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Waste Cooking Oil Flow Meter Entrained Gas: Quick Fixes That Work on Site
Quick Answer
When your waste cooking oil flow meter reads unstable or fails to measure, trapped gas is the first thing to check. Degas the oil upstream with a bubble trap or air release valve. If you need a meter that handles gas without breaking, consider a Coriolis mass flow meter from Silver Automation Instruments, it measures mass flow directly and ignores most gas slugs up to 10% by volume.
The Classic Symptom: Flow Value Jumps While the Pump Runs Smooth
One of our dealers in Indonesia sent us a video last month. A positive displacement oval gear meter on a 2 inch waste oil line was jumping from 12 L/min to 28 L/min and back to zero within three seconds. The pump speed was fixed. The gear meter clicked loudly. The LCD display went blank twice a day. The operator kept resetting the totalizer by hand.
We asked for a quick site check. They bypassed the strainer for ten seconds and collected a sample in a glass jar. The oil looked cloudy, small bubbles clung to the jar wall. Entrained gas was the root cause. The meter was counting gas voids as oil and the internal gears were spinning free.
Why Waste Cooking Oil Holds So Much Gas
Used oil from restaurants and fryers picks up water and fine food solids. During collection, trucks pump the oil into tankers and air gets mixed in. Heating the oil to 60°C or 80°C in a settling tank releases some gas, but a lot stays trapped as micro bubbles. The viscosity at 80°C can be around 20 cP to 45 cP, viscous enough to keep bubbles locked in. Most volumetric meters, oval gear and turbine types, measure volume and treat a gas pocket as oil. That gives you false totals and hits batch accuracy hard.
First Fix: Mechanical Gas Removal Before the Meter
Install a continuous air release valve or a top mounted degassing vessel on the pipe. Size the vessel so the oil residence time is at least 30 seconds. For a 5000 L/h line, a DN150 vessel 800 mm tall works in most plants. Add a PT100 temperature sensor after the vessel. If you keep the oil above 70°C, gas separates faster.
Second step is back pressure. Placing a spring loaded check valve after the flow meter, set to 1.5 bar to 2 bar, keeps the line full and reduces gas breakout. We have seen this alone fix 60% of erratic readings on a vane pump system in a Malaysian biodiesel plant.
When a Meter Technology Swap Solves the Problem
Some waste cooking oil lines feed reactors or centrifuges where bubbles are part of normal operation. You cannot always install big degassing tanks. In that case, switch to a mass flow meter instead of fighting the gas. Silver Instruments Coriolis mass flow meter, model SICMF, handles gas/liquid mixtures up to a
For a 2 inch line running at 3000 kg/h, we recommend the SICMF-DN15 or DN25 depending on flow range. Density output also gives you an online gas content check. If the density drops from 910 kg/m³ to 840 kg/m³, you know gas is getting worse. The transmitter sends a 4-20 mA HART density alarm to the PLC.
Real Case: Biodiesel Plant in Thailand
A biodiesel plant near Rayong used gear meters on three skids. Entrained gas from a centrifuge feed pump caused daily meter faults. They moved to SICMF-DN25 meters with local display and Modbus output. The totalizer drift stopped. Four months later they ordered five more meters for other lines. The engineer told us, they saved two hours a day on manual recalibration.
Sizing Tips for Waste Cooking Oil Flow Meters
Always share the actual viscosity at pumping temperature, not just cold oil specs. Oil at 40°C can be 70 cP, at 80°C it drops to 20 cP. That changes meter selection a lot. Oval gear meters need a viscosity under 100 cP for accurate repeatability. Silver Instruments oval gear flow meters with PPS gears work well for clean degassed oil. If the oil contains particles, a Coriolis meter without moving parts is a better long term investment.
Frequently Asked Questions
Can I use an electromagnetic flow meter for waste cooking oil? No, electromagnetic flow meters need a conductive liquid with minimum 5 µS/cm. Waste cooking oil is non-conductive.
What is the fastest way to check if gas is the problem? Collect a sample at the meter inlet in a clear bottle. Let it settle for two minutes. Bubbles on the wall or a layer of foam on top confirm gas.
Does a Coriolis meter always ignore gas? Most Coriolis meters maintain accuracy up to 2% to 5% gas volume fraction. Silver Instruments SICMF can handle up to 10% with stable mass flow readings. Beyond that you still need gas removal.
How do I size the degassing vessel for my line? Calculate the actual flow rate in L/min. Multiply by a dwell time of 0.5 minutes. That gives the vessel volume in litres. Then pick a standard pipe size like DN150 or DN200 to match that volume with enough height.
Can I use a vortex flow meter instead? Vortex meters work for steam and gases but struggle with high viscosity oils. At 20 cP the turndown drops and the signal jitter makes it unreliable.
Send us your flow rate (kg/h or L/min), oil temperature (°C), pipe size (DN), and viscosity (cP) if known. We will propose a meter and degassing setup. Email our support team with your process data to get a direct quote.