Keeping Welsh lake under control
Nine years after a project to automate sluice gates on a Welsh lake, control-technologies firm Denca has upgraded the system to include every conceivable safety feature, writes senior engineer Andy Vogen
Through the efforts of a team of river officers, and the control technologies of Widnes-based Denca Controls, however, Bala Lake is able to accommodate watersports and leisure activities, as well as deliver carefully measured volumes of water for abstraction further down the River Dee. This has been achieved via a technically complex problem being translated into a reliable, simply operated solution.
Denca's relationship with Bala Lake stretches back to 1999, when several members of today's Widnes-based team were involved in the project to automate control of the four sluice gates managing water flow into the River Dee.
The sluice gates were installed in the 1950s, and built to replace a set of earlier sluices engineered by Thomas Telford in the 19th century to supply water to the Llangollen canal. The new gates enable better water management for abstraction and flood alleviation.
Bala's four main sluice gates were operated manually and can rise and fall by up to 4m depending on environmental conditions. Operation involved having a duty controller plus two staff on site on a 24-hour shift roster in potential flood situations.
Inevitably, this manual process relied on operator expertise and only allowed limited control of flow rates into the River Dee. Due to the mid-river location of the sluice gates, and for health and safety reasons, the Environment Agency Wales required that the new system should be fully automated and operated by a single duty controller whether on site or at a remote location.
The original 1950s Bala Lake Scheme subsequently increased storage capacity. And the Environment Agency Wales and its predecessors embarked on a river management system that would embrace environmental safety, fish maintenance, river ecology and leisure requirements, as well as underline its responsibility to maintain abstraction requirements at Chester's Huntingdon WTW and other locations.
A functional yet user-friendly approach to lake level control and flow rate management had to be achieved. Following a two-year study of all environmental factors, ecological issues, abstraction requirements and leisure activities surrounding Bala Lake, as well as all safety and management criteria, the contract to automate the four main sluice gates (incorporating two stepped fish protection gates with V-notch weirs) was placed in 1999.
In developing their solution, which involved motorised actuator control of the flow gates, the Denca team focused on the principle of precisely regulating gate control rather than delivering a package that simply provided fixed levels of flow.
This was achieved by mathematically calculating varying flows under given conditions and then computing the flow differential between upstream and downstream levels to arrive at a formula that would enable accurate flow prediction per 1mm variance in sluice gate height.
Although a more conventional means of measuring flow, in this instance it was not possible to use established control algorithms to calculate water flow as any point of stabilisation would be at least half a mile down the River Dee, and would take around half an hour to achieve. Arriving at a formula that would predict a resultant flow from a given set of conditions, the Denca team was then able to map ideal predicted flow levels for almost any given situation.
With an automation criteria engineered to work between set lake levels of 0.8m and 1.8m, Denca then devised a programme complying with the six modes of operation required by the agency:
- Manual - PLC set to enable manual gate operation when flow criteria is outside pre-established flow levels
- Fixed flow control - enables a predicted flow-set point for constant flow whatever the water pressure (as the differential on either side of the sluice gates increases, predicted flow is maintained by reducing gate aperture)
- Programme/profile control - enables flow to be increased/decreased over a given time period (ie to allow for abstraction or storm water and mitigate effects downstream)
- Constant lake level - holds the lake at a given position via variable flow control (as with all other modes, this level will default to manual with operator alert in changing circumstances)
- Rising/falling lake mode - for whenever higher/lower lake levels are required. Enables the lake to rise/fall at a managed rate via regulation of flow. (Optional module)
- Design of the automated system to accurately measure/control sluice gates' position and water levels within the different modes and parameters of river management
- Design of remote viewing and control system, to allow staff to monitor, interrogate and adjust all parameters from a remotely located laptop
- Manufacture of specialised control equipment
- Coding of control programmes for the programmable logic controllers (PLCs), human machine interface (HMI), remote viewing and remote diagnostics
- Assembly of motor control, emergency shut down (ESD) and instrument + control access (ICA) cabinets; full system installation and commissioning
This ensured that such eventualities as lake level warning signal failure would be verified by a dual checking procedure. At the same time, a system override was also implemented to prevent the accidental manual setting of a flow value that was outside pre-set safety criteria. Ge Fanuc Simplicity, a SCADA system with advanced
client/server architecture, well suited to discrete applications, and able to handle large amounts of digital signals and alarm bursts, was used.
As part of the Denca design, all SCADA controls are housed offsite at the Bala Water Resource Office in Bala town. To provide River Dee duty officers with instant manual control of the sluice gates (to accommodate any occasions when rapidly changing climatic conditions mean immediate decisions have to be made), a remote, password-protected external link was also created.
All system alarms were also routed to a central control facility in Cardiff where full remote monitoring was also enabled.
From all remote access points, Denca provided Dee river officers with exactly the same management capabilities as those that could be found at the sluice gates. Control options include: gate mode selection, alarm settings, sluice gates positioning, river flow and level status.
To facilitate a measured extraction process, in automatic mode, only one sluice gate moves at a time over a 20-second period. If two or more gates move simultaneously, or a gate moves for longer than the specified time, a hard-wired trip logic solution removes power from the gate drives.
Prior to switch-over from manual to automated sluice control, detailed systems testing and user training programmes were implemented. Factory trial simulations of all control panels were carried out, only to be followed by a rigorous nine-month monitoring programme to ensure the sluice gate control system was fit for purpose in all environmental conditions.
River Dee duty officers were then trained to use the new system, understand and operate the remote access procedures and recognise which conditions could be changed and those that could not be influenced. Finally, extensive maintenance training was provided - including alarm criteria and telemetry - to duty officers and control room staff.
Since winning the original automated control contract, a technically complex problem that required a reliable, simply operated solution, Denca has also provided maintenance to ensure continued operational reliability of the Bala sluice control system.
In March 2007, Denca Controls carried out a legacy upgrade of the SCADA in place at the Bala Sluice Gate System. The upgrade was required to ensure continued security from both a hardware and software perspective. The process involved the integration of two new PCs running upgraded software to monitor the sluice gate system, with one remaining on cold standby. Prior to commissioning, Denca also carried out full IO software testing to ensure all upgraded SCADA software was 100% corruption free.
Additional system upgrades at Bala have included the supply of a trip mechanism to help prevent the mechanical drive operation of the fish gates from lowering, when becoming jammed with river debris.
The team had to meet demanding compliance requirements at every stage of the project. To ensure operator safety in extreme weather or potential flood conditions, it was clear that an automated solution had to be implemented.
Engineers also had to make certain that under no circumstances would any malfunction of the automated processes lead to a situation where adverse discharges from the lake could occur. For this reason, every conceivable safety feature was included to prevent out-of-limits operation and ensure the system would always function at 100% as required.