Düsseldorf, Nordrhein-Westfalia's state capital, has two water treatment plants, taking in waste water from both the domestic and business sector, as well as precipitation. During periods of heavy rainfall, increased volumes of water in the drains are collected in underground reservoirs. The excess water is stored and not let into the water treatment plants until later. Hell Industrie-Elektrik KG, Krefeld, designed, planned and constructed 100 entirely computer-aided state-of-the-art pumping stations for the Düsseldorf drains system, with products from PHOENIX CONTACT.

To prevent the entire drains network and the associated water treatment plant from becoming overloaded or even flooded after heavy rainfall, structural relief was included in the planning in the form of a rainwater collecting tank. As soon as the drains system is able to accept more water, these tanks must be emptied rapidly in order to be armed for the next rainfall. In many cases, the discharge sewers from the rainwater collecting tanks to the water treatment plant cross streets or stretches of water. Such topographical conditions mean that the sewers have to be laid deeper, resulting in the ideal gradient to the water treatment plant being lost. The pumps must therefore be particularly efficient for discharging the water into the water treatment plant inlet, the so-called main sewer. In the main pumping station in the Mecumstraße, there are therefore three pumps with a capacity of 75 KW each, and one residue discharge pump with a capacity of 15 KW. This high capacity is necessary to guarantee the inlet volume of 8,000 liters per second to the Kläranlage Süd, the downstream water treatment plant in the south.

There are numerous I/O nodes in each pumping station. Level, through-flow and pressure, for example, are measured in each sewer. The conversion of these process variables is to analog standard signals 0..20 mA. Since space-saving in the switch cabinet is a major criterion when making the choice, Hell opted for the slim 12.5 mm MCR-C-I/I-00-DC 3-way isolating amplifier from PHOENIX CONTACT for electrical isolation, filtering and amplification of the analog signals before the controller. Each of these standard signal circuits is connected with UFBK-M-24AC-ST pluggable surge voltage protection to protect against surge voltages. Not only the slim design, but also the fact that the modular arrester can be removed without interruption and without influencing the impedance, were decisive criteria.

Not only the process variables, but also the pump functions, such as "Manual/Automatic changeover" or "Pumps on/ off" must be controlled, making it necessary to include electrical isolation and level and capacity adaption. This is where the 6.2 mm slim, pluggable PLC-RSC relay interfaces from PHOENIX CONTACT with their modular structure come into play. They can be used in all N/C, N/O or PDT applications.

A total of 100 pumping stations were erected in Düsseldorf, the capacity of the pumps depending on the location. 15 of these stations are main pumping stations that are connected directly to the process control system of the Kläranlage Süd water treatment plant in the south via the telephone network, and to a certain extent, via leased line. In the main pumping station in the Mecumstrasse, reliable transmission of signals is guaranteed by 32 MCR isolating amplifiers and 32 Plugtrab surge voltage protection modules; 128 PLC relays perform all the pump control tasks.

Due to the smaller number of I/O nodes, the remaining 85 pumping stations are smaller and are to be found to a certain extent directly below the streets. A modem connects them to the process control system.

Modularity, reliable transmission characteristics and the minimum width were decisive factors for using Interface and surge voltage protection modules from PHOENIX CONTACT in this sensitive environment.