from the specification):
(1)
"A lavatory pan is in practice connected behind the trap to a waste pipe for conveying away matter flushed from the
pan. This waste pipe is normally provided with an extension pipe extending upwardly above the roof of the
Page 4 of [1996] 1 All SA 1 (A)
building (at least two metres above the location of the pan). This extension pipe provides a source of air to the waste
pipe when matter is flushed down the pipe to prevent there being an airlock in the waste pipe or the syphoning dry of
the water trap. Furthermore the upper extension of the waste pipe
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provides a safety feature for the plumbing in case there should be a blockage in the waste pipe. The feature of the
waste pipe extension is technically satisfactory and is widely and extensively used in plumbing. However the provision
of this extension has its disadvantages because of the extra costs involved in the erection of the extension, especially
in tall buildings, and the fact that roofs have to be modified to accommodate these extensions and whenever a
building is being decorated or redecorated the extension pipe must itself be additionally painted separately from the
wall beside which it is located."
The specification then proceeds to describe the two "aspects" of the invention (to which I have already alluded) in
the following terms:
(2)
"According to one aspect of the invention there is provided a plumbing arrangement including a toilet pan
incorporating a water trap, a waste disposal pipe connected to the pan and a waste pipe extension extending above
the water trap, characterised in that the waste pipe extends only a short height above the pan flood level and there
has a normally closed two way valve means leading to atmosphere. The valve means is preferably a differential valve
opening to the atmosphere, the arrangement being such that only a small pressure difference is required to permit air
to flow into the waste pipe extension and a larger pressure is required to allow air to escape. A cover is preferably
provided to permit the air flow to and from the valve but still to prevent the ingress of leaves or dirt which could have
a detrimental effect upon the working of the valve."
(3)
"According to another aspect of the invention there is provided twoway valve means for use in an arrangement as
set out above, the valve comprising a housing, which is preferably cylindrical, having a crossplate thereacross, at
least two apertures through the crossplate and a pair of oneway valves opening in opposite directions and carried in
the crossplate, the oneway valves being preferably arranged to open at different pressures. The oneway valves
preferably each comprise a seat, a flexible disc which can seal on to the seat but which when there is excess pressure
through the seat moves off the seat to permit gas flow therethrough and spring means holding the disc on to the seat,
the strength of the spring means determining the said excess pressure. There are preferably more than one such
aperture and a oneway valve therein."
The specification continues by describing various embodiments of the invention with reference to certain
accompanying drawings. These show the general plumbing arrangement, including lavatory pan, water trap, waste
pipe and an upward waste pipe extension; and also, in detail, the twoway valve means. From the drawings it
appears that the "flood level" of the pan, referred to in the specification in the context of the height of the waste
pipe extension, is located at the top of the rim of the pan. It is claimed in the specification that the valve
arrangement provides a satisfactory seal in normal circumstances to prevent gases with foul smells escaping from
the waste pipe, while at the same time permitting air ingress to the waste pipe extension to prevent (a) air locks
being formed in the waste pipe or (b) the syphoning dry of the pan trap. It further permits gases to escape should
there be a blockage in the waste pipe.
Page 5 of [1996] 1 All SA 1 (A)
Claims 1 and 9 of this patent read as follows:
"1.
A plumbing arrangement including a toilet pan incorporating a water trap, a waste disposal pipe connected to the pan
and a waste pipe extension extending
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above the water trap, characterised in that the waste pipe extends only a short height above the pan and there has a
normally closed two way valve means leading to atmosphere ... .
9.
Two way valve means for use in an arrangement as claimed in any one of the preceding claims, the valve means
comprising a housing having a crossplate thereacross, at least two apertures through the crossplate and a pair of
oppositely opening oneway valves carried in the crossplate."
The "arrangement as claimed in any one of the preceding claims" referred to in claim 9 consists of the general
plumbing arrangement described in claim 1 and the body of the specification (including the twoway valve) and
various permutations thereof referred to in claims 2 to 8.
According to its specification the 1989 patent, which is entitled "Plumbing Equipment", describes an invention
relating to "vent systems for use in the sanitary plumbing field". The specification explains the background to the
invention in terms identical to those appearing in the 1987 patent. (See the abovequoted paragraph (1) of the
1987 specification.) It then refers to the plumbing arrangement provided for in the 1987 patent; and continues to
describe "the present invention", viz. a twoway valve means, in terms similar, but by no means identical, to those
appearing in the abovequoted paragraph (3) of the specification relating to the 1987 patent. There follow
embodiments of the invention which use exactly the same drawings as those appearing in the specification of the