Principles of waste water discharge
Principles of waste water discharge
Stacks can often be cleaned from the roof provided the vent cap can be removed. With high buildings it is recommended that an access fitting be provided every 3 to 4 storeys. Ventilation lines, including secondary ventilation, may become (partly) blocked when rainwater gets in or by deposition of dry matter from the environment. Vent lines should therefore also be provided with access fittings.
water flowing away too quickly, which can give rise to hydraulic sealing.
Equal discharge and flow are assumed. This will not be the case immediately downstream of the fittings, but the wave of water will be equal after some time (and some length of pipe) has passed. 2.3 Fall A certain flow rate is required to correctly remove foul water. A lower limit of 1:200 has been established for this purpose. 1:400 or 1:500 may be possible with shorter runs, on condition that a calcula- tion is provided. In practice the installa- tion will then have to be carried out with great care. Further, any hydraulic distur- bances must be minimised: a fitting upstream that is frequently used and/or has a fairly large baseline discharge is an advantage in this case. In general a maxi- mum fall of 1:50 is maintained to prevent
above the level of the fittings. An air admittance valve may also be useful in solving problems associated with sepa- rate discharging fittings. However they cannot take the place of the (primary) vent pipe. A separate direct discharge system (or direct parallel discharge system) may be employed to solve problems where addi- tional fittings are connected to an existing system. This may facilitate air movement, while hydraulic sealing has less effect and the capacity of the stack may increase. Such a discharge pipe will be connected exclusively to the stack, that is, no branch pipe will be connected (illustration 2.4). In places in the system where hydraulic sealing may be caused occur because of bends, transitions from vertical to horizontal or where insufficient
airflow is possible, a balancing line may be provided. This line allows the balan- cing of overpressures and underpres- sures. One common example arises if the stack is not fully vertical but has horizontal sections (illustration 2.5). The uppermost example, a restricted secon- dary discharge system, is the most common solution. The recommendations in this manual are based upon the presence of a primary discharge system. 2.2 Filling of pipework In order to allow a free flow of air the pipework must be designed to take account of the quantities to be removed, the fall (= gradient or hydraulic slope) and the diameter, so that the pipes are no more than 70% full (water depth is 0.7 x the internal pipe diameter).
2.4 Access fittings Blockages may occur even in well-
designed discharge systems, for example as a result of deposits (e.g. solidified fat) or improper use of the drain (paint residu- es, food remnants, cat litter, potting com- post, small objects, etc). Access fittings should therefore be provided at strategic locations in the pipework, taking account of the fact that clearing equipment can only cope with limited changes in direc- tion. Access fittings should be easily accessible and placed in such way that hoses or springs can easily be inserted. The access fitting should where possible be located on top of the pipe so that in the case of blockages the whole pipe does not empty if the cap is removed.
Acces Fitting
alternative
Illus. 2.4 Principles of the direct separate discharge system (the stack discharges directly).
Illus. 2.5 Examples of balancing pipework.
Illus. 2.2 Principles of secondary discharge system.
Illus. 2.3 Example of restricted secondary discharge system.
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The uppermost method is most commonly applied.
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