Using an Hitachi micro PLC from HID Limited enabled controls consultants Trax Circuits to solve a pumping station monitoring problem for Anglian Water Services in Beccles, Suffolk.

A problem encountered by just about all the water companies is the cycling of pumps at the remote pumping stations used to move waste water and sewage through the system. At the Beccles pumping station there are four pumps, of which one or two are used at any time. According to demand, first one pump switches on then a second activates if demand increases. The remaining two pumps are standby units. All the pumps are driven by standard AC induction motors. The problem is how to utilise all the pumps in rotation so as to ensure no pump has a higher duty cycle than any other.

Traditionally, an engineer from the water company has to visit the site to switch pumps over and to check for any faults that may have occurred. This is time consuming and a strain on the limited resources of most water companies. Trax' engineers developed a simple PLC controlled panel which fits within the existing control panel of the Beccles pumping station.

The levels which dictate the pump duties are measured using level sensing transducers. There was also a question about what happens if one sensor was to fail. By incorporating a Dual Level Controller MC10053 - also available from HID Limited - within the new PLC panel, Trax was also able to accommodate a level detection failsafe.

The PLC deployed is an Hitachi H28DRP with 28 I/O. A program has been written by Trax to enable pumps one and two to be used when twin pumps are required, but, the next time dual pumps are used, pumps three and four switch in. Hence, pump life is regulated across all four pumps.

The program was written using Hitachi's Actwin PLC programming software - an IEC1131/3 compatible program that enables ladder, SGC, and instruction set languages such as Grafcet to be used.

To enable maintenance work and allow for manual override, Trax has incorporated a simple switch to allow pumps to be manually shut down. The system still operates the remaining pumps in rotation while replacement or repair to the shut down pump takes place.

The panel developed by Trax is low cost and involved no major replacement of existing control hardware. There was little downtime in the installation and the result has been more efficient pump rotation without the need for manual intervention. It is anticipated that other water companies will express interest in the development.

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