Over 2 000 PRVs in the Valley of a Thousand Hills

EThekwini is unlike SA's other metros. Approximately 60% of its population lives in rural areas, with over 560 informal settlements. It also operates across a varied topography – from 0 m to 1 000 m above mean sea level.

By Kirsten Kelly

Our metro has one of the biggest pressure management programmes in the country. Between

R30 million and R40 million has been invested annually over the past 12 years in pressure management. The programme cuts across a number of departments and teams within the municipality and is highly complex, given our hilly terrain and large rural areas,” says Bhavna Soni, department deputy head, eThekwini Water and Sanitation (EWS).

Consulting engineering teams are appointed to actively review the existing water distribution systems – with the aim of having a minimum 250 kPa and a maximum 600 kPa supply pressure to customers.

If there are any places that breach the 600 kPa measurement, steps are taken to install a pressure-reducing valve (PRV).

“However, EWS is starting to reach a saturation point with the retrofitting of PRVs, and we are now focusing on the pressure management of new reticulation systems. These are constructed according to design guidelines where, according to pressures, control valves are installed at key points,” explains Nischal Nundlall, acting senior manager: Water Design and Non-Revenue Water at EWS.

Asset management

“Asset management and pressure management go hand in hand. EWS has established a prioritisation model when it comes to pipe replacement that is purely based on the performance of the system. We are currently conducting a case study in Chatsworth, where an entire pipe network (including the service connections) up to a metering point will be replaced. This area has experienced a high number of water leaks. The performance of the new system will be benchmarked against the old system. The new system will need to meet certain parameters regarding leakages in order for the contractor to receive full payment. It is a performance-based contract,” adds Nundlall.

The operations team engages with EWS Planning and Design teams whenever pipes repeatedly burst in a supply area. Before pipes are replaced, the existing supply pressure is measured. If pressure exceeds 600 kPa, steps are taken to reduce that pressure in the area, which will then, as a consequence, lessen the frequency of burst pipes.

Retrofitting PRVs

Nundlall adds that a works contractor is currently retrofitting the distribution system with PRVs or control valve installations. This involves installing pipework and building chambers/kiosks to house the PRVs. To date, over 130 sites have been handed over to eThekwini Municipality.”

The municipality’s water distribution system is divided into roughly 300 reservoir supply zones, with over 12 500 km of distribution mains. Each reservoir zone is evaluated based on the level of the reservoir and the extent to which that reservoir feeds into an area.

The reservoir zones are further broken down into district metered areas that comprise a maximum of 2 000 households. These district metered areas are then divided into pressure management zones. The pressure management zones cover approximately 40% of EWS’s total distribution system. Serving no more than a couple hundred households each, the pressure management zones do not have multiple feeds – there is a distinct supply point where a PRV is installed. This makes it easier to operate and maintain control.

ETHEKWINI MUNICIPALITY STATS

• 1 050 Mℓ/day water in the system • Population: 3.6 million people • Non-revenue water: 49.2% • Pressure-reducing valves: 2 341 • Control valves: 265 • Break pressure tanks: 94

“It is very difficult to calibrate hydraulic models when there are so many variables in the existing distribution system. After a desktop study, on-site simulations are conducted where a certain supply area has been modelled and pressure control measures are simulated in the field. If that simulation has passed all tests, the operations teams will sign off on the implementation of PRVs.” says Nundlall.

Maintenance

Jabulani Mayise, a civil engineering technologist at EWS, conducts regular site visits to verify whether the PRVs are working at the prescribed settings. “We have over 2 300 PRVs and 500 piloted control valves that comprise inlet control valves or pump control valves. Most of the PRVs are hydraulic, with about 10% being time controlled and used for pressure management at night. Each PRV is examined at least once a year, or every few months if located in problematic areas.”

Criminal activity (theft and vandalism) is also an issue and often causes burst pipes and water loss.

Mayise adds that each PRV is put on to EWS’s Geospatial Information System, which can clearly show where they are installed and what zone they service.

Drought

Due to the severe drought experienced in 2015/16, EWS implemented a drought mitigation plan to address water shortages. Part of the plan involved reviewing the existing pressure management zones and reducing the pressure via PRVs (where there were no design limitations) to 150 kPA.

The effect of pressure reduction within the areas was varied, and properties located at a higher elevation within the pressure management zone were impacted more than those at lower elevations. “As the pressure management zones cover 40% of EWS’s total distribution system, we also had to install reducers at the metering points. Approximately 250 000 devices were installed in areas of the municipality most affected by the drought, including Verulam, Tongaat and KwaMashu. That had a significant impact on the demand reduction (almost 30%),” says Nundlall.

Presently, EWS’s systems have recovered and have reverted to their normal operations. EWS’s drought action plan has been shared with other municipalities.

View from the street of an underground PRV installation

Informal settlements

The implementation of a community ablution blocks programme has played a significant role in formulating a pressure management plan within informal settlements.

“Fires are common in informal settlements so, therefore, the fire department was

Jabulani Mayise, civil engineering technologist, EWS Bhavna Soni, department deputy head, EWS Nischal Nundlall, acting senior manager: Water Design and NonRevenue Water, EWS Underground PRV installation

reluctant to have restricted pressure in these areas – especially because vehicle access is often difficult. We therefore decided to install fire hydrants next to the ablution blocks and not to reduce water pressure to these points,” explains Nundlall.

Lessons learnt from water pressure management

“There are many different types of PRVs and control valves (time-based, flow modulators, hydraulically actuated). Typically, municipalities will use hydraulic control valves and will then look into optimising them with technology. But once layers of technology are added, the payback period is significantly extended. We have found that a simple hydraulic operation will sometimes yield the best results,” explains Nundlall.

He goes on to say that there are circumstances where there are high flow rates and extreme changes in demand (like in central business districts) where it is feasible to install electronically controlled PRVs, but mostly hydraulically operated pressure control systems are suitable and have the lowest payback periods.

“It is also important to remember that technology has to be backed up with the necessary support, and teams must be equipped with the skills to operate and maintain sophisticated electronic devices. There is little point in having the best technological systems in place but not having the operational capacity to rectify problems as they arise,” Nundlall concludes.