Measurement principle of Ross Mont mass flowmeter and its application in petrochemical industry（一）
Ross Mont mass flowmeter is widely used in petrochemical and other fields. It is one of the most advanced flow measurement instruments in the world.
First, the principle of mass flow measurement
The measuring system of a mass flowmeter includes a sensor and a transmitter for signal processing. Rosemount mass flowmeter is based on Newton's second law: force = mass * acceleration (F=ma).
As shown in Fig. 1, when a particle with mass m is moving at a speed V in a pipe rotating at the angular velocity of the P axis, the particle is subjected to the acceleration and force of the two components:
(1) normal acceleration, that is, centripetal acceleration R, whose value equals 2 Omega R and faces P axis;
(2) tangential angular velocity alpha T, that is, Coriolis acceleration, whose value equals 2 Omega V, and its direction is perpendicular to the alpha R. Because of the complex motion, the Coriolis force Fc=2 Vm is Vm in the direction of the alpha t of the particle. The pipe acts on the particle with a reverse force -Fc=-2 Omega Vm.
When a fluid with a density of Rho is flowing at a constant velocity of V in a rotating pipeline, any length of a length of X will be subjected to a tangent Coriolis Force Delta Fc: Delta Fc=2 Omega V P A delta x (1).
In the form of the A - the cross section of the pipe.
Due to the relation of existence: mq= Rho VA
So: Delta Fc=2 Omega QM delta x (2)
Therefore, directly or indirectly measuring the Coriolis force of the flowing fluid in the rotating tube can measure the mass flow rate.
In the sensor, the flow tube is U. When no fluid flows through the flow pipe, the flow pipe is driven by an electromagnetic drive coil installed at the end of the flow pipe. The amplitude is less than 1mm and the frequency is about 80Hz. The flow pipe is forced to accept the vertical movement of the flow pipe when the flow flow is flowing into the pipe. In the half cycle of the flow pipe upward, the fluid resists the upward movement of the pipe and exerts a downward force on the flow tube; conversely, the flow of the flow pipe exerts an upward force on the flow tube to resist the downward movement of the pipe and reduce its vertical momentum. This leads to the distortion of the flow pipe. In the second half of the vibration, the flow tube vibrate downward, and the twist direction is opposite. This distortion is called the Coriolis (Coriolis) phenomenon, that is, the Coriolis force.
According to the Newton's second law, the size of the flow pipe distortion is directly proportional to the mass flow of flow tube, and the electromagnetic signal detector installed on both sides of the flow tube is used to detect the vibration of the flow tube. When the flow pipe is not flowing through the flow pipe, the flow pipe does not distort, and the detection signal on both sides of the electromagnetic signal detector is in the same phase; when the fluid flows through the flow pipe, the flow pipe produces distortion, resulting in the phase difference between the two detection signals, and the phase difference is directly proportional to the mass flow through the flow tube.
Because the mass flow meter mainly relies on the vibration of the flow tube to measure the flow rate, the vibration of the flow pipe and the force of the flow through the pipe produce the Coriolis force, which causes the torsion of each flow pipe, and the volume of the torsional flow is proportional to the mass flow rate flowing through the pipe in the vibration period. Because the distortion of a flow tube lags behind the distortion of the other first class tube, the output signal of the sensor on the quality tube can be determined by the circuit comparison.
The lag time between the left and right detection signals is measured by the time difference detector in the circuit. This "time difference" delta T has been digitally measured, processed and filtered to reduce noise and improve resolution. Time difference multiplying the flow calibration coefficient to represent mass flow. Because temperature affects the steel property of the flow tube, the distortion of Coriolis force will also be affected by temperature. The measured flow is continuously adjusted by the transmitter, which detects the output of the platinum resistance thermometer adhered to the surface of the tube at any time. The transmitter uses a three-phase resistor thermometer bridge amplifying circuit to measure the temperature of the sensor. The output voltage of the amplifier is converted to frequency and the counter is digitized to read the micro processor.