|Irrigation Reference Manual (Peace Corps, 1994)|
|Chapter 5 - Farm water delivery systems|
|5.2 Pipeline hydraulics and design|
Air vents or air relief valves are necessary for allowing air to escape from pipelines as they fill with water, thus preventing air pockets, which restrict flow. Vacuum relief valves are necessary to prevent vacuum pressures (sub-atmospheric pressures) from collapsing pipelines (especially plastic lines) at times when pressures may fall below atmospheric.
Air and vacuum relief are often combined into one valve and are often called air/vacuum relief valves. Air and vacuum relief valves should be installed at the beginning and end of a pipeline, at high points or summits in a line where air could be entrapped, or periodically where air could otherwise be entrained by the water.
In low pressure lines, the air and vacuum relief can be accomplished simply with a stand pipe that is taller than the expected maximum operating pressure, expressed as the height of a column of water with a freeboard of 30 cm (1 ft). (See Figure 5.20 for dimensions.) For higher pressures, a commercial valve for air/vacuum relief is available.
The sudden escape of large amounts of air from a pipeline as it fills can cause high momentary pressure rises - as high as 4 times the operating pressure - when automatic air relief valves close suddenly. Thus, it is important that pipelines be filled slowly. The velocity at which an empty pipeline should be filled should not exceed 0.7 m/sec, or 2 ft/sec. The orifice of a pressure/vacuum relief valve should be at least 1/2 inch for pipelines of 5 inches or less, and 1 inch for pipelines of 6 to 10 inches.
Pressure relief valves prevent pressures that would be significantly above normal operating pressures from building up. This helps to protect against breakages in the pipeline from pressure build-up due to blockages.
Open vent stands can be used for pressure relief; however, pressure relief valves are often used for pressurized systems. They are often made for a specific pressure, which should be marked on the valve.
Some pressure relief valves are adjustable and can be set for different pressures. The valves should be capable of discharging the design flow rate without elevating pipeline pressures by more than 50% above the working pressure. One valve at the lowest point in a pipeline, or the point of maximum expected pressure, is usually sufficient to provide necessary protection.
Pressure relief valves do not provide sufficient protection against water hammer. The Soil Conservation Service suggests that pressure relief valves be no smaller than 1/4 inch nominal size for each diameter inch of the pipeline. Pressure relief valves should be set to open at pressures no greater than 5 psi above the design working pressure of the pipe.