Frisian drinking-water filtered with PROFIBUS (Netherlands)

Every year Nuon Water pumps more than 10 million cubic metres of water from the ground in Noordbergum, Friesland, the Netherlands. However this water needs to undergo a series of treatments before it can enter the mains network in the form of drinking-water. Within the scope of a large-scale automation operation this pumping station opted for the incorporation of the entire process from catchment to distribution in a PROFIBUS network.
Drinking-water is produced using a process that involves a series of unit operations. In essence the water is pumped out of the ground, aerated, filtered, softened, re-filtered, and finally brought to pressure for supply to the mains network. The various filtration stages, in particular, are readily observable operations at the pumping station in Noordbergum.

Nuon Water is in the possession of an extensive network of pipes for the filtration processes. This network also includes a number of pumps linked to frequency converters, aerators or blowers and valve-positioners ? and, of course, the instruments required to measure flow rates, levels, and pressures. In the new design for the process control Nuon opted for a plant network comprised, for the time being, of two Nyquist PLCs, which serve as the masters for instruments, including two remote I/Os for the motor-control centre, six Vacon frequency converters, two ABB flow meters, two Festo valve islands and, in conclusion, 28 KS98 controllers for the filter lagoons. Each KS98 controller is also equipped with a CAN bus connection to provide for operation in the field. An ordinary RS 232 links the PCs to the PLCs, which are in turn linked to the field by the well-known purple PROFIBUS cable. A PROFIBUS fibre-optics cable in turn links the PLCs to each other via six Hirschmann media converters. The link with the next hierarchical level is guaranteed by the ethernet fibre-optics cable. This fibre-optics cable also ensures for a later opportunity for vertical integration using the Linux platform. In the longer term Nuon intends to make use of an ERP package for an evaluation of the process and machinery data in an endeavour to achieve a more efficient deployment of the production equipment.

The knowledge Nuon acquired during a number of training courses at the PROFIBUS Center in The Hague contributed to the firm's ability to make full use of the benefits offered by the design. Nuon particularly appreciates the increased flexibility of the control system and the accuracy of the process control. It has now become possible to monitor a large number of process parameters on a permanent basis. However it should be realized that an additional benefit is the saving in cabling ? a benefit which is visibly manifested in the form of a mountain of cabling and scrapped switching cabinets on the site (see figure). An additional benefit is provided by the integration of the network in Nuon's WAN, as a result of which operators and maintenance staff can now examine the system in the evening or the weekend from their homes ? and assess whether a given alarm warrants the implementation of corrective measures. This additional facility makes a direct contribution to further cost savings.