Field work needs a valid permit, isolation and the site's procedures. Tell operations before you touch a loop that feeds control or a safety system, and put the loop in manual or bypass only with their agreement. Device menus, limits and tolerances vary by make and model: always check the manufacturer's manual.
Checks for every flow meter
- Datasheet vs nameplate: tag, size, flange rating, material, range, units, output and power all match.
- Flow direction: the arrow on the body points with the flow.
- Range and units are the same in the meter and in the control system (and mass vs volume vs standard volume is clear).
- Output: loop check the 4–20 mA and pulse/frequency outputs with the meter's simulation function. See Loop Checking.
- Low-flow cut-off is set so the reading is zero at no flow, but not so high that it hides real low flow.
- First run: compare with a second measurement: tank level change, a check meter, or a pump curve.
Magnetic flow meters
| Check | Why |
|---|---|
| Liquid is conductive (typically at least about 5 µS/cm; check the data sheet) | A mag meter cannot measure oils, hydrocarbons or very pure water |
| Pipe always full at the meter | A part-full pipe reads high and noisy; mount in a rising line or low point |
| Earthing / grounding rings fitted for plastic or lined pipe | Without a reference to the liquid the signal is noisy or offset |
| Straight run (typically about 5D upstream, 2D downstream; check manual) | Bends and valves distort the profile |
| Electrodes horizontal (not at top or bottom) | Air at the top or solids at the bottom cover the electrodes |
| Empty-pipe detection set | Gives a clear alarm rather than a false reading |
Field example: mag reads 40 % with the pump off
A new mag meter on a vertical downward line read flow with the pump stopped. The line drained, the meter was part-empty, and the electrodes picked up noise. Moving the meter to the rising section of the line fixed it.
A new mag meter on a vertical downward line read flow with the pump stopped. The line drained, the meter was part-empty, and the electrodes picked up noise. Moving the meter to the rising section of the line fixed it.
Coriolis meters
- Install as the manual shows: for liquids usually tubes down (so gas can escape), for gas tubes up (so liquid drains).
- Support the pipe on both sides; don't let the meter carry pipe stress.
- Fill the meter completely and run flow for a while to purge gas and reach process temperature.
- Stop the flow with a tight-shut valve downstream (meter stays full and pressurised).
- Run the zero adjustment. Never zero with flow running or with the meter part-empty.
- Check density reads sensibly (for example about 1000 kg/m³ for water at 20 °C). A wrong density points to gas in the liquid.
Entrained gas or bubbles are the most common Coriolis problem: unstable flow and low density readings, and the drive gain rises. Fix the process (back-pressure, location) rather than the meter.
Vortex meters
- Needs a minimum velocity / Reynolds number. Below it the meter reads zero, so it suits steady, moderate-to-high flows, not low flows or start-up.
- Needs longer straight runs than most meters (often 15–20D or more upstream after bends and reducers; check the manual).
- Pipe vibration can create a false signal at no flow. Check the reading with the line isolated; adjust the filter/low-flow cut-off per the manual.
- Gasket must not stick into the bore: it disturbs the vortex street.
DP flow (orifice, venturi, nozzle)
- Orifice orientation: sharp edge facing upstream; tab marked "inlet" on the upstream side.
- Bore and β match the sizing calculation. Check the stamped bore on the plate tab.
- Impulse lines: liquids: taps on the side, transmitter below; gas: taps on top, transmitter above; steam: condensate pots at equal height, legs filled.
- Square root in one place only (transmitter or DCS).
- Zero check with the manifold equalised.
Q = Qmax × √(ΔP / ΔPmax)
Example: ΔP range 0–250 mbar, Qmax 100 m³/h, measured 62.5 mbar → 100 × √0.25 = 50 m³/h
Size or check the element with the DP Primary Element app; troubleshoot the transmitter with Transmitter Troubleshooting.
Quick fault table
| Symptom | Mag | Coriolis | Vortex | DP |
|---|---|---|---|---|
| Reads flow at no flow | Part-full pipe, noise, poor earth | Zero done badly, pipe stress | Vibration | Zero shift, passing equaliser |
| Reads low | Electrode coating | Gas entrainment | Below minimum velocity | Leaking high side, worn orifice edge |
| Noisy | Solids, air, poor earth | Gas / two-phase flow | Swirl, short straight run | Pulsation, gas in liquid legs |
| Reads zero | Empty pipe | Empty tube, drive fault | Below cut-off | Equaliser open, plugged lines |
Q&A quiz
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