In drip irrigation, especially subsurface drip irrigation, vacuum prevention is essential, even at very low negative pressure, for the prevention of suction of dirt through drippers, as well as for the prevention of damage to piping and accessories. There are three major causes for the formation of vacuum cavities in manifolds from which dripper laterals emanate (distribution manifolds) and in manifolds to which they drain (collection manifolds).
1. At sudden pump stoppage or valve shut-off, water column separation occurs after the inline isolating valve and at peaks, because water supply is suddenly stopped, yet, the existing water mass continues to flow, driven by the forces of inertia. Vacuum cavities are, thus, developed, exerting sub-atmospheric (negative) pressure and suction. 2. At system drainage, if air is not admitted at the rate water is drained, vacuum cavities form, exerting sub-atmospheric (negative) pressure and suction. In extreme cases, this can result in pipe or accessory collapse.
3. At pipe or accessory burst (blind flanges, risers, or on-line isolating valves breaking off, for instance), water is drained, sometimes at great flow rates. If water supply is
VALVE HEAD IN SUB-SURFACE DRIP IRRIGATION
SHOWING DISTRIBUTION MANIFOLDS
SUCTION OF DIRT INTO DRIPPER
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slower than the rate of drainage, and air is not admitted into the pipe, vacuum cavities form, which cause suction and, sometimes even pipe or accessory collapse. For the above reasons, large orifice air/vacuum valves (vents) are required in the following locations along the pipeline:
a. After inline isolating valves at valve heads, and in distribution and collection manifolds,
b. At peaks along distribution and collection manifolds, and
c. On tops of risers at the ends of the manifolds.
Sizing of the air valves should be determined according to the maximum water flowrate at
water column separation.
– If the field is relatively flat, without serious elevation differences and/or significant
slopes, yet the operating flow rate is significant, air valve sizing should be determined
according to the operating flowrate. The reason for this is that at sudden valve closure, the
water column continues flowing at the operating flow rate, at least for a very short time.
Thus, air intake should be equal to the operating flow rate.
– If the field has a varied topography, with differences in elevations and/or significant
slopes, air valve sizing should be determined in accordance to the maximum drainage flow
rate at controlled drainage or at pipe burst (the higher of the two). Air intake should be
equal to maximum drainage flow rate.
To prevent suction even at very low negative pressures, air intake requirements should be
determined at low vacuum (negative) pressure, say 1.45 psi (1 meter of water).
Following, are five graphs to help in determining the number and the size of “Guardian” air
valves (vents) required for vacuum protection in plastic manifolds at 1.45 psi vacuum
pressure.
The main graph covers slopes from 0% to 500% and flowrates from 0 cfm to 850 cfm.
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The other graphs zoom-in on the main graph, showing more and more detail. The second graph covers slopes from 0% to 100% and flowrates from 0 cfm to 600 cfm.
The third graph covers slopes from 0% to 50% and flowrates from 0 cfm to 400 cfm.
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The fourth graph covers slopes from 0% to 500% and flowrates from 0 cfm to 220 cfm.
The last graph covers slopes from 0% to 10%, and its flowrates range from 0 to 170 cfm.
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The slope to be considered in determining air valve requirements is the steepest slope of any section of the manifold sub-main, from the location of the air valve, to the lowest point on each side of the air valve, which is not protected by another air valve. If the Air Intake Flowrate determined by the slope is lower than the operating flowrate at the particular section of the manifold, use the operating flowrate (the values on the x-axis) to determine air valve requirements. Example: A manifold submain is 4 in. in diameter and the slope of its steepest section is 3% (0.03 ft/ft). According to the graph, the air intake flowrate demand, as determined by the slope, is about 29 cfm. If the operating flowrate is, say, 40 cfm, sizing should be determined by the operating flowrate, since it is higher than the flowrate that would be driven by he slope. According to the flowrate determined by the slope (29cfm), one 3/4 in. “Guardian” would be sufficient for vacuum protection. But, as can be seen from the graph, at 40 cfm operating flowrate, one 1 in. “Guardian” is required, and this is the air valve that should be installed. The graphs’ plot areas are color coded according to the number and sizes of “Guardians” required, in order to make the graphs easier to read. The versatility of the A.R.I. Guardian AV-010 Kinetic Air/Vacuum Valves play a major role in the prevention of vacuum formation in drip irrigation Systems under varying field conditions. System clogging, flow blockage and damage to pipes, fittings, and accessories are prevented, ensuring constant and uniform water flow.
Clarification
It must be pointed out that this paper and its graphs deal in a specific application of a specific accessory, under very specific conditions. The A.R.I. “Guardian” is what is officially called an “Air/Vacuum Valve” by the AWWA, and is sometimes called a “Large Orifice Air Valve”. In agricultural application, the word “Valve” is often replaced by the word “Vent”, and “Air Valve” becomes “Air Vent”. This type of air valve is a very important component in any water system, and even more so, in drip irrigation systems, especially subsurface drip systems. The two major purposes or functions of this type of air valves are:
1. To discharge large volumes of air from the system when filling with water.
2. To intake large volumes of air into the system for vacuum protection, thus, acting as a vacuum breaker.
When the system is under pressure, this type of air valve will not continue releasing air, and other types of air valves, like A.R.I.’s “Barak” or “Segev” are required for completing the air control functions. This paper deals only with the vacuum breaking function of the “Guardian”, which is of extreme importance in drip irrigation systems. This does not mean that these “Guardians” will not, also, greatly improve system operation by discharging large volumes of air at system filling, nor, does it mean that other types of air valves are not necessary for improving system efficiency in air release under pressure.
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The graphs describe specific conditions, only in plastic pipes (or pipes of similar friction properties, represented by their Hazen-Williams Coefficients), and only under vacuum pressure of 1.45 psi and above. The use of a vacuum pressure of 1.45 psi assumes that the drippers and laterals have in-built measures for protection at vacuum pressures below 1.45 psi. “Guardians” provide excellent protection in other pipes and at much lower vacuum pressures, but other, similar graphs must be used for sizing.