Manufacturer DFC Water has supplied valves to three reservoir rehabilitation projects for the bulk water supply to Sandton, north of Johannesburg.

The company reports that more than 100 valves have been installed at the Linbro Park, Marlboro, Morningside, Illovo and Bryanston reservoirs.

Most of the valves that were supplied were Vosa wedge-gate valves. A large number of Cla-Val pressure reducing valves, Cla-Val combination level and flow control valves, and Vent-O-Mat air relief valves have also been supplied.

The company reports that pressure management in the bulk water supply to Sandton would also enable the Bryanston reservoir to operate effectively.

Gauteng water utility Johannesburg Water has introduced a series of pressure reduction chambers along the Sandton water supply pipeline which, after supplying Linbro Park and then Marlboro, branches to feed the Illovo reservoir complex, the Morningside resevoir and the Bryanston reservoir.

The first chamber is nearing completion at Linbro Park, where the pressure is reduced from 20 bar to 11,5 bar.

A second chamber at Marlboro is also nearing completion and the pressure has been 
reduced further to 9,5 bar. A third chamber has been added on the inlet line to the Bryanston reservoir complex, where the pressure is reduced to 3,5 bar.

The section of pipeline at Linbro Park was overseen by consulting engineer Civec Consulting. Engineering company Themba Consultants was appointed at the Marlboro and Bryanston reservoir complexes.

The Linbro Park reservoir marks the point at which the 20-bar high-pressure supply from Rand Water is received by Johannesburg Water. This point incorporates a connection that the utility claims is the largest on the Rand Water system, as it delivers a peak flow rate of 394 Mℓ/day in the area between Marlboro and Dainfern. 

At Linbro Park, a maximum upstream pressure of 2 200 kPa needed to be reduced to 1 000 kPa 
downstream. However, the 
design of the pressure reducing chamber was complicated by an unknown minimum required flow rate that needed responsive valve operation to prevent upstream pressure surges and hunting conditions downstream of the control valve station.

The company says that the control valve regulation of the large volumes flowing through these pipes was only feasible if demand remained within 25% to 80% of control-valve capacity. Above 80%, the valve response would become slow and cause inconsistent downstream pressures. Below 20%, there would be overcompensation during opening and closing, which would 
result in hunting. Problems could, therefore, be expected during off-peak times.

Civec carried out a detailed economic comparison of the available valves, the installation and maintenance costs, valve 
efficiencies and spares availability.

The solution was a division of the two large supply pipelines into eight, 400-mm-diameter smaller pipelines. This allowed the installation of control valves with best possible flow-to-size characteristics and low-as-possible flow resistance values. The company says that an added benefit of this solution would be flexible operation, with any one valve easy to isolate and refurbish without shutting down.

Valve protection was provided 
by the installation of large- 
capacity 2 500-kPa pot strainers equipped with removable heavy-duty stainless steel cartridges and manual backwashing facilities.

More than 40 VOSA wedge-gate isolating valves were 
installed at Linbro Park, either as replacements for existing units controlling the reservoir level, or as isolating valves associated with the pressure reducing valves (PRV) chamber.

At the Bryanston reservoir complex, similar work was 
carried out on a smaller scale with pipelines being similarly isolated by VOSA valves for the installation of the PRVs.

The levels of both the Marl-boro and Illovo reservoir complexes cannot have supply pressures that are less than those at the Linbro Park.

Themba Consultants positioned the Marlboro pressure reduction chamber downstream of the branches leading off to the reservoirs, on the pipeline feeding the Morningside and Bryanston reservoir complexes.

The new PRV chamber was constructed in the existing roadway at Philo road, with a height restriction to accommodate the existing pipeline and new isolating and PRVs below the existing road level.

The main challenge in achieving this was the height of the 
400-mm wedge-gate valves, the tops of which needed to be 
accommodated within the roof slab of the chamber.

Another challenge that had to be resolved was to ensure that, in the event of failure of the PRVs in this chamber, the pressure experienced at the downstream Bryanston reservoir would not suddenly exceed that which the PRVs there could handle. The solution to this was to fit the Marlboro PRVs to ensure that any failure would close the valves.

The company reports that, during construction, shutdowns of the existing pipeline were care-
fully managed to prevent the complete drawdown of water in the downstream reservoirs.

At the Bryanston reservoir the inlet pressure was found to average 15 bar, with the static pressure being as high as 20 bar.

The average inflow into the complex was estimated to be 
708 ℓ/sec, for which the original manifold had been designed. An existing peak of about 1 200 ℓ/s 
was expected to increase to about 1 300 ℓ/s as demand increased over time.