2019-11-26 11:24:27

Instruments that measure fluid flow are collectively referred to as flow meters or flow meters. The flowmeter is one of the important instruments in industrial measurement. With the development of industrial production, the accuracy and range of flow measurement requirements are getting higher and higher, and the flow measurement technology is changing with each passing day. Various types of flow meters have been introduced to suit various applications. More than 100 flow meters have been put into use. From different perspectives, flow meters have different classification methods. There are two commonly used classification methods. One is to classify according to the measurement principle adopted by the flowmeter: the second is to classify according to the structural principle of the flowmeter.

Sort by measurement principle

a. Mechanical principle:

Instruments belonging to such principles have differential pressure type, rotor type using Bernoulli's theorem;

Impulse type using the momentum theorem, movable tube type;

Direct mass equation using Newton's second law;

a target using the principle of fluid momentum;

Turbine using the angular momentum theorem;

Vortex type using vortex principle of fluid oscillation;

Use the total static pressure difference of the pitot tube type as well as volumetric and sputum, trough and so on.

b. Electrical principle:

Electromagnetic,

Differential capacitor type,

Inductive,

Strain resistance type, etc.

c. Acoustic principle:

Ultrasonic.

d. Thermal principles:

Thermal type,

Direct calorimetry,

Indirect calorimetry and so on.

Several problems of solutions

Recalculating the differential pressure scale

Temperature and pressure compensation can only reduce the measurement error, not only can not solve the problem fundamentally, but also the measurement signal exceeds 20mA, resulting in steam leakage measurement. The transmitter measurement signal exceeds 20 mA, indicating that the actual measured differential pressure signal ΔP exceeds the design differential pressure value.

Increase temperature and pressure compensation

When the temperature and pressure of the steam change, the density of the steam changes, and the steam flow measurement produces an error. Measurement error can be reduced by temperature and pressure compensation. Since the temperature of the saturated steam is a single-valued function of the pressure, the temperature and pressure compensation of the saturated steam can be pressure compensated or temperature compensated. Because the pressure signal detection is sensitive and the compensation accuracy is high, it is compensated by pressure and realized by DCS.

Conclusion

Steam is a special medium. As the pressure and temperature change, the density of steam changes. Therefore, it is necessary to compensate for temperature and pressure. When the pressure and temperature fluctuation of the steam are not large, that is, when the operating condition parameters deviate from the design parameters and the influence on the measurement is small, the temperature and pressure compensation measures can achieve the purpose of accurate measurement. However, when the operating parameters deviate too much from the design parameters or the operating parameters fluctuate frequently and are too large, even with the temperature and pressure compensation, it is difficult to meet the measurement accuracy requirements. At this point, only differential pressure or flow can be recalculated for a particular throttling element.

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.