Pressure Management in Water Distribution Networks

A Water Distribution Network (WDN) is an essential infrastructure, meant to supply indigenous, fresh water within boundaries of a city or designated area. A distribution system that supplies drinking water from the points of supply, to the points of consumption, consists of pipelines, valves, tanks, pumps etc. and supplies water to consumers, under certain hydraulic conditions that are often difficult to control mainly due to constant growth of urban population. The purpose of the WDN is to provide water to the consumers, not only of adequate quantity and optimal pressure but also of the quality in compliance with local regulations. However, smooth running of large-scale water supply networks is still a major engineering challenge due to many factors affecting such as:

• The topographic complexity of the network

• Complexity of the network connections

• Different functional regulatory systems

• Temporal and spatial variations in water demand

• The friction between water and the internal wall of the pipelines

• The existence of Non-Revenue Water in high percentages

Significant portion of the treated water is lost and does not reach the end user. Leakage is one of the reasons of water loss and comprises of physical losses from pipes, joints and fittings. Overflows from service reservoirs also adds to the losses. Quantity of the leakage depends on water pressure, pipe age, the standard of fittings, the type of the soil around the pipe etc. Theoretically, water leakage from the distribution network occurs, when the residual resistance of the pipe cannot bear the impact of the water pressure. Therefore, approaches towards water leakage control can basically be classified into two categories, improving pipe resistance and reducing water pressure.

The WDN is meant to deliver adequate water to the consumers, with the minimum acceptable pressure throughout the operation time. Increase in the leakages are often observed during the off-peak hours, when the consumer demands are lower and system pressure tends to be higher. Water utilities must continuously make efforts in order to reduce the inevitable water losses through leaking pipes in the WDN by maintaining the pressure as per requirements. Water loss not only affects the revenue generation of water utility providers, but also results in wastage of an outsized amount of energy required to treat raw water and distribute potable water. It is not an easy task and requires the development of a proactive leakage detection technique as well as speedy repairs of leaky pipes.

In WDNs, leakage outflows are sensitive to pressure in the pipe. Minimization of excessive pressure is performed using a system of the Pressure Reducing Valves (PRVs), Throttle Control Valves (TCVs), Flow Control Valves (FCVs) and using Variable Speed Pumps (VSPs). These valves are variable closure devices that allows water to flow through it until a set pressure is achieved downstream. PRVs are often controlled with different approaches like hydraulic or electronic controllers. Electronic controllers are often used perfectly in supervisory control and data acquisition (SCADA) systems, consistent with the momentary operation conditions. VSPs are pumps with Variable Speed Drive (VSD). The VSD regulates the rotational speed of the pump’s motor by changing the frequency of the input power. Changing the speed of the electric motor can change the hydraulic performance of the pump (such as power consumption, outlet flow, and pressure).

Another approach is dividing the WDN in different operational zones, called District Metered Areas (DMAs) or Pressure Managed Areas (PMAs) which can be independently monitored and managed. The water pressure in these areas is regulated by installing network elements such as control valves at the entry point of the zone(s).

Factors Affecting Pressure in Water Distribution System

1. Inlet Pressure: Higher inlet pressure is always considered to be conducive to improve the outlet pressure, reducing the pressure drop along the pipeline.

2. Flow Rate: As the flow rate increases, pressure also increases.

3. Transport Distance: The outlet pressure of the pipeline gradually decreases as the transport distance increases.

4. Roughness of Pipe: Under certain inlet pressure conditions, the pressure drop in pipe increases with the rise in pipe wall roughness.

Pressure Management

Water pressure regulations in pipes are proven to be an important tool, for long term reduction of losses in WDNs. Therefore presence of pressure management schemes is an important aspect of water networks. In most networks, active pressure control for loss minimisation, through the reduction of excess water pressure is essential. There are a number of methods for regulating pressure in the WDNs. These include the use of variable speed pump controllers and application of the concept of DMAs/ PMAs. Several control valves can be used either to control the water pressure or flow at some specific points in the networks. These control valves include, but are not limited to:

• Pressure Reducing Valves (PRVs), used to limit the pressure in the pipe links

• Pressure Sustaining Valves (PSVs) , used to maintain pressure at a specific value

• Pressure Controlling Valves (PCVs) , used to control the pressure in a specific zone in the water networks

• Pressure Breaker Valves (PBVs) which is used to force a specified pressure loss across the pipeline.

Methods of controlling Pressure

1. Fixed outlet pressure control It typically employs a PRV, with no additional equipment. It is the most straightforward way of pressure management. PRVs are easy to install, operate and maintain. However, most PRVs lack the pliability to regulate water pressures at different times of day. Excessive pressure in pipes downstream of the PRV, increase the prospect of leakage or breakage if pressures are not decreased when necessary. This results in water loss, potential contamination and expenditures associated with repairing leaks and breaks.

2. Time Modulated Pressure Control (TMPC) The TMPC approach offers a greater flexibility of pressure adjustments at specific times of the day achieved with the help of the controller. The controller is a low cost type and relatively easy to set up. A notable limitation of TMPC is that of its poor response to water demand requirements, such as the demand for firefighting. During the firefighting demand period, full pressure is usually required to tackle fire outbreak. Higher level of expertise is required to operate and maintain the installation of the devices used in this approach compared to the fixed outlet pressure control approach.

3. Flow Modulated Pressure Control (FMPC) It provides greater control and adaptability than time-modulated pressure control. This method uses an electronic controller which interacts with a properly-sized meter and PRV to make sure adequate water pressure within the event of needed fire flow. The increased flexibility of flow modulated pressure control, offers more savings than fixed outlet and time-modulated control methods. These savings could also be counterbalanced by the increased equipment expense related to the electronic controller and properly-sized meter.

4. Closed Loop Pressure Control (CLPC) In this type, pressure control is achieved by adjusting the settings of the PRVs based on the pressure at critical point(s) in the PMAs. In this technique, a pressure sensor, placed at the critical point(s) of the network is used to provide live data to the pressure controller at the inlet of the PMAs. This pressure control technique is more complex and expensive, but its potential for maximizing the benefit of the pressure management cannot be overlooked. It provides the ultimate level of control but the major disadvantage is that there is a greater opportunity for equipment to fail using this technique.

5. Parameter-less Pressure Controller It is another efficient controller to adjust the pressure; which is based on the flow in a pressure control valve (PCV) and is easy to implement. This method has advantages over the fixed outlet and time modulated pressure control approach. In this method, the controller used is easy to set up and has the capability to respond to changing water demand conditions.

6. Optimisation Approach Optimisation is a powerful tool used to achieve optimal opening adjustment and settings of the PRVs and PCVs. The optimal location as well as the opening adjustment of these valves is vital for effective pressure regulation. Artificial Neural Networks and Genetic Algorithm optimisation technique has been used to develop a model for reducing pressure in urban WDNs. It was observed that the network leakage can be reduced by more than 30%.

Benefits of controlling pressure in Water Distribution Networks

• It reduces surges and excess pressure

• Lowers pipe failure rate

• Extends pipe service lives

• Improves water distribution management

• Lowers water loss through leaking pipes

• Saves water and energy cost

Water loss through leaking pipes, have been a major threat to water utilities around the world. It is a general agreement that reducing pressure will reduce the leakage flow rate as well as the possibility of pipe burst. Frequent variations in pressure, are associated with higher frequency of new leaks. There is no doubt that pressure management is a fundamental tool in any leakage management strategy. Several pressure management strategies are available for leakage reduction in water distribution systems. The operational performance of the parameter less controller, as well as the optimisation approach gives an edge above other pressure control strategies.

With recent advancements in technology, which led to the development of electronic and hydraulic controllers coupled with PRVs and a probable combination of both approaches, that is optimisation and parameter less controller, could be best suited for pressure reduction in smart water networks.

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