Flow Measurement & Control


Umgeni Water brown field project

April 2003 Flow Measurement & Control

Umgeni Water, the largest water authority in KwaZulu-Natal, treats and supplies water to a population of about 9,3 million people – around 20% of the country’s total population. The area of supply covers approximately 24 000 km2, with the main boundaries being the Indian Ocean in the east, the Tugela and Mooi Rivers in the North, the Drakensberg Mountains in the west and the Mkomazi and Mzimkuku Rivers in the south.

A significant increase in the area of supply from 7000 to 24 000 km2 in 1994 (with proposals in hand for a further increase) has resulted in additional demands but also additional potential resources.

The growth in water demand from existing customers has been approximately 7% per annum for the past 10 years. This high growth rate has been largely due to the recovery after the drastic curtailment measures implemented during the 1982/1983 drought. The projected growth rate for the future is lower and generally falls between the 4 and 5% levels.

Like other government-controlled utilities, Umgeni is mandated to assure consistent quality and distribution and cannot afford operational problems. However, ageing equipment and an outdated control system were contributing to periodic downtime and poor quality. Partly at issue were the limitations and operational problems associated with older Square-D PLCs, which would need to be replaced.

Although familiar with Umgeni's operations through prior equipment sales, Rockwell Automation's local GMS (Global Manufacturing Solutions) and Sales groups formed a team to compete in developing and ultimately winning the project.

Competition was steep, with both Siemens and Group Schneider having significant installation experience at other Umgeni sites throughout the country. However, to level the playing field, Umgeni Water insisted that all competitors would be required to meet its needs for a new control system along with new control strategies, an Ethernet network and new documentation processes. In addition, suppliers would be asked to submit proposals for replacing the ageing PLCs with a state-of-the-art system, provide new loop drawings, and supply a new network infrastructure, all with little or no risk to a plant that had no planned shutdown dates.

As part of its approach to a total solution, Rockwell's Johannesburg-based GMS office developed a structured approach to brown-field projects called 'RAMap'. As a methodology, RAMap has value because it highlights the specific difficulties that suppliers have when handling large brown-field projects, as opposed to relatively simple green-field projects. It also ensures that the methodology is visible and manageable, which in a brown-fields situation, is a prerequisite for success.

To enhance its local presence, the GMS/Sales team also joined forces with two local companies that had significant control systems experience between them. This skilful combination of strategies ultimately won the team the project, valued at over R6m.

"Our combined deliverable includes a simple CLX solution consisting of 21 processors with 6000 I/O points, including all the network infrastructure, cabling, loop diagrams, functional design specification, engineering and commissioning services across three sites to control water treatment plants and bulk distribution," said Sean Smith, GMS manager. "Rockwell Automation also provided technical training through its SAQA-accredited automation training centre - the only one in the country - reinforcing the holistic approach that GMS has to provide a complete solution that not only addresses the project requirement, but also ensures that the solution has the lowest total costs of ownership (TCO) possible," said Steven Jeffery, Rockwell Automation KwaZulu-Natal.

The ControlLogix will be responsible for controlling the local water plants - Wiggins and Durban Heights - and involves complex chemical dosing PID loops to ensure that the final product is within quality limits. The ControlLogix will be interfaced with the Adroit 4.2 Scada package that Umgeni is currently using. The system also involves interfacing - using the new ControlLogix information that comes from the Umgeni Water's Moscad telemetry infrastructure that controls remote sites like Inanda Dam - for starting and stopping remote pumps.

"Our RAMap methodology also helps engineer the system and manage the complete changeover. This methodology will result in a far more stable, proven control system that will assure system uptime, but not expose the customer to the downtime risks associated with brown-field changeovers," added Smith.

"The solution, coupled with our solid brown-field experience and RAMap methodology, provided the incentive needed to turn this order in our favour," said Richard Przybyl, GMS's Engagement Manager. "In fact, this project was not sold on the basis of our superior hardware - it was sold as a superior solution based on service, expertise and confidence, all of which was supported by our hardware and approach to complete automation."

The changeover from the Square D PLCs to ControlLogix will take place over a period of three years. As part of the Contract Participation Goals or Targets, Rockwell will achieve this by using its Durban-based empowerment system integrators - Thuthuka Computer Services - to do all the PLC programming, and CSK Electrical for the installation of the fibre network and the new ControlLogix PLC.

For more information contact Jeff Sandison, Rockwell Automation, 011 654 9700, jksandison@ra.rockwell.com

Umgeni Water contacts: Sean Smith or Richard Przybyl, 011 654 9700.

The process - Wiggins Waterworks

Wiggins Waterworks, situated in the Cato Manor area of Durban, is designed to treat water from the Mgeni River intake, and also water supplied from Inanda Dam. The waterworks gravitates potable water from its 124 Ml storage reservoir to Durban and adjacent areas. It has a present design capacity to treat up to 350 million litres of raw water per day.

Head of works

Raw water arriving at the works is monitored by in-line pH, dissolved oxygen and temperature meters. Generally this water varies in pH from 7,5 to 8,9; turbidity 3 to 150 NTU and chlorophyll 0,3 to 8,5 µg/l.

Facilities exist to dose powdered activated carbon (PAC), bentonite and chlorine gas in solution or sodium hypochlorite (NaOCl) in solution before passing through a covered canal to the aeration tank. Just prior to this tank, PAC, bentonite, lime, chlorine, or NaOCl can be added.

The aeration chamber allows for 1600 m3 of air per hour to be injected into the raw water through 28 diffusers with a 12 minute contact time at a flow of 250 million litres per day.

Ozonation

The pre-ozone contact tank, fitted with porous diffuser discs, allows for the addition of ozone. Currently, ozone is produced from air-fed Degremont ozonators capable of producing 20 kg/h and the ozone can be dosed either at this point or after filtration. The general performance of the ozone contactor is monitored by an arrangement of in-line meters, which includes ozone production, destruction and residual meters. The primary reasons for using pre-ozonation at Wiggins Waterworks are for the oxidation of iron and manganese THM precursors and taste and odour compounds such as geosmin and 2-methylisoborneol. It also aids in the reduction of the colour of the final water, enhancement of algae removal and possible reduction of coagulant demand. After ozonation, provision is made to dose PAC, lime, chlorine and primary coagulant (blended polyamine and polyaluminium chloride).

Clarifiers

The four Degremont clarifiers, each with a surface area of 995 m2, presently allow for a rise rate of 4 m/h. Sludge blanket depth is approximately 2 m and time-based dischargers release sludge at approximately 0,3% solids. In-line instruments, which include ion charge, pH, turbidity and dissolved oxygen meters, record the performance of the clarifiers.

Filters

Chlorine can be dosed prior to the clarifier water entering the Degremont Aquazur 'v' type filters, each of which has a surface area of 112 m2, allowing for a maximum filtration rate of 6 m/h. The media consists of silica sand (0,9 mm ES) with a depth of 900 mm. While backwashing of filters can be done manually, this is normally done automatically on a time-based and/or head loss initiation. Water used during backwashing is returned to a point upstream of the clarifiers.

Disinfection

Chlorination can be undertaken directly after filtration followed by pH correction if necessary, by the addition of caustic soda or soda ash. Post-ozonation can be carried out after filtration. This is effected in the post-ozonation contact tanks where the ozone/air mixture is introduced at the bottom of the tanks through porous carborundum diffusers. pH adjustment as well as chlorination can occur after the post-ozone contact tank - before the treated water enters the storage reservoirs. In-line instrumentation records the pH, chlorine and turbidity.

If necessary, chlorine is added after the reservoirs on the final outlet and controlled automatically to a required set point. In-line analysers record the pH, free chlorine and turbidity of the final water, with pH being maintained between 7,8 and 8,2; turbidity being kept below 0,5 NTU and a chlorine residual range of 0,6 to 1,2. This may be adjusted seasonally.

Sludge treatment

Sludge drawn off the clarifiers passes into a homogenising tank from which it is pumped into a dissolved air flotation (DAF) unit from where the thickened sludge (7% solids) gravitates into a de-aeration tank before it is pumped to the centrifuges. Air-saturated water from a 5 m3 saturation tanks is dosed with coagulant and fed into the DAF unit. Coagulant for the centrifuges is introduced into the sludge feed. Sludge cake (20-30% solids) is removed off site, and a sludge lagoon stores any surplus sludge. Recovered water from the DAF and centrifuges is gravitated into the wash water recovery tank.

Central control

The control room houses a central controller that interfaces with various outstations in the field. Monitoring of the status of all items of works and equipment is made possible through a computer that interfaces with the central controller. This also includes safety features such as alarms for chlorine and ozone leak detection. To a large degree, the plant can be operated from the terminal with live mimics screening the different units of the plant. Scada is used on the plant to ensure that digitals and analogs are processed, many of which are data-logged at half-hour intervals to assist with short-term operational decisions as well as long-term planning.

General

The main chemical building provides storage, batching, and distribution systems for the various chemical systems used around the plant.



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