Orifice flow meter

Orifice plate flowmeter consists of a flat plate with a precisely machined sharp edge hole, which is concentrically placed in the pipeline. When the liquid flows through the pipe, the flow suddenly shrinks when it is close to the orifice, and then suddenly expands back to the entire pipe diameter after the orifice. This will form a constrictor vein or throat at the orifice. The reduction of the flow pattern at the constructor leads to an increase in velocity, thus reducing the throat pressure, similar to a Venturi flowmeter.

Orifice plate flowmeter consists of orifice plates, fixed device, upstream and downstream flow meter pipeline and pressure tap. So far, the most critical part of the flowmeter is the orifice plate, especially the widely used square edge concentric plate. These standards define the edge, flatness, thickness (and bevel details if required), and aperture limits of the plate.

The most common fixing system is a pair of orifice flanges. However, various accessories are used for more accurate measurements. These simplify the installation/removal of plates to change the flow range and facilitate inspection. In any case, the orifice must be installed concentrically in the pipe within the range specified in the standard.

The installed orifice plate does not have the specified upstream and downstream pipe lengths, controlled within tight tolerance and/or the pressure tap (usually flange) without proper fabrication is not a "legal" flow meter; It must run specific tests to determine its calibration. Because it is not economical, almost all orifice systems are built to meet the standards. This allows calculation using the specified tolerance. The control of orifice metering accuracy comes directly from the data in the standard and must be observed without exception.

Electromagnetic flow meter 

The electromagnetic flowmeter is an instrument which is composed of a magnetic circuit system, a measuring pipe, an electrode, a shell, a lining and a converter, and measures the flow of conductive fluid according to the electromotive force generated when the conductive fluid passes through an external magnetic field. Electromagnetic flowmeter is the product of the progress of the times.

Comparison between orifice flow meter and electromagnetic flow meter

On the one hand, from the perspective of the structure and working principle, the orifice flow meter is composed of orifice plates and various transmitters. The flow of gas and liquid is measured by measuring and comparing the pressure difference. The invention of the electromagnetic flowmeter came into being with the change of electronic technology. The principle is Faraday's electromagnetic induction law, which is basically composed of electrodes, coils, sensors and microprocessors. On the other hand, from the perspective of application fields, they are widely used in petroleum, chemical industry, metallurgy, electric power and other industries.

Advantages and disadvantages of orifice flow meter

Advantages of orifice flow meter

• The standard throttling element is fully used and recognized by the International Standards Organization. It can be put into use without real flow calibration and is unique among flow sensors;
• The structure is easy to copy, simple, firm, stable and reliable in performance and low in price;
• It has a wide range of applications, including all single-phase fluids (liquid, gas, steam) and some mixed phase flows. The pipe diameter and working conditions (temperature and pressure) in the general production process can be measured;
• The test pieces and differential pressure display instruments can be produced by different manufacturers separately, which is convenient for specialized scale production.

Disadvantages of orifice flow meter

• The repeatability and accuracy of measurement are at the medium level  flow sensors. Due to the complexity of many factors, it is difficult to improve the accuracy;
• The range is narrow, because the flow coefficient is related to Reynolds number, the general range is only 3:1~4:1;
• It is generally difficult to meet the length requirements of long straight pipe sections. Especially for larger pipe diameters, the problem is more prominent;
• Large pressure loss;
• The acute angle line of the hole in the orifice plate is used to ensure the accuracy, so the sensor is sensitive to corrosion, wear, scaling and dirt. It is difficult to ensure the accuracy in long-term use, so it needs to be removed for mandatory inspection once a year;
• The flange connection is easy to cause the problems of running, emitting, dripping and leaking, which greatly increases the maintenance workload.

Advantages and disadvantages of electromagnetic flow meter

Advantages of electromagnetic flow meter

• The transmitter of the electromagnetic flowmeter is simple in structure, without movable parts or any throttling parts that hinder the fluid flow, so when the fluid passes through, it will not cause any additional pressure loss, and it will not cause problems such as wear and blockage. It is especially suitable for measuring liquid-solid two-phase fluids such as mineral slurry  solid particles, sewage, and various viscous slurries. Similarly, since there are no moving parts in its structure, it can be used for measuring various corrosive media by attaching a corrosion resistant insulating lining and selecting corrosion resistant materials to make electrodes.
• The electromagnetic flowmeter is a volume flow measuring instrument. During the measurement, it is not affected by the temperature, viscosity, density and conductivity (within a certain range) of the measured medium Therefore, the electromagnetic flow meter can be used to measure the flow of other conductive liquids without additional correction after being calibrated with water
• The measuring range of the electromagnetic flowmeter is very wide, and the measuring range ratio of the same electromagnetic flow meter can reach 1:100. In addition, the electromagnetic flowmeter is only proportional to the average velocity of the measured medium, and has nothing to do with the flow state (laminar flow or turbulent flow) under the axisymmetric distribution
• The electromagnetic flow meter has no mechanical inertia and is sensitive. It can measure the instantaneous pulsating flow with good linearity Therefore, the measured signal can be directly converted into a standard signal output by a converter, which can be used for local indication or long-distance transmission.

Shortcoming of electromagnetic flow meter

• It cannot be used to measure gas, steam and liquid containing a large amount of gas;
• It cannot be used to measure the liquid medium with very low conductivity, and the electromagnetic flow meter cannot measure the medium such as petroleum products or organic solvents;
• It cannot be used to measure high temperature medium. At present, it is limited by the lining materials and electrical insulating materials of the measuring pipe of the general industrial electromagnetic flow meter;
• The electromagnetic flowmeter is vulnerable to external electromagnetic interference. Electromagnetic flowmeters are widely used, large diameter instruments are mostly used in water supply and drainage projects, and small and medium diameters are often used in high requirements or difficult to measure occasions. The electromagnetic flow meter must be installed at the lowest point of the pipeline or the vertical section of the pipeline to ensure the use of the electromagnetic flow meter and the accuracy requirements.

Because the design and processing of the orifice plate of the orifice flow meter are based on international standards, and the sensor is a single crystal silicon intelligent differential pressure sensor, the orifice flow meter has high measurement accuracy, strong stability, a wide range of use, self diagnosis function, timely maintenance, simplicity, long service life, and relatively economic price; The low-frequency rectangular wave excitation of the electromagnetic flow meter increases the stability of the measurement and reduces the power loss. 

The microprocessor adopts a 16 bit embedded system, which is fast in calculation and high in accuracy. The anti-interference double-layer cable is used, which has strong anti-interference ability and wide measurement range. It also has the function of self detection and diagnosis. It can report instrument problems at the first time without affecting the measurable progress. However, the electromagnetic flow meter should avoid close contact with ferromagnetic objects and strong magnetic field equipment to prevent the operation of electromagnetic field in the sensor from being affected.

Of course, we should also maintain and overhaul them in the process of use. We should try our best to make their working environment dry and ventilated, free from wind and sun, and the ambient temperature should not change too much. Whether it is an orifice flow meter or electromagnetic flowmeter, a good working environment can increase the accuracy of their service life measurement. The measurement of different media also requires different grades. We need to select the most appropriate measuring instrument according to the product requirements.