Pressure is a critical factor in the operation of an irrigation system. Conservation of pressure energy is perhaps the most important thing in design. Ideally, a constant pressure throughout an irrigation system would be great for all concerned, except the owner who would pay a high price for oversized components. Designers must use an economic consideration in selecting components. Pressure variations are caused by elevation change and pressure loss due to friction of water moving through the system.
Static Pressure is the state at which the system is full of water but no water is moving. The weight of a column of water or the force of pump induced pressure in a pressure tank causes pressure through out the system. Variation in this pressure is caused by elevation differences. Figure 1.5a illustrates the creation of pressure by the weight of water column. Figure 1.5b illustrates the effect of water column (elevation change) on the water pressure in an irrigation system.
Dynamic pressure is the pressure in a system when water is flowing. Friction loss is now a factor along with the influence (elevation head) of the static pressure. During dynamic flow the water moves over the small rough features on the wall of a pipe and a drag occurs that slightly slows the water flow, causing a loss in energy.
Friction loss is the loss of pressure energy in an irrigation system. Water velocity contributes to water turbulence and energy loss. Water moving over the component surface or around turns in components drags against the component surface and loses energy. Water flow in pipes is the greatest loss usually. Velocity is a key to the situation; the greater the water velocity, the greater the turbulence, the greater the energy loss (pressure loss) will be. Figure 1.5c illustrates how water velocity in a pipe affects the friction loss in a pipe and the loss of pressure in a system.
System design must balance pressure loss and component cost in an effort to achieve an efficient system. Components, including pipe lines, must be selected on the basis of allowable pressure losses to keep costs within reason. The system designer uses experience and economic guidelines to guide component selection.
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