Pressure control to fight water shortages in Eastern Cape

Nelson Mandela Bay Municipality (NMBM) is experiencing a drought, with its largest supply dam, Kouga Dam, dropping to its lowest level since it was commissioned in 1969. Water pressure management is considered as the most effective real loss and water demand management intervention by NMBM.

NMBM is a water services authority (WSA) and water services provider (WSP). It encompasses the Gqeberha (formerly Port Elizabeth), Kariega (formerly Uitenhage), Despatch and KwaNobuhle areas, and supplies water to Kouga Municipality (its largest water consumer).

High water pressure contributes towards an increased number of burst pipes and a greater volume of water lost due to underground leaks. The aim of pressure management

Lyle Francis, acting deputy director: Water Demand Management, NMBM

St Georges Strand PRV during construction Pieter Joubert, professional engineer at Engineering Advice & Services

is to reduce pressure to an acceptable level and, therefore, actual water losses. Water pressure management also decreases water consumption and increases the asset life.

NMBM has appointed two service providers – Re-Solve Consulting and Engineering Advice & Services – to formulate and implement a water pressure management plan.

“The NMBM region is divided into supply zones, with plans to install one or more pressure-reducing valves (PRVs) into as many zones as possible. Currently, there are 68 pressure-managed zones within NMBM, which covers approximately 25% of the water network,” explains Lyle Francis, acting deputy director: Water Demand Management, NMBM.

PRVs

Zones within NMBM are divided into gravitysupplied areas (no PRVs) and pressuremanaged areas (with PRVs). The plan is to convert as many gravity-fed areas as possible into pressure management zones.

Pieter Joubert, professional engineer at Engineering Advice & Services, explains that implementing pressure management is an intricate exercise. “We cannot use a blanket approach across all zones. Each zone is individually analysed and the PRV installations are designed and sized according to zone characteristics, as well as to ensure adequate water supply to all consumers in that zone. Water meters and remote monitoring loggers are installed at the majority of the zones. We, therefore, know the minimum and peak flow rates, volume of water supplied each day, and consumption patterns of each zone. All of these parameters are affected by the type of consumer in each zone.”

Industrial zones, or zones with a large number of businesses, will typically have a fairly ‘flat’ consumption pattern if they operate 24 hours a day, as they are constantly using water. A residential zone will typically have high and low points in consumption throughout a normal day.

Due to the topography of NMBM’s supply areas, certain parts of the water reticulation system have high pressure and other parts have lower pressure. A zone with minor to minimal changes in elevation throughout the terrain will most likely have similar pressures throughout the zone and is an ideal area to reduce pressure (especially if supplied from a water source situated much higher than the zone).

“We analyse each zone individually according to their high, low and critical points. In most cases, the critical point is either the highest point in the zone, or the point furthest away from the proposed PRV position. A theoretical calculation is completed taking into account the number and type of consumers in each zone, as well as information from the water meters and loggers, before completing a detail design for PRV installations,” says Joubert.

During the setting of the PRV, it is important to ensure that the critical point has at least the minimum predetermined water pressure. This pressure is often determined by design guidelines used by engineers, or in consultation with the WSP,” adds Joubert.

Local by-laws give NMBM the discretion to reduce water pressure to levels they deem acceptable. This is particularly important during a drought, as it provides NMBM with the ability to manage the usage of the water sources more effectively.

In Bluewater Bay, Port Elizabeth, NMBM saved over 650 kℓ per day by dropping water pressure and thus reducing leakage flow rates.

Most of the PRVs used by NMBM supply a constant downstream pressure, and are mechanically and hydraulically controlled, negating the need for external power.

Amsterdamhoek PRV – pipework before installation

Maintenance

Joubert maintains that whenever new technology or infrastructure is implemented, its maintenance must be considered. “At NMBM, we are using simple PRVs and focusing on getting the basics correct. We are applying PRVs to areas where the most water savings can be achieved, and also take a look at additional areas where PRVs can be utilised to reduce leakage rates and pipe failures, while at the same time prolonging the infrastructure’s operating lifetime.” Technology is available to implement advanced pressure management, whereby WSP Consulting can manage the supply pressure remotely. However, within NMBM, most of the zones have not been fitted with this type of technology as yet.”

A dedicated team from the service providers, along with support from NMBM, focuses on the optimal running of PRVs. Similar to any mechanical equipment, parts of the PRVs need to be serviced or replaced from time to time. The performance of the valves is constantly assessed, and minor or major services are performed at regular intervals. This ensures that the PRVs operate as intended and thus reduce the chances of burst pipes and continue to reduce water lost by means of leaks. “If a PRV is not maintained, and thus not functioning as required, there will be an increase in leaks, burst pipes and water consumption. Rather have well-maintained old infrastructure than new infrastructure that will not be maintained,” adds Joubert.

Impact of drought on water pressure

In zones where water pressure is governed by the level of a water reservoir, water pressure will reduce as the water level drops within the storage reservoir and starts to drain the water network.

“Should no effort be made to reduce water consumption, the ongoing drought will make it increasingly more difficult to keep the reservoirs full. This has already been experienced within NMBM and consumers felt the effects of low to zero water pressure. Unfortunately, once the network is drained due to a disruption in supply, the water network first needs to fill before pressure can build up to a point where all consumers have adequate supply pressure,” says Joubert.

“The installation of PRVs is geared towards optimising NMBM’s water distribution system, not removing a consumer’s access to water. We also believe that it will change consumer behaviour. When one opens a tap and a reduced amount of water runs out of it, one will be reminded to use water sparingly. Consumer behaviour is key when dealing with a drought,” concludes Francis.

Fember Street PRV