Thames Water pumps up investment

01Nov

Thames Water is investing millions of pounds to renew the water supply infrastructure around London. As part of this huge programme it is upgrading the controls at a series of pumping stations to Rockwell Automation’s Allen-Bradley CompactLogix controllers

Thames Water and its predecessors have been providing water to Londoners for over 400 years, and now part of the multi-national RWE utilities group Thames, is today the largest water company in the UK. To
guarantee safe, reliable water supplies for its seven million customers in London and the Thames Valley
region, it is investing £1 million a day in its UK infrastructure.

A key element of this renewal is the replacement of obsolete control systems at a London water ring main
shaft pumping station that supplies water to the network of mains running below the streets of South London.

One of Thames Water’s approved systems integrators, Innecto Projects Ltd of Farnham in Surrey, secured the contract to replace controls at the Streatham Shaft pumping station with a new system based on Allen-
Bradley CompactLogix controllers. Following the successful completion of this installation, similar systems
are to be installed at the shafts at Brixton and Battersea.

The Streatham station has two 395kW pumps, two 465kW pumps and one 680kW pump linked via a series of six electrically operated valves to the water main and the nearby Norwood reservoir. By running the appropriate number of pumps the station maintains pressure in the main in response to varying demand and diverts surplus water to storage at times of low consumption.

The station pumps and valves are remotely monitored and controlled around the clock from a remote
control centre using an existing SCADA system running on a rackbased CPU linked to the programmable controllers by interface modules. Local control of each pump is also provided by pushbuttons on the front of the control panels.

The existing controllers and I/O racks were 15 years old and support and spares were getting more difficult to obtain, so the decision was taken to replace the control hardware with state-of-the-art Allen-Bradley equipment.

“After reviewing the available options, Thames Water selected Rockwell Automation as one of its preferred controls suppliers, because it is able to provide a complete range of proven, reliable and well supported hardware,” says Trevor Stewart, a director of Innecto.

The five new controllers and associated Allen-Bradley 1769 I/O modules were easily installed in the existing control panels with only minor changes to the wiring. A feature of CompactLogix that makes it well suited to control system upgrades is its compact, chassisless design. This means modules are only added as needed and the panel space required is kept to a minimum. Despite its small size, the powerful processor is able to handle complex control tasks, making it ideal for small to medium-size standalone
applications requiring up to 1024 I/O or part of a network of programmable controllers.

ACompactLogix controller and five 1769 I/O modules provide all the processing power needed to control each pump set at the Streatham pumping station, while occupying a fraction of the space taken by the legacy controllers they replaced.

Innecto completely rewrote the existing programming code for the new controllers using Rockwell Software’s RSLogix 5000 editor. To simplify maintenance and reconfiguration, the code was split into
sections, with separate routines for alarms and the SCADA interface.

When programming with RSLogix 5000, all tag names can be user-defined, rather than pre-determined
by the hardware. This removes any maximum limit on the number of addresses available and enables the
system developer to produce programs and addresses with logical names that will be easily understood by another engineer.

The CompactLogix tag database also allows the use of aliases to simplify the definition of tag names for I/O locations. As a result, the PLC code can be written before the physical wiring locations of the controller or the I/O are known, as it is quick and easy to add to an existing tag an alias that defines its connection later.

When it comes to reusing code at another pumping station, Innecto will also be able to simply re-alias the
existing tags to a new I/O configuration, reducing the time taken to generate the control program for subsequent upgrades.

Aliases are also useful for giving I/O commands descriptive names such as “pump_on”, which are more
meaningful than pre-defined addresses, and make it simple to match tags in the software to physical
tags on the control wiring.

Each 400V three-phase pump is brought on-line by a soft starter that gradually increases speed to a pre-set
operating level, the starters receiving start or stop signals from the hardwired 24V dc digital I/O modules.
Each of the four smaller pumps is linked to its controller by three 16-point input modules and four 8-point output modules, while five 16-point input modules and six 8-point output modules are used for the largest pump. The electrically operated valves are signalled directly from the PLC outputs to open or close and supply status feedback of Open, Closed or Available.

CompactLogix offers a wide range of I/O options, and is able to “mix and match” up to 30 high density digital,
analogue or speciality modules without running out of backplane capacity. Modules can be mounted in up to
three banks vertically or horizontally, providing maximum flexibility in control panel layout.

While replacing the I/O modules, Innecto took the opportunity to switch from ‘source’ to ‘sink’ input voltages. With a sinking input circuit (negative common), the input module is at zero volts while the field device - in this case the motor starter contactor - is at +24V dc. This means there are zero volts across the input module when the contacts are open. While the Allen-Bradley 1769-IQ16 I/O modules enable the use of both source and sink, Stewart explains that Thames Water’s preferred option is now to use sinking inputs.

“Sink modules are now more widely available and are being adopted in most industries,” says Stewart. “The electrical circuit is also more easily understood in terms of electrical flow.”

The combination of the CompactLogix controller, 1769 I/O modules and RSLogix 5000 programming software provided the ideal solution for Thames Water’s pumping stations.

“The small size of the controller and wide range of onboard technical options - including the high density of
I/O and the wide range of I/O modules available - were major factors in the selection of CompactLogix,” says Stewart. “In addition, the tag aliasing that enables us to reuse code makes future upgrades more cost
effective for Thames Water.”