Design of Agricultural Irrigation Systems.in Florida Water Hammer To avoid water hammer problems in pipelines, water velocity should be kept low, normally less than 5 feet per second (fps) for other than experienced irrigation system designers. Under no conditions should velocities ever exceed 10 fps. Velocities in the range of 5 to 10 fps are sometimes used if: * it is economical when both fixed and operating costs are considered pipe is properly pressure rated startup velocities are controlled by slow-opening valves thrust blocks are installed as required at tees, elbows, valves, or other points where hydraulic shocks may occur air relief valves are installed at all high points along the pipeline as required valves at the irrigation system subunits are not normally all closed at pump startup. For more information on water hammer, see IFAS Ext. Cir. 828, Water Hammer in Irrigation Systems (7). Pressure Rating Mainline pipe must be properly pressure rated to withstand the normal system operating pressure plus that due to hydraulic surges (water hammer). Also, the pressure due to surges should normally not be allowed to exceed 28% of the pipe pressure rating. Class 160 (160 psi pressure rated, or SDR 26) is normally adequately pressure-rated when velocities are kept below 5 feet per second (fps). Higher pressure ratings may be required if higher velocities are used. Cost Mainline pipe sizes should be selected based on cost and the previously discussed water hammer considerations. The cost of the energy consumed by friction losses should not exceed the amortized cost of the next larger-sized pipe. The economic analysis should consider the additional pumping costs associated with smaller pipe sizes, fuel cost escalation Page 7 for the life of the system, as well as any anticipated expansions of the system that would require greater flow rates in existing mainline pipes. Friction loss tables that are used to estimate pressure losses for the selection of mainline pipes assume that the pipes flow full. Air relief valves should be installed at all high points along the pipes to ensure that air will not be trapped at these points, causing the pipes to flow less than full. Trapped air may also lead to water hammer problems when the air is suddenly displaced. For more information on irrigation pipelines, see the appropriate ASAE, FIS and SCS standards. Also, see Agricultural Engineering Fact Sheet AE-69, Fittings and Connections for Flexible Polyethylene Pipe Used in Microinigation Systems (14). Irrigation Distribution Systems Irrigation distribution systems are those components of irrigation systems which apply water to the irrigated areas. In seepage and flood irrigation systems, these are the lateral ditches or pipes that distribute water throughout the fields. In pressurized systems, these are the system subunits or zones. Subunits are the groups of emitters and lateral and manifold pipelines that operate at the same time. Subunit design criteria are 1) uniformity of water application; and 2) economic considerations. Lateral ditches and pipes must be large enough and spaced closely enough that water is applied uniformly, but ditches and pipes must be small enough and widely enough spaced to be affordable. Absolute (100%) uniformity is impossible. Extremely high uniformities are costly. Tradeoffs of benefits from uniformities versus cost must be made. For more information on uniformity, see Agricultural Engineering Fact Sheet AE-43 (65), and Bulletins 256 (2), 265 (41) and 266 (42). For more information on system costs, see IFAS Ext. Cir. 821 (34), Bul. 276 (39), and Fact Sheets AE-30 (53) and AE-74 (35). Pressurized (Sprinkler and Microirrigation) Systems Pressure is normally regulated at the entrance to a subunit, especially for large field-scale systems. Control components located here are normally a valve (manual or automatic), pressure regulator, pressure gauge, and, for microirrigation systems, sometimes a secondary (screen) filter.