How they work, where they excel, advantages, and limitations.
How they Work:
Coriolis flow meters maintain a certain amount of mystique due to the complex theory behind their operation. Although they are a comparatively newer technology, they are a fast-growing segment of the market. They are quickly becoming the industry standard for certain application areas, like custody transfer, due to their high accuracy and immunity to changes in density, and ability to measure many other process parameters in a single instrument.
There are two key points to understanding how they work. The first point, and the element they are named after, is the Coriolis principle. It can simplistically be summed up as the idea that movement causes inertia. For example, kids on a merry-go-round not only experience the force moving them in circles, they experience a force pushing them off the merry-go-round as well. This is an example of a Coriolis force. The second key point to understand is that the Coriolis force is the cause of the inertia within the pipe that causes deformation/movement of the pipe. It is not what is technically being measured in and of itself.
With those two points clarified, the operation becomes a bit easier to understand. For common dual-bent-tube architecture, the flow is immediately split into two separate tubes. There is an excitor coil on these tubes that causes artificial oscillation and causes the tubes to vibrate in opposition to each other. On either side of the excitor coil are two magnetic sensors. The media moving through the bend of the tubes causes inertia and the tube twists. This tube movement is picked up by the sensors as a phase shift in the sine waves formed between the two sensors. When there is no flow, the two sensors are in phase with each other. When flow is present, they are not. The motion of the pipes in relation to the magnetic sensors is directly proportional to the mass flow rate.
Anatomy of a Coriolis Meter
Dual-Tube Coriolis Motion
Image Credit: Wikipedia
No-Flow
Flow
Advantages:
Extremely accurate and repeatable
Can be used for sanitary applications
Not affected by changes in media density
Good for applications where media properties aren’t already well known
Do not require conversion factors
Do not require straight piping runs
Some models can also measure density, temperature, volumetric flow, or viscosity
Common Applications:
Pulp and Paper
Chemical Processing
Water and Wastewater
Custody Transfer
Mining and Mineral Processing
Power Generation
Petrochemicals
Sample Media:
Water
Caustics
Chemicals
Acids
Cleaning agent and solvents
Fuels
Silicon oils
Limitations:
Cannot tolerate bubbles
High initial cost
Pressure drop must be considered
Can be very heavy
Direct, in-line versions only
Not good for dual-phase fluids
Considerations:
Requires completely full pipes
Needs to operate in upper range of flow rate
Installation must minimize vibration/noise
High viscosity media causes pressure drop
Typical turndown ratio of 100:1
Curved tube designs offer better accuracy and are more immune to vibration