PE drainage explained

Basic principles

Waste water systems encompass the system of drainage and pressure relief pipes within a building and terminate 0,5 m outside the external wall. Waste water systems are based on the primary pressure-relief system in which water and air-flow occurs in the same pipe. The waste water system must be separated from the roof drainage system.

Pipe system and flow conditions
For a properly functioning interior sewer system, the flow conditions of the various types of pipe must be known so that they can be taken into account in the design. In the pipework for a soil & waste system, the following pipes are defined as indicated in the illustration at the right.

Connectors
A connector links a drain fixture to the collector, down or underground pipe. Only one drain fixture is to be connected to a connector. In an internal sewer, a connector can be both horizontal and vertical and may, in contrast to collectors, become completely full. To prevent stench traps from being siphoned empty, certain parameters are normally imposed on connectors. These parameters relate to:
- minimum pipe diameter
- maximum pipe length
- maximum height difference

Collectors
A collector is a horizontal pipe that links connectors to down pipes.

Down pipes
A down pipe is a drain pipe that deviates no more than 45º from vertical. A down pipe connects a collector to a ground pipe and is outfitted with a pressure-relief pipe.

In a soil & waste system, the design must focus on preventing large pressure difference in down pipes. Pressure differences in a down pipe can be reduced by implementing joint-free zones, appropriate types of joints for connectors and the proper sizing of the down pipe.
Underpressure arises in a down pipe as a result of a connector draining
waste water into the down pipe. Depending on the flow volume, the drainage water will gradually spread over the entire pipe wall. Due to gravity and pipe friction, the drainage water will reach a maximum velocity and carry the air in the core along with it. This creates a pressure difference that transmits up the pipe to a point above the input of the drainage flow.

Overpressure is created at the base of the down pipe where the drainage flow is forced to slow down substantially in transitioning to the horizontal collector or underground pipe. The air in the horizontal pipe can only escape to a limited extent, causing the air at the base of the down pipe to be pressed together, giving rise to overpressure. The transition to a collector or underground pipe must therefore occur using 2 x 45º bends so that the air in the horizontal pipe can flow away.

 

Pressure-relief pipes
A pressure-relief pipe must ensure proper air intake and venting of the internal sewer. The pressure relief is the part of the down pipe that extends above the highest collector joint and is open to the air above the roof surface. This is the principle of primary pressure relief, and it is associated with pressure differences as described in the chapter on down pipes. In some cases, secondary pressure-relief pipes are required to prevent large pressure differences from occurring. Such secondary pressure-relief pipes are directly or indirectly connected to the primary pressure-relief pipes.

Ground pipes
A ground pipe is a horizontal pipe that collects drainage flows from connectors, collectors and down pipes and discharges them into the external sewer. The ground pipe remains inside the 0,5 m boundary of the internal sewer.

Flow conditions in horizontal pipes
Flow conditions in horizontal pipes depend on the nature of the individual discharges that can, in turn, partly or fully coincide. In practice, draining occurs in waves that quickly reach their maximum level of drainage and subsequently subside to nil due to the friction of the pipes and fittings. Flow calculations are therefore extremely complex but are simplified into a system with 70% water filling of the pipe during the maximum expected flow volume. The remaining 30% air is unrestrictedly in contact with the pressure-relief pipe so that no large positive or negative pressure differences arise.

Pressure differences can be created in the pipe system if the drain flow hydraulically seals the pipe, causing the system to become completely full. Pressure differences can seriously disrupt the system, causing an odour problem due to the emptying of siphons. It is also possible that a noise nuisance can arise due to the enclosed air and air that is pressed through the stench traps.

A completely full system is more likely to occur when there is large incline because the high flow velocity tends to cause build up in the bends. The standard limits the incline to 1:50 (20 mm/m). A risk of hydraulic sealing also may occur where pipes join together, both in the case of two horizontal pipes and in the transition to a stand pipe. The standard therefore specifies requirements for the installation of drain pipes.