Electromagnetic Insertion Flowmeters

TECHNOLOGY

The Electromagnetic Insertion Flowmeters are based on the same technology as the more traditional full-bore electromagnetic flowmeters (usually called mag meters) namely Faraday's Law of Electromagnetic Induction:

When a conductive liquid moves through a magnetic field, it produces a voltage directly proportional to the velocity of the medium.

The essential differences between the two types of flowmeters are their installation and the positioning of the electrodes that allow this voltage to be measured:

• Full-Bore Flowmeters are installed on a section of pipe and their 2 electrodes are positioned symmetrically on each side of the pipe;
• Insertion Flowmeters are inserted into the pipe through a hole drilled in one side of the pipe and their electrodes are positioned along the body constituting these flowmeters.

TWO MODELS

• Single Point Flowmeters equipped with a single pair of electrodes; the pair of electrodes placed at the end of the flowmeter body is positioned in the center of the pipe (or 1/8th in the case of very large diameter pipes);
• Full Profile Flowmeters equipped with several pairs of electrodes; these flowmeters are inserted to the bottom of the pipe, and the different pairs of electrodes are positioned across the entire flowmeter body (2, 3, 5 or 7 pairs of electrodes depending on the diameter of the pipe).

CONDITIONS OF USE

• Clean or raw water, without fiber or matter
• Pipes under pressure
• Maximum pressure: 15 to 20 Bar (depending on products)
• Maximum speed: 2 to 10 m/sec (depending on products and pipe diameters)
• Respect of downstream/upstream distances from sources of hydraulic disturbance, with different requirements depending on the type of flowmeter (D = pipe diameter):
  — 10D/5D on average for single point flowmeters; possibility to introduce velocity profile coefficients to limit the impact on accuracy
  — 5D/2D or less for full profile multi-electrode flowmeters

ADVANTAGES

• Reduced installation cost (no pipe cutting, no flanges, hot tap installation)
• No service interruption
• No temporary connection, no draining of the pipes
• Can be installed in small spaces (vertically or laterally)
• Moveable from one site to another (removal and insertion under pressure by ball valve)

PRECISION

Contrary to some ideas sometimes held, insertion-type electromagnetic flowmeters offer excellent accuracy, sometimes even better than that offered by full-bore flowmeters.

• In the case of Single Point Flowmeters, this accuracy requires that installation instructions be followed, and often that velocity profiles be produced during commissioning.
  This operation consists in carrying out punctual velocity measurements, by progressively raising the probe from the bottom to the top of the pipe; it then makes it possible to apply to the measurements which will be obtained from the flowmeter a 'profile' coefficient taking into account the dispersion of velocities inside the pipe, to improve the accuracy of the average velocity.
  These flowmeters may also require more frequent checks and re-calibrations, but these operations are facilitated by the concept of mounting insertion flowmeters.

• Full Profile Flowmeters, which have several sets of electrodes from the bottom to the top of the pipe, have the advantage of permanently taking into account this dispersion of velocities and the variability of profiles according to hydraulic regimes (turbulent, laminar or transient regimes).
  It is also understandable that this multi-point measurement can offer, especially in cases of disturbed and non-uniform regimes, a better accuracy than that of full-bore flowmeters.
  However, the arrangement of the electrodes along the flowmeter body, as well as the method of calibration, are important to ensure this accuracy, especially at low velocities/flow rates.

---

Our partner, Flow-Tronic, has favoured more demanding design choices in manufacturing, which allow to optimize the accuracy of the measurement of the average flow velocity:

• Individual calibration of each pair of electrodes in velocity, rather than a global calibration in flow;
• Arrangement of the electrodes with equal areas rather than equal distances, which allows a better consideration of the velocity profile, especially low velocities at the edges of pipes.