Flow, pressure, and temperature are the three fundamental parameters for detecting objects, widely applied in measurement. With the rapid development of China's industry, the demands for flow measurement in various automated control systems have grown increasingly stringent, leading to the extensive adoption of flow meters.
Requirements for Flow Measurement Instruments in Process Control
Flow meters are extensively employed in process control. Their role involves detecting fluid flow rates within sealed pipelines. When necessary, flow measurement instruments are integrated with control instruments and actuators to form regulation systems, stabilizing flow within appropriate ranges to ensure process stability. Given this specific function in process control, flow measurement instruments must meet the following requirements.
1. Performance Stability
① The output of flow measurement instruments should exhibit excellent stability. If the flow signal itself contains noise, internal damping adjustments should stabilize the reading for easy interpretation. When integrated into a control system with a regulator, the regulator output should remain free of noticeable oscillations.
② The influence of ambient temperature on the instrument's displayed value should remain within specified technical parameters.
③ The instrument should demonstrate excellent long-term stability.
2. Reliability Requirements
① Instruments must exhibit high reliability. Modern industrial installations trend toward large-scale continuous processes where instrument failure can easily destabilize operations. Since flow meters installed on pipelines cannot be repaired by halting the process, reliability must be prioritized in both instrument manufacturing and system design-including the reliability of thermistors used for temperature compensation. Some manufacturers implement redundancy for components prone to failure and difficult to repair. Others design methods for sensor replacement without interrupting flow. Electromagnetic flowmeter manufacturers provide techniques and tools for non-disruptive electrode replacement, all contributing to enhanced reliability.
② Fault Diagnosis. Upon instrument failure, the diagnostic system should automatically indicate the location and nature of the fault to minimize repair time. When diagnostic data is transmitted digitally to a computer, the computer can monitor instrument operation, trigger alarms during failures, display fault details, and even implement necessary safety measures.
3. Strong Anti-Interference Capability
① Vibration resistance.
Most flow sensors are mounted on pipelines in harsh field environments where vibration is a major disturbance. Therefore, flow sensors, transmitters, and other components must possess strong anti-interference capabilities. Some vortex flow meters and Coriolis mass flow meters perform poorly in the field due to inadequate vibration resistance, exhibiting phenomena like "false readings" or "excessive readings."
② Resistance to Radio Frequency Interference
Industrial sites housing flow meters contain multiple interference sources. For instance, overhead cranes passing overhead, forklifts operating nearby, or personnel using walkie-talkies can cause elevated readings on certain flow measurement instruments. This occurs when radio frequency electromagnetic waves emitted by crane electrical systems, forklift spark plugs, or walkie-talkie antennas enter the instrument through various pathways, disrupting its operation. In recent years, the impact of RF interference has gained significant attention. Measurement instruments now incorporate RF interference resistance specifications and employ numerous measures to enhance interference immunity.
4. Short Response Time
Many flow measurement instruments form control systems with regulators, requiring response times under 1 second. In setpoint flow control systems, a total time constant exceeding 1 second in the flow measurement segment may significantly degrade control quality. In severe cases, this can cause system oscillations and operational failure.
5. Diverse Output Signals
① Analog Output.
Flow measurement instruments should feature a 4–20mA analog output with constant current characteristics.
② Frequency Output.
Flow transmitters (converters) transmit flow signals to display instruments or controllers via frequency, preserving accuracy with minimal loss-a key advantage of this method.
③ Digital Output.
Flow measurement instruments connect to computers via communication ports like RS485. With dedicated software support, they not only transmit measured parameters to computers but also send fault information, configuration data, and instrument status indicators. Furthermore, operators can modify field instrument configurations, perform inspections, calibrations, maintenance, and management tasks remotely from control rooms via computers.
Flow Detection and Flow Meter Usage
Flow measurement is a common industrial measurement method widely applied in sectors such as power generation, metallurgy, chemical engineering, petroleum, and food processing. Any process involving changes in mass necessitates flow measurement. Flow meters serve as the tools for this measurement. Based on differing measurement principles, these instruments can be categorized into numerous types. With the advancement of modern measurement techniques, flow meters have evolved from early differential pressure, positive displacement, and electromagnetic types. They now feature not only simpler structures but also increasingly diverse functionalities. The accuracy of flow meter measurements directly impacts the correct and stable execution of industrial control processes, holding direct relevance to China's national economic development. Therefore, mastering the principles of common flow meters and understanding the application of typical flow meters in automation systems is crucial for enhancing industrial automation levels and instrumentation equipment standards.
Application of Flow Meters in Automated Control Systems
1. Application of Flow Meters in Oilfield Automated Measurement Systems
Oilfields represent one of the most extensive industries for flow meter application, primarily used for oil production measurement, statistics, and analysis, including daily well output monitoring. Advanced measurement and process technologies facilitate timely understanding of oilfield development status and reservoir changes, enabling analysis of dynamic shifts in oil and gas production to further guide oilfield development strategies. In oilfield sub-unit metering, raw produced fluids first pass through three-phase separators to be divided into three streams: one directed to the compressor station via a regulating valve, another routed to the settling tank via an electromagnetic flowmeter, and the third sent to the buffer tank via a mass flowmeter.
The oil-water mixture passes through a mass flow meter sensor that collects parameters such as flow rate, temperature, and density within the oil pipe. These signals are transmitted to the processor, where relevant microcomputer algorithms analyze and calculate the collected parameters of crude oil and water. After undergoing a transmission stage, the data is sent to the monitoring host via TCP/IP Ethernet communication. This enables comprehensive management functions including data display, storage, reporting, and printing, thereby achieving monitoring of multiple oil-water measurement systems.
Additionally, at oilfield well-turning sites, addressing the widespread issues of high-density drilling fluids and heavy material waste in deep wells, the flowmeter sensor collects and analyzes changes in drilling fluid viscosity, density, and centrifuge performance parameters. After the control system calculates the centrifuge's operating speed and corresponding processing capacity, computer-output control establishes the control system workflow. This effectively enhances the recovery rate of heavy materials and reduces their usage costs.
2. Flowmeter Applications in Power Plant Process Systems
2.1 Application in Boiler Air Supply Processes
In power plant boilers, flow meters primarily measure the flow rates of air, steam, and boiler air supply. The most commonly used flow meter is the vortex flow meter. Operating on the velocity principle, it utilizes the regular vortex shedding phenomenon to measure flow. When fluids like steam or air flow past the sensor, a high-pressure zone forms ahead of the sensor, where pressure exceeds the static pipe pressure. As the fluid accelerates through the pipe's acceleration section, a low-pressure zone forms where pressure is lower than the pipe's static pressure. A vortex-induced vacuum zone then develops behind this low-pressure zone, creating pressure fluctuations. The frequency of these fluctuations is directly proportional to the gas flow rate. By measuring this vibration frequency and applying appropriate conversion and compensation, the fluid velocity can be calculated.
Taking vortex flowmeter measurement in boiler air supply flow as an example: boiler air supply flow is a critical parameter reflecting the operational status of power plant boilers and fans, playing a vital role in the automatic control system for boiler combustion. Actual power plant air supply ducts are predominantly rectangular in cross-section, making precise measurement challenging with conventional flowmeters. The vortex flowmeter demonstrates superior performance in this application.
When employing vortex flow meters for boiler air supply flow measurement, the system comprises a sensor, converter, and control center. The sensor consists of a vortex generator and a vortex detector, primarily responsible for measuring the air supply flow and converting it into a corresponding frequency signal. This frequency signal undergoes shaping and amplification within the converter, outputting a 4–20mA DC control signal to the control center. There, the measured airflow is displayed, recorded, and analyzed, serving as a critical reference for boiler operational status within the power plant.
When employing vortex flowmeters for boiler airflow measurement, careful consideration must be given to instrument range selection and temperature/pressure compensation. Maintaining the measured fluid flow within 1/2 to 2/3 of the vortex flowmeter's capacity ensures accuracy remains within acceptable limits. Additionally, appropriate temperature and pressure measurement instruments must be selected to complement the vortex flowmeter, establishing a precise and accurate boiler automation control system. With advancements in computer and microelectronics technology, intelligent vortex flow meters have become widely adopted. Featuring flow calibration and self-diagnostic capabilities, they enable more flexible control based on power plant boiler operating conditions and perform error correction, representing a more mature technology.
2.2 Application in Flue Gas Desulfurization Processes
Flow meters are also extensively used in power plant flue gas desulfurization processes. Due to high dust content, elevated temperatures, and corrosive properties of flue gas emissions, coupled with turbulent and swirling conditions in boiler flue ducts, accurate flow measurement is challenging. Consequently, multiple measurement points are required to calculate average values. The numerous measurement points in power plants-including primary air, secondary air, boiler feed gas, and desulfurized flue gas-present significant challenges for flue gas monitoring. Flue gas flow meters for desulfurization employ a unique principle based on thermal dispersion. They convert the relationship between temperature difference across the sensor RTD and flow rate into a linear flow signal output. Combined with specialized flow data models and fuzzy control theory, they generate control signals. System control is achieved through dedicated charge-sensing probes and scraping devices.
3. Flow Meter Applications in Wastewater Treatment Process Systems
Pharmaceutical Plant Wastewater Treatment Systems
With the rapid advancement of modern industry, the importance of municipal wastewater treatment continues to grow. Flow meters have found extensive application in automated wastewater treatment plants. Wastewater contains substantial suspended solids, effluent, impurities, pathogens, etc. Different monitoring points have varying requirements for flow meters. Electromagnetic flow meters and ultrasonic flow meters are both applied, with ultrasonic flow meters gaining more usage in recent years due to their high accuracy, good integration, and compact size.
Taking the application of ultrasonic flow meters in wastewater treatment as an example: by integrating ultrasonic flow meters with a Parshall flume, effluent flow is monitored to control inflow and bypass valves, thereby achieving flow regulation in wastewater treatment. Within the ultrasonic flow automatic control system, ultrasonic sensors detect flow information. By measuring the distance from the zero position to the sensor diaphragm and the full-scale range, the actual wastewater flow corresponding to the height is determined and transmitted to the central microprocessor of the control system. After conversion, a 4–20 mA pulse signal is output to the programmable controller in the central control room. Following communication, the management terminal displays information including instantaneous flow rate, maximum value, minimum, and average values. It supports flow statistics and printing, and operates based on fault diagnosis logic.
When system faults or abnormal flow occur, it outputs alarm information, prompting operators to adjust the inlet valve and bypass valve for flow control, thereby meeting the production requirements of the wastewater treatment process. More advanced control systems can treat flow as a variable input into the PLC in the central control room. This enables direct programmatic calculation and control of the adjustment increments for the inlet valve and bypass valve. Simultaneously, converting these valves to electric drive eliminates the need for manual operator intervention, further enhancing system efficiency.
Beyond these applications, flow meters are extensively utilized in desulfurization processes, direct current power supply systems, coal gasification wastewater treatment, energy metering, environmental protection, and other fields, permeating every energy conversion stage of industrial production. With the continuous advancement of industrial automation and the rapid development of computer microelectronics technology, flow meters have evolved from mechanical to electronic designs. New types of flow meters continue to emerge, playing an increasingly significant role in China's national economy and demonstrating promising development prospects.