The invention of the Coriolis mass flow meter is the result of decades of painstaking research in the scientific and technical community. It is accurate, repeatable, and stable and has no flow blocking elements or movable parts in the fluid path, making it reliable and long-lasting. It can also measure the flow of high viscosity fluids and high-pressure gases.
The clean fuel for cars, compressed natural gas (CNG), is measured in petroleum, chemical, metallurgy, building materials, paper, medicine, food, bioengineering, energy, aerospace, and other industrial sectors, it is also increasingly widely used. Moreover, it has brought about a profound change in fluid measurement technology. As a result, experts hail it as the mainstream flowmeter of the 21st century.
1. Principle of Coriolis mass flowmeters
Coriolis mass flowmeter mass measurement principle is Newton’s second law F = Ma.
A Coriolis force proportional to the mass flow rate is generated as the fluid flows through the vibrating tube. Therefore, when no fluid flows through, the vibrating box is not distorted, and the signals detected by the electromagnetic signal detectors on both sides of the vibrating tube are in phase.
When fluid passes through, the vibrating tube is distorted by the torque, and there will be a phase difference between the two detectors. The transmitter measures the lag time between the left and proper detection signals, and this time difference is multiplied by the flow calibration factor to determine the mass flow rate.
The density measurement principle of Coriolis Mass Flow Meters is that the frequency of vibration is inversely proportional to the square root of the density of the fluid. So we determine the thickness of a liquid by measuring the vibration frequency. So the mass flow meter can achieve the measurement of the mass flow of the fluid, but also the measurement of the density of the fluid.
2. Components of a Coriolis mass flow meter
The Coriolis Mass Flow Meter consists of a sensor and a transmitter for signal processing connected via a special 9-core cable.
2.1 Sensor types
The sensor is available in various configurations such as a straight tube, U-tube and Ω-type. Today, we chose the U-shaped tube mass flow meter to explain.
When controlling the feed to the cracker, we use a U-tube sensor, which has two open ends connected to the pipe, with fluid flowing in through one port and out through the other.
An electromagnetic device is fitted to the top of the U-tube to excite the U-tube to vibrate on its center axis at an inherent self-oscillation frequency, perpendicular to the plane in which the U-tube is located. As a result, the fluid flows into the U-tube and then moves vertically with the pipe as it flows along the line. In addition, the liquid in the flow through the U-shaped tube will produce a Coriolis acceleration and in the form of Coriolis force back to the U-shaped line.
Fluid in the U-tube inlet and outlet on both sides of the flow direction opposite, forming a moment of action, resulting in the U-tube twist, the size of the twist angle and through the U-tube mass flow of fluid related to the electromagnetic detector will measure the tube twist into an electrical signal, sent to the transmitter for further processing.
2.2 Transmitters for Coriolis mass flowmeters
The transmitter is a kind of intelligent flowmeter with a microprocessor. It sends the low-level signal from the sensor after conversion and processing, output two ways with flow and density proportional to the 4 ~ 20mA standard signal. As shown in Figure 3, terminals 17 and 18 are connected to the first level mA output signal. And terminals 19 and 20 are connected to the second level mA output signal, indicating mass flow, volume flow, mass accumulation, volume accumulation, medium density, medium temperature, etc.
3. Features of Coriolis Mass Flow Meters
3.1.1 Coriolis Mass Flow Meters measure mass flow directly and have high accuracy.
3.1.2 Mass flow meters can measure a wide range of fluids. It includes a wide range of liquids with high viscosity, slurries with solids, liquids with traces of gases, and medium to high-pressure gases with sufficient density.
3.1.3 The vibration amplitude of the measuring tube of the mass flow meter is small and can be regarded as a non-moving part, and there are no obstructions or moving parts in the measuring line.
3.1.3 The mass flow meter is not sensitive to the onward flow velocity distribution, so there is no upstream and downstream straight pipe section requirement.
3.1.4 The measured value of the mass flow meter is not sensitive to the fluid’s viscosity. Therefore, fluid density changes on the measured value of the impact are small.
3.1.5 Mass flow meters can be used for multi-parameter measurements, such as simultaneous measurement of density and, as a derivative of this, measurement of the concentration contained in the solute in a solution.
3.2 Disadvantages of Coriolis mass flowmeter
3.2.1 Coriolis mass flow meter zero-point instability to form a zero-point drift, affecting its accuracy further. Making many instrument models will only divide the total error into the fundamental error and zero-point instability metric.
3.2.2 Coriolis Mass Flow Meters cannot be used to measure low-density media and low-pressure gases; liquids containing gas above a specific limit (varying by model) can significantly affect the measured value.
3.2.3 Coriolis Mass Flow Meter is more sensitive to external vibration interference. Most Coriolis Mass Flow Meter flow sensor installation models fixed high requirements to prevent the impact of pipeline vibration.
3.2.4 Mass flow meter can not be used for larger pipe diameter, is still limited to 150 (200) mm below.
3.2.5 Wear and corrosion of the inner wall of the measuring tube or deposition of scale will affect the accuracy of the measurement, especially for thin-walled tube measuring line Coriolis Mass Flow Meter more significant.
3.2.6 Pressure loss is high, comparable to volumetric meters. Some Coriolis Mass Flow Meters models are even 100% larger than volumetric meters.
3.2.7 Most Coriolis Mass Flow Meters are heavy and bulky.
3.2.8 Expensive. Foreign prices 5000 ~ 10000 U.S. dollars a set, about the same caliber electromagnetic flowmeter 2 ~ 5 times; domestic prices for electromagnetic flowmeter about 2 ~ 8 times.
4. Installation of Coriolis mass flowmeters
4.1 Installation location
A mass flow meter is based on the principle of vibration work, in its work and the use of electromagnetic technology for vibration excitation, torsion angle detection, and detection signal processing. So the mass flow meter installation location can not have a significant source of vibration but can not be installed in transformers, motors, and other equipment generating strong magnetic fields near to prevent external interference affecting its regular work.
4.2 Sensor and pipe installation
Mass flow meter installation without straight pipe section requirements, but the sensor and the pipeline connection should avoid the presence of stress. For example, suppose there is a deviation in the concentricity of the centerline of the sensor and pipe during the mass flow meter installation. In that case, this will result in stress on the vibrating line, which will form pressure, tension, or shear forces affecting the alignment of the pipe, causing an asymmetry in the detection probe, leading to a change in the zero point and affecting the measurement accuracy.
4.3 Mounting method
If the concentricity deviation between the pipe and the sensor is too large, there is a risk that the zero point cannot be adjusted. Reinforcement measures should therefore be taken to stabilize the piping near the meter. We should try to take a soft connection during the flow sensor installation, and the flange connection screws should be tightened evenly and symmetrically. To reduce the impact of residual stress, the flow sensor should also be zero-point calibrated after installation. To facilitate the installation and zeroing of the mass flow meter, the front and rear ends of the mass flow meter should also be installed with shut-off valves.
4.4 Mounting the transmitter
When connecting the sensor and transmitter cables, we should use the manufacturer’s special cables. The transmitter receives a low-level signal, so the line should not be too long. When the transmitter is installed, we ensure that the bulkheads used to separate the intrinsically safe sensor cable from the power and output cables are in place. We ensure that the sensor junction box and transmitter housing are well sealed to avoid short circuits that could lead to measurement errors or flowmeter failure.
4.5 Installation direction
Depending on the nature of the fluid, choose a different sensor installation orientation, always keep the sensor flow tube in the liquid in an entire state. Gas feed measurement mass flow meter U-shaped tube facing up to avoid the accumulation of condensate in the measurement tube; liquid feed measurement mass flow meter U-shaped tube facing down to prevent the collection of gas in the measurement tube.
The mass flow meter is technologically advanced, stable, and easy to install and maintain. Although the price is relatively high, the subsequent almost no maintenance costs, high accuracy, long cycle of regular and reliable operation will bring long-term benefits to enterprises, and various application fields will be more widespread.