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Radar level in foaming tanks needs more than a high-level setting. Liquid can continue entering after a stop command, while foam can occupy space above the liquid. Before increasing batch size, establish what the radar follows and how much room remains during the complete stopping sequence.
Separate liquid rise from foam expansion
A liquid-level signal and a foam-top signal answer different questions. Foam may expand while the amount of liquid changes very little. Conversely, a sensor that follows liquid beneath the foam may give a useful batch reading without indicating the approach of foam to a vent. Use a target-specific foam trial to identify the actual surface before calculating remaining space.
The METRAVON MV-RD12 has a published liquid measuring interval of 0.1-30 m. That interval is a distance specification, not a permitted filling height or a guaranteed foam-penetration depth. Its lens, connection and operating conditions still need to suit the vessel.
Radar level in foaming tanks: account for the stopping sequence
For a vessel section with constant horizontal area A, additional liquid rise is approximately Q x t / A when inflow Q remains constant for time t after the relevant command. Use consistent units. If flow changes during valve closure, use the accumulated inflow volume instead. Pipe drainage or a pump rundown can add liquid after the electrical command has already changed.
As an illustrative calculation, 6 cubic metres per hour continuing for 15 seconds adds 0.025 cubic metres. In a 0.5 square metre section, that means 0.05 m of extra liquid height. These are assumed values, not a METRAVON response specification or a recommended safety allowance. Foam growth, surface movement and measurement delay require separate assessment.
Choose one start event for the time interval. Measuring from the stop command to actual flow cessation covers the stopping behaviour after that command, not the earlier measurement and controller-decision delays. If the assessment starts when the liquid reaches a defined high-level condition, account for those earlier delays once as well. Do not add valve travel time again if it is already included in the observed command-to-flow-stop interval.
| Input | Evidence needed |
|---|---|
| Inflow | Flow during rundown, including gravity drainage after the valve or pump stops. |
| Delay | Elapsed time through measurement, controller action and the physical stopping mechanism. |
| Foam | Observed maximum occupied height during the relevant recipe and mixing stage. |
| Space | Distance to the first limiting opening, plus the radar's usable near-distance limit. |
Verify the sequence without approaching overflow
Use an approved test at a lower, controlled fill level to record the command, actual flow cessation and final settled liquid position. Where the geometry changes with height, convert the measured extra volume using the correct tank section. Do not transfer a cylindrical-section height allowance into a narrow neck unchanged.
Keep foam observations separate from that liquid calculation. If the required foam envelope cannot be established, the allowable batch fill remains unresolved. A qualified process review must determine protective measures; this calculation does not qualify the radar as an independent overfill device.
Questions about usable tank capacity
Q1. Can the high-level setpoint be raised if the reading is stable?
Stability does not establish remaining capacity. Confirm the detected surface, actual rundown volume and nearest limiting opening before changing a setpoint. A calm final reading says little about foam height during mixing.
Q2. Does a faster sensor eliminate the reserve space?
No. Valve travel, residual pipe contents and process expansion remain. Faster measurement may reduce one delay, but the installed stopping sequence must be evaluated as a whole.
Summary
Set usable fill capacity from measured stopping behaviour and the foam envelope, not from the radar's maximum range. Send the vessel section, fill rate and stop-sequence record for an MV-RD12 application review before revising the operating limit.
