drain waste and vent.pdf

8/10/2019 Drain Waste and Vent.pdf

1/5

Flow of waste in a drain system

Solid and liquid waste flows in vertical pipes much differently than it does in hor-

izontal systems. In near horizontal pipes we talked about maintaining the slope ofthe pipe typically between 14 inch and 12 inch per foot to have enough velocity to

move the liquids and solids along the pipe, but not so much velocity that the liquids

would pass the solids by and let the solids get hung up. With a vertical pipe, the

water tends to attach to the walls of the pipe, creating a cylinder of water around

the perimeter. The center of the pipe often remains filled with air. Most vertical

pipes are designed to have no more than about a third of the pipe carrying waste.

Turbulence At At the bottom of the pipe, there is a point of considerable turbulence and high

Bottom Of pressure where the waste stops falling and starts to move horizontally. These

Stack pressures can push sewer gases back up through traps ahead of it in the system.

The goal is to minimize the turbulence and pressure here with fittings that make

gentle turns, rather than sharp turns.

Changes In When we change from a horizontal to vertical direction or vice versa, we want to

Direction maintain the momentum of the flow and not allow material to get hung up at inter-

sections. It goes beyond our scope to explore all of the fittings that are appropriate. As

a general principle, we want to make the change in direction gradual, and encourage

the material to flow in the intended direction.

D R A I N , W A S TE A N D V E NT P L U M B I N G

Copyright 2000 Carson Dunlop & Associates Limited. All rights reserved.12

S 1


2/5

Drain, Waste There are different names for pipes on the waste side of plumbing systems, based

& Vent Pipes on their functions.

Soil pipes carry liquids and solids. The pipe from a toilet is a soil pipe.

Waste pipes carry only liquids according to some authorities and liquids and

solids according to others. The terms drain and waste are often used inter-

changeably.

Drain pipes are intended to carry liquids and solids (some say liquids only).

Vent pipes carry air only (no liquids or solids).

Note: Most pipe materials are described as DWV (Drain, Waste and Vent),

which means they are suitable for all three functions.

2.2.3 TRAPS

Must Deal Lets make it even more complicated. If we attach a pipe to the bottom of all the

With Vapors plumbing fixtures that allows the solid and liquid waste to run out of the house

Too without clogging, it sounds like we have accomplished our goal. However, thereare vapors (gases) that pose odor and health problems. We need to keep the sewer

gases from backing out of the sewer pipes into the house. You are all probably a

step ahead, and have already designed a trap.

Function The main function of traps is not to catch rings that you have dropped down the

drain, although that can be an important side benefit.

Traps Seal The main purpose of a trap is to prevent sewer odors from coming back into the

Odors Out Of house. The trap under the fixture creates a low spot that is kept filled with water.

Home This water gets pushed out of the trap and down the drain every time more water

flows through the fixture, and as water flows out of the fixture, the last bit of water

is left in the trap, forming a new seal to prevent odors from coming back up.

Definitions We told you we dont like definitions, but wed better give you a few more.

trap a fitting that provides a water seal against sewer gases

trap weir (crown weir) the inside bottom of the highest part of the trap

(trap outlet)

trap dip (upper dip) the inside of the lowest part of the trap

trap seal vertical distance from trap dip to weir

tail piece the pipe from the fixture to the trap; also called fixture outlet pipe

trap arm the pipe from the trap to the vent; also called fixture drain

D R A I N , W A ST E A N D V E NT P L U M B I N G

Copyright 2000 Carson Dunlop & Associates Limited. All rights reserved. 13

S 1


3/5

Clogging Its strange that a couple of minutes ago we were talking about having exactly the

Traps right slope for waste piping, and now we put a low spot in it. Wont this collect

junk? Traps may accumulate sludge and can clog up. Over the years, there have

been several design considerations to minimize the problem.

Self-Scouring Traps are engineered systems, and their shape is not an accident. Traps are

Traps designed to be self-scouring, so the waste that flows through them keeps moving.

There are also rules as to how close the trap must be to the fixture and what size the

trap is. As a last resort, traps have either a cleanout or some easy way to remove thetrap for cleaning, such as slip joints.



S 1


4/5

Water Shouldnt The velocity of water moving through a trap is important to its function. This is

Move Too Fast why they have decided that putting two traps in series (double trapping) is not a

Or Too Slowly good idea. The water velocity to the second trap is so low that clogs are likely. Its

also why there are limits to how far below the fixture the trap can be placed. We

dont want the water moving too fast when it reaches the trap. It may siphon out the

water and then wed have no seal. The pipe coming out of the trap has to be hori-

zontal so we dont speed up the water and siphon the trap. Thats why we like

P-traps and S-traps are not allowed.

No Traps For Good home inspectors check under each fixture to make sure there is a trap. They

Toilets also notice that toilets never have traps. You would think that such a smelly plumbing

fixture definitely needs a trap. However, the toilet bowl itself is a trap. Water flows out

of the toilet by spilling over a high point, down into the drain. Water coming from the

toilet tank flushes the water in the bowl over the crown of the bowl (trap) and into the

drainage system. If the toilet is well designed, the water takes whatever solids are in

there with it. Toilets use siphons to help get rid of their solid and liquid waste. Toilets

are the only self-trapped fixtures in the house and are the only traps designed to

siphon as they flush.

D R A I N , W A ST E A N D V E NT P L U M B I N G

Copyright 2000 Carson Dunlop & Associates Limited. All rights reserved. 15

S 1


5/5

Slope, Traps, Things look pretty easy so far. We should be looking for waste pipes that are nearly

Leaks And horizontal, or are vertical (45 degrees or more off the horizontal). We look for traps

Freezing under every fixture (including floor drains) except toilets. We check for leaks. In

cold climates, we check to see if traps are subject to freezing.

Check For Look into floor drains. If you see water in the trap and do not smell anything, thats

Water In Floor good. If there is no water in the trap, or there is an odor, the water may haveDrain Traps evaporated or leaked out of the trap. Water evaporates from floor drains because

they hardly ever get used. Traps may be cracked by building settlement or heaving,

tree roots or freezing for example.

Water Pouring a bucket of water down the drain usually tells you if the trap is cracked,

Evaporates Or unless its a small crack. If the water stays in the trap, it probably just evaporated. If

Leaks Out of the water disappears, the trap is probably cracked or broken and will have to be

Drains replaced (this can be several hundred dollars to replace). Some people recommend

using a little mineral oil on top of the trap water to keep it from evaporating so quickly.

Trap Primers In new houses, they have a solution. They use a trap primer. This is usually a

38 -inch diameter plastic tube attached to a fixture that is used regularly. Every timethe fixture is used, a little water flows through this primer and into the floor drain.

This replaces any water in the trap that may have evaporated. Obviously, you cant

overfill a trap, because the water just flows out over the crown weir (bottom of the

trap outlet) of the trap and down the drain.



S 1

drain waste and vent.pdf

Documents