knowledge
How to Size an Electromagnetic Flow Meter Using Flow Rate and Velocity
This guide explains how to select the correct electromagnetic flow meter size based on pipe diameter, flow rate and liquid velocity.
In most applications, the meter is selected in the same nominal size as the pipeline. However, when the pipe is relatively large and the operating flow rate is low, a smaller meter may be installed with reducers to achieve a more suitable velocity.
The sizing process can be completed in four steps:
- Confirm the pipe size and flow range.
- Calculate the velocity at the existing pipe size.
- Check the minimum, normal and maximum velocities.
- Compare a smaller meter if the velocity is too low.
Step 1: Confirm the Pipe Size and Flow Range
Collect the following information:
- Existing pipe size
- Minimum flow rate
- Normal flow rate
- Maximum flow rate
- Liquid type
Normal flow is the main operating condition. Minimum flow is used to evaluate low-flow performance and sedimentation risk, while maximum flow is used to check high velocity, wear and pressure loss.
Do not select the meter based only on maximum flow. A large meter may handle the peak flow but still operate at an unsuitable velocity during normal conditions.
Meter size is only one part of the selection process. If you have not yet chosen the appropriate meter type, see our electromagnetic flow meter selection guide by application and operating conditions.
Step 2: Calculate the Flow Velocity
The relationship between flow rate, pipe area and velocity is:
For a circular pipe:
When flow rate is expressed in m³/h and internal diameter in millimetres:
Where:
(v) = velocity, m/s
(Q) = flow rate, m³/h
(D) = internal diameter, mm
Calculate the velocity at minimum, normal and maximum flow.
Step 3: Check Whether the Velocity Is Suitable
A velocity of approximately 1–3 m/s is often used as a preliminary sizing range for general water and conductive liquids.
However, this is not a fixed requirement. The appropriate velocity depends on the liquid and application.
|
Application |
Preliminary consideration |
|---|---|
|
Clean water |
Lower velocities may be acceptable |
|
General conductive liquids |
Approximately 1–3 m/s is a useful starting point |
|
Wastewater or sludge |
Sufficient velocity may be needed to reduce sedimentation |
|
Liquids that cause buildup |
Higher velocity may help reduce deposits |
|
Abrasive slurry |
Lower velocity may reduce liner wear |
Low velocity does not automatically mean that an electromagnetic flow meter cannot measure accurately. Many modern meters can operate well below 1 m/s. However, very low velocity may make zero-point effects and flow fluctuations more significant and can increase the risk of solids deposition or electrode buildup in some applications.
Excessive velocity may increase liner wear, erosion, pressure loss through reducers and pumping requirements.
The final velocity should therefore be checked against the selected meter model, liquid properties and operating conditions.
Step 4: Compare a Smaller Meter if Necessary
If the existing pipe size provides suitable velocities at minimum, normal and maximum flow, a meter with the same nominal diameter can normally be selected.
If the minimum or normal velocity is too low, compare one or more smaller standard meter sizes. For a DN200 pipeline, for example, DN150 or DN125 may also be evaluated. Recalculate the flow velocities for each option and confirm that the reduced size does not create excessive maximum velocity, pressure loss, liner wear or pumping demand.
Installation conditions should also be considered, including the available space for reducers, sufficient straight pipe length and the additional pressure loss caused by the reduced meter size. The objective is to achieve a practical operating velocity, not the highest possible velocity.
Electromagnetic Flow Meter Sizing Example
Consider a municipal wastewater pipeline with the following conditions:
Existing pipe size: DN200
Minimum flow: 30 m³/h
Normal flow: 100 m³/h
Maximum flow: 180 m³/h
DN200 Velocity
|
Flow condition |
Flow rate |
Velocity |
|---|---|---|
|
Minimum |
30 m³/h |
0.27 m/s |
|
Normal |
100 m³/h |
0.88 m/s |
|
Maximum |
180 m³/h |
1.59 m/s |
The maximum velocity is suitable, while the minimum velocity is relatively low.
This does not necessarily make DN200 unsuitable. The decision depends on how often low-flow operation occurs and whether sedimentation is a concern.
DN150 Velocity
|
Flow condition |
Flow rate |
Velocity |
|---|---|---|
|
Minimum |
30 m³/h |
0.47 m/s |
|
Normal |
100 m³/h |
1.57 m/s |
|
Maximum |
180 m³/h |
2.83 m/s |
DN150 provides a more suitable normal operating velocity, but it requires reducers and creates additional pressure loss.
Therefore:
Choose DN200 when installation simplicity and low pressure loss are the priority.
Evaluate DN150 when low-flow operation is frequent or sedimentation is a concern, provided the pump can handle the additional pressure loss and the velocity at maximum flow remains acceptable.
The final choice should balance low-flow measurement performance, pressure loss, wear risk and installation space.
Electromagnetic Flow Meter Size and Flow Rate Chart
The following chart shows approximate flow rates at velocities of 1 m/s and 3 m/s.
Conclusion
Start electromagnetic flow meter sizing with the existing pipe diameter and calculate the velocity at minimum, normal and maximum flow.
Use the same meter size when the resulting velocities are suitable. Compare a smaller meter only when low operating velocity or sedimentation is a concern, and confirm that the reduced size does not create excessive velocity, pressure loss or wear.
If complete flow data is unavailable, sizing can be estimated from the available operating conditions. Contact our engineers for a recommendation.
Need Help Selecting the Right Meter Size?
Send us your pipe size, minimum, normal and maximum flow rates, liquid type, operating pressure and temperature. ARTang can help evaluate the appropriate meter size, liner, electrode material and installation configuration.
