PRESSURE MANAGEMENT
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 the lowest level since it was commissioned in 1969. Water pressure management is considered by NMBM as the most effective real loss and water demand management intervention.
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MBM 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 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. Fember Street PRV
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“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 gravity-supplied areas (no PRVs) and pressure-managed areas (with PRVs). The plan is to convert as many gravityfed 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 furthest point away from the proposed PRV position. A theoretical calculation is completed taking into the 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 established 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. Maintenance Joubert maintains that whenever new technology or infrastructure is
