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Troubleshooting Common Operational Anomalies in a Coriolis Flow Meter
Quick Answer: Most Coriolis meter anomalies trace back to entrained gas, zero drift after a temperature change, or installation stress. Start by checking the drive gain and pickoff voltage on the local display. On Silver Automation Instruments meters, a drive gain above 20% points to two-phase flow or coating on the tube wall.
1. Zero Drift After Temperature Swings
Zero drift will throw off low-flow measurement every time. After a plant shutdown or steam tracing warm-up the meter zero can shift without notice. A water treatment station in Saudi Arabia saw a DN15 Coriolis meter drift 0.12 kg/h after ambient temperature moved from 25 °C to 45 °C. The fix was simple: let the sensor stabilize at process temperature for 30 minutes, then run an automatic zero calibration with the line full and valves closed. Most engineers skip this part because they rush the startup. Do a zero check under process conditions, not just with water at room temperature. Silver Instruments meters store the last zero value in onboard EEPROM, so you can compare the drift over months. If the zero value changes more than 0.05% of full scale between calibrations, inspect for tube damage or debris.
2. Erratic Flow Output from Two-Phase Flow or Slugging
In chemical batching a small amount of entrained air can make the Coriolis transmitter hunt between flow rates. A paint manufacturer in Vietnam had a DN25 meter measuring solvent at 20 L/min. The analog output swung from 16 mA to 4 mA in seconds. The culprit was a pump suction leak that introduced air slugs. Drive gain shot up to 85% and the density reading dropped below the expected 0.87 g/cm³. Here is the thing: the Coriolis meter tries to maintain tube oscillation, but gas makes the vibration uneven. The quick diagnostics on the local screen showed “drive gain overrange” and “pickoff amplitude low”. The plant then installed a backpressure regulator set to 2.5 bar at the meter outlet. This dissolved the fine bubbles and brought the drive gain back to 12%. In practice, for a Silver Instruments CMF series, keep the drive gain under 30% during normal operation. If it spikes, check for cavitation, leaking pump seals, or an empty pipe segment before the sensor.
3. Density Reading Errors at Elevated Temperatures
A petrochemical terminal in Nigeria used a DN50 Coriolis meter to measure fuel oil density at 120 °C. The indicated density was 0.845 g/cm³, but the lab sample gave 0.855 g/cm³. The offset was consistent. The PT100 sensor inside the meter was reading 5 °C lower because of a poor thermal contact in the thermowell pocket. After they re-seated the PT100 with heat sink compound, the density error dropped to 0.002 g/cm³. Silver Instruments factory calibrates each meter with a multi-point density correction from 0 to 150 °C. The transmitter has a built-in curve for temperature compensation. Still, always verify the internal temperature reading against an external thermowell. A 3 °C error can shift density by about 0.001 g/cm³ for hydrocarbons. Configure the correct reference temperature for your density contract, usually 15 °C or 20 °C.
4. Low Flow and Zero Cutoff Misconfiguration
Many users set the low-flow cutoff value too high and then wonder why the totalizer does not count small batches. A food plant in Brazil installed a Silver Instruments Coriolis meter on a chocolate syrup dosing skid. Production wanted to record each 2 kg shot, but the batch total was zero. The factory default cutoff was set to 0.5% of the nominal flow rate, which was 0.4 kg/h. The short dosing pulses were below this threshold. The engineer lowered the cutoff to 0.01 kg/h and enabled the “smart mass totalizer” algorithm in the transmitter. Afterwards, even a 0.5 kg dose registered in the PLC. Here is a quick rule: set the mass flow cutoff to about 1% of the lowest batch flow rate you want to capture. But do not set the cutoff to zero, or the meter may accumulate noise when the line is static.
5. Output Signal Spikes from
A water injection skid on an offshore platform in Malaysia reported 4-20 mA spikes that triggered the DCS alarm. The Coriolis meter used a 4-20 mA HART output, but the current loop was sharing a common ground with a VFD pump. The loop resistance measured 480 ohms, slightly above the 450-ohm HART limit. They added an isolated barrier with active output, and the spikes stopped. For Silver Instruments meters, keep the loop resistance under the maximum load listed in the manual, typically 500 ohms at 24 V DC. Use twisted pair shielded cable, and ground the shield only at the control room end. Do not carry the 4-20 mA signal alongside AC motor cables for more than 2 meters. If you see random pulses on the display that do not match the actual flow, check the cable routing first.
6. Installation Stress and Pipe Vibration
Coriolis meters are sensitive to external vibration because they work on oscillation. If the pipe support induces a mechanical frequency close to the meter tube frequency, you get a noisy signal. An LNG metering skid in Australia had a DN80 twin-tube Coriolis meter that resonated whenever a nearby reciprocating compressor started. The pickoff voltage showed a 50 Hz noise component. The fix was adding flexible piping connectors and a vibration dampener on the upstream pipe. The meter was also re-supported at the factory-recommended support points. Silver Instruments includes a flexible installation sketch with each meter delivery. Do not hang the meter from the pipe only. Use the base frame mounts if the meter is heavy. After installation, record the drive gain and pickoff voltage at zero flow; any value that jumps 5% before and after the mount change signals stress on the sensor.
Frequently Asked Questions – Coriolis Anomaly Troubleshooting
Q: Why does my Coriolis meter show a flow rate when the valve is closed?
A: This is often a zero offset caused by small bubbles trapped in the tube or a very slow thermal drift. Purge the line to remove gas, let the temperature stabilize, and perform a zero calibration with the isolation valves closed. On Silver Instruments transmitters, the zero procedure takes 30 seconds and the screen confirms the result.
Q: What drive gain value is too high for a healthy meter?
A: A clean, liquid-filled tube usually shows a drive gain between 5% and 20%. Values above 30% warrant investigation. Above 50%, the meter may fault out. Common causes include two-phase flow, thick coating, or a frozen tube vibrator. Check the process fluid condition first.
Q: How often should I re-zero a Coriolis meter?
A: For custody transfer, once per month is typical. For process control with stable temperature, once every six months. Always re-zero after maintenance that involves opening the line, or after a change from one product to another with a large temperature difference. Silver Instruments meters have an automatic diagnostic that can alert you when the zero drifts beyond a user-set limit.
Q: Can a Coriolis meter measure gas-liquid mixtures accurately?
A: No, a standard Coriolis meter cannot measure mixed phases with high accuracy. The density and flow readings will fluctuate. You need to ensure single-phase flow. If you cannot guarantee it, consider a Coriolis meter with advanced entrained gas management, like the Silver Instruments CMF-G series, which can handle up to 5% void fraction and still give a mass flow reading with 2% repeatability.
Q: How do I get a quote for a replacement Coriolis meter or technical support?
A: Send us your application details: fluid name, pressure in bar, temperature in °C, pipe size (DN), flow range in kg/h or L/min, and your required accuracy. Our team will select the right Silver Instruments Coriolis model and send a quotation within one working day.
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