PLCs are popular in plant and machine automation because of their flexibility and reliability. In normal operation, PLCs are unlikely to fail, and it is possible to run a backup PLC if required. But how far do you go in backing up a reliable PLC control system?

The engineer at one Severn Trent wastewater treatment realised he could use the ABB VSD to act as its own PLC backup. The new control system takes over if the primary PLC-based wash water control system fails.

The wash water is a critical part of the wastewater treatment process at Severn Trent’s Stoke Bardolph to clean the screens at the inlet works. If no wash water is available, the screens become blocked and the water level in the inlet channel rises. If this happens, the inlet screw pumps turn off to prevent the screen house from flooding.

Should the PLC fail, there is no wash water to clean the inlet screens and the treatment process stops. With sewage arriving at 4000 litres per second, the emergency storage capacity only lasts about eight hours. Zac Dorn, electrical and instrumentation maintenance technician for the site says: “The last failure saw the sewage tank filling to within a couple of inches of its capacity. An overflow of sewage into residents’ homes would be very undesirable and expensive.”

Dorn looked for a simple alternative that avoided the cost of installing and programming a complete standby PLC backup system. The emergency system only needed to handle the basic wash water control system. Control of the wash water filter’s backwash system and duty rotations could be wait for restoration of PLC control.

PLC backup

Dorn designed a system that would act as a back up to the main PLC while it was out of service. It uses an ABB machinery drive, ACS355, acting as a PID controller. The drive uses a watchdog relay to check pulses from an output on the PLC. If pulses are not detected because of a PLC failure, the drive takes over.

The drive controls two wash water feed pumps that pump water into a buffer tank. It monitors the level of water in the buffer tank and starts the pumps when the water level becomes too low. At the same time, it opens an inlet valve into the tank from the pump lines.

Four booster pumps maintain the pressure of the water leaving the buffer tank at five bar. This is important as the buffer tank is 500m from the screen house and the booster pumps ensure that the water is at the correct pressure when it arrives. Four ACS550 drives control the speed of the four booster pumps using a speed reference signal from the ACS355.