Coriolis flowmeters are mass flowmeters, which measure the mass of fluid passing through a pipeline unlike the other volumetric flow meters, which measures the volume of the fluid passing through the pipeline. Coriolis mass flowmeter provides direct, in-line and accurate mass flow measurements that are independent of temperature, pressure, viscosity and density. Mass flow, density and temperature can be accessed from the one sensor. They can also be used for almost any application when calibrated.
Principle of Coriolis mass flowmeter:
When a fluid flows into a pipe and is accelerated by Coriolis by mechanically inserting apparent rotation into the pipe, The amount of deflective force generated by the inertial effect of Coriolis will depend on the fluid’s mass flow rate. If a pipe rotates around a point while the liquid flows through it (toward or away from the centre of rotation), the fluid can produce an inertial force (acting on the pipe) at the right angles to the flow direction.
The basis of the Coriolis meter is Newtons’ Second Law of Motion, where:
Force = Mass x Acceleration
The conventional way to measure the mass of an object is to weigh it. In weighing, the force is measured with a known acceleration (9.81m/sec2). This type of measuring principle is not easy or possible with fluids in motion, particularly in a pipe.
The Coriolis effect causes a retarding force on a rotating section of pipe when a flow is moving outward, conversely producing an advance on the section of pipe for flow moving towards the axis of rotation.
Coriolis flowmeter Construction and working:
Coriolis effect is the principle used to determine the acceleration due to the torque (the amount of twisting). Sensors are used to measure the amount of twist in the flow tubes within the meter as a result of the flow tube vibration and deflection due to the mass flow. The amount of twist measured is proportional to the mass flow rate and is measured by magnetic sensors mounted on the tubes.
The flow through the pipe is separated to flow through two different pipelines in Coriolis flowmeter. The fluid entering the tubes generates oscillation and vibration in a tube at their resonant frequency and sensors are used to detect the movement of the pipe.
When there is no flow, the sensor output for both the tubes will be in same phase. when liquid flows there is a difference between the oscillations of the two pipes. This is caused as the flow is accelerated on the inlet and decelerated on the outlet. The pipes twists, there will be a phase difference in oscillation of two pipes, this difference in the phase of the oscillations is proportional to mass flow.
Tube Design of Coriolis flowmeter:
A tube can be curved or straight, and when placed vertically certain designs can also be self-draining. When the design consists of two parallel channels, the flow is separated into two streams by a splitter at the inlet of the meter and recombined at the outlet. The flow inside the meter is not broken in the single continuous tube configuration (or in two series tubes). Which are illustrated in the below picture:
In either case, the tubes are vibrated by passengers. Such motors are a wire attached to one tube and a magnet connected to the other. The transmitter applies an alternating current to the wire, which draws and repels the magnet by degrees, pulling the tubes to and away from each other.
The sensor will sense the tubes ‘ location, distance, or acceleration.
Applications & Advantages:
Coriolis Mass flowmeters can measure the flow of all liquids, both Newtonian and non-Newtonian, as well as relatively dense gases. For sanitary applications that satisfy clean-in-place criteria, self-draining designs are available.
Some meters are equipped with circuits that are intrinsically safe between the flow tube and the transmitter. Therefore, there is a limited amount of moving force that can be supplied to the flow pipeline.
- Direct, in-line mass flow measurement.
- Independent of temperature, pressure, density, conductivity and viscosity.
- Sensor capable of transmitting mass flow, density and temperature information.
- High-density capability.
- Conductivity independent.
- Suitable for hydrocarbon measurements.
- Suitable for density measurement.
Disadvantages:
- Cost.
- Affected by vibration.
- Installation costs.
- Adjustment of zero point
