Thread Content
1 Working principle of the turbine transmitter A turbine flowmeter consists of a turbine, bearings, a pre-amplifier, and a display instrument, as shown in Figure 1. The fluid being measured impacts the turbine blades, causing the turbine to rotate; the rotation speed of the turbine changes with the flow rate – that is, the higher the flow rate, the faster the turbine rotates. A magnetoelectric conversion device then converts the turbine’s rotation speed into electrical pulses of corresponding frequency. These pulses are amplified by the pre-amplifier before being sent to the display instrument for counting and display. The instantaneous flow rate and cumulative flow rate can be determined based on the number of pulses per unit time and the total number of pulses accumulated. The working principle of a turbine flowmeter is that as the fluid flows along the axis of the pipe and strikes the turbine blades, a force proportional to the product of the flow rate qv, the flow velocity V, and the fluid density ρ acts on these blades, causing the turbine to rotate. As the turbine rotates, the blades periodically cut through the magnetic field lines generated by the electromagnet, thereby changing the magnetic flux in the coil. According to the principle of electromagnetic induction, a pulsating potential signal is induced within the coil. The frequency of this pulsating signal is proportional to the flow rate of the fluid being measured. That is: where qv represents the total volume of fluid, and N represents the total number of pulses generated by the transmitter ； ξ is the flow coefficient. It is an important characteristic parameter of turbine transmitters; different instruments have different values of ξ, and these values change over time due to wear and tear resulting from long-term use of the instrument ； It refers to the number of pulses output by the transmitter per unit volume flow rate as it passes through the transmitter. The pulse signal generated by the turbine flowmeter is amplified by a pre-amplifier and then sent to the display instrument, thereby enabling the measurement of flow rate. 2 Selection of turbine flowmeters: (1) It is advisable to use 316 stainless steel for the flowmeter body in order to ensure corrosion resistance; if it is used in an explosion-proof area, it must also have explosion-proof properties. (2) Bearings generally come in three types: tungsten carbide, polytetrafluoroethylene, and carbon graphite. Tungsten carbide offers the highest precision and is used as a standard component in industrial control applications ； Polytetrafluoroethylene and carbon graphite provide corrosion resistance, and they are generally given priority in chemical processing facilities. The lifespan of bearings is inversely proportional to the square of the flow rate; therefore, it is better to use a flow rate that is 1/3 of the maximum flow rate. (3) The induction probe detects the movement of the rotating body and converts it into pulse digital electrical signals. The voltage output of its electromagnetic coil follows a near-sine curve, and the frequency of the pulse signals varies linearly with the flow rate being measured; typical ratios are 10:1, 25:1, and 100:1. The resistance of an electromagnetic coil is generally less than 2000Ω; values higher than this may cause damage. 3. Installation of the turbine flowmeter: (1) The power cable of the transmitter should be a metal-shielded cable, with reliable and proper grounding. The power supply is 24V DC with an impedance of 650Ω. (2) The transmitter should be installed horizontally, avoiding vertical installation, and appropriate straight pipe sections should be ensured in front of and behind it, generally 10D in front and 5D behind. (3) Ensure that the flow direction of the fluid is consistent with the arrow direction on the instrument enclosure; it must not be installed in the reverse direction. (4) The medium under test must not be corrosive to the turbine, especially at the bearings; otherwise, measures should be taken. (5) Be careful not to collide with the magnetic induction section. 4. Configuration and calibration of turbine flowmeters The standard calibration method is the ten-point water calibration method; however, the calibration values vary depending on viscosity, so it is usual to create a viscosity calibration curve. 5. Display instrument of the turbine flowmeter The task of the display instrument is to convert the number of pulses output per unit time and the total number of pulses output into the instantaneous flow rate and total flow rate, and to display them. The pulse signals output by the pre-amplifier have irregular amplitudes and waveforms. Upon entering the display instrument, they must first be shaped by a shaping circuit into regular rectangular pulse signals with a fixed amplitude. Subsequently, a frequency/current conversion circuit is used to transform the frequency signal into the corresponding current signal (4–20 mA), from which the instantaneous flow rate can be determined; the total volume is then obtained through conversion and integration circuits. Some display instruments show the readings on-site, while others send the data to the DCS for display. 6 Precautions (1) The pipeline must be cleaned before installing the turbine flowmeter. A filter should be added when the medium under test is not clean. Otherwise, the turbine and bearings are prone to getting stuck, making it impossible to measure the flow rate. (2) When disassembling or assembling the flow meter, do not collide with the magnetic induction section. (3) Set the instrument coefficients before commissioning. Carefully check to ensure that the instrument wiring is correct and the grounding is proper before powering it on. (4) When installing a turbine flowmeter, the flanges at the upstream and downstream ends of the pipeline must be level; otherwise, pipeline stress will have a significant impact on the flowmeter. 7 Future Prospects With the continuous advancement of science, today’s turbine flowmeters have evolved into compact, highly integrated modules equipped with powerful software, as well as RS232 standard computer communication interfaces, which facilitate maintenance and repair. It can communicate with a DCS, which replaces the display instruments; thus, flow meters such as HOFFER flow meters are more convenient and practical in industrial production processes. In summary, a turbine flowmeter is a type of velocity-based flow meter. Thanks to its advantages such as high measurement accuracy, fast response time, wide measurement range, low cost, and easy installation, it is widely used in chemical manufacturing. For more information, please visit the company’s official website at http://www.yb1518.com/. Please retain this link when reproducing the content!