Wessex Water undertakes control system upgrade

31 May 2011

Wessex Water has undertaken a major process control system upgrade to deliver increased reliability and improved operational efficiencies.

Upgrading a process control system without disrupting operations presents a challenge in terms of logistics, planning and execution. For an essential public service such as a sewage treatment plant this leaves is no room for error. With the help of Siemens Industry Automation the in-house engineering team at Wessex Water has succeeded in upgrading a legacy control system at a major sewage works in Poole on time and to budget, with no interruption during the upgrade to the essential service the site performs.

Wessex Engineering and Construction Services (WECS) Automaton Team is an in-house group that provides the majority of control and monitoring systems for Wessex Water, including detailed design, PLC software, SCADA / HMI configuration, telemetry and commissioning.

In 2009 Wessex Water entered into a framework agreement with Siemens Industry Automation for the provision of control and monitoring system components, including PLCs, HMIs, SCADA and other associated networking and communication equipment.

The equipment that provided the control and monitoring functions for the Biological Aerated Filtration plant (BAF), digestion and inlet works at Poole STW had been in use for over 20 years and had started to suffer from reliability issues. It was also becoming difficult to support in terms of software, maintenance and component supply. With a need to continually strive for process optimisation, deliver operational efficiency savings and ensure compliance with all legislative requirements, a replacement control and monitoring system would need to provide increased security, high functionality, high reliability and allow the easy identification and implementation of process improvements.

The replacement control and monitoring system comprised redundant WinCC SCADA servers, industrial panel mount PCs as SCADA clients, redundant S7 400H PLCs, and an S7 319 PLC. Full use was made of redundant Profibus remote I/O. The fibre network infrastructure was enhanced to form a fault tolerant redundant ring. IT security was provided by the use of independent networks and firewall technology.

As well as replacing the existing control and monitoring system, it was decided to investigate the additional functionality of the equipment to provide a wide range of benefits. In particular, secure remote access to the SCADA system and the PLCs directly, better accountability through a system audit trail to show what was modified, when and by whom; automatic and regular backups to the system configuration and software, and the ability to automatically export data from the SCADA database to a remote corporate system.

Before the works could commence, the WECS Automation Team, led by David Mining spent a number of months addressing the development of standards for equipment, software and graphical user interfaces to be used with Siemens equipment, to support this project and other concurrent and future upgrades across the Wessex Water estate.

As an essential works, the Poole sewage treatment processes had to be kept running at all times, as the major upgrade to the control system took place. To enable this, the in-house engineering team with support from Siemens and Operations Technicians devised an implementation strategy to minimise any disruption. The upgrade was staged to avoid having more than one area of the plant disturbed at any one time. Temporary control systems and partial manual operation of the plant were also employed

Bringing in the expertise of General Panel Systems - Bristol-based specialist panel builders - to work alongside the Wessex Water team and Siemens, the existing control panels were re-worked and extended to make room for the replacement equipment. Many hundreds of cables were disconnected from one control system and re-terminated in another, presenting a significant logistic challenge.

The existing documentation for the plant was not sufficient to allow the detailed development of the new control and monitoring system, hence much of the software was produced by careful reverse engineering of the existing software. The site SCADA system was completely replaced with a system that now contains nearly 30,000 tags, over 4,500 alarms and over 300 mimics.

A Siemens S7 400H PLC system was utilised to increase reliability and to reduce risk. To minimise process disruption, the S7 400H system was developed as two independent S7 400 PLCs. Once completed, the two processors were brought together as a true redundant pair, so that each now offers full support for the other.

Software was developed to interface the new control and monitoring system with the standard Seprol telemetry outstation, as well as the PAKSCAN Rotork actuator network controller: approximately 200 valve actuators are controlled and monitored.

Each of the plant areas controlled and monitored by the new system were completely re-commissioned, including the testing of all SCADA and telemetry signals.

The development and installation of the new process and control system at Poole was delivered within the expected timeframes and on budget. The company now has a fully supportable control system that is driving efficiencies through its operation. Wessex has already implemented an improved inlet flow control regime at Poole, and is currently using the new control and monitoring system to analyse energy consumption for each process area with a view to carrying out further optimisation work.

The new operator ‘overview’ for the entire system ensures a more holistic visualisation for the entire plant, and quick and easy monitoring of essential parts of the plant’s operation such as inlet works, digesters and BAF is now to hand. The operations team also has speedy access to essential diagnostic information as and when they require it to ensure operations are maintained at all times and faults identified and rectified without undue delay.

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