Application of Thermal Mass Flow Meter in Nuclear Power Plant

Abstract: The application information of thermal mass flow meter in nuclear power plants is provided by excellent flow meter and flow meter manufacturers and quotation manufacturers. Flow is one of the important process parameters in the operation of nuclear power plants. Commonly used flow measuring instruments include orifice flowmeters, ultrasonic flowmeters, electromagnetic flowmeters and thermal mass flowmeters. Thermal mass flow meters are currently mostly used for gas flow in industry. More flowmeter manufacturers choose models and price quotations. You are welcome to inquire. The following are the details of the application of thermal mass flowmeters in nuclear power plants. Flow is one of the important process parameters in the operation of nuclear power plants. Commonly used flow measuring instruments include orifice flowmeters, ultrasonic flowmeters, electromagnetic flowmeters and thermal mass flowmeters. Thermal mass flow meters are currently mostly used for gas flow measurement in industry, and are generally used in ventilation system flow measurement occasions in nuclear power plants. In a nuclear power plant using a shaft-sealed main pump, the leakage flow of the third-stage seal of the main pump is a typical small flow condition. Orifice flowmeters and ultrasonic flowmeters are usually not suitable or can not achieve good measurement results. Thermal mass flowmeters have been successfully applied in the ventilation system of nuclear power plants since their inception, and thermal mass flowmeters also have certain applicability in the measurement of micro flow of liquids. 1 Principle of thermal mass flowmeter ① Thermal mass flowmeter is an instrument that uses the principle of heat conduction, that is, through the heat exchange between the fluid in flow and the heat source, to measure the flow of the medium. It can be mainly divided into two categories: the use of fluid flow Thermal distributed flowmeters that transfer heat to change the effect of the temperature distribution (ie, heat conduction distribution) of the measurement tube wall, also known as calorimetric mass flowmeters; intrusive mass flowmeters that utilize heat dissipation effects (based on Guin's law), also known as It is a thermal conductivity mass flowmeter or an insertion mass flowmeter. The working principle of the thermal distributed mass flowmeter is that two sets of heating/detection coils are wound on the outer wall of the measuring tube and downstream respectively, and the two sets of coils are heated by a constant current. , The downstream temperature distribution is in a symmetrical equilibrium state, and the resistances of the two sets of detection coils are equal; when there is fluid flow, the fluid will take away the heat from the upstream pipe wall and transfer it to the downstream pipe wall, destroying the original equilibrium state, and the coil resistance will be different, and the difference will be detected. value to obtain the fluid mass flow: (1) where A———Thermal conductivity; cp———Constant pressure specific heat capacity of the measured medium; K———constant. Heat dissipation effect (based on Kim's law) The working principle of the intrusive mass flowmeter is to place two temperature sensors (usually platinum thermal resistance) in the fluid in the pipeline, and one of the platinum resistance measures the temperature T of the fluid itself , another platinum resistor is heated by a certain power, and its temperature TV is higher than T. When the fluid is static, its temperature is the highest. With the increase of the fluid flow in the pipeline, the fluid flow takes away more heat to reduce the TV, and it can pass through The temperature difference is used to obtain the flow value. The heat dissipation rate of the hot wire based on Guin's law is: (2) where cv———The constant volume specific heat capacity of the fluid; d———Hot wire diameter; H/L———Heat dissipation rate per unit length; V———the flow rate of the fluid;ρ———the density of the fluid;λ———Thermal conductivity of the fluid. Intrusive mass flow meters are divided into constant power type, constant temperature difference type and constant ratio type according to the heating method of platinum thermal resistance. The constant power type is to use a constant power electric energy to heat the platinum resistance on the heating circuit. When the fluid medium is static, the temperature difference between the heated platinum resistance and the unheated platinum resistance is the largest, with the flow of the fluid medium. , the temperature on the heated platinum resistance decreases, and the temperature difference between the two platinum resistances decreases. The constant power thermal mass flowmeter obtains the change of fluid medium flow by measuring the change of temperature difference. The constant temperature difference type is to heat a platinum resistance, making it a constant temperature higher than the unheated platinum resistance. With the flow of the fluid medium, the temperature of the heated platinum resistance decreases due to heat dissipation, and the current (or voltage) of the heater is fed back to the processor through the feedback circuit to increase the current (or voltage) of the heater to keep the temperature difference at a constant value, and then by detecting the change The current (or voltage) to obtain the change value of the flow. The constant ratio type is based on the principle of constant temperature difference, by adjusting the heating current applied to the heating end thermal resistance to ensure that the resistance of the heated platinum resistance and the resistance of the unheated platinum resistance are in a constant ratio. Like the constant temperature differential mass flowmeter, the constant ratio mass flowmeter also obtains the flow value of the fluid in the pipeline by detecting the changing current. 2 Measurement of micro flow in nuclear power plants and application of thermal mass flow meter 2.1 Typical micro flow measurement conditions in nuclear power plants In nuclear power plants using shaft-sealed reactor coolant pumps (main pumps), the third Stage seal leakage flow is a typical micro flow condition. The shaft seal system is the key component of the shaft seal main pump, and its long-term reliable operation is not only related to the normal operation of the nuclear power plant, but also directly affects the safety of the nuclear power plant.

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