CC-Link helps to ensure reliability of pumping station

A water pumping station serving a major population centre now maintains its water reserves to precision levels so that water inflow and outflow are accurately matched. At the heart of the station are six variable-speed pumps, two rated at 130kW, two at 250kW and two at 630kW. These pump water from the nearby treatment works that has gravitated into a contact tank and extract water from a reservoir, ready to feed through on demand to the mainly residential area served by the station.

Without regulation there is a risk that the tank will run dry or, more likely, overflow and flood nearby roads. If the tank were to overflow, it would almost certainly be necessary to reduce significantly the flow through the treatment works - thereby causing inconvenience, pollution and possible disruption to supply. The main goal of the automation project, therefore, was to ensure that the site had a secure pumping capability at all times, day and night.

Hot-standby architecture

To ensure availability of control it was decided to use a dual-redundant, hot-standby architecture for both the master PLC and the associated communication networks to the pump PLCs. If the active master PLC processor fails for any reason, the standby PLC processor takes control immediately with a 'bumpless' changeover.

The master PLC includes dual processors, two power supplies and two CC-Link open communications protocol cards, with each pair being configured in a hot-standby architecture. This doubling of key components makes the likelihood of systems failure virtually nil.

Each of the six pumps has a local PLC, fitted with two CC Link cards, which receives commands from the master PLC and transmits pump status information.

The operating regime is selected from an HMI communicating with the master PLC; from this the combination of pumps and their required operating speed are sent via the network to the individual pumps. If an operating pump fails, the master PLC decides which available pumps can be operated to maintain the selected operating regime requirements.

Displayed on the HMI are the status of all the pumps, the flows and levels. Also the flows and levels are trended to enable managers to monitor the history of the plant over the previous twelve hours.

The whole network also links to an existing plant-wide DCS (distributed control system) running the Modbus II protocol. The interconnectivity of CC-Link automation products proves itself on applications such as this where there is existing equipment that would become redundant if interfacing was impossible.