Here’s a look at the range and applications of level measurement technology
Donald Koeneman and William Sholette, AMETEK Drexelbrook
More than one level of measurement technology has proven viable for the range of applications found in coal-fired power plants. Because different coal-handling processes exist within a single power facility - along with various and often extreme, operating conditions - no single technology is best suited for every application.
Here’s a users’ guide to level measurement technology for coal-handling and related fly ash operations. Its purpose is to broaden end users’ understanding of available technologies, whether the customer is selecting instrumentation for the coal yard, power plant or fly ash facility.
Point Level vs. Continuous Measurement
Regardless of application, level measurement instrumentation can be divided into two major classifications: point level and continuous level measurement.
Point level (also known as on/off) measurement indicates the absence or presence of material whether within a surge bin or storage silo or on a conveyor belt. Point level switches are used as high and low level and as spill prevention devices. They either turn on or turn off pumps and conveyors. Continuous level (also known as proportional) measurement indicates the level in a storage vessel or bin over the full span of measurement. These devices typically are used for process control and inventory management.
The technologies used to measure level are affected by different process conditions. The technologies most commonly used in a typical coal-handling facility are:
RF Admittance - This employs a radio frequency signal. A change in RF admittance indicates either the presence or absence of material or how much material is in contact with the sensor. This makes it a highly versatile choice for a wide range of conditions and materials for point or continuous level measurement.
Radar - Radar uses frequency modulated continuous wave (FMCW) transmitted through the air that allows for an accurate non-contact reading of reflected electromagnetic signals.
Vibration - Vibration technology uses a rod or fork, vibrating at a fixed frequency. As the fork is covered with material, its frequency shifts. This shift is detected by the internal oscillator and translated into a switching command.
Ultrasonic (Point Level) - Point level ultrasonic measurement electronically resonates a crystal at a fixed frequency to generate sound waves that travel across an air gap to a second crystal. As the gap between the two crystals fills, the second crystal begins to resonate with the first.
Ultrasonic (Continuous Level) - Continuous level ultrasonic measurement uses a transmitter to generate an ultrasonic pulse and measures the time it takes for a reflected signal to return to a transducer, which determines the level.
Time Domain Reflectometry (TDR) - This method takes a highly focused electronic wave guided by a metallic rod or flexible cable to the surface of a liquid or other material and reflects it back along the rod or cable to determine the level.
Typical Point Level Applications
The figure shows the typical locations in a coal-fired power plant where point and continuous level measurements are taken.
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Typical Point Level Applications
(LS-1) Empty Belt Detection - Coal is transported using conveyor belts. In order to ensure continuous operation, the belt should have coal on it at all times. A lack of coal would typically be caused by a blockage or plugging in the upstream process that needs correction. An RF sensor with a flexible tip is mounted above the belt and the flexible section rides on the coal. The sensor detects an absence of coal and initiates an alarm.
(LS-2) High-Level Alarms for Silos, Bunkers and Surge Bins - Point level switches used for high-level indication have two purposes. First, they provide an alarm to indicate when a silo or bin is full. Second, they prevent overfilling, which can result in lost material, clean up, and in some cases, regulatory fines. Often two switches are used for these purposes: one to indicate a high-level condition and a second to serve as an overfilling alert.
(LS-3) Low-Level Alarm - A low-level point switch indicates a vessel that is empty or almost empty. It sounds an alarm when there is no material on the sensing element. Often these switches are positioned in the silo or bin to indicate when the level has reached a point to provide just enough material to allow refilling.
(LS-4 & LS-6) Plugged Chute Alarm - Coal that blocks and backs up a chute will cause an interruption in the process and possibly cause material to spill. A special RF sensor that mounts flush inside the chute wall is calibrated to be in the “normal” condition when coal flows freely. It provides an alarm, allowing the operator to take corrective action when coal backs up in the chute.
(LS-5) High-Level Alarm on Pulverizer Mill - Coal is fed into a pulverizer mill where it is ground into a fine powder to allow more efficient burning. The coal powder is typically blown from the pulverizer into the boiler furnace. An RF point level switch is mounted in the top of the mill to indicate a high level of coal. A high-level indication allows the operator to manage the coal feed into the pulverizer and reduces blockages and backups.
(LS-7 & LS-8) Fly Ash High-Level/Low-Level - Electrostatic precipitators (ESPs) or filter bags are used to remove particulate matter from stack gases exhausted from burning coal. This ash is collected in hoppers under the ESP or filter. The ash must be removed before it causes the ESP to short out or damage the filter bags. An RF point level switch is mounted at a point below the ESP or filter bag to indicate when the ash needs to be removed. Another switch is often used to indicate a low-ash condition when emptying. RF sensors are suitable for this abrasive and hot environment in which temperatures can reach 1,500 F.
(LS-9) High-Level Boom Control - The coal boom conveyor deposits coal in piles in the coal yard. When the pile reaches the height of the boom, the boom is moved and starts a new pile. An RF point level switch mounted so the sensor is just below the boom indicates when the pile has reached the high point. This allows the operator to maximize stockpiling space in the coal yard.
(LS-10) Diesel Fuel High-Level Alarm - Most power plants maintain a backup fuel supply the primary fuel is interrupted. For coal-fired power plants, diesel is often used as a secondary fuel supply. Point level switches indicate when tanks are full to prevent spilling. For diesel fuel, either an RF point level switch or a vibration technology, such as a tuning fork, are suitable for high-level detection.
Typical Continuous Level Applications
(LT-1) Continuous Indication, Coal - Continuous level systems are used in storage silos to ensure enough coal is available for the boilers. Coal silo storage and blending silos are monitored to ensure there is enough coal in storage or in the blending silos. The blending of coals allows the power plant to feed its boilers with a blend of coal that produces a constant Btu output.
Low Btu coal is blended with higher Btu coal in an effort to produce a constant output power level. Continuous level systems in coal silos must be mechanically strong to withstand the forces and abrasion from the coal. Non-contact systems must be able to read through the vapor and dust that may be in the headspace above the coal.
In bunkers and surge hoppers, the level indication also serves as an alternate means to indicate any upstream, or possibly downstream, blockages that can occur in conveying systems.
(LT-2) Continuous Level, Fly Ash Silo - As different types of coal are burned and the fly ash is removed to storage silos, stratification occurs. The first few feet of a strata may be one type of ash, while the next strata is from a different coal/ash type. Each layer has a different dielectric constant and other characteristics. Time Domain Reflectrometry (TDR) technology has proven the best available technology for continuous level measurement in fly ash.
TDR technology is unaffected by differences in dielectric constants, dust conditions and most sensor coatings. However, all TDR systems are not equal. Many lack the sensitivity to measure the low dielectric values of fly ash. Many also don’t have a single cable sensor design or the ability to hold up to the mechanical forces and abrasion associated with fly ash. What’s more, fly ash can generate a high amount of electrostatic discharge (ESD). A suitable TDR system should be able to handle ESD of up to several thousand volts.
(LT-3) Diesel Fuel, Backup - Continuous level systems are used to ensure that the on-hand volume of fuel oil in storage is available for situations when the primary fuel source is interrupted.
(LT-4) Scrubber - Scrubber systems help remove toxic gasses such as sulfur from stack gases prior to being emitted into the atmosphere. This is primarily accomplished by washing the spent gases in a bath that contains neutralizing agents.
(LT-5) Feed Water Heater - A continuous level device is used to ensure that enough feed water volume is in the boiler to aid in the removal and distribution of heat and prevent a loss of water that would lead to boiler burn out.
Operators of coal-handling processes need to consider a range of options in selecting a level measurement solution. No single technology is best suited for every possible application. The best choice for finding the right solution is to choose a solution provider familiar with all technologies and experienced with a full range of level measurement technologies and applications typically found in a coal-fired plant.
Authors
Donald W. Koeneman is Continuous Level Product Manager for AMETEK Drexelbrook with more than 28 years of marketing, regional sales, customer service and support experience.
William Sholette is Point Level Product Manager for AMETEK Drexelbrook, responsible for all point level and wireless interface level measurement products. He has more than 26 years experience with Drexelbrook in process control instrumentation, manufacturing, sales and marketing.