Troubleshooting Guides · Free Tool
DP Flow Transmitter Fault Finder
Choose the flow measurement symptom and get the likely causes for DP flowmeters such as orifice plates and venturis, with checks and fixes.
Reads high or lowNo flow shownNegative flowUnstable
SymptomFlow reads too high
Most likely causeLP impulse line leaking or partly blocked
First checkIsolate and check the LP line; vent and look for free flow
Causes in order of likelihood
| Cause | How to check | Fix |
|---|---|---|
| LP impulse line leaking or partly blocked | Isolate and check the LP line; vent and look for free flow | Repair the leak, clear the LP line |
| Orifice installed backwards or wrong bore | Check plate tab marking and bore against the data sheet | Reinstall correctly or replace the plate |
| Actual density differs from design (gas or steam pressure or temperature changed) | Compare actual pressure and temperature with the design values | Apply pressure and temperature compensation |
Advertisement
Before You Start
- Work under a permit, inform the control room and put the loop in manual or bypass the trip before touching a live instrument.
- Check whether square root is applied in the transmitter or the DCS, not both.
- Note the DP in mbar and the expected flow; use the square root relation to compare.
- Check the manifold: both block valves open, equaliser closed.
DP Flow Transmitter Fault Finder: All Symptoms
Flow reads too high
| Likely cause | How to check | Fix |
|---|---|---|
| LP impulse line leaking or partly blocked | Isolate and check the LP line; vent and look for free flow | Repair the leak, clear the LP line |
| Orifice installed backwards or wrong bore | Check plate tab marking and bore against the data sheet | Reinstall correctly or replace the plate |
| Actual density differs from design (gas or steam pressure or temperature changed) | Compare actual pressure and temperature with the design values | Apply pressure and temperature compensation |
Flow reads too low
| Likely cause | How to check | Fix |
|---|---|---|
| Equaliser valve passing or open | Close equaliser and see if reading rises | Close or replace the manifold |
| HP impulse line partly blocked or leaking | Vent and check flow from HP line | Clear or repair the HP line |
| Worn or damaged orifice edge | Inspect the plate at the next opportunity | Replace the plate |
Shows zero flow while process is flowing
| Likely cause | How to check | Fix |
|---|---|---|
| Low flow cut off set too high | Check cut off value in transmitter or DCS | Set cut off to a few % of flow |
| Both block valves closed or equaliser open | Check manifold valve positions | Restore correct valve positions |
| Transmitter in fixed current mode | Check HART status | Return to normal mode |
Negative flow or reading below zero
| Likely cause | How to check | Fix |
|---|---|---|
| HP and LP impulse lines swapped | Trace lines from tapping to transmitter | Swap the lines |
| Zero shift from trapped liquid or gas | Equalise and read the zero | Vent or drain lines, zero trim |
| Reverse flow in the process | Check pumps and valves | Process issue, not instrument |
Unstable or noisy flow reading
| Likely cause | How to check | Fix |
|---|---|---|
| Turbulence from upstream fittings (short straight run) | Check straight lengths against ISO 5167 tables | Add a flow conditioner or relocate the meter |
| Square root amplifying noise at low flow | Noise worse near zero | Set a low flow cut off, add damping |
| Gas in liquid lines or liquid in gas lines | Check impulse line slopes and condensate pots | Correct slopes, fill pots, vent |
Key insight: because flow follows the square root of DP, a small zero error in mbar becomes a large flow error at low flow. Always check the zero first.
Advertisement
Frequently Asked Questions
Why does my orifice flow read low?
Common causes are a passing equaliser valve, a blocked HP line or a worn orifice edge.
Can a DP flow transmitter show negative flow?
Yes, if HP and LP lines are swapped or the zero has shifted; reverse process flow is also possible.
What is a low flow cut off?
A setting that forces the flow to zero below a small percentage, because the square root amplifies noise near zero.
You May Also Like
Learn more on the blog
5 costly DP flow measurement errors
A practical, field focused guide on instrumentationblog.in that explains the theory behind this tool and how engineers apply it on real plants.
Read Full ArticleMore Troubleshooting Guides
Pressure Transmitter Fault FinderNo output, Stuck readingMagnetic Flowmeter Fault FinderEmpty pipe alarm, Noisy readingLevel Transmitter Fault FinderRadar level, DP levelThermocouple and RTD Fault FinderOpen sensor, Reads high or lowControl Valve Fault FinderNot moving, Hunting4 to 20 mA Loop Fault Finder0 mA, Low currentpH Sensor Fault FinderLow slope, Offset error
More Instrumentation Tools
Instrument Tag Decoder2 toolsIP, NEMA and Ex Rating Decoder4 toolsWire and Cable Color Codes4 toolsPID Tuning Tools6 toolsP&ID Symbols4 toolsPipe and Flange Charts3 toolsThread Size Charts4 toolsCommunication Protocols8 toolsPractice Tests8 toolsCalculators184 engineering calculatorsUnit Converters76 converters, 700+ units