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42
an inspection chamber and a cleansing arm. To be effectual the
following requirements should therefore be fulfilled:—
1. There should be a sufficient water seal.
2. The trap should be self-cleansing.
3. The cap of the raking or cleansing arm should not be forced
by the pressure of air in the sewer, and it should admit of
easy removal should the trap become blocked.
The first requirement is generally fulfilled. A water-seal of
¾inch is usually sufficient and most traps have a seal of an inch
or over.
It was stated in a previous paragraph that most of the 6 inch
traps were not self-cleansing. There are very few houses in Acton
where a 4 inch trap would not be ample, and if this is properly laid
it can be made self-cleansing. Under existing conditions, the best
type is that which has a 2 inch or 2½ inch cascade,but occasionally,
the " fall " is not sufficient to allow of this arrangement. The
opinion prevails among many people that a 4 inch trap is more liable
to become blocked than a six inch one. If the drain is properly
laid it is difficult to imagine why this should be so. Anything
that will pass through a closet trap or through the grid of a yard
trap will easily pass through a 4 inch intercepting trap. Moreover,
in a trap which is not self-cleansing any solid article acts as a nucleus,
and thus sets up an obstruction.
The other objection to a 4 inch trap is the possibility of siphonage.
Experimentally, it is quite easy to siphon a 4 inch trap, but
in practice this must be a very rare occurrence, and if it did occur,
the seal would only be broken until one of the contributory drains
is used.
Where an inspection chamber is provided, the cap of the
raking arm should be able to resist any rise of pressure in the sewer,
and at the same time admit of easy removal in case of obstruction
at the trap. There are many types now on the market which fulfil
these conditions, and local authorities should have the power to
veto the use of an unsatisfactory type of trap.