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DP Flow Measurement Fault Finding Guide
A one page guide to DP flow problems: impulse line faults, square root and cutoff settings, orifice condition and installation effects.
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A one page A4 guide to print for the workshop or keep on your phone in the field. The full content is also shown below.
Download PDF A4 · 54 KBBefore you start
- Permit, isolation and control room informed.
- Put the loop in manual or inhibit trips as authorised.
- Use certified test equipment in hazardous areas.
- Record as found readings before changing anything.
Quick checks
- Equaliser closed, HP and LP open?
- Legs full (liquid) or drained (gas)?
- Square root in one place only?
- Plate direction and bore correct?
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Symptoms, causes and fixes
| Symptom | Likely causes | Checks and fixes |
|---|---|---|
| Reads low | Gas in liquid legs, HP side leak or plug, worn orifice edge, orifice installed reversed | Vent legs; check HP valves and tap; inspect plate edge and direction |
| Reads high | LP side leak or plug, liquid in gas legs, wrong bore or range | Check LP side; drain gas legs; verify bore and range |
| Flow shown at zero | No low flow cutoff, equaliser passing, zero shift | Set cutoff; check equaliser; zero at line pressure |
| Flow far too high at low DP | Square root applied twice (transmitter and DCS) | Apply square root in one place only |
| Noisy | Pulsation, two phase flow, short straight run | Damping; check process; verify straight run |
| Slowly drifting | Plugging taps, condensate level changing in steam legs | Blow down; keep condensate pots full and equal |
| Wrong after rerange | DCS scaling or totaliser not updated | Update all settings and data sheet |
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Rules of thumb
- Flow % = √(DP %) × 10.
- 25 % DP = 50 % flow = 12 mA (square root output).
- Liquid service: transmitter below taps; gas service: above taps.
- Steam: condensate pots at equal height.
Related Tools, Procedures and Calculators
Frequently Asked Questions
Why does my orifice read low?
Gas trapped in liquid impulse lines or an HP side problem are the usual causes.
How do I check square root settings?
At 25 % DP the flow should be 50 %; if it shows about 71 %, square root is applied twice.
Does a worn orifice read high or low?
A rounded edge raises the discharge coefficient, so the meter reads low.
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