Preventive and Predictive Oxygen Probe Maintenance

Note that, if not burned off, soot will gather around the probe tip and eventually cause sensor failure.

This morning, we visited a commercial heat treatment supplier to follow up on a staff training session we performed around 1 year ago. The Maintenance Manager asked us if we had noticed that their probe consumption had dropped since the training. He is responsible for 6 Ipsen sealed quench furnaces with SSi Redundant Probe Systems fitted, i.e. 12 oxygen probes in total on site. The furnaces tend to run at pretty high carbon potentials (near to the soot limit), so probes were regularly filling with soot and being returned to us for cleaning, testing and re-certification.  Dani, in the workshop, had been advising during his test reports that the burn-off did not seem to be effective, so during the training we advised that burn-off air pumps were inspected. Some of them were replaced in order to get higher flow during burn-off.

We also advised that regular burn-off checks are performed. During the burn-off, it is important to measure the response of the zirconia element and the probe’s temperature rise or fall. Air should pass the tip in significant volumes (typically between 3-6 l/min), so the mV should drop to around 200 mV (remember, as air/oxygen % goes up, mV goes down). If this is not happening, there will not be enough oxygen present to burn-off the soot. The temperature of the tip (only measurable from an internal TC) is a good indication that burn-off is occurring, and that the flame tip is beyond the probe sheath. You will see a rise of between 5 and 25 deg C if soot is being burned correctly. If the temperature rise is higher than this, the flame tip is too close to zirconia element. Over time this will affect the probe readings and in the worst case scenario, cause a material failure, so burn-off airflow should be adjusted.

Extreme example of damage to the tip of an oxygen probe sheath due to overheating during burn-off.

By training operators, replacing a few burn-off pumps, and regularly checking the burn-off performance, the heat treater has significantly reduced the furnace running costs (by thousands of £s), but more importantly, reduced reworks and unplanned furnace downtime.

Those are a couple of simple items. By combining those with quarterly impedance and recovery time tests (which are performed automatically in SSi controllers but can also be performed manually), we can start to analyse the level of deterioration/degradation of the sensor. Those values (see image below) can be recorded and charted in a SCADA system. When the trend begins to accelerate, or the values exceeds pre-defined tolerances, we know that the probe is beginning to deteriorate so know when to return it for inspection.

Probe data, recorded and charted in a SCADA system as part of predictive maintenance plan.

Many furnaces have inadequate reference and burn-off air supplies, which may be reducing the life of the oxygen probes. It is definitely worth checking them to ensure that they are not the cause of furnace problems or quality issues. Let us know if we can help with training or advice!