A047 Air Vent Selection Guide

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Air Vent Selection and Placement Guide Large Capacity Air Vents Provide Maximum Protection for Irrigation Systems 1 The Importance of Air Vents Why do we need to control air in irrigation systems? The most efficient way to control air in irrigation systems is by proper use of air valves. Control of air is very important and, depending on the circumstances, both the presence of air and its absence can cause severe problems and damage to the system. Problems and damages due to presence of air i
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  1 Large Capacity  Air VentsProvideMaximumProtection forIrrigationSystems    Air Vent Selection and Placement Guide  2  Why do we need to control air in irrigationsystems? The most efcient way to control air in irrigation systems is by properuse of air valves. Control of air is very important and, depending on thecircumstances, both the presence of air and its absence can cause severeproblems and damage to the system.Problems and damages due to presence of air in pipelines are:    Impedance of ow in pipelines - obstruction up to completestoppage, at times.    Serious head losses resulting in energy losses.     Water hammer damage to pipes, accessories and ttings.    Inadequate supply of water to sections of crops due to ow impediment and accumulation of pressure losses at the end of systems.    Inadequate water supply to crops due to inaccurate meter andautomatic metering valve readings.    Serious damage to spinning internal parts of meters, metering valves,sprinklers and sprayers.    Corrosion and cavitation.    Physical danger to operators from air-blown ying parts and fromvery strong streams of high velocity escaping air.Problems and damages due to the absence of air, when and where it isneeded:    Vacuum enhanced problems and damages- Suction of mud and dirt through drippers and tricklers.- Suction of seals and gaskets, in-line drippers and other internalaccessories of pipes, into the pipelines.- Uncontrolled suction of injected chemicals or fertilizers into thesystem.    Pipe or accessory collapse due to subatmospheric (negative)pressures. Absence of an air cushion can increase the damages of surge and slamoccurrence. The Importance of Air Vents  3  How air enters the water network  Water contains 2% - 3% soluble air. As water temperature rises and/orpressure in the line changes, this soluble air is released from the water.These bubbles grow and rise to the top of the pipe and accumulate atelbows and high points in the system. If not released, air pockets areformed, reducing the effective diameter of the pipe. Velocities higher than5 ft/s move the air bubbles towards the end of the pipe. Note: The frictionof the water along the air layer can be much higher than the friction along the walls of the pipe, especially when the air moves in the opposite direc-tion to the ow of water. Air is compressible, stores energy and reacts like a spring, causing localwater hammer, greatly affecting the hydraulic characteristics of the PVCnetwork.If not released, air can cause the PVC to burst. Under vacuum conditionsthe pipe has the potential of collapsing. When using pipes with gaskets,dirt can be sucked under the gaskets and when pressurized again, a leak may occur. Three stages of operation (air and an irrigation system) a. At system start-up the network is full of air. As water enters thenetwork, it pushes the air to the nearest opening.b. During normal operation of the system, dissolved air is released fromthe solution and free air accumulates and must be released.c. At the end of the irrigation cycle when the pump is stopped, vacuumconditions may occur, and air needs to be introduced into the system.  4  How do Air Vents control air in irrigation systems? There are three major types of air vents. Air/Vacuum Relief Vents are also known as kinetic air valves, largeorice air valves, vacuum breakers, low pressure air valves and air relief (notrelease) valves. These air vents discharge large volumes of air before a pipe-line is pressurized, especially at pipe lling. They admit large quantities of air when the pipe drains and at the appearance of water column separation. Air Release Vents are also known as automatic air valves, small oriceair valves and pressure air valves. These vents continue to discharge air,usually in smaller quantities, after the air vacuum valves close shut as theline is pressurized. Combination Air Vents, also known as double orice air valves, ll thefunctions of the two types of air vents described above, admitting andreleasing large quantities of air when needed, and releasing air continuously when the lines are pressurized. Rolling Seal Mechanism The 1” Automatic Continuous Acting Air Vent and the 2” Combination Air/Vacuum Relief and Continuous Acting Air Vents were both developedusing a revolutionary concept, the Rolling Seal Mechanism. Thecombination of the rolling seal and the hyrodynamic and aerodynamic oatdesign make these air valves much more efcient and resistant to prematureslamming and shut-off of the air valve.    The unique rolling seal mechanism allows gradual opening andself-cleaning of the air vent.    The unique aerodynamic design of the oat ensures that the oatwill remain open and allow the venting of air at differential airpressures of up to 12 psi. The result is continuous venting of airuntil water reaches the vent and closes the oat.This design enabled the introduction of small, light, and cost effective airvents. How Air Is Controlled
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