Types of Networks and Pumping Systems

Although and infinite number of network topologies are possible, the possibilities can be reduced to the following key characteristics:

Network characteristics—A water system usually consists of several main transmission pipelines (from pumping stations to reservoirs, elevated tanks, or booster stations) and many branches and loops to distribute water to local water-demand points.
Piping characteristics—These include pipeline length (L), diameter (D), roughness (C or f), elevations or profile (based on topography), water levels at suction and receiving water bodies, flow (Q), pressure head (H) at nodes, and pressure wave speed (a).
Pressure wave speed—This varies from as low as 340 m/s to as high as 1,438 m/s for water in thin-walled plastic pipes to thick steel pipes, respectively. Pressure wave speed is also affected by pipe installation due to bedding, anchorage, and soil conditions.
Modeling complexity—In the past, networks were usually reduced to a few key water mains, taking the flow distribution, pipeline profiles, and kinetic energy of the system into consideration. This usually provided conservative results for these main lines, but the transient energy transmitted from the main lines to the distribution network (or vice versa) was overlooked. Modern computer models, such as Bentley HAMMER CONNECT, can simulate networks with thousands of pipes and dozens or hundreds of boundary conditions.

For the purpose of transient analysis, pumping systems can be grouped as follows:

Open pumping system—An open-water system consists of upstream reservoirs, pump stations, and downstream reservoirs or elevated tanks. Transient pressure-wave travel is confined to a single system and transient energy cannot be transmitted to another system. With a favorable pipeline profile (e.g., concave upward), no significant vapor cavity occurs and the water columns do not separate. The maximum upsurge pressure seldom rises 50% higher than the steady pressure head. However, an irregular pipeline profile can result in a large water-column separation and severe transient pressures. Vapor or air pockets will eventually collapse due to flow reversing from the upstream reservoir or tank.
Closed system—In a closed system, the pump supplies water and maintains adequate pressure for the whole system. There is neither a reservoir nor a standpipe in the system. Closed systems usually service a small water supply zone. Pumps employed in a closed system often have flat pump curves that are undesirable from a transient perspective because rapid flow alterations can occur. After a power failure, the downsurge likely results in more vapor cavities than in an open system, while the upsurge is relatively small in comparison. Upon pump startup, higher transient pressures can be expected due in part to the greater number of air cavities that are trapped and remain in the system, and in part due to inherently rapid flow acceleration. The air trapped at local high points should always be released.
Boosted system—For some water systems, water may be delivered directly to a booster pumping station that resupplies water to another system on its discharge side. Normally, no reservoir or suction well is installed upstream of the booster pumping station; consequently, the hydraulic performance of one side of the booster pumping system can be significantly affected by the transient conditions of the other side. From a hydraulic point of view, all possible combinations of power failure should be considered, including:
- All the pump stations fail while the booster continues to operate.
- Only the booster fails while all others continue to operate.
- A global power failure occurs at all pumping stations for both systems.
Because of flow continuity, the booster pump stops soon after a power failure in the upstream system and the resulting transients may be similar to a power failure at both pumping stations. In cases where the booster pump fails while the upstream pump continues to operate, a worse transient may result in part of the water system.