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Module 16: Water Distribution Systems

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Le valvole antiriflusso e valvole

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Backflow
Backflow is any unwanted flow of used or non-potable water back into the potable water distribution system. This reverse flow occurs as a result of cross-connection, which is a direct link between a contaminated liquid and a potable water supply.

Backflow can occur as a result of an improper or altered plumbing hookup or a garden hose being left in a pool of contaminated water.

These conditions do not in themselves cause a significant hazard, but they create the potential for serious hazards to the public, especially in terms of waterborne diseases.

Danger of backflow occurs when there is a break in the water main or other vacuum potential exists somewhere in a water line. If this happens, the water could be drawn backward through the water supply line.

Backflow prevention is required in many plumbing installations to keep contaminated water or other liquids from flowing back into the potable water system.

Water supply lines should be protected from back siphonage by air gaps and by the overflow level.

Air gaps are measured vertically from the lowest end of the potable water outlet to the flood level rim of the fixture into which it discharges.

The minimum required air gap is two times the size of the potable water outlet. Consult your local code for minimum air gap requirements.

Basic backflow prevention devices are designed to safeguard against dangerous cross connections. These devices must be well maintained to work effectively.

Example 1: In a bathroom sink the faucet line is set above the sink’s flood-level rim (the point at which water begins to overflow the top of the sink). In addition, the top drain returns water to the bottom drain before the water can rise to the faucet.

Example 2: The vacuum breaker in a hose bibb connection acts as a backflow preventer by interrupting the vacuum that could cause a backflow.

Example 3: There must be a physical separation between wells and municipal water. Wells must not be connected to municipal water supplies.

Some backflow preventers protect against both back pressure and back siphonage, while others can handle only one type of backflow.

Click on each button to know what causes back pressure and back siphonage:

Back Pressure
Back pressure occurs in the water distribution system when a pressure higher than the supply pressure causes a reverse flow into the potable water piping.

Back Siphonage
Back siphonage occurs when contaminated or polluted water flows from a plumbing fixture back into the potable water piping. Back siphonage is caused by a negative pressure in the plumbing fixture.

The five most commonly used mechanical backflow preventers are as follows:

• Atmospheric vacuum breakers

• Pressure-type vacuum breakers

• Dual-check valve backflow preventers

• Dual-check valve assemblies

• Reduced-pressure zone principal backflow preventers

Valves
A valve or faucet regulates the flow of water in a water distribution system.

Valves may be used to turn water service on and off, act as a throttling device that controls the rate of water flow, regulate the pressure, or prevent a reversal of flow through a line.

Types of Valves
Plumbers must be familiar with a wide variety of valves. Click on each type of common valve used in the water distribution system to know more:

• Gate valves

• Globe valves

• Angle valves

• Ball valves
• Check valves

• Pressure regulator valves

• Supply stop valves

• Temperature and pressure (T and P) relief valves
Gate Valve
A gate valve is a valve in which the flow is controlled by moving a gate or disc that slides in machined grooves at right angles to the flow. The gate is moved by the action of the threaded stem on the control handle.

Globe Valve
In a globe valve, the flow is controlled by moving a circular disc against a metal seat that surrounds the flow opening. The disc is forced onto the seat or withdrawn from it by screw action as the handle is turned.

Angle Valve
The angle valve is similar to the globe valve, but it can serve as both a valve and a 90-degree elbow. Because flow changes direction only twice through an angle valve, the angle valve is less resistant to flow than the globe valve, in which flow must change direction three times.

Ball Valve
The ball valve is used to control the flow of gases and liquids. Ball valves are installed in piping systems where quick shutoffs for in-line maintenance may be necessary, or in lines used for mixing various liquids and gases. The ball part of the valve is rotated into the open or closed position by a handle on the outside of the valve body. These valves allow quick action in controlling the flow in piping systems.

Check Valve
A check valve is used to prevent reversal of flow in a piping system. Pressure in the line keeps the valve open. The valve is automatically closed by the reversal of flow or by the weight of the disc mechanism. Three types of check valves are available: the ball-check valve, the swing-check valve, and the lift-check valve.

Pressure Regulator Valve
The pressure regulator valve is used to reduce water pressure in a building. The valve is activated by changes in pressure within the system. As the pressure changes, a spring located in the dome of the valve acts on the diaphragm to move the valve up or down.

The valve is in the open position when the diaphragm is pressed down away from the valve seat, and it remains open until the pressure in the building reaches a set level. The valve then closes and remains closed until the pressure in the building begins to drop.

Supply Stop Valve
Supply stop valves, or supply valves, are commonly used to disconnect the hot or cold water supply to water closets and sinks. These valves make it easy to control the water connection at an individual fixture for repair work. They are available in either right angle or straight design.

T and P Relief Valves
Temperature and pressure (T and P) relief valves are normally used for liquid service, although safety valves also may be used. Ordinarily, pressure relief valves do not have a chamber or a regulator ring for varying or adjusting blow down, so they operate with a relatively lazy motion. As pressure increases, they slowly open; as pressure decreases, they slowly close.