Automatic control valves regulate the levels

Amanziflow Projects’ pressure-sensitive, purpose-designed diaphragm valves (DVs) open and close intuitively to keep reservoir levels and water system flows constant.

By Peter Townshend*

The programme for the City of Mthatha’s bulk water conveyance master plan started in 2012 within the O.R. Tambo District Municipality, as well as the surrounding villages. This entails the construction of a new bulk water supply system, including rising and gravity mains, as well as reservoirs to transfer the clear water pumped out of the Thornhill and Highbury water treatment works (WTWs).

The review, design, project management and implementation phases were awarded to GIBB consulting engineers. The implementing agent is Amatola Water.

There are 25 concrete reservoirs ranging in size from 0.5 Mℓ to 12 Mℓ in capacity positioned in rural settings, some in remote areas. Each reservoir required self-regulating inlet control valves that open to fill and close off when full. Amanziflow Projects’ locally developed and manufactured DVs by were selected because: • They do not require any external electrical source or telemetry to operate them. • They will close off when the reservoir is full to prevent overflow and will open automatically as the water level drops to fill the reservoir. • They close over a short period of time

to prevent water hammer surges in the pipelines. • They are totally submerged inside the reservoir to prevent vandalism. • They are cost-effective and are supplied in sizes from 150 mm to 400 mm, as

Large 1 350 mm diameter DV as a dam outlet at Ncora Dam, Eastern Cape

was the case for this project, but can be supplied in larger sizes up to 1 350 mm in diameter. • They maintain the top water level in the reservoir with as little as 0.5 m of head loss across the valve. These were fitted as upward-facing control valves for the larger reservoirs, with the inlet pipe coming up under the reservoir floor. For the smaller reservoirs, the valves are downward-facing control valves with a top entry into the reservoir. A float valve placed at the full supply level closes the DV when the reservoir is full.

The DVs were constructed in stainless steel grade 316L and can operate in the pressure range of 0.05 bar to 6 bar. Larger pressures can be accommodated with the use of ratio-reducing valves.

400 NB upward facing control DV Complete installation of a 250 NB automatic control DV with float valve at the top to maintain a top water level in the reservoir

WTW flow control

A 900 mm diameter DV is also used at the Thornhill WTW to maintain a constant flow over a series of cascading weirs through the plant. The water level at the Mthatha Dam – the source for the incoming raw water – can fluctuate and the DV automatically regulates to maintain a constant water level and therefore a constant flow into the WTW. A DV will also be used at the Highbury WTW, where the pressure head may be as low as 0.5 m during droughts.

Ncora Dam

Also, in the same area, but for the Department of Water and Sanitation, two large outlet DVs will be fitted to the Ncora Dam on the Tsomo River to replace obsolete and dysfunctional Neyrpic control valves. The right-hand DV is already fitted, with the other one to be installed early in 2021. The two DVs are 1 350 mm in diameter each and, under 20 m head, deliver in the order of 7 m3/sec.

The DVs will automatically control the water level in the DV chamber, which then delivers controlled flows to an irrigation scheme and a small Eskom hydropower plant. These large DVs were fabricated by Amanziflow Projects in 3CR12 material with good corrosion protection. They are certain to provide long-term durability and efficiency.

*Peter Townshend, Pr Eng, is a director at Amanziflow Projects.

500 NB automatic control DV for water treatment works inlets and holding dams

Typical 400 NB top entry DV for reservoirs and dams

250 NB upward facing control DV