How a Coriolis Mass Flow Meter Works ~ Learning Instrumentation And Control Engineering Learning Instrumentation And Control Engineering

How a Coriolis Mass Flow Meter Works

Custom Search

The vast majority of flow meters in use today are volumetric. However, there are a few other applications where what is required is a mass flow meter. One of such mass flow meters is the Coriolis Mass Flowmeter that can measure both the mass of liquids and gases.

Today, commercial Coriolis flowmeters are gradually gaining prominence in flow measurement applications. Steady technical improvements on these meters since they first came into the markets in the 1970s have greatly increased their accuracy and acceptance in the process industries. Currently in the process industries, direct mass flow measurements represent a substantial and fast-growing percentage of worldwide flowmeter applications (15 to 20%). The diagram below shows the construction a U-shaped Coriolis mass flow meter:
Construction a Commercial Coriolis Mass Flow Meter. Photo Credit: Micro Motion


How a Coriolis Mass Flow Meter Works
A Coriolis flowmeter requires a force acting on a tube carrying a flowing fluid. This force deforms tubes through which the fluid flows. The amount of deformation depends directly on the mass flow rate through the tubes. Signals from sensors measuring this deformation provide a direct indication of the mass flowrate.
In a Coriolis meter measuring process mass flow rates, the flowmeter must rotate the fluid. In practice they rotationally oscillate the fluid, which produces equivalent Coriolis forces. Tube designs are U-shaped, S-shaped, or straight. 

Design and Working Principle of a Commercial Coriolis Meter
Inside U-shaped sensor housing, the U-shaped flow tube is vibrated at its natural frequency by a magnetic device located at the bend of the tube. The vibration is like that of a tuning fork, covering less than 0.1 in. and completing a full cycle about 80 times/sec. As the liquid flows through the tube, it is forced to take on the vertical movement of the tube as shown in the diagram below. When the tube is moving upward during half of its cycle, the liquid flowing into the meter resists being forced up by pushing down on the tube.
Operating Principle of the Coriolis Mass Flow Meter. Photo Credit : Micro Motion

Having been forced upward, the liquid flowing out of the meter resists having its vertical motion decreased by pushing up on the tube. This action causes the tube to twist. When the tube is moving downward during the second half of its vibration cycle, it twists in the opposite direction.

Having been forced upward, the liquid flowing out of the meter resists having its vertical motion decreased by pushing up on the tube. This action causes the tube to twist. When the tube is moving downward during the second half of its vibration cycle, it twists in the opposite direction. The amount of twist is directly proportional to the mass flow rate of the liquid flowing through the tube. Magnetic sensors located on each side of the flow tube measure the tube velocities, which change as the tube twists. The sensors feed this information to the electronics unit, where it is processed and converted to a voltage proportional to mass flow rate. The meter has a wide range of applications from adhesives and coatings to liquid nitrogen.

Commercial Coriolis flowmeters design incorporate identical dual tubes oscillating in opposite directions. The flow from process piping splits in two as it enters the flow meter. This provides a more balanced design, making the meter more resistant to external vibrations and temperature swings. Sensors mounted on the tubes measure their relation to each other rather than to a fixed plane.

Straight tube designs operate in a similar manner. The vibrating tube is fixed at its ends, creating two rotating reference frames. The rotations at the inlet and outlet sides are in opposite directions, creating opposing Coriolis forces that distort the tube.

Merits of Using a Coriolis Mass Flow meter

1. Used for direct, in-line and accurate mass flow measurement of both liquids and gases.

2. Can achieve accuracies as high as 0.1% for liquids and 0.5% for gases.

3. Mass flow measurement ranges cover from less than 5 g/m to more than 350 tons/hr.

4. Flow measurement is independent of temperature, pressure, viscosity, conductivity and density           of the medium.

5. Can be used for direct, in-line and accurate density measurement of both liquids and gases.

6. Multi-variable capability as mass flow, density and temperature can be accessed from the one             sensor.

7. Can be used for almost any application irrespective of the density of the process.

Demerits of Using a Coriolis Mass Flow Meter

1. They are very expensive.

2. Many models are affected by vibration.

3. Current technology limits the upper pipeline diameter to 150 mm (6 inches).

4. Secondary containment can be an area of concern.