Magnetic Flow Meter Straight Run Requirements
Flow Meter Installation
A magnetic flow meter does not have one universal upstream and downstream straight-run requirement. Use the instructions for the exact sensor design and the actual nearby disturbance. A general 5D chart can be a starting point for planning; it cannot approve an installation, and a “0 x DN” description does not remove every layout condition.
Why the published numbers differ
A straight run is the pipe length before or after the meter, usually expressed as a multiple of a stated diameter. Bends, valves and changes in pipe size can disturb the velocity distribution. Different measuring designs and installation conditions therefore need different checks. KOBOLD’s general guide explicitly distinguishes general guidance from the instrument’s own manual.
For example, ifm’s SM magmeter installation page describes 3 pipe diameters upstream and 1 downstream as good practice. That is guidance for its SM range, not a replacement for another manufacturer’s requirements.
Read the configuration and the installation row together
Endress+Hauser’s Promag W special documentation SD02761D, July 2021, pages 2–3 provides a useful documented example:
| Layout in that document | Design options C, H and I | Design options J and K |
|---|---|---|
| Before or after a bend | 0 x DN permitted in this documented case | 0 x DN permitted in this documented case |
| Downstream of a pump | 0 x DN permitted in this documented case | At least 2 x DN inlet run |
| Upstream of a valve | 0 x DN permitted in this documented case | At least 1 x DN outlet run |
| Downstream of a valve that remains 100% open | 0 x DN permitted in this documented case | At least 2 x DN inlet run |
Option C is not listed for Promag W 10 in this document.
This table is an example of how one “0 x DN” family can have configuration-specific conditions. It is not a universal Promag installation table. Match the supplied device’s design code and applicable operating instructions before using it; the cited special document supplements those instructions. A partly closed valve does not satisfy the last row.
Convert the required run into a layout check
Use the diameter definition and measurement endpoints in the applicable drawing. Do not silently change a drawing’s sensor-size or DN reference to pipe outside diameter.
Here is a hypothetical arithmetic example, not a rating for a named meter. If the applicable drawing requires 5D upstream and 2D downstream and defines D as 100 mm, the required lengths are 500 mm and 200 mm. Available lengths of 400 mm upstream and 300 mm downstream give 4D and 3D. The extra downstream length does not make up the upstream shortfall.
Record four items on the sketch: the exact sensor/design, the disturbance, the required distance and the available distance. If several fittings occur together, compare the combined arrangement with the manufacturer’s guidance rather than approving each fitting in isolation. Yokogawa’s AXW general specification, April 2017, page 13 explicitly warns that its individual piping examples may be insufficient when conditions are combined.
What to do when the space is insufficient
Compare relocating the measuring point, changing the pipe arrangement, or selecting a design whose documented conditions fit the available space. Submit the actual layout for a model-specific review before accepting a reduced run. Do not create an accuracy correction factor from the missing length.
Straight-run acceptance is only one installation check. Full-pipe conditions, air bubbles, liquid suitability and the measurement reference remain separate. For probe installations, use our insertion electromagnetic flow meter guide; for interpreting the stated performance, see flow meter accuracy calculation.