--- title: "Pipe material factors; CI vs steel" subject: "Water Supply Engineering" chapter: 8 key: "semester_5/water_supply_engineering:pipe-material-factors-ci-steel" source: https://www.bctengineer.com/q/semester_5/water_supply_engineering/pipe-material-factors-ci-steel site: BCT Engineering --- # Pipe material factors; CI vs steel Water Supply Engineering · Chapter 8: Conveyance of Water - Asked in: 2068 Baisakh (old course), 4+4 marks - Video: [Ductile Iron Pipe vs Steel Pipe | Which Is Better for Water & Infrastructure?](https://www.youtube.com/watch?v=dOTnKFwcETo) — Yasmin Trading - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/water_supply_engineering/pipe-material-factors-ci-steel - Whole chapter: https://www.bctengineer.com/md/semester_5/water_supply_engineering/chapter-8 ## Question Describe the factors that should be considered while selecting pipe materials for water supply schemes. Give comparative merits and demerits of cast iron and steel pipes. ## Short answer Factors include pressure, corrosion, friction, cost, weight, jointing ease, availability, and life. CI: strong in compression, brittle, long life, corrosion-resistant, heavy, socket joints. Steel: strong in tension, ductile, needs coating against rust, lighter, welded joints, cheaper in large diameters. ## Answer #### Factors for selecting pipe materials | Factor | Consideration | |---|---| | Pressure | Working and surge pressure decide strength and class | | Corrosion | Soil and water quality; need for lining or coating | | Friction | Smooth interior gives a smaller pipe size | | Cost | Material, transport, laying and repair; life-cycle cost | | Weight and handling | Affects labour and transport in hills | | Jointing | Ease and reliability of the joint | | Availability | Local supply of pipes and fittings | | Life and maintenance | Durability, ease of repair and tapping | #### Cast iron versus steel pipes | Point | Cast iron | Steel | |---|---|---| | Strength | Strong in compression, weak in tension and impact; brittle | Strong in tension, ductile; high pressure and shock | | Corrosion | Resists corrosion better; life 100+ years | Rusts quickly if unprotected; needs paint, coal-tar or cement lining; life 25–50 years | | Weight | Heavy | Lighter for the same pressure (thin wall) | | Diameter available | Up to about 1000 mm | Large diameters (above 2 m possible) | | Joints | Socket-spigot, flanged; easy | Welded, riveted, flanged; welding needs skill but gives a continuous line | | Cost | Costly | Cheaper in large diameters | | Repair | Easy to cut and drill; breaks by hammer | Easily cut and welded; flexible against ground movement | | Uses | Distribution mains | Transmission mains under high head, river crossings, exposed lines | ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why is soil and water quality a selection factor? A. It decides the pipe colour B. It decides corrosion risk and whether lining or coating is needed C. It fixes the pipe length D. It sets the jointing gasket thickness Answer: B. Aggressive soil or water corrodes pipes, so material or protective coating must be chosen accordingly. (Key point: Corrosion factor) 2. How does a smooth interior help the scheme? A. Removes need for valves B. Makes the pipe stronger in tension C. Raises pressure D. Allows a smaller pipe size for the same flow Answer: D. Less friction loss means a smaller diameter can carry the flow. (Key point: Friction) 3. Which describes cast iron compared with steel? A. Lighter than steel for same pressure B. Strong in tension, ductile C. Strong in compression, weak in tension and brittle D. Cannot resist corrosion Answer: C. CI is brittle: good in compression, poor in tension and impact. (Key point: CI strength) 4. Why is steel preferred for transmission mains under high head and river crossings? A. It is ductile, strong in tension and available in large diameters B. It is brittle and heavy C. It never rusts D. It is cheaper than CI in small sizes only Answer: A. Steel's tensile strength, shock resistance and large sizes suit high-head, exposed lines. 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