Coriolis mass flow meter

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Dibujo Coriolis flow meter.PNG

A Coriolis mass flow meter (CMD) is a flow measuring device that measures the mass flow of liquids or gases flowing through it. The measuring method is based on the Coriolis principle .

construction

Two-pipe version measuring principle

A mass flow meter was made from two metallic tubes that were in the shape of an arc, semicircle, delta shape, or even a full circle.

The current generation of Coriolis mass flow meters uses the straight tube geometry in the one - tube or two-tube version .

function

The pipe bend is made to vibrate by means of actuators. The axis of rotation is the basis of the arch, i.e. the direction of the inlet and outlet. The legs swing and thus complete a partial circle. The path speed of a point on the leg is greater, the greater the distance from the axis of rotation.

Without flow

without flow

The two legs of the pipe bend, left and right, swing the same without time difference. Seen from the front of the arrangement, they move congruently one behind the other. The deflection of the pipe due to the oscillation is not recognized by the eye. The amplitude is too small, but it can be felt by hand.

The frequency of the output signal is here , where k is the torsion spring constant (the stiffness compared to the acting deflection torque) of the pipe and J is the moment of inertia (which is proportional to the mass of the substance in the pipe). The density of the measurement object can thus be determined.

With flow

with flow

If there is flow through the pipe, with the entry of the fluid, e.g. B. in the left leg of the arch, the mass can be brought to an ever greater speed. According to the law of inertia , the fluid in the left thigh is therefore delayed = lagging. The cause is the Coriolis force or which arises from the Coriolis acceleration. The medium at the top of the sheet has reached the maximum web speed.

The medium, which flows back towards the axis of rotation in the right arm, constantly reaches places of lower path speed. The inertial force of the Coriolis acceleration now pushes the liquid in the limb in the direction of oscillation. The fluid in the right thigh is leading.

If you look at the moving pipe loop from the front, the two legs no longer move one behind the other. The time difference depends on the oscillation frequency, the mass of the medium and the flow speed and also on the (temperature-determined) modulus of elasticity of the pipe. This method is therefore able to measure the mass flow rate directly instead of determining it indirectly via other properties (volume, density, viscosity).

Simplified representation

Coriolis force on the garden hose

The influence of the Coriolis force becomes visible when a garden hose is held between the hands and gently stretched.

Signal conversion

Sensors that generate sinusoidal signals are mounted on the vibration system on the inlet and outlet sides. Without flow, both signals are in phase. With mass flow, the different Coriolis force in the inflow and outflow results in a phase shift of the two signals. This phase shift is proportional to the mass flow.

application

Depending on the medium, whether gases or liquids, pipes with different internal pipe diameters are used. The measuring device is only suitable to a limited extent for multiphase flow, since the uneven density distribution in the measuring tube can have an unfavorable effect on the oscillating system. If, on the other hand, the phases are homogeneously mixed, two phases can be calculated proportionally using density measurement and knowledge of the fluid properties (e.g. dissolved sugar in water, can be converted directly into Brix and output). The pressure loss varies depending on the design, the inside diameter of the pipe and the properties of the medium. The sensors have no gaps and can therefore also be used excellently in the food industry or pharmaceuticals. In addition to the mechanical properties of the measuring tube (or tubes), the resonance frequency of the oscillating system depends on the density of the medium to be measured. Temperature expansions in the measuring tube (s) and support structure also change the frequency. Therefore, the temperature on the measuring tube and support structure is usually measured and used for temperature compensation. The density can be precisely recorded with an accuracy of ± 2 kg / m³ and, if the material data is known, it can also be outputted in a temperature-compensated manner.

Devices for high temperatures and pressures are available. Use in hazardous areas is also possible. Versions for custody transfer are also available.

Other areas of application include:

  • Natural gas pumps
  • Pipeline billing measurements
  • Dosing systems
  • Truck loading and test bench construction.

The measuring devices should not be confused with similarly constructed density measuring devices based on the flexural vibration principle.

application areas

Since this technology is independent of the properties of the medium such as: conductivity, flow profile , density, viscosity, etc. Almost all substances can be measured such as oils and fuels, cleaning agents and solvents, fats, silicone oils, alcohol, methane, fruit solutions, starches, colors, vinegar, ketchup, mayonnaise, beer, milk, sugar solutions, gases, liquefied gases.

Product quality

Many users particularly value density measurement and use it specifically for quality control. Due to the simultaneous recording of the density and the temperature of the medium, a simultaneous quality assessment of the medium is possible. If the density of the medium deviates from the target size, this indicates quality problems in the process. Air inclusions can also be identified directly from the density signal. Conventional density meters often cost a lot more with similar accuracy.

advantages

In summary, a Coriolis mass flow meter has the following advantages:

  • Universal measuring system for mass, density and temperature, independent of
    • conductivity
    • Inlet and outlet sections
    • Flow profile
    • Medium density and thus pressure and temperature
  • Direct mass flow measurement
  • Very high measurement accuracy (typically ± 0.15% of reading, special sensors up to ± 0.05% of reading)
  • Multivariable measuring principle, simultaneous measurement of
    • Mass flow
    • density
    • temperature
  • No moving parts (the measuring tubes move a maximum of 30 µm, therefore one speaks of no movement, thus wear-free)
  • a measurement uncertainty analysis is much easier to set up, since only one device has to be considered

disadvantage

  • Relatively high purchase price
  • There are application limits with multiphase media or a high gas content.
  • Deposits can lead to errors, especially in density measurement.
  • Limited material selection for wetted parts, corrosion must be checked in particular.
  • The medium to be measured must be homogeneous.

Web links

Commons : Coriolis Mass Flow Meter  - collection of images, videos, and audio files