Fieldbus based system answers real-time demands

After looking at many network solutions, automation system supplier Pro/Automatic proposed the latest in fieldbus technologies for a new PLC-controlled water network in Denmark.

Since 1979 the Klinting Vandvaerk water company in Denmark has been a supplier of the highest quality drinking water for the western region of West Jutland.

The normal population of this picturesque part of Denmark is 3000 households, but as tourism has increased in the area, and supply was extended to service the local community of Lunde and Nr Nebel (1500 new users, giving altogether 4500 users), the water station has had to increase production drastically to keep pace with new current and future demands.

The challenge set to the engineers involved with the project was not only to increase the output capacity, but also how to increase the number of boreholes which provide the water, and to increase the whole system including the reservoir capacity (from 700 to 2200m3) without effecting supply to existing customers.

The existing water system consisted of five remote boreholes, a 700m3 reservoir, green sand filter system and an outlet pipeline.

The boreholes are located 1.2km from the main pumping station (in the middle of a local forest) with each borehole able to extract up to 30m3 of water per hour, with the boreholes going down to a depth of 100m.

Controlling this was a system of differential pump controllers and relays which were used to switch the appropriate pumps and valves.

It was totally electromechanical control, and had run very reliably for 25 years, but was no longer able to give the interactive control and monitoring capability now required by the modernised water station.

Another issue was that the old system did not allow for remote maintenance, so whenever a problem arose the operators had to travel to site to correct minor faults or reset/switchover duty pumps.

The first part of the project tackled was the reservoir, and a new 2200m3 capacity tank was installed at the station site.

The next step was to increase the number of boreholes from five to 11.

After the mechanical elements were installed the engineers involved set about the task of updating and connecting the new system together.

Henne-Outrup EL Service was the chosen main contractor for the control system, and Leif Kirchheiner of Henne-Outrup EL Service in conjunction with Jens Kruse Madsen of Klinting Vandvaerk put the overall control system requirements together.

When the overall concept was decided, specialist automation system supplier Pro/Automatic was called in to manufacture the control system panels and program the newly chosen PLC controlled solution.

The first problem to overcome was how to connect the remote boreholes pumps (up to 1.2km away) back to the water station's new PLC controlled system.

After looking at many network solutions specialist automation equipment supplier, Louis Poulsen proposed the latest in fieldbus technologies, CC-Link.

CC-Link can use standard cabling up to 1200m, but on this system the area is very flat and lightning can cause transients in communications, and so a fibre-optic system with repeaters was chosen to avoid any possible network errors.

The master of the CC-Link network is a Mitsubishi Electric Q series which resides at the main water station.

This connects via the fibre-optic cable to remote Wago CC-Link I/O modules which are connected directly to the borehole pump controllers.

On each of the remote Wago CC-Link I/O is a 4-20mA analogue input connected to every borehole, which is used to measure borehole pressure using pressure sensors (at the level of the ground-water).

This allows the system to detect level changes of the ground-water when a borehole is pumping.

Every month each borehole is tested.

This is done only at night time by pumping from a borehole for exactly half an hour.

The ground-water level sinks and the amount of flow from the pump is logged.

This gives the possibility to view the conditions of both the borehole and the pump.

Each borehole normally runs for a 2h duty cycle then another takes over.

Up to four pumps work at any time during normal operation, but up to six boreholes are used in peak water demand times.

CC-Link is also used to connect the 10 newly installed variable speed drives (VSDs) which are used throughout the whole water control process in the main pumping/filtration station back to the central control PLC.

On the VSDs CC-Link is used to communicate to the central PLC alarms, frequencies, allows the setting of speeds, and even permits changing the VSDs status from manual mode and switching them automatically into network mode and visa versa.

All the VSDs still need to have manual control as a backup in case anything happens to the main controller, enabling the plant to be operated on a semimanual basis, ensuring continuity of supply even in the event of any major control problems.

The use of VSDs has the additional effect of saving large amounts of energy over the old direct online (DOL) pump starting system.

They do this by optimising energy usage by reducing the frequency to match instantaneous changes in pumping pressures.

Using VSDs also has the advantage of extending considerably the pump's life expectancy, and are under the real-time control of the central PLC via the CC-Link's 10Mbaud network.

CC-Link was chosen as the site fieldbus standard for several reasons.

The main reason is that it was very easy to set up and install and extremely noise resistant and robust.

Another reason is its unique I/O disconnect mode allows stations to be removed from the network without affecting network performance or data loss.

Jean Petersen of Pro/Automatic says of using CC-Link: "It was wonderful to use such a simplistic but powerful fieldbus solution".

"It was so easy to create the network systems and it has some very powerful unique features such as the station removal".

"Another aspect we liked was that there are a growing number of automation manufacturers that now offer CC-Link connectivity and we know that by installing this network solution we are using cutting edge fieldbus technology that is supported worldwide".

The advantages of the new system are huge.

Now the entire site can be controlled remotely.

The new system has improved the safe operation of the overall system, with the new system having duplex backup control of all major water pumping elements.

The system is also designed to be very flexible and can be expanded without any fuss, and allows for any changes in legislation and needs that may arise from local authorities without having to renew the control system.

Jens Kruse Madsen of Klinting Vandvaerk is rightly proud of his new water station which is extremely clean and very well managed.

"Water is food", he says and insists that only the highest standards of cleanliness are used in the station, and that the control equipment is installed to the same levels as any food factory.

He continues: "We started the project in February 2003 and everything was completed without any hitches by the official opening on the 1st October 2003, with all work completed and without any noticeable water supply stoppages to the served communities".

The new water control system is so flexible it can now easily accommodate the variable supply requirement.

Now average water consumption ranges from 1500 to 2000m3 per day in winter and soars up to 4000-5000m3 in the summer (due to tourism), with the CC-Link controlled system easily able to respond to real-time demands.

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