Principle analysis of various types of flowmeters
(1) Principles of mechanics: Instruments belonging to such principles have differential pressure and rotor type using Bernoulli's theorem; impulse type and movable tube type using momentum theorem; direct mass type using Newton's second law; The target type of the momentum principle; the turbine type using the angular momentum theorem; the vortex type using the principle of fluid oscillation, the vortex type; the pitot tube type using the total static pressure difference; the volumetric type, the enthalpy, the trough type, and the like.
(2) Electrical principle: The instruments used for such principles are electromagnetic, differential capacitive, inductive, strain resistant, etc.
(3) Acoustic principle: Ultrasonic type, acoustic type (shock wave type), etc. are used for flow measurement using the acoustic principle.
(4) Thermal principle: The heat, direct thermal, indirect calorimetry, etc., which measure the flow using the thermal principle.
(5) Optical principle: laser type, photoelectric type, etc. are instruments belonging to such principles.
(6) Originally based on physical principles: nuclear magnetic resonance, nuclear radiation, etc. are instruments of this type.
(7) Other principles: Marking principle (trace principle, NMR principle), related principles, etc.
Vortex flowmeter installation requirements for straight pipe sections:
It is very important that the vortex flowmeter installation meets the requirements for straight pipe sections. Its detailed requirements are as follows:
The flowmeter has certain requirements on the upstream and downstream straight pipe sections at the installation point, otherwise it will affect the measurement accuracy.
If there is a tapered pipe upstream of the installation point of the flowmeter, there should be a straight pipe section of not less than 15D (D is the pipe diameter) upstream of the flowmeter, and a straight pipe section of not less than 5D in the downstream.
If there is a diverging pipe upstream of the installation point of the flowmeter, the upstream of the flowmeter shall have a straight pipe section of not less than 18D (D is the pipe diameter), and the downstream shall have a straight pipe section of not less than 5D.
If there is a 90° elbow or down joint upstream of the installation point of the flowmeter, there should be a straight pipe section of not less than 20D upstream of the flowmeter, and a straight pipe section of not less than 5D downstream.
If there is a 90° elbow on the same plane upstream of the installation point of the flowmeter, there should be a straight pipe section of not less than 25D upstream of the flowmeter, and a straight pipe section of not less than 5D downstream.
The flow regulating valve or pressure regulating valve should be installed as far as possible downstream of the flowmeter 5D. If it must be installed upstream of the flowmeter, the upstream of the flowmeter should have a straight pipe section of not less than 25D, and the downstream should have a straight pipe section of not less than 5D. .
If there is a piston type or plunger type pump in the upstream of the flow meter, a piston type or a Roots type fan and a compressor, the upstream of the flow meter should have a straight pipe section of not less than 25D, and the downstream should have a straight pipe section of not less than 5D.
Special attention: If the valve is installed near the upstream of the installation point of the vortex flowmeter, the valve is continuously opened and closed, which has a great influence on the service life of the flowmeter, and it is very easy to cause permanent damage to the flowmeter. The flowmeter should be avoided to be installed on the very long pipelines in overhead. After a long time, the leakage of the flowmeter can easily cause the leakage of the flowmeter to the flange. If it has to be installed, it must be in the upstream and downstream of the flowmeter. Pipe fastening devices are provided separately.
How the turbine flow meter works
The working principle of the turbine flowmeter: the fluid flows through the sensor housing. Since the blade of the impeller has a certain angle with the flow direction, the momentum of the fluid causes the blade to have a rotational moment. After the friction torque and the fluid resistance are overcome, the blade rotates, and the rotational speed is stabilized after the torque balance. Under certain conditions, the rotational speed is proportional to the flow rate. Due to the magnetic permeability of the blade, it is in the magnetic field of the signal detector (composed of permanent magnet and coil). The rotating blade cuts the magnetic field lines and periodically changes the coil. Magnetic flux, so that the two ends of the coil induce electricity
Pulse signal, which is amplified and shaped by the amplifier to form a continuous rectangular pulse wave with a certain amplitude, which can be transmitted to the display instrument to display the instantaneous flow rate and cumulative amount of the fluid. Within a certain flow range, the pulse frequency f is proportional to the instantaneous flow rate Q of the fluid flowing through the sensor. The flow equation is: Q = 3600 × f / k
In the formula:
F——pulse frequency [Hz];
K——the meter factor of the sensor [1/m], given by the checklist. If [1/L] is used, Q=3.6×f/k
Q——the instantaneous flow rate of the fluid (under working condition) [m3/h];
3600 - conversion factor.
The meter factor of each sensor is filled in the verification certificate by the manufacturer, and the k value is set in the matching display meter to display the instantaneous flow rate and the cumulative total amount.