Peripheral requirements for removal of waste water
3. Peripheral requirements for removal of waste water
As well as proper calculation and selec- tion of the appropriate diameter, a num- ber of other factors are important to achieve good operation and prevent hydraulic seals, fouling of pipes and excessive overpressures and under- pressures. This chapter deals with these peripheral requirements. For this purpose we now follow the pipework from the source of the discharge onwards. 3.1 Traps Each discharging fitting must be provided with a trap. A certain diameter will be required, depending on the baseline discharge level (see illustration 1.2). If too small a diameter is chosen then emptying of the fitting will take too long. Self- siphoning (emptying of the trap through suction when the fitting is discharged) may occur, and noise will increase. On the other hand, an excessive diameter will lead to a lower speed of flow. This results in greater difficulty in carrying away soiling and blockages may result. There must be at least 50 mm of water (500 Pa) in a trap. The seal will remain intact if a maximum underpressure of 300 Pa arises in the system, even if some of the water in the trap has evaporated. Traps with rubber seals are to be preferred because of easier cleansing and replacement. This also prevents problems arising from the use of different materials. There are two main types of trap, the tubular (M,D,P,S types) and the bottle type (which includes floor-mounted varie- ties). Each type has its advantages and disadvantages. The bottle types are more readily blocked, but also more easily cleared. The bottle types lose less water to evaporation and are less sensitive to pressure differences (there is more water to be put into motion). A tubular trap is less prone to blocking because the speed of flow is higher and the shape allows free flow.
ted using a connecting pipe. During discharge the connecting pipe must be completely filled with water. No require- ments are therefore set down for the minimum fall. Some fall is certainly requi- red however as the pipe must empty fully. If the pipe is completely filled a plug of water is created which can cause under- pressure. This empties the trap by suc- tion and gives rise to gurgling sounds (illustration 3.1). Complete filling of the pipe must therefore make way as rapidly as possible for partial filling. Significant here are the length of the connecting pipe, its diameter, whether the pipe runs vertically or horizontally and the number of changes in direction. The required dia- meter is stated in illustration 1.2. If the total length of the connecting pipework from the trap to the common stack is greater than 3.5 metres then a diameter required for the excess must be determi- ned as if it were a collecting stack, using 3.3 and illustration 4.1. The total length of a connecting section may not exceed 12 metres. The diameters of parts a and b in illustration 3.2 may be smaller provided the following conditions are met: total length less than 3.5 metres
3.3.1 General
Additionally, the maximum permitted sum of changes in direction is 135°. Where these conditions are met the flushed contents of the pan can reach the vertical stack or under- ground pipe in one go and blockages and deposits are prevented. Length is taken to mean the horizontal length from the toilet to the vertical stack or underground pipe. All reductions in the collecting pipe must be eccentric (top of pipe at a single level).
requirements The diameter must be determined on a section by section basis (from connecting section to connecting section). Collecting pipes must be as straight and as short as possible. Bends must not be tighter than 45°. To prevent soil being left behind, the maximum sum of changes in direction depends on the fall (illustra- tion 3.3). If the sum of changes in direction is larger, the next largest diameter should be selected for the extra length, unless discharge takes place upstream. For a section with no toilet, the maxi- mum length may be 12 metres, if two fittings are connected, where the Q¡ for one of these is no greater than 0.5 l/s (illustration 3.4). For a section including a toilet and other fittings with Q¡ less than 0.75 l/s, the permitted maximum length is dependent on the fall (illustration 3.4).
a < 0,5 m b < 1,5 m; if b > 1,5 m calculate as a stack
a + b + c < 3,5 m; if > 3,5 m calculate as a collecting pipe
Illus. 3.2 Conditions for reduction in
diameter of a connecting pipe, see also illustration 1.2.
part a less than 0.5 metres part b less than 1.5 metres (total of the vertical sections).
Illus. 3.1 Self-siphoning or suction emptying of the trap.
The diameter thereby permitted is stated in illustration 1.2. In view of the possibility of damage and for maintenance it is recommended that a rubber sleeve should be used in the floor (at location x). 3.3 Collecting pipes Many different factors may affect collec- ting pipes. This accounts for the many conditions set down to achieve the problem-free removal of waste water. These requirements serve two main aims: To ensure that free-flowing air remains available above the water. To ensure that no fouling takes place.
from
to
max sum of changes of direction
example: 1:200 section d larger Ø 1:150 section d larger Ø
1:50 1:75
1:75 1:100 1:140 1:180
221/2 °
45°
1:100 1:140 1:180
671/2 °
1:100 section c en d larger Ø 1:50 section b,c en d larger Ø
90°
1:200 1121/2 ° Where these limits are exceeded, the next diameter up should be selected for the extra length, unless discharge takes place upstream.
3.2 Connecting pipes Every fitting must be separately connec-
Illus. 3.3 Maximum permissible sum of changes of direction for a particular fall in collecting pipes.
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