Lesson Notes By Weeks and Term v5 - Grade 11

Lesson Video

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Performance objectives

• Explain the principles of gravity drainage.
• Identify different types of drainage pipes and their uses.
• Sketch and label common drainage fittings like traps.
• Describe correct installation procedures for pipes.
• Calculate the required fall for drainage pipes.

Lesson summary

This week, we delve deeper into the essential systems that ensure hygiene, health, and comfort in our built environment: drainage, plumbing, and sanitation. These systems, often hidden within walls and under floors, play a critical role in our daily lives, especially in a South African context where access to clean water and effective sanitation can be a significant challenge. Imagine life without reliable running water or a functioning toilet – the impact on health, hygiene, and overall quality of life would be devastating.

Lesson notes

2.1 Principles of Gravity Drainage The core principle behind most drainage systems is gravity. Wastewater flows downhill from fixtures (sinks, toilets, showers) to the sewer or septic tank. This requires a carefully designed system with adequate slope or "fall" in the pipes. The gradient must be steep enough to ensure that solids are carried along with the liquid, preventing blockages, but not so steep that the liquid flows too quickly, leaving solids behind.

Fall/Slope: This is the vertical drop of the pipe per unit length. It's typically expressed as a ratio (e.g., 1:40) or as millimeters per meter (e.g., 25mm/m).

A fall of 1:40 means that for every 40 meters of horizontal distance, the pipe drops 1 meter vertically.

Velocity: The speed at which the wastewater flows through the pipe. Too slow, and solids settle; too fast, and it can cause erosion and noise.

Self-Cleansing Velocity: The minimum velocity required to keep solids in suspension and prevent blockages. This is typically around 0.75 meters per second. Full Bore Flow vs.

Part Bore Flow: Drainage pipes are designed for part bore flow. They should never run completely full, as this can create pressure issues and increase the risk of backflow. Part bore flow allows air to circulate, preventing siphoning of water seals in traps. 2.2 Types of Drainage Pipes Several types of pipes are used in drainage systems in South Africa, each with its own advantages and disadvantages: PVC (Polyvinyl Chloride): Lightweight, durable, easy to install, and relatively inexpensive. Resistant to corrosion and chemicals. Widely used for both above-ground and underground drainage. Available in various sizes and thicknesses (SABS mark is crucial for quality).

HDPE (High-Density Polyethylene): Very strong and flexible, resistant to impact and chemicals. Often used for underground drainage, especially where ground movement is expected. Can be joined by welding, creating a leak-proof joint. More expensive than PV

C. Clay Pipes (Vitrified Clay): Traditional material, very durable and resistant to chemicals.

However, they are heavy, brittle, and more difficult to install than PVC or HDPE. Less commonly used in modern construction but may still be found in older systems. Require skilled labor for jointing.

Cast Iron: Very strong and durable, but heavy and expensive. Primarily used in older buildings or for specific applications where high strength and noise reduction are required. Susceptible to corrosion.

Example: Comparing PVC and HDPE: | Feature | PVC | HDPE | |--------------------|--------------------------------------|-----------------------------------------| | Cost | Lower | Higher | | Flexibility | Less flexible | More flexible | | Jointing | Solvent welding, rubber ring joints | Welding, mechanical fittings | | Impact Resistance | Lower | Higher | | Common Use | Above-ground and shallow underground | Underground, areas with ground movement | 2.3 Drainage Fittings Fittings are essential for connecting pipes, changing direction, and providing access for cleaning and maintenance.

Common fittings include: Bends (45°, 90°): Change the direction of the pipe run. Gradual bends (45°) are preferred to sharp bends (90°) to reduce flow resistance and the risk of blockages. Junctions (Y-junctions, T-junctions): Connect branch pipes to the main drain. Y-junctions are preferred because they allow a smoother flow path. Traps (P-traps, S-traps, Bottle Traps): Water-filled fittings that prevent sewer gases from entering the building. The water seal in the trap acts as a barrier. The depth of the water seal is critical (typically 50-75mm).

Couplings: Connect two straight lengths of pipe.

Reducers: Connect pipes of different diameters.

Access Points/Inspection Chambers: Provide access to the drainage system for cleaning and inspection. Typically located at changes in direction, junctions, or at regular intervals along the pipe run.

Sketch of a P-Trap: ``` Inlet Pipe | | ------- | | | | Water Seal | | ------- | | Outlet Pipe ``` 2.4 Installation Procedures Proper installation is crucial for the long-term performance of a drainage system.

Key considerations include: Bedding: Pipes must be laid on a firm, even bed of granular material (e.g., sand or gravel) to provide support and prevent damage. The bedding should extend along the entire length of the pipe and should be compacted properly.

Jointing: Pipes must be joined correctly to create a watertight seal. For PVC pipes, solvent welding is commonly used. The surfaces to be joined must be clean and dry, and the correct solvent cement must be used. For HDPE pipes, welding or mechanical fittings are used.

Fall: The correct fall must be maintained throughout the pipe run to ensure efficient drainage. This can be achieved by using a spirit level and adjusting the bedding as necessary.

Testing: After installation, the drainage system should be tested to ensure that it is watertight.