specific integrated system. No special tools are required for installation. The mount that locates the ceramic window on the bin wall also mounts the upper portion of the waveguide for proper alignment. This mount is adjustable to accept any angle of wall slope. The mounting flange on the remote sensing element provides a lower support point. The electronics are remote from the sensor and detector, which allows easy user access to the switch, and it can be located conveniently on a handrail or column support.
In use, the maximum range of these switches is more than 300 feet (91 meters). Sensitivity adjustments are used for higher dielectric materials, such as paper bales or plastic film, or very small ranges (less than 3 feet). In the fly ash system, the detector is set to maximum sensitivity. This allows for measuring the pile inside the hopper, not the minimal material buildup on the wall. The alarm relay has an adjustable time delay for make/break of the alarm to minimize false trips from material surges. Figure 2 illustrates how a two-point system could be utilized.
|2. High-temperature application. The setup shown here with waveguides can be used up to 700C (1,292F). The maximum ambient temperature where the sender and receiver are located is 70C (158F). Courtesy: Hawk Measurement Systems|
Making the Right Choice
Fly ash hoppers present plant managers with unique challenges. Making the best decision in order to optimize the tradeoff between risks and costs is a challenge. Without accurate information it becomes even more problematic.
Microwave-based systems extend the performance of capacitance and RF probes greatly through use of an extremely stable oscillator core, which exhibits almost no drift with process temperature changes. High stability allows higher sensitivity to be used when setting switch points. That greatly improves the ability to reliably detect products having lower dielectric constants. High temperature ceramic insulation is used in the construction of the heavy-duty probes for fly ash applications, and the rugged 36-millimeter stainless steel sensing element can withstand heavy impact loads without bending or sustaining damage. Lighter-duty versions and Teflon-insulated versions are also available for less demanding process conditions. ■
—Jack Evans is president of Hawk Measurement America and director of global sales and marketing for Hawk Measurement Systems.