The operation of high-quality field devices and process control systems can only be as effective as the signal they receive. However, the signal lines are exposed to numerous sources of interference. Galvanic isolation of the signal circuit between the field and process control system offers effective protection at a relatively low cost. It avoids the risks involved with a direct connection between field devices and the controller.
“Fish killed by malfunction in sewage treatment plants” — “Failure in biological treatment stage due to faulty controller” — “Untreated sewage flows into river after malfunction”— faults in the water/wastewater industry are expensive and dangerous. Along with uncontrollable influences such as discharges from third parties, the plant’s process controller is another potential source of faults, and the effects of malfunctions can be severe.
One such example is incorrect pH values: If these figures deviate too far from the setpoint value, this inevitably leads to a die-off of bacteria in the biological stage and therefore to a failure of this treatment stage. This means wastewater that is only mechanically treated flows into rivers. Another example is pump failure: If pumps shut down due to a malfunction, the wastewater can no longer be fed into sedimentation tanks. However, if new wastewater is continually being fed into the sewage treatment plant, external capacity must be arranged to compensate for the output of the failed pumps. This may have to be achieved by an expensive operation by the fire department.
What’s more, investigations show that, particularly in older sewage treatment plants, energy consumption and CO2 emissions could be reduced by up to 60 %. This would mean significant reductions in operating costs. Signal conditioners can play a role here because they can prevent malfunctions and process deviations that may occur due to distorted signals.
Disturbances Jeopardize Process Control
Faults have many causes. Damage to machinery cannot be avoided, nor can extraordinary discharges. These place a heavy burden on the biological stage in particular. In contrast, measuring and control circuits can be controlled effectively. To minimize the chance of problems occurring — including total power outages — as well as the enormous cost of repairs and replacements associated with them and the damage to an organization’s image, it is essential to keep at least the avoidable risks within limits.
The process control components are exposed to a number of influences. However, although sensors, actuators, and the process control system are usually well protected, this is often not the case with signal paths.
Water management plants are typically very large, which means that, due to their length, signal paths are particularly susceptible to interference caused by coupling. In addition, there are frequently a large number of powerful components in the vicinity of the signal lines, which can lead to potential shifts and therefore to compensating currents in the resulting ground loops. Other sources of interference include wireless transmitters, which induce interference currents in signal lines. If signals are transmitted directly between the field and the controller, this interference can lead to malfunctions and, in the worst case scenario, to a plant failure.
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