Electromagnetic flow technology is continuously evolving, and Emerson hopes to provide transparency and understanding to installation requirements and why they matter to a user. There has been a lot of buzz lately about zero diameter (0D) inlet/outlet installations which means there is no straight run before or after the electromagnetic flow meter. Our users want to know more about this type of installation and its impact on sensor accuracy.
What is an Electromagnetic Flow Meter?
Let’s begin with how an electromagnetic flow meter works. Coils located on the sensor generate a magnetic field. As a conductive fluid flows through this field, the charged particles are separated creating an induced voltage. Two electrodes located in the sensor pick up the induced voltage caused by the shifting ions that the transmitter then converts into a flow rate. This flow measurement is possible without any pipe obstruction or changes, and no pressure drop.
Reasons for Installation Requirements.
When installing a magnetic flow meter, an upstream and downstream straight run is specified by the manufacturer. An industry standard is five pipe diameters (upstream) from the inlet and two pipe diameters (downstream) from the outlet, as depicted in the figure below.
When flowing through a straight pipe, the conductive incompressible fluid has a more symmetric flow profile, and thus a more consistent velocity, that allows for the most accurate flow rate reading. In a short-run installation, the upstream disturbance disrupts the flow profile and thus creates variability in the fluid velocity as it moves through the meter. This variation creates an offset in the flow measurement accuracy; however, the meter will still provide a consistent and repeatable measurement. More details as to why this shift in accuracy occurs with a short-run installation is covered in the next section.
Effects on Accuracy.
The shift in accuracy of an electromagnetic flow meter following an obstruction is due to a change in the fluid flow profile. Upstream disturbance such as a close coupled elbow leads to swirling of a fluid which results in an asymmetric flow profile. This causes a slightly different induced voltage to be generated when the fluid moves through the magnetic field than what would occur in an ideal installation. A visual depiction of a fluid’s swirl and asymmetry after an elbow is shown below.
A five-pipe diameter upstream and two-pipe diameter downstream ideal installation allows the fluid to regain a more symmetric flow profile, so users can trust the flow meter’s readings accurately represent what occurs in their facilities. For applications where a less-than-ideal installation is necessary, the user’s readings will still provide a consistent measurement. Depending on the user’s needs, a consistent reading with slightly lower accuracy meets the needs of the application. However, if a higher accuracy without a straight run is required, modifications such as additional electrodes, changes to the sensor design, or installation specifications may be required.
Types of Zero Diameter (0D) installations.
Manufacturers provide specific installation instructions in association with the stated accuracy of the electromagnetic flow meter. However, it is difficult to include every type of installation. If a user installs a meter outside of the specified installation for an accuracy, it is possible that the performance may not be as expected. Factors to take into consideration are:
- Sensor Direction: Horizontal, vertical or diagonal
- Obstacles: Close-coupled valves, elbows, T-fittings, build-up in the pipe, insertion devices, and nominal diameter differences
- Electrode Orientation: In the plane or out of the plane
Emerson’s Installation Accuracy.
With a better understanding of how flow profiles and disturbances impact magnetic flow meter accuracy, users can make better decisions about their unique installations. The real world doesn’t always offer an ideal installation, and an installation of five pipe diameters (upstream) from the inlet and two pipe diameters (downstream) from the outlet is not possible. For applications that require a zero diameter (0D) implementation, Emerson has evaluated their meters internally and in third-party labs, and determined they meet these specifications for a flow rate of 1 m/s to 10 m/s:
In some cases, specific installations have a better accuracy than listed above with a zero diameter (0D) installation. For example, a zero-diameter installation where the electrodes are out of the plane of a single upstream elbow give results in a shift of a quarter percent which gives the results as seen below:
*With high accuracy calibration
To Sum it up.
In summary, electromagnetic flow meters are designed for optimal performance with straight inlet and outlet runs of 5 diameters and 2 diameters respectively. However, in many cases, especially in larger line sizes, achieving this amount of straight run is expensive and impractical. As a result, close coupling elbows and other upstream disturbances are commonplace. And while the repeatability of the magnetic flow meter was still very high, the absolute accuracy was in question. Emerson has done extensive testing and empirical analysis to quantify the impact that short-run installations have on our Rosemount magnetic flow meter products. This data now enables users to confidently apply this technology in their applications regardless of installation constraints.
To learn more, go to www.Emerson.com/RosemountMagneticFlow