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Features, Installation, and Use of Turbine Flow Meters 1. Features of Turbine Flow Meters As discussed earlier, turbine flow meters are flow measurement instruments with many advantages. In summary, they possess the following features: (1) High accuracy The accuracy of turbine flow meters is around (0.5–0.1)%. Within the linear flow range, even if the flow rate changes, the accuracy of the cumulative flow measurement remains unchanged. Moreover, the repeatability of turbine flow meters can reach 0.05% within a short time period. (2) Wide range ratio The range ratio of turbine flow meters can be as high as 8–10. For the same diameter, the maximum flow rate that can be measured by turbine flow meters is higher than that of many other flow meters. (3) Strong adaptability Turbine flow meters can be designed with a closed structure, and their rotation speed is measured in a contactless manner; therefore, it is easy to design them to withstand high pressures. If the turbine and bearings of the flow meter are made of materials resistant to high temperatures and with a low coefficient of thermal expansion, it can be used over a wide temperature range. In such cases, it is necessary to adjust its meter coefficient (primarily due to changes in its flow cross-section): K = K0 – (R + 2H)(t – to) (Equation 3-23). Here, K and K0 represent the meter coefficients at the time of use and during calibration, respectively ； t, t0 —— Fluid temperature during use and during calibration ； R and H represent the material expansion coefficients of the turbine and the casing, respectively. (4) Digital signal output: The turbine flowmeter outputs pulse digital signals that are proportional to the flow rate. This approach offers the advantages of maintaining accuracy during transmission, facilitating accumulation of data, and enabling easy integration into computer systems. 2. Installation and use of turbine flowmeters: To fully utilize the advantages of turbine flowmeters, it is necessary to pay close attention to their installation and usage. The following outlines some key points related to this aspect. (1) Medium being measured: The liquids measured by turbine flowmeters are generally those with low viscosity (usually less than 15×10⁻⁶ m²/s) and low corrosivity. Although there are now turbine flowmeters available for measuring various types of media, careful consideration must still be given when measuring substances with high temperatures, high viscosity, or strong corrosiveness, and appropriate measures must be taken. When the viscosity of the medium exceeds 15×10⁻⁶ m²/s, it is necessary to calibrate the meter using actual fluid samples; otherwise, significant errors may occur. Turbine flowmeters cannot be used to measure gas-liquid two-phase flows, gas-solid two-phase flows, or slurry flows. (2) Requirements for installation piping: The way the flowmeter is installed has a significant impact on its measurement accuracy. ① Uneven velocity distribution and secondary flows within the pipe are important factors that affect the accuracy of turbine flowmeters. Therefore, turbine flowmeters require certain lengths of straight pipe upstream and downstream. For industrial applications, it is generally recommended to have 20D of straight pipe upstream and 5D downstream. To eliminate secondary flows, it is advisable to install a flow straightener at the upstream end. If a 20D length of straight pipe can be ensured upstream, along with the use of a flow straightener, the measurement accuracy of the flowmeter can reach the level specified during calibration. ② Turbine flowmeters require high cleanliness of the fluid; therefore, a filter must be installed before the flowmeter to ensure the fluid remains clean. Funnel-shaped filters can be used, and their cleanliness can be determined by measuring the pressure difference across them. ③ To ensure that the fluid passing through the flowmeter is single-phase, i.e., to prevent air or vapor from entering the flowmeter, an air eliminator should be installed upstream if necessary. For liquids that are prone to vaporization, a certain backpressure must be maintained downstream of the flowmeter. This backpressure can be calculated as twice the pressure drop across the flow sensor at maximum flow rate, plus 1.2 times the vapor pressure of the liquid at the highest temperature. (3) Signal transmission lines: To ensure that the display instrument can detect the pulse signals generated by the turbine sensor accurately, it is necessary to improve the signal-to-noise ratio. To achieve this, various electrical interferences such as electromagnetic induction, static electricity, and capacitive coupling must be prevented during installation. Therefore, when configuring the signal transmission lines, the following points must be taken into account: ① Limiting the maximum length of the signal lines. The maximum length of the signal lines is L = dV ； Here, V is the effective value of the output voltage of the sensing coil at the minimum flow rate, in mV ； d is a coefficient, in units of m/mV, and its value can be: when V < 1000 mV, d = 1.0 ； When 1000 mV＜d＜5000 mV, d=1.5 ； When V > 5000 mV, d = 2.0. ② Shielded cables should be used for signal transmission lines to prevent induction noise from external sources. It is required that the transmission cable be shielded and grounded at the display instrument end. The transmission cable must not be placed near strong electromagnetic devices, nor should it be arranged parallel to power lines. (4) Operation and maintenance ① When the pipeline of the turbine flow meter needs to be cleaned, a bypass must be activated; the cleaning liquid must not pass through the flow meter. ② A bypass must be turned on first when starting up the pipeline system, in order to prevent a sudden increase in flow rate that could cause the turbine to spin too fast and get damaged. ③ The bearings of the turbine flow meter should be replaced regularly; their wear level can generally be assessed by observing changes in performance at low flow rates