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The schematic diagram of the principle of the turbine flowmeter is shown in Figure 3-1. Place a turbine at the center of the pipe, supported by bearings at both ends. As the fluid flows through the pipe, it impacts the turbine blades, generating a driving torque on the turbine that enables it to rotate by overcoming the frictional torque and the fluid resistance torque. Within a certain range of flow rates, and for a given viscosity of the fluid medium, the rotational angular velocity of the turbine is proportional to the fluid flow velocity. Thus, the fluid flow rate can be determined from the rotational angular velocity of the turbine, allowing the fluid flow rate through the pipe to be calculated. http://img70.ybzhan.cn/9/20191204/637110497295290193577.jpg The rotation speed of the turbine is detected by sensor coils installed outside the casing. When the turbine blades cut through the magnetic flux lines generated by the permanent magnets inside the housing, it causes a change in the magnetic flux in the sensing coil. The sensing coil sends the detected periodic changes in magnetic flux to a pre-amplifier, which amplifies and shapes the signal to generate pulse signals proportional to the flow rate. These pulse signals are then sent to a unit conversion and flow accumulation circuit, where the cumulative flow value is calculated and displayed ； At the same time, the pulse signal is also sent to the frequency-to-current conversion circuit, where it is converted into an analog current value, thereby indicating the instantaneous flow rate. The structure of the turbine flowmeter: The fluid flows in through the inlet of the casing. A pair of sleeve bearings are fixed on the central axis of the tube using brackets, and the turbine is mounted on these bearings. Radial flow straighteners are installed on the supports upstream and downstream of the turbine to guide the fluid, thereby preventing it from spinning and altering the angle at which it impacts the turbine blades. Sensing coils are installed on the outside of the casing above the turbine to receive signals of magnetic flux changes. The main components are as follows: (1) Turbine – The turbine is made of magnetically conductive stainless steel and is equipped with spiral blades. The number of leaves varies depending on the diameter, ranging from 2 to 24. To enable the turbine to respond well to flow velocity, the mass must be as small as possible. The general requirements for the structural parameters of turbine blades are: blade inclination of 10°–15° (for gases) and 30°–45° (for liquids) ； The leaf overlap degree P is 1–1.2 ； The gap between the blade and the inner shell is 0.5-1 mm. (2) The bearings of the bearing turbine generally use cemented carbide bearings with a sliding fit, requiring good wear resistance. Since the fluid exerts an axial thrust on the turbine as it passes through it, this increases the frictional torque on the bearing, accelerating its wear. To eliminate this axial force, hydraulic balancing measures must be implemented in the design; the principle behind this method is shown in Figure 3-3. Since the diameter DH at the turbine is slightly smaller than the diameter Ds at the upstream and downstream supports, the flow cross-section in the turbine section increases, the flow velocity decreases, and the hydrostatic pressure rises by P. This hydrostatic pressure P acts to counteract part of the axial thrust. http://img70.ybzhan.cn/9/20191204/637110498820570632399.jpg (3) Pre-amplifier: The pre-amplifier consists of a magnetoelectric induction converter and an amplification and shaping circuit; the schematic is shown in Figure 3-4. In China, magnetoelectric converters generally use the reluctance type, which consists of permanent magnets and an induction coil wound around them. As the fluid passes through and causes the turbine to rotate, when the blades are directly below the permanent magnet, the magnetic resistance is low; whereas when the gap between the two blades is below the magnet, the magnetic resistance is high. The rotation of the turbine continuously changes the flux in the magnetic circuit, generating a varying induced voltage in the coil, which is then sent to an amplification and shaping circuit to be converted into a pulse signal. The frequency of the output pulses is proportional to the flow rate passing through the flow meter, with the proportionality constant K given by K = (3 – 1). Here, f represents the frequency of the output pulses from the turbine flow meter ； qv--The flow rate through the flow meter. This coefficient is also known as the instrument coefficient of the turbine flow meter. http://img68.ybzhan.cn/9/20191204/637110499982198754349.png (4) Signal reception and display: This function is carried out by components such as coefficient correctors, adders, and frequency-to-electricity converters within the display unit; it converts the pulse signals sent from the pre-amplifier into cumulative flow rates and instantaneous flow rates, which are then displayed.