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Module 15: Drain, Waste, Vent (DWV) Systems

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P-Trap
The P-trap is the most commonly used trap. The P-trap can be a one-piece or two-piece plumbing unit, with a union nut. It can also have several styles of cleanouts.

Made of brass, brass with chrome plating, plastic, copper, cast iron, malleable iron, or glass, the P-trap is designed to be attached directly to fixtures. A floor drain may be designed with a P-trap.

Parts of Traps
The following are the basic parts of a trap. Click on each part to know more:

Inlet
Inlet is the part where water enters from the fixture.

Top Dip
The inside curve of the pipe under the inlet is called the top dip.

Bottom Dip
The bottom dip is the bottom of the lowest curve of the pipe beneath the inlet.

Crown Weir
Sometimes called the trap weir, it is the highest point in the seal of the trap. Crown weir and trap weir are often referred to simply as weir.

Fixture Drain
Also called the trap arm, the fixture drain is the point where the waste water leaves the trap and goes into the drainage piping.

Trap Installations - Requirements
There are some typical trap installation requirements.

1. The vertical distance from the fixture outlet to the crown weir may not exceed 24 inches.

2. The horizontal distance from the crown weir to the trap vent varies depending on the diameter of the trap.

3. The total drop (or fall) in the horizontal pipe from the crown weir to the vent may not exceed one pipe diameter.


Trap Malfunction
When a trap functions properly, waste from the fixture flows into and through the trap. The trap is refilled with the last of wastewater to leave the fixture. This water provides the necessary liquid seal.

For the trap to function this way, the pressure on both sides of the trap must remain nearly equal. Water tends to flow in a level line, called the hydraulic gradient. The crown weir must always be installed lower than the top of where the fixture drain enters the vent line.

A trap may lose its seal in a number of ways. This can be through siphonage, aspiration, momentum, oscillation (wind effect), back pressure, evaporation, capillary attraction, or cracks.

Properly designing and installing the DWV system can prevent siphonage and back pressure.

The next slide presents more information on the causes of a trap losing its seal.

Click on each term to find out what it means and how it contributes towards a trap losing seal:

• Siphonage
• Aspiration
• Momentum
• Oscillation
• Back Pressure
• Evaporation
• Capillary Attraction
• Cracks
Siphonage
If the trap is not properly vented, it is likely to siphon. Siphonage occurs when there is negative pressure inside the DWV piping. This pressure difference pushes the water that is normally held in the trap into the DWV piping system.

Generally, siphonage occurs when the DWV piping is improperly vented or the vent is blocked. As the waste leaves the traps, an area of reduced pressure is created in drainage piping. Because of the difference in the pressure, the water is forced from the trap. This destroys the trap seal.

Aspiration
The term aspiration means the drawing in, out, or up of something, usually a fluid.

In piping, aspiration takes place when a large volume of water flows near the trap, creating negative pressure. This negative pressure draws the water from the trap causing the seal to fail.

Because of aspiration, a trap seal can absorb gases and odors. When a trap seal is saturated with gas or odors, it will emit the same, often unpleasant, odor into the building.

Momentum
The momentum, or speed, of water rushing through a pipe can force the standing water out of trap and empty it, thus breaking the seal. Water can gain enough speed to empty a trap when the vertical distance between the fixture outlet and the trap is too long.

Oscillation
Oscillation, or wind effect, is one of the least likely ways a trap can lose its seal.

Where there are strong upward or downward air currents, the pressure or section of the moving air may cause the water in the trap to rise or fall. If it rises enough to spill over into the waste pipe, less water remains in the trap and the seal is weakened. Lower than normal back pressure could break the seal, too.

Back Pressure
Back pressure can cause a trap seal to break.

Back pressure is pressure inside the DWV piping that is greater than atmospheric pressure. If enough wastewater from a fixture enters the stack so that slug of water forms a moving plug, the air in the stack below the plug in compressed. This excess pressure tries to escape through the trap.

To prevent normal back pressure from destroying the trap seal, the stack must be properly sized, and the trap must be properly sized and protected by a vent.

Evaporation
A trap may lose its seal as a result of evaporation. This is most likely to happen in traps that are seldom used.

The water evaporates, causing the seal to break. If the DWV piping is properly designed, evaporation will become a problem only during long period of non-use. When sewer gas enters a structure, unused floor drains are often the cause.

Capillary Attraction
Capillary attraction may cause a trap seal to break if a porous material, such as string or paper is caught in the trap. The porous material acts as a wick and draws the water out of the trap by capillary action. Cleaning the trap will solve this problem.

Cracks
A more common cause of waste and sewer gas leaking into a building is a crack in the trap. Cracks can be caused by a worn washer or by a broken nut, solder joint, or glue joint.