The Basics of Flow Measurement with Coriolis Meters: Part 2
In the first part of this blog series (The Basics of Flow Measurement with Coriolis Meters: Part 1), I summarized Coriolis technology advantages and the flow meter’s main components. Now, I’ll discuss how the Coriolis Effect is used to measure mass flow by looking inside the sensor. Finally, we’ll compare the overall cost of ownership with other types of flow meters.
In a basic Coriolis meter, the fluid passes through one or more slightly bent tubes that are stimulated to oscillate in opposition to one another (or to a reference frame, for a single tube meter) to create an environment for the Coriolis Effect to occur. As the fluid flows through the tubes, the Coriolis Effect causes the downstream side of the oscillating tubes to slightly lead the upstream side, creating a minuscule twist in the tubes, as shown in Figure 2. The rate at which the tube twists differs based on how fast or slow the fluid is flowing through the tubes, as shown in Figure 3. The twisting is directly proportional to mass flow rate.
Figure 2. The same sensor in a no-flow state and during process flow. Notice the twist in the flow tubes under process conditions – that’s the Coriolis Effect.
The twisting tube motion is measured by sensors located on the tubes. The signals from these sensors are processed to quantify the amount of twisting (in units of time) by comparing the time difference between the motions of the tubes detected at different locations on the tubes. The transmitter processes the time delay with other measurements such as temperature, to deliver a mass flow rate.
Figure 3. The amount of twisting (in units of time) is quantified by comparing the time difference between the motions of the tubes detected at different locations on the tubes. Note the time difference in the sensor readings.
In a Micro Motion precision dual-tube Coriolis meter, the tubes are designed in a “U” shape for low frequency operation and optimal sensitivity, which results in the highest accuracy and measurement performance even when entrained gas is present in a liquid fluid (two-phase). The design is capable of measuring time delays in the range of several nanoseconds (one trillionth of a second) with high repeatability, thus providing unmatched measurement performance. Therefore, Micro Motion meters are the standard for applications in critical process control, custody transfer and demanding environments.
The initial cost of a Coriolis meter is higher than most flow meter technologies. However, when looking at the lifecycle cost of ownership, including the reduced maintenance, higher measurement performance and lower installation costs, the return on investment over time is superior. The chart below compares the cost of calibration, installation costs, maintenance, engineering costs as well as initial costs.
|Straight pipe runs||0||1000||1000|
|Straight pipe runs||0||500||500|
|Maintenance cost (10 year life)||Meter repair*||1000||2000||5000|
|Meter flow calibration||0||2000||5000|
* Periodic meter zeroing
There are many options when choosing a device for flow measurement. Fortunately, Coriolis meters are capable of being used for applications spanning a variety of industries. The meters tend to be on the higher end of the price range, but they prove to be more valuable over time with minimal calibration and maintenance costs compared to other technologies. And, there are differences between Micro Motion Coriolis meters and other basic Coriolis meter designs that result in the best installed measurement performance and confidence.
I encourage you to look closer at Emerson’s technologies and services. We welcome the opportunity to learn about what’s challenging your enterprise and discuss how we can help.
For more information about Coriolis flow and density measurement, click here for a video overview.