Self-cleansing versus scouring in sewers: In which sewer material is the highest non-scouring (erosion-safe) velocity permissible?

Introduction / Context:
Sewer design balances two limits: a minimum self-cleansing velocity to prevent deposition and a maximum non-scouring velocity to prevent erosion of the pipe wall. Different materials withstand different upper velocities before scouring becomes critical.

Given Data / Assumptions:

• Comparing typical non-scouring limits among common sewer materials.
• Standard practice values: cast iron has the highest abrasion resistance among options listed.

Concept / Approach:
Non-scouring velocity is the maximum safe velocity that does not erode the internal surface. Materials with high strength and smooth, durable interiors tolerate higher velocities. Cast iron is more resistant to abrasion and impact than masonry or concrete in sewer service.

Step-by-Step Solution:
1) List materials by erosion resistance: cast iron > vitrified clay ≈ concrete > brick.2) Identify which allows the greatest non-scouring velocity: cast iron.3) Select the corresponding option.

Verification / Alternative check:
Design manuals commonly specify higher permissible velocities for metallic pipes relative to concrete or brick due to better abrasion resistance and surface hardness, confirming the choice.

Why Other Options Are Wrong:
Stoneware: Smooth but brittle; non-scouring limit lower than cast iron.Cement concrete: Prone to surface wear under high velocity, especially with grit.Glazed brick: Mortar joints and brick faces are less resistant to high-velocity abrasion.Asbestos cement: Not listed in the stem originally and generally has lower abrasion resistance than cast iron.

Common Pitfalls:

• Confusing self-cleansing (minimum) with non-scouring (maximum) velocities.
• Assuming smoothness alone governs scouring; material strength also matters.

Final Answer:
Cast iron sewers