Furnace glass level control is, like many other furnace process controls, a delicate and important task of a process control system. There are several methods of measuring glass level like glass contact probes, probes you detect the ionized air layer just above the glass bath, probes using isotopes and vision systems. All faces the same problems; they need to be very precise and they will have to operate, over a very long time, in very harsh environments.
Once we have a stable and precise measurement the furnace process system has to take it from there. One of the problems such a control has to face is the reversals taking place in regenerative end-port or side-port furnaces. Switching off and on the combustion will almost unavoidable lead to furnace pressure fluctuation which have their effects on the glass level. The glass level control should not be affected by level fluctuations which are coursed by abnormal furnace pressure changes or reversals. Special reversal strategies will have to take care for that.
Especially in end-port furnaces and large side-port furnaces we often find multiple batch chargers who have to be controlled by the glass level control strategy. Some customers require specific batch charging strategy in which they would like to run more batch to the furnace fire side synchronized with the reversals. Since batch charger’s charge volumes are difficult to measure such strategies could become very complex, taking silo weight measurements into account.
Also special strategies are required in case of a batch charger failures in which unfortunately the glass level could have dropped potentially. To get the furnace back to its normal glass level it would be necessary to increase the batch charge speed and charge tonnage. However, the glass level control needs to take the melting capacity of the furnace into account and should not try to fill up the furnace with batch as quickly as possible which will, eventually result in unwanted glass quality problems.
Taking all these constrains and consideration into account it becomes obvious that glass level control can be a complex issue and in most cases cannot be achieved by a “standard” controller. Foxboro Programmable Automation Controller (PAC) is perfectly capable of running these complex strategies in a redundant environment, it is easy, on the fly adaptable as soon as a change of strategy is required and it can be seamlessly integrated into the furnace control system.