1. Volumetric flow meter
Working principle: The mechanical measuring element divides the fluid flowing into the flow meter continuously into unit volume and sends it to the outlet. During this process, the fluid drives the rotating part of the flow meter to rotate. As long as the number of rotations of the rotor is measured, it can be passed. The accumulated value of the fluid volume of the flow meter.
Features of volumetric flow meter:
- Accuracy: general: ±0.5%; high: 0.2%; generally used in the field of measurement;
- Wide range, generally 10:1~30:1 or greater;
- There is no need for front and rear straight pipe sections, and there is no influence on the measurement accuracy when the flow velocity field is distorted;
- Direct-reading instrument, without external energy, can directly obtain the cumulative total, clear and easy to operate;
- Regular verification is required, large volume, and heavy: it has been gradually replaced by turbine, electromagnetic, and vortex flowmeters in the measurement of large flow and large pipe diameters;
- Generally, filters need to be installed and cleaned regularly, and the maintenance workload is heavy; the most common failure is the flow of dirty media, causing the flow meter to operate inflexibly or even stuck.
- The influence of pressure and temperature: avoid the water hammer effect caused by the sudden switch of the valve; the reduction of the structural gap will block the moving parts, so when it is used at a higher temperature, it is necessary to reserve a size gap to compensate. At present, the usable temperature range is rough -30~+160°C and the pressure is up to 10MPa.
- Large pressure loss: 20KPa-100KPa
- Effect of liquid viscosity: Compared with differential pressure, float, and turbine flow meters, the influence is smaller.
2. Turbine flow meter
Working principle: When the measured fluid passes through the turbine flow meter, the fluid impacts the turbine blades through the deflector. Since there is an inclination angle "settle" between the turbine blades and the flow direction of the fluid, the impact force of the fluid generates a rotational torque on the turbine, so that the turbine overcomes the mechanical friction. The resistance distance and the flow resistance distance are rotated. Within a certain range, for a fluid medium with a certain viscosity, the rotational angular velocity of the turbine is proportional to the flow through the turbine. Therefore, the flow rate can be measured by measuring the rotational angular velocity of the turbine.
Features turbine flow meter:
- High precision: 0.15%~0.5% for liquid, 1%~1.5% for gas;
- Good repeatability: 0.05%~0.2%;
- Range ratio width: 10~50:1;
- Pulse signal output, strong anti-interference ability;
- High-pressure resistance, small pressure loss, and fast response;
- There are movable parts, which are easy to wear;
- There are certain cleanliness requirements for the medium;
- The front and rear straight pipe sections are required to be 10D and 5D respectively.
Working principle: The measuring body of the Rotameter is composed of a vertical conical tube that expands from bottom to top and a float that can move freely along the axial direction of the conical tube. When the measured fluid flows into the flow meter from the lower end of the conical tube, due to Under the action of the fluid, a differential pressure is generated on the upper and lower ends of the float, and the sum of the differential pressure and the buoyancy on the float is the lifting force of the float. When the lifting force is greater than the weight of the float immersed in the fluid, the float begins to rise. As the float rises, the annular area between the maximum outer diameter of the float and the conical tube gradually increases, the flow velocity of the fluid decreases accordingly, and the lift force acting on the float gradually decreases until the lift force is equal to that of the float immersed in the fluid. When the weight is lifted, the float is stabilized at a certain height. At this time, the height h of the float in the conical tube has a corresponding relationship with the passing flow.
Features of Rotameter:
- High reliability, used in the process control system, suitable for small and medium flow measurement, range ratio 10:1, accuracy 1.5%~2.5%;
- The front 5D and rear 3D straight pipe sections should generally be installed vertically, and the medium should be from bottom to top. If necessary, bypass pipes and magnetic filters should be installed.
- For glass tube Rotameter: simple structure, low cost, intuitive, but the measured medium needs to be transparent, poor reliability, can not measure high temperature and high-pressure medium.
- Metal tube flow meters are the most widely used at present;
- Common faults: the float is stuck.
4. Vortex flow meter
The vortex flow meter uses the principle of fluid vibration to measure flow. That is, under certain flow conditions, a part of the kinetic energy of the fluid generates fluid vibration, and the vibration frequency has a certain relationship with the flow velocity (or flow rate) of the fluid. Velocity or flow is measured by measuring the frequency of vibration.
Features of vortex flow meter:
- High precision, liquid 0.5%~1%, gas 1%~2.5%
- Wide detection range, up to 100:1
- The output frequency signal is proportional to the flow rate and has a strong anti-interference ability
- It is not affected by working parameters such as temperature, pressure, density, viscosity, etc. It does not need to be re-calibrated when replacing the detection element.
- Small pressure loss, especially suitable for medium and large diameter measurements.
- It has a wide range of applications and can be used for flow measurement of liquid, gas, and steam, and it is universal for gas and liquid.
- No moving parts, simple and solid structure, easy installation, less maintenance, and less failure.
Disadvantages: front and rear straight pipe sections (front 10D, rear 5D) are required, and the shock resistance is poor.
5. Throttle flow meter
Working principle: If a throttling member with a flow area smaller than the cross-sectional area of the pipe is fixedly placed in a pipe full of fluid, the flow in the pipe will cause local contraction when passing through the throttling member. At the constriction, the flow rate increases and the static pressure decreases, so there will be a certain pressure difference before and after the throttle. For a throttling piece of a certain shape and size, a certain pressure measuring position, and a straight pipe section before and after, under certain fluid parameters, there is a certain functional relationship between the differential pressure "delta P" before and after the throttling piece and the flow rate Qv. Therefore, flow can be measured by measuring the differential pressure across the orifice.
Features of throttle flow meter:
- Simple structure
- Ease of use
- Long life
- Work reliably
- Low cost
- No calibration required (referring to the standard throttling device)
- Wide adaptability
- Large pressure loss (generally refers to orifice plate)
- Strict installation requirements
6. Electromagnetic flow meter
Working Principle of electromagnetic flow meter:
Electromagnetic flow meters are flow meters that measure flow according to Faraday's law of electromagnetic induction. The advantage of the electromagnetic flow meter is that the pressure loss is extremely small and the measurable flow range is large. The ratio of the maximum flow rate to the minimum flow rate is generally more than 20:1, and the applicable industrial pipe diameter range is wide, up to 3m. The output signal is linear with the measured flow rate, and the accuracy is high. It can measure conductivity ≥ 5μs/cm Fluid flow of acids, alkalis, salt solutions, water, sewage, corrosive liquids, and mud, ore pulp, paper pulp, etc. But it cannot measure the flow of gas, steam and pure water.
Features of electromagnetic flow meter:
- The sensor has a simple structure, no moving parts and flow blocking parts;
- No pressure loss;
- Can measure dirty, corrosive, liquid-solid two-phase conductive medium, and only conductive liquid can be measured;
- High accuracy, 0.2%~1%;
- The range ratio can reach 50~100:1, and the caliber ranges from 2mm~3m.
- The place of use should not have strong magnetic field interference, the pipeline is filled with fluid, the first 5D straight pipe section.
- Determination of lining: mainly consider corrosion resistance, wear resistance and fluid temperature;
- Selection of electrodes: Consider corrosion, pressure conditions, etc.;
- Use of non-metallic pipes should be grounded rings.
7. Ultrasonic flow meter
When the ultrasonic wave propagates in the fluid, it will carry the information of the fluid velocity. The flow velocity of the fluid can be detected by receiving ultrasonic waves passing through the fluid, and converted into flow (volume flow).
Features of ultrasonic flow meter:
- The only flow meter that can realize non-contact measurement (clip-on type);
- It can be made into a portable instrument;
- No blocking parts, no pressure loss;
- Accuracy 0.1%~5%; (note the meaning of accuracy)
- The inner and outer walls of the pipeline are clean, and a straight pipe section of sufficient length is required;
- The flow meter of the propagation velocity difference method needs to have a clean medium, while the Doppler flow meter needs to have scattering particles, which is suitable for sewage and liquids containing air bubbles;
- It is especially suitable for large pipe diameter measurement, and its cost has basically nothing to do with pipe diameter.