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Coriolis Meter Troubleshooting: Entrained Gas Fixes
Quick Answer: Entrained gas makes a Coriolis meter report zero flow, spike to unrealistic highs, or show density values that jump without reason. Raise back pressure on the meter outlet, reduce flow velocity inside the tubes, or install an inline air eliminator upstream. If the gas enters as slugs, the sensor drive gain swings hard and stops stable measurement within seconds.
What Entrained Gas Does Inside a Coriolis Tube
Gas bubbles disrupt the oscillation of the flow tubes. A Coriolis meter needs a uniform liquid mass to generate a clean phase shift. Even 1 percent gas volume in a DN25 tube can push the drive gain from 10 percent to above 80 percent. The transmitter then flags over-range or tube not vibrating faults.
We have seen a paint manufacturer in central Vietnam batch a solvent-based resin with a DN40 Silver Instruments Coriolis meter. Halfway through the batch the flow reading dropped to zero and the density jumped from 0.92 g/cm³ to 1.15 g/cm³. The operator stopped the pump and vented the line. A quick inspection showed a loose hose clamp on the suction side pulling air. Tighten that clamp and the meter returned to a stable 45 litre per minute within two minutes.
Root Causes That Repeat on Customer Sites
Most gas problems do not come from the meter. They come from upstream conditions. A pump sucking against a nearly empty day tank pulls air through the vortex. A return line dropping into an open tank without a dip tube traps foam. In food plants, CIP cycles leave wet tubes that dry out and let air rush in when the next product starts. In diesel loading skids, a slow pump ramp-up fails to flush vapour out of a long horizontal pipe run before the meter.
Sometimes the process itself generates gas. Hydrogen peroxide dosing, yeast propagation in a brewery, or a flashing liquid that drops below its vapour pressure right at the meter inlet. In those cases back pressure control becomes the only fast fix.
How to Detect Gas Before the Meter Alarms
Do not wait for a zero-flow trip. Watch the live density value on the 4-20 mA HART loop. If density fluctuates more than 0.005 g/cm³ every update cycle, you have free gas. Drive gain is another tell. On Silver Instruments transmitters, you can read it through Modbus register 2200. A stable, full-pipe liquid typically sits below 15 percent gain. Gas kicks it above 50 percent, and slug flow pushes it beyond 90 percent.
In most cases the temperature reading from the PT100 inside the tube stays steady while the density dances. That mismatch tells you the problem is not a temperature shift but a second phase inside the tube.
Step-by-Step Fixes That Field Engineers Use
Raise the back pressure on the meter. Install a spring-loaded check valve or a back pressure regulator 3 to 5 pipe diameters downstream of the meter outlet. For a DN15 to DN25 meter moving solvent or light fuel, 0.3 to 0.5 bar back pressure keeps dissolved gas in solution. For a DN50 line with viscous resin, 1.0 to 1.5 bar is often needed. Use a gauge right at the outlet flange.
Lower the flow velocity through the meter. Large Coriolis tubes act like a centrifuge. High velocity pushes the heavier liquid outward and leaves a gas core in the center. If you can drop the flow rate by 20 to 30 percent without hitting the meter minimum, try it. A DN80 meter in a Saudi Arabian desalination brine line stopped dropping out after the operator throttled the downstream valve from 80 m³/h to
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Put an air eliminator or a small vapour pot ahead of the meter. Install it at a high point in the pipe. For batch sizes below 500 litres, a simple float-type air eliminator with a DN15 vent line works. For continuous processes, a vortex separator with a drain trap is more reliable. One food factory in Peru running yogurt through a DN15 meter used a small vertical trap just before the inlet and eliminated daily morning start-up alarms.
Fix suction-side leaks. Inspect every hose clamp, O-ring, and pump gland within 3 metres of the pump inlet. A 1 mm gap on a camlock fitting can pull more air than most people expect. Use soapy water on joints while the pump is running but the meter is stopped. Bubbles appear quickly if there is a vacuum leak.
When Meter Replacement or Re-Ranging Helps
Some lines always carry a small amount of entrained gas by design: wellhead crude, spent yeast slurry, or lime milk. In those applications, a standard Coriolis with tight density cutoffs may never stay online. Silver Instruments can supply a two-phase tolerant transmitter firmware that ignores short density spikes and holds the last good mass flow reading for a programmable interval of 1 to 5 seconds. This option is common on our meters shipped to palm oil estates in Indonesia and latex batching plants in Nigeria. Tell us the expected gas fraction and we pre-configure the damping before dispatch.
FAQ: Entrained Gas Fixes for Coriolis Meters
Q: Does entrained gas permanently damage the flow tubes?
No. The tubes can withstand short slug events. However, repeated hammering from liquid-gas slugs over months can stress the manifold and lead to a zero drift. Fix the root cause and re-zero the meter at process temperature.
Q: What minimum back pressure does a DN40 Coriolis need?
Assume 0.5 bar above the liquid vapour pressure at the meter outlet. For diesel at 40 degrees Celsius, 0.3 bar may be enough. For acetone at 30 degrees Celsius, you need at least 0.8 bar. Provide the process temperature and fluid name and we calculate the exact value.
Q: Can I use a Coriolis meter on a line with intermittent two-phase flow?
Yes, with the right transmitter setup. Silver Instruments meters accept a two-phase alert relay output that can open a recirculation valve and send a 4-20 mA signal to the PLC. The meter does not need to trip the process.
Q: How do I confirm the fix on site without a calibration rig?
Run a dense, bubble-free liquid like cold water at a known temperature and check the density on the local display. If the value matches the expected density within 0.002 g/cm³, the meter is healthy. Then repeat with your process fluid. Compare drive gain before and after the fix to verify the improvement.
Q: We see gas only during start-up. Is that normal?
Yes, especially after a pipe drain or a filter change. Install a manual vent valve at the highest point before the meter. Open it for 10 seconds at pump start until liquid flows out without sputtering. This simple step avoids nuisance alarms on many batch lines.
Email us your pressure in bar, temperature in degrees Celsius, pipe size in DN, and expected flow range. Our engineers size a Coriolis meter with the correct tube geometry and back pressure recommendation for your fluid. Reach Silver Automation Instruments at the enquiry address on our website flow-meter.com.au and ask for a project-specific data sheet.
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