Self Cleaning, Recovery Time, and Sooting Alarm
May 29,
1996
OXYGEN PROBE SELF CLEANING
The airways in oxygen probes are often prone to sooting due to the dirt and fumes present in the furnace atmosphere. The resulting build-up of deposits on the surface of the zirconia cell can cause the probe to give an inaccurate reading. To overcome this, the probes are cleaned by forcing compressed air through them at regular intervals, which burns off the soot deposits. Typically, this is carried out every four to eight hours during a firing cycle. The 900EPC's probe cleaning function performs this automatically using a logic or relay output configured as CLEAN VALVE.
| The probe cleaning setup display, which is found in Level 2 or Level 3, is shown in figure 1. To operate the cleaning function, AUTOMATIC must be set to ON. The CLEAN VALVE output will then be activated for TIME seconds every FREQ hours. Cleaning can also be activated by a digital input configured to CLEAN PROBE. After a manual or automatic cleaning, the automatic cleaning timer will be reset to zero and the instrument will wait for another FREQ hours before cleaning again. | |
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RECOVERY TIME ALARM While the oxygen probe is being cleaned it is unable to function because no furnace gas will reach the Zirconia cell. The controller therefore freezes the atmosphere PV during the cleaning cycle and will only resume calculating the PV when the probe output returns to 95% of its previous value. A recovery alarm can be configured as PROBE HEALTH, which compares the time taken for the recovery with a user definable value. This is illustrated in figure 2. If the recovery time exceeds the alarm time, the alarm output will be activated. |
SOOTING ALARM
Carburizing furnaces are particularly prone to sooting because if the atmosphere becomes saturated, excess carbon is deposited throughout the furnace. This can damage the furnace and the load being fired, so an alarm on the instrument can be configured to SOOTING on PV1 to detect carbon saturation. This feature is available on all DRAYTON, ACCUCARB and DEW POINT instruments.
The calculation which is used to detect sooting has been defined by oxygen probe manufacturers. It compares the carbon potential calculated using the CO gas correction value and the probe mV and temperature signals with a deviation calculation based only on the probe mV signal. If the calculated carbon potential falls outside the allowable band it indicates that the probe has deteriorated outside its tolerance limits, so the sooting alarm will be activated.