Page - 51 - in Clean Water Using Solar and Wind - Outside the Power Grid

Water supply 51 The dynamic pressure depends on the speed of the water in the pipe, in other words on the flow rate. The higher the speed the higher the dynamic pressure. If the pipe is wide, then the friction losses in the pipe are small and the dynamic pressure increases only slowly as the flow rate increases. Conversely, if the pipe is narrow then the water speed increases much faster when the flow rate increases. Consequently, the dynamic pressure will increase faster with the flow rate. Actually, the dynamic pressure and the friction losses depend on the square of the water velocity v, in other words on the square of the flow rate Q. So, if the flow rate is doubled, then the power to provide the dynamic pressure must increase by a factor of four. The static and dynamic pressures are illustrated in Figure 4.1. Figure 4.1 Different system curves that represent the load to the pump. If the water is lifted only a small height then the static head is small, and vice versa. If the pipe is narrow, then the losses are relatively high and the required dynamic pressure increases rapidly with the flow rate. The pump curve, called the QH curve, describes the pressure that a pump can produce as a function of the flow rate, Figure 4.2. This is Downloaded from https://iwaponline.com/ebooks/book-pdf/520710/wio9781780409443.pdf by IWA Publishing user