Milk of lime carrying grit up to 2 mm is a hard density application. The sensor has to track a small density span of a few hundredths of a g/cm³ while the slurry deposits carbonate scale on wetted surfaces and abrades them. Coriolis meters can measure it, but the tube's resonant frequency is the measurement, and scale and wear both change it. Microwave inline analyzers are becoming the common choice for this service because the reading tracks water content and does not depend on particle size.
Solids concentration from slurry density
Lime slaking and dosing are controlled on solids concentration, and density is the proxy for it. For a two-phase suspension of hydrated lime in water, the solids mass fraction follows from a mass balance:
w = rho_s * (rho - rho_w) / ( rho * (rho_s - rho_w) )
rho = measured slurry density [g/cm3]
rho_w = density of the carrier water at process temperature [g/cm3]
rho_s = particle density of the solids [g/cm3]
w = solids mass fraction [-]Worked example, with assumptions labeled: take rho_w = 1.000 and rho_s = 2.24 g/cm³. Hydrated lime is roughly 2.2 g/cm³, so use your own lab pycnometer value. A reading of 1.100 g/cm³ then gives w = 2.24 × 0.100 / (1.100 × 1.24) ≈ 0.164, or 16.4 wt%. The sensitivity at that point is dw/dρ = rho_s·rho_w / (rho²·(rho_s − rho_w)) ≈ 1.49. So every 0.001 g/cm³ of density error moves the computed solids by about 0.15 wt%.
That sensitivity sets the budget. Coating, wear, air and temperature errors of a few thousandths of a g/cm³ all show up as solids error of the same order as normal process variation. A density instrument for this service has to hold its zero over months of exposure to the slurry, not just meet a clean-water accuracy figure.
How lime slurry degrades inline density sensors
Four physical effects cause drift on this medium. They are heat and material faults, not logic faults. Instrument configuration will not fix them.
- Carbonate scaling. Dissolved Ca(OH)2 reacts with CO2 and deposits CaCO3 on wetted walls. On a Coriolis tube this adds mass that vibrates with the tube, lowering the resonant frequency. The meter then reads density high and the error grows over time.
- Abrasion. Grit up to 2 mm erodes the tube wall, especially at bends. Removing wall mass raises the frequency, so density reads low. It also thins the pressure-containing wall, which is a containment risk and not only a measurement error.
- Settling. At low velocity, coarse particles drop out in horizontal runs and in the low points of bent tubes. The sensor then sees a stratified mix instead of the bulk slurry.
- Entrained air. Air from slakers and agitated tanks damps tube vibration and displaces water. Coriolis readings get noisy and the drive signal rises. Microwave readings shift toward higher apparent solids, because air has far lower permittivity than water.
Coriolis, microwave and alternative principles compared
| Principle | What it senses | Response to 2 mm particles | Response to scale / wear | Main limitation on lime slurry |
|---|---|---|---|---|
| Coriolis mass flowmeter | Tube resonant frequency (density) and phase shift (mass flow) | Small-bore and bent tubes plug or erode; settling in bends | Coating reads high; wear reads low and thins the wall | Needs a large, straight measuring tube, velocity control and periodic cleaning |
| Microwave inline analyzer | Microwave phase/transit time through the medium, driven by water permittivity | Insensitive to particle size; GHz wavelengths are in the centimetre range, much larger than 2 mm grit | Full-bore spool with no vibrating element; heavy coating on the windows still needs checking | Needs site calibration against lab solids; air bubbles bias the reading |
| Dedicated densimeter / insertion concentration sensor (e.g. Sensotech) | Physical property correlated to concentration | Depends on sensor geometry and principle | Wetted sensing surface is exposed to deposits | Confirm suitability for coarse abrasive solids with the supplier |
| Gamma (nuclear) density gauge | Radiation attenuation across the pipe | No wetted parts | Pipe-wall scale adds attenuation and shifts zero | Radioactive-source licensing and handling |
Recommendation: use an inline microwave analyzer for milk of lime with coarse grit. It has no vibrating element whose mass balance scale and wear can upset, and particle size does not affect the reading. Microwave measurement specialists such as Pro-M-Tec supply this class of instrument. Keep Coriolis for cases where you also need mass flow from the same device. In that case specify a straight, full-bore tube sized for the grit, and budget for scheduled cleaning and wall-thickness checks.
Installing a microwave density analyzer on a lime line
- Locate the spool in a vertical run with upward flow. The pipe stays full, air rises through instead of collecting, and solids cannot settle against the sensing path.
- Place it downstream of the slurry pump and upstream of any throttling valve. This avoids the low-pressure zones where air comes out of solution.
- Match the spool bore to the line so there is no step, reducer or dead leg where grit and scale accumulate.
- Set line velocity high enough to keep the 2 mm fraction suspended. Stay within the supplier's abrasion guidance for the window material; read that limit from the analyzer datasheet.
- Install a sampling valve directly beside the spool so lab samples represent the same slurry the analyzer sees.
- Wire the process temperature input or enable the internal temperature compensation. Water permittivity changes with temperature, so an uncompensated reading drifts with slaker temperature.
- Provide a flush connection (water, or dilute acid where the supplier permits) to clear carbonate from the windows during shutdowns.
Calibrating against lab solids and confirming the reading
A microwave analyzer reports water content or a derived concentration. Calibrate it to your slurry with paired samples.
- Pull samples at the sampling valve while logging the analyzer output with a timestamp.
- Measure lab density on each sample with a density cup or pycnometer. Measure dry solids by oven drying.
- Collect points across the full operating span, from the lowest to the highest dosing concentration, not only at the normal setpoint.
- Enter the pairs into the analyzer's calibration table or fit a linear or polynomial correction. Convert to density with the mass-balance formula above if the DCS expects g/cm³.
- Repeat a single check sample weekly for the first month, then set the interval from the observed drift.
| Quantity | Limit / expected behavior | Where to read it |
|---|---|---|
| Analyzer vs lab solids | Offset within your control tolerance (≈0.15 wt% per 0.001 g/cm³ at 16 wt%) | Lab log vs historian trend |
| Microwave signal attenuation | Stable at constant concentration; a steady rise indicates window coating | Analyzer diagnostic screen |
| Reading noise | Spikes toward high solids indicate air | Fast-sampled trend, compared with tank level and agitation state |
| Coriolis drive gain (if Coriolis is used) | Rise over baseline indicates air or coating | Transmitter diagnostics |
| Coriolis tube frequency on water | Shift from commissioning baseline indicates scale (lower) or wear (higher) | Transmitter diagnostics during water flush |
Drift signatures that separate process faults from sensor faults
A slow one-directional offset that clears after an acid or water flush is coating. A Coriolis offset that keeps moving low and does not recover after cleaning is wall loss; take the meter out of service for a thickness check. Noise that follows pump start or agitator speed is air, not concentration. An offset that tracks slaker temperature points to missing temperature compensation. Recalibrating over any of these errors hides the fault until it gets larger.
FAQ
What happens if lime scale builds up inside a Coriolis meter?
The carbonate layer adds vibrating mass, lowers the tube resonant frequency, and the meter reads density high. Solids concentration drifts upward over time. Check the tube frequency on clean water against the commissioning baseline to confirm, then flush.
What happens if 2 mm grit runs through a bent-tube Coriolis meter?
Grit erodes the bends and settles at low points, so density reads low and wall thickness drops. Use a straight full-bore tube and keep velocity within the supplier's abrasion limit, or switch to a microwave spool with no vibrating element.
What happens if air is entrained in the milk of lime?
On Coriolis, air damps the tube, raises drive gain and makes density noisy. On microwave, air displaces water and reads as higher solids. Mount the sensor in a vertical upflow run downstream of the pump to minimize both effects.
How do I convert lime slurry density to percent solids?
Use w = rho_s(rho − rho_w) / (rho(rho_s − rho_w)) with your lab-measured solids density. With rho_s = 2.24 and rho_w = 1.000 g/cm³, a density of 1.100 g/cm³ gives about 16.4 wt% solids.
What happens if the analyzer still disagrees with lab samples after flushing and recalibration?
Stop adjusting the calibration and send the paired lab and analyzer data, the diagnostic readings and the installation details to the instrument manufacturer's support. A persistent unexplained offset, a Coriolis reading that keeps drifting low, or suspected wall loss needs the manufacturer's evaluation before the device stays in service.