AWWA M51 PDF

The safe operation and efficiency of a pipeline are dependent on the continual removal of air from the pipeline. This chapter includes an explanation of the effects of air and the sources of air in a pipeline. Therefore, when water is pressurized, its capacity to hold air is greatly magnified. The bubbling in soft drinks occurs after they are opened because the pressure over the fluid is reduced, and the excess carbon dioxide gas rapidly escapes. Once out of solution, air will not readily return to solution and will collect in pockets at high points along the pipeline. Air comes out of solutions in a pipeline because of low pressure zones created by partially open valves, cascading flow in a partially filled pipe, variations in flow velocity caused by changing pipe diameters and slopes, and changes in pipeline elevation.

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The safe operation and efficiency of a pipeline are dependent on the continual removal of air from the pipeline. This chapter includes an explanation of the effects of air and the sources of air in a pipeline. Therefore, when water is pressurized, its capacity to hold air is greatly magnified. The bubbling in soft drinks occurs after they are opened because the pressure over the fluid is reduced, and the excess carbon dioxide gas rapidly escapes.

Once out of solution, air will not readily return to solution and will collect in pockets at high points along the pipeline. Air comes out of solutions in a pipeline because of low pressure zones created by partially open valves, cascading flow in a partially filled pipe, variations in flow velocity caused by changing pipe diameters and slopes, and changes in pipeline elevation.

An air pocket may reduce the flow of water in a pipeline by reducing the cross sectional flow area of the pipeline and may, if the volume of the air pocket is sufficient, completely air bind the pipeline and stop the flow of water Karassik, Generally, the velocity of the flow of water past an enlarging air pocket is sufficient to prevent complete air binding of the pipeline by carrying part of the air pocket downstream to collect at another high point. Although the flow velocity of water flow may prevent the pipeline from complete air binding, air pockets will increase head loss in the pipeline Edmunds, Additional head loss in a pipeline decreases the flow of water and increases power consumption required to pump the water.

Air pockets may also contribute to water hammer problems, pipeline breaks, pipeline noise, and pipeline corrosion, and can cause erratic operation of control valves, meters and equipment. These conditions exist in the vortex at the pump suction, at pump glands where negative pressure occurs, and all locations where the pipeline lies above the hydraulic grade line.

Finally, vertical turbine and well pumps start with air in pump column, which may pass by the check valve and flow into the pipeline. A typical air release valve mechanism is shown in Figure 2 1. Air release valves are characterized by outlet orifices, which are much smaller than the inlet connection or pipe size. When received, the valve is normally open and will vent air through the orifice. As water enters the valve, the float rises, closing the orifice.

When air, which has accumulated in the piping system, enters the valve, it replaces the water, causing the float to drop and allowing the air to vent through the orifice. An air release valve designed with the proper float weight and leverage mechanism will allow the valve to open at any pressure up to the maximum working pressure of the valve. The negative pressure may be caused by column separation, pipeline draining, pump failure, or a break in the pipeline.

As a pipeline fills with water, the air in the pipeline must be expelled smoothly and uniformly to minimize pressure surges. Likewise, after a power failure or as a pipeline drains, air must be admitted to the pipeline to prevent the formation of a vacuum, which may collapse some pipelines or cause surges in the system.

As water enters the valve during filling the system, the float will rise closing the orifice. Combination air valves can be supplied in a single body configuration or a dual body configuration as shown in Figure 2 3. The information in this chapter is intended to apply generally to transmission pipelines but may also apply to other situations. This manual does not address the location or use of air valves for downsurge and column separation control, which should be considered for some systems.

An improper location can render the valve ineffective. The following guidelines are recommended for the general location and corresponding types of air valves. However, there may be other locations where valves may be deemed necessary. A sample pipeline profile illustrating typical valve locations is shown in Figure 3 1. The horizontal axis is the running length of the pipeline, usually expressed in station points. The vertical axis is the elevation of the profile stations relative to a specified horizontal datum.

Air valves are typically used in transmission pipelines where raw water is being transported to a treatment plant or where finished water is transported to a distribution system, or similar applications. Air valves may not be needed on smaller piping in distribution system piping grids where hydrants and service connections can provide sufficient removal of air in terms of both performance and cost. High Points. Combination air valves should be installed at pipeline high points to provide venting while the pipeline is filling, during normal operation of the pipeline, and for air inflow and vacuum protection while the pipe is draining.

A high point is defined by the hydraulic gradient and is considered the upper end of any pipe segment that slopes up to the hydraulic gradient or runs parallel to it. Mainline Valves not illustrated in Figure 3 1.

Increased Downslope. A combination air valve should be considered at abrupt increases in downslope. Decreased Upslope. Long Ascents. Long Descents. Horizontal Runs. It is difficult to evacuate air from a long horizontal pipeline at low flow velocities. Air release valves should be installed upstream of Venturi meters to eliminate measurement inaccuracies caused by trapped air. Deep Well and Vertical Turbine Pumps. Air valves mounted on these type of pumps may require special consideration in selection because of the violent changes in flow rate during pump cycling.

Air release valves are often used with time delayed, power actuated check valves to release the air in the pump column slowly under full pump pressure Val Matic Valve, Siphons not illustrated in Figure 3 1. To maintain a siphon on a section of pipeline that extends above the hydraulic gradient and that constantly runs under negative pressure, install an air release valve on the high point of the siphon to vent the air.

However, the air release valve must be equipped with a vacuum check devie on the outlet to prevent admitting air into the pipeline. An antisiphon valve is designed to vent air during start up, close tight during flowing conditions, and open to break the siphon during reverse flow conditions using a flow paddle.

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An improper lceation ean render the valve ineffective, The following guidelines are recommended for the general location and corresponding types of air valves, However, there may be other lacations where valves may be deemed necessary. The vertical axis is the elevation of the profile stations relative to a specified horizontal datum. Description No. A high point is defined by the hydraulic gradient and is fonsidered the upper end of any pipe segment that slopes up to the hydraulic gradient or runs parallel to it. Airfvacuum valves or combination air valves can be used on the draining side of mainline valves tw facilitate draining of the pipeline.

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