Loss Of Seal By Momentum
Theoretically, a trap may lose its seal by momentum. If a trap
is placed directly beneath a fixture, but some distance below it, a
flush of water might acquire sufficient momentum to carry it
through the dip of the trap and into the waste pipe beyond. As a
matter of fact, however, there are modifying conditions to prevent
such loss. Most fixtures as now made have outlets so obstructed by
strainers or cross-bars that the outlets are of less area than that
of the waste pipe, consequently the pipe could not fill full bore
and the velocity would hardly be sufficient to acquire the
necessary momentum. If it did, no harm would result, as sufficient
water would adhere to the long inlet pipe and to the sides of the
fixture to again seal the trap. Nevertheless, traps should be
placed as close to fixtures as possible, not only to prevent
possible loss of seal by momentum but also to avoid a long stretch
of untrapped waste pipe.
Answers (1)Manage My LifeTraps may lose their seals by leaking,
but there are other, more subtle causes to look at. Traps can lose
their seals by
direct siphonage
. We've talked about this. It's the-siphonage that occurs if we
don't have a vent just downstream of a trap. There are other ways
traps can lose their seals.
A fixture on a second floor can cause a fixture on the first
floor to siphon. The liquid falling through the stack can draw in
air from the trap arm downstream of a first floor trap. As this air
is pulled into the stack, the air pressure on the fixture side of
the trap is greater than the pressure in the trap arm. Atmospheric
pressure pushes the water out of the trap and the seal is lost.
This is called
indirect siphonage
. It's because of indirect siphonage that plumbing fixtures on
the first floor are not allowed to be wet-vented by the soil stack
from fixtures on the second floor. A separate vent is required for
each floor level.
Back pressure
is caused by air being pushed ahead of a slug of waste. The back
pressure can sometimes push through the water in the trap back out
into the fixture, allowing sewer gases to enter the house. This can
happen on a multistory house with a slug (mix of solid and liquid)
from the top-floor-level fixtures falling through a stack and
pushing air up through the traps on lower fixtures. Separate
venting of lower-level fixtures prevents this back pressure from
going through the trap and allows the positive air pressure to
escape through the vent. This is another reason why wet-venting is
not permitted from one floor to the next.
The seal at traps can be lost because of
evaporation
. This is common on floor drains and other fixtures that are not
used regularly.
Capillary action
is a less common method by which traps lose their seal. Foreign
material such as a combination of hair, lint and soap film gets
hung up in a trap and extends over the crown weir of the trap. In
much the same way that a paper towel will draw liquid up off a
counter top, the hair or lint, for example, will draw the moisture
out of the trap, up over the crown weir. The water will either
evaporate out of the hair or will run down the trap arm and into
the drainage system. Eventually, the seal will be lost.
When the trap seal is very small (less than 1.5 inches), the
wind pressure
blowing over the top of the vent stack can cause water levels to
fluctuate in the traps. As the water level moves up and down, water
will be spilled over the weir of the trap. Eventually, enough water
may be spilled over that the trap seal is lost. Wind effect,
evaporation and capillary action, for example, may work together to
defeat trap seals
Read more:
http://chestofbooks.com/home-improvement/construction/plumbing/Principle-Practice/Self-Siphonage-Of-Traps.html#.VGzYEDSUfZ8#ixzz3JXPs0D00
