Peripheral requirements for removal of waste water
Peripheral requirements for removal of waste water
3.4.1 Connections to stacks
3.4.2 Angled sections Angled sections must be avoided. It was previously believed that angled sections were useful in reducing the speed of fall. If an angled section is nevertheless unavoidable because of cables or other services, then the length of the branch may be 1.50 metre at most and the bends used may be a maximum of 45° (illustration 3.12).
3.3.3 Connection
separations and sequence (illustration 3.8)
- Method
Connections to the stack should be at right angles to prevent hydraulic sealing in the collecting pipes. With connections at less than 45°, pressure differences in the stack are lower but overpressure is created in the collecting pipe, which then requi- res additional ventilation. At high rates of discharge from the collecting pipe hydraulic sealing may occur in the stack immediately below the connec- tion. A 90° swept T-piece may be used to reduce the likelihood of this. The precondition is that the top side is straight, otherwise the solution is contra-productive. A variety of possible situations are shown in illustration 3.10. Where a toilet is directly connected to the stack, it is recommended that the upper surface of the water seal is at least 100 mm higher than the under- side of the pipe connecting to the stack. In the case of a connection to 110 mm that will mean that the upper surface of the water in the trap seal will be at least up to the upper surface of the connecting pipe. This prevents back-flowing foul water appearing in the pan. Mutual separations and angles of connection. In order to limit interac- tions the distance between pipes connecting to the stack must be greater than 0.5 m if the included angle is greater than 90° (illustration 3.11).
In order to prevent interactions as much as possible there are requirements set down for separation distances. The dis- tances are so selected that a subsequent connection smoothes out the flow from the previous connection, allowing ade- quate air to remain above the water level. The separation must be at least 5 x D of the collecting pipe. If the collecting pipe is at least 110 mm and the calculated Q¡ of the pipe furthest upstream is no more than 0.75 l/s then the distance may be 2 x D. If a toilet, dishwasher or washing machine is connected upstream of a floor-gully shower connection, then the separation must be at least 1 metre to prevent hydraulic sealing through the large quantities of water and siphoning of the shower trap seal. No other connection may be made for a length of 1 metre downstream of a toilet connection unless that other connecting pipe is provided with a discharge. Only toilets and no other fitting may be connected upstream of a toilet connection, unless the connecting pipe is provided with a discharge. The upstream fitting (not being a toilet) may be connected (at least 1 metre) downstream of the toilet connection (illustration 3.9). Strive to connect fittings with a high Q¡ to the upstream section of the col- lecting pipe, so as to prevent fouling and deposition as much as possible in that section. In order to prevent interactions as much as possible there are requirements set down for separation distances.
Illus. 3.9 Situation 6 from illustration 3.8.
A
B
Fitting
Fitting
L
1 any, unless a toilet
any, unless a toilet
5 x D
3.4.3 Stack foot and
any, unless a toilet
2 x D, if D 110 mm
2 any, if Q¡ 0,75 l/s
connection-free zones Overpressure is created by compression at the transition from the stack to a horizontal pipe or underground pipe. In order to prevent this, two 45° bends with an intermediate section of at least 0.25 metres should be used in place of a right-angle bend. This will smooth out the flow (illustration 3.13). Also noticeably less noise will be generated. Where a horizontal branch is required on a stack, connection-free zones should be established alongside both bends (illustration 3.13). 1 Metre connection-free zones are required for each 10 metres of stack length (between the horizontal pipe and the highest connection) where the stack joins a horizontal pipe. Where the stack length is between 10 and 20 metres the connection-free zones must be 2 metres long. Connection- free zones of 1 metre are required at the horizontal/vertical transition (illustration 3.13). The diameter required for the horizontal section is of course calculated as for common or underground piping, and the largest calculated diameter is used throughout. An alternative solution must be found where connections are nevertheless required in connection-free zones (illustration 3.14).
3 toilet, dishwasher or washing machine
floor gully shower
1 m (L1 )
4 toilet
toilet
5 x D
any, (no floor gully shower)
5 toilet
if with secondary discharge to B
5 x D
if connected
6 any (not a toilet)
toilet
1 m downstream of B, illustration 3.9
Illus. 3.8 Connection separations and sequences.
3.4 Stacks Since the upwardly extended section of the stack also serves as the vent line in a primary discharge system, great care must be taken to ensure that no hydraulic sealing or excessive pressure differences arise. A free flow of air, including the col- lecting pipework, is of crucial importance. The vulnerable areas are: the in-flow from the collecting pipework and any bends, including that at the foot of the stack where this enters the underground pipe, and any angled bends or horizontal branches in the stack. When waste water is discharged into the stack it will flow down, after passing a short length, more or less along the pipe walls. A column of air with some water will also be drawn downwards. The air and the pipe wall will slow the falling water. Depending on the diameter and the quantity of water, the
speed of flow will be between 7 and 11 m/s after 10 to 15 metres and will not increase beyond this. 5 to 12 times as much air as water is required. The supply of air via the ventilating pipe is therefore essential. It is important here that the stack is as straight as possible. An underpressure will be created in the upper section of the stack. This may not exceed 300 Pa for stacks used for remo- val of waste water (traps must be 500 Pa = 50 mm). The determination of diameter as shown in illustration 4.2 is based on this. The diameter of the stack may not be smaller than that of any one of the collecting pipes connected upstream. Overpressure will exist lower in the stack. In general no attempt is made to limit this, but its effect (the forced emptying of traps) is eliminated by establishing zones where no connections may be made.
Illus. 3.10 Various connections to a stack.
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