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 such as glass contact probes, probes that detect the ionized air layer just above the glass bath, probes using isotopes and vision systems. All face 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 stable and precise measurements are established the furnace process system is of importance. One of the problems related to control is the reversals taking place in regenerative end-port or side-port furnaces. Switching off and on the combustion will likely lead to furnace pressure fluctuation which affects the glass level. The glass level control should not be affected by level fluctuations which are caused by abnormal furnace pressure changes or reversals, which will be managed by special reversal strategies.
In end-port furnaces and large side-port furnaces we often find multiple batch chargers, which have to be controlled by the glass level control strategy. Some customers require a specific batch charging strategy in which they would run more batches to the furnace fire side, synchronized with the reversals. Because batch chargers charge volumes are difficult to measure, such strategies could become very complex, taking silo weight measurements into account.
Special strategies are required in case of a batch charger failure in which the glass level could have dropped. 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 constraints and considerations 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. Eurotherm Programmable Automation Controllers (PAC) are perfectly capable of running these complex strategies in a redundant environment. It is easy to make on-the-fly alterations as soon as a change of strategy is required, and it can be seamlessly integrated into the furnace control system.