Keeping a level head
A Water Pumping Station serving a major population centre now maintains its water reserves to precision levels, following automation of its CC-Link based control system so that water inflow and outflow are accurately matched.
At the heart of the station are six variable speed pumps, two of 130kW, two at 250kW and two 630kW. Their job is to pump water from the nearby treatment works that gravitates into a contact tank and to extract water from a reservoir, ready to feed through on demand to the mainly residential area the station serves.
Without regulation there is a danger that the tank will run dry or, rather more likely, overflow and flood nearby roads. If the tank overflows, it would almost certainly be necessary to significantly reduce the flow through the treatment works – causing massive inconvenience, pollution and possible disruption to supply. The main goal therefore was to ensure that the site had a secure pumping capability at all times, day and night.
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. Thus 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 up of key components makes the likelihood of systems failure to 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 will be sent via the network to the individual pumps. If an operating pump fails the master PLC will decide which available pumps can be operated to maintain the selected operating regimes requirements.
The status of all the pumps, the flows and levels are displayed on the HMI. 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 Modbus II protocol. The interconnectivity of CC-Link automation products proves itself on applications such as this where there is already-installed equipment that would become redundant if interfacing was impossible.
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