Networked plant sets standard

On the Central Coast of NSW, a 60ML per day state-of-the-art water treat ment plant has been constructed for MidCoast Water to serve major populations in the greater Taree and Great Lakes areas.

Raw water enters the plant from the Bootawa Dam or is pumped direct from the Manning River.

In the flash mixing tank the water is dosed with aluminium chlorohydrate to facilitate coagulation of suspended matter before proceeding to the flocculation tank to allow time for the suspended particles to form flocs, which are then filtered by four micro filtration cells.

The filtered water is then pumped to the clear water tank and then to an ozone contact tank to break down odour causing compounds in the water. Ozonised water then flows to biologi cally activated carbon filters to remove organics, and then to a chlorine contact tank to disinfect the water.

The chlorinated water passes over a weir at the outlet of the tank where it is dosed with soda ash to correct the pH and also dosed with fluoride.

Following this process, the water flows via an underground pipeline to an 8.5ML treated water reservoir and is then pumped to the distribution network utilising the newly built pump station next to the treatment plant.

The water treatment plant starts and stops based on the level in the treated water reservoir, but there is also a capa bility to adjust the plant production flow set-point to give greater continuity of operation.

Process design
UGL Infrastructure’s senior control and instrumentation engineer, Arjan Koolhaas, says the company was respon sible for process design of the plant, development of P&IDs, mechanical and electrical design and installation, control system design and installation, control philosophy design, software program ming and testing (FAT and SAT), and commissioning of the plant.

“The entire water treatment plant is automatically controlled using a redun dant ControlLogix PLC, and the Citect SCADA system provides an operator interface, data recording (trends/histor ical data) and reporting.

The MCW communications infra structure allows remote access to the control system (SCADA and PLC) via Internet,” Koolhaas explained.

“The PLC is configured as a redun dant main rack, with CPU, Ethernet, ControlNet and Redundancy Modules, and two remote racks with ControlNet, DeviceNet, Profibus and Ethernet Modules, while the control system consists of a fibre optic ring network (multimode 62.5um) around the plant with communication breakout boxes (CBBs) in strategic locations.”

Protocols
Koolhaas says that in relation to network protocols used, each CBB contains an Ethernet switch and is configured to support three Virtual LANs as follows:

• VLAN 1: Data transfer between the PLC and the SCADA;

• VLAN 2: Data transfer between AS-i master modules and the PLC;

• VLAN 3: A general purpose Ethernet network which could be used to connect to the office network from different locations on the plant.

“Most of the analogue instruments and all control valves are connected to Profibus. Where an instrument would not be available for Profibus (DP or PA), 4-20 mA AS-i units have been used. There are three Profibus networks on site and where required, a communica tion breakout box has a fibre optic to copper converter.

The Profibus masters reside in the extension racks of the PLC, and where necessary, Profibus DP/PA converters have been installed,” he said.

“All digital signals from level switches, flow switches, and process valves etc are connected to several AS-i networks.

“The ControlNet network is used for communications between the main PLC rack and the remote racks (two off) between the main PLC rack and remote ControlNet to DeviceNet converters, and between the main PLC and the PLC in pump station PS2B.

“Several DeviceNet networks are installed around the plant, mainly for communication between the main PLC and smart motor starters (for DOL motors) in each MCC room, VSDs in each MCC room, local MCCs for fan control, and IO over DeviceNet.”

Benefits of AS-interface
The AS-interface for the water treatment plant, provided by ifm efector, is an open standard patented protocol design optimised for digital signals.

According to the company’s network and controls manager, Dan Buzatu, a key advantage with AS-i is that you are not restricted to using one product/brand — you can ‘mix and match’.

“With traditional wiring methods (parallel wiring), there are many connec tions, junction boxes, and input cards, which means long installation time, complex fault finding, lots of documen tation and longer time to commission. AS-i enables fewer connections, there are no input/output cards, installation is quicker, diagnostics are clearer, plan ning is easy, there is less chance of errors, and commissioning time is shorter,” Buzatu said.

“Installation and wiring of AS-i is where the main cost savings occur due primarily to fewer connections required and less cabling.

“Another advantage of the AS-i system provided by ifm is that local diagnosis in the field is enabled using our AS-i nodes, rather than just in the central cabinet.

“With AS-i there are no limits to the structure, which is a benefit specific to AS-i compared to other field bus systems.”

Buzatu points out that the clients’ preference was to use Allen Bradley PLCs so that they didn’t have to learn a new program, and that due to its compatibility, AS-i was a good solution, with use of the Ethernet IP gateway.

“Ethernet enabled AS-i to be run over long distances to isolate parts of the plant by creating ‘islands’, so that the plant has a series of separate but linked systems.

“Unscreened two-wire cable has been utilised for data and entry, sensors and actuators connected to AS-i nodes do not have to be any specialised instru ment, and no specialist software or hard ware was required.”

Ongoing support
Koolhaas emphasises the strong support provided by ifm efector throughout the project, from pre-sales to after-sales.

In particular, during the development of the PLC configuration, UGL found that the AS-i modules and the Ethernet card in the ControLogix system were not communicating.

“In a non-redundant PLC configura tion, the AS-i modules are communi cating through the Ethernet processor in the primary PLC rack.

“Because of the redundancy, this method could not be used and commu nications to the AS-i modules had to be configured for CIP messaging using a separate Ethernet processor in one of the remote PLC racks,” he said.

“Even in this mode, the modules wouldn’t communicate. ifm provided technical support to UGL, and after going through several test configurations with UGL with no result, ifm took the matter to its head office in Germany.

“The ifm engineers analysed the configuration and discovered that the AS-i modules are configured for sending/receiving 512 bytes in every communications cycle.

“The Rockwell Ethernet processors are configured for a maximum of 450 bytes, so ifm provided a new version of the system software which utilises the AS-i modules for sending/receiving 450 bytes instead of the original 512 bytes.

“After downloading the software to the AS-i modules, the communication between PLC and AS-i modules worked perfectly.”

Project manager at MidCoast Water, Matthew Koller, notes that the new plant is delivering water of world class standard and that customer feedback has been very positive.

“The approach taken to this project has provided a simplified solution to complex requirements, and delivered time and money savings. A water treat ment plant has been produced that could be a model for other water author ities,” he said.