--- title: "Design steps and criteria of distribution system" subject: "Water Supply Engineering" chapter: 7 key: "semester_5/water_supply_engineering:distribution-design-steps" source: https://www.bctengineer.com/q/semester_5/water_supply_engineering/distribution-design-steps site: BCT Engineering --- # Design steps and criteria of distribution system Water Supply Engineering · Chapter 7: Reservoirs and Distribution System - Asked in: 2072 Kartik, 8 marks; 2070 Chaitra, 8 marks; 2070 Asar, 8 marks (asked 3 times) - Video: [How to Design Water Supply System - Part I](https://www.youtube.com/watch?v=FBu_DU-hK04) — Phil 4 Science - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/water_supply_engineering/distribution-design-steps - Whole chapter: https://www.bctengineer.com/md/semester_5/water_supply_engineering/chapter-7 ## Question State the factors you would take into consideration and the procedure (design steps and design criteria) you would follow in designing a distribution system for the water supply of a (city) community. ## Short answer Factors: population, demand, topography, road pattern, available head, cost, future expansion. Procedure: collect data, forecast demand, select system and layout, locate reservoir, zone area, compute pipe flows, size pipes using Hazen-Williams or Hardy-Cross, check velocity (0.6-3 m/s) and pressure (10-15 m residual), add appurtenances, prepare drawings and estimates. Criteria: minimum 100mm for fire lines, hydrants every 100-150m, 0.75-1m pipe cover. ## Answer Distribution system design is the process of finding the layout, pipe sizes and appurtenances that supply the required quantity of water at the required pressure to all consumers at the least cost. #### Factors to consider 1. Population now and in the design year; growth rate and settlement pattern. 2. Per capita demand, peak factors, fire demand and institutional/commercial needs. 3. Topography and ground levels; position of the source and the reservoir. 4. Road pattern and the town plan. 5. Available head (gravity or pumping) and required residual pressure. 6. Cost, availability of pipe materials, and ease of operation and maintenance. 7. Future expansion and water quality (to avoid stagnation). #### Procedure 1. Collect data: map with contours, road plan, population, source yield and level. 2. Forecast the population for the design period and find the demand: average daily, maximum day (1.5–2 × average) and peak hour (about 2.5–3 × average, i.e. about 1.5 × maximum day). 3. Select the system: gravity, pumping or dual; continuous or intermittent. 4. Choose the layout (dead end, grid iron, ring or radial) to suit the town. 5. Fix the location and capacity of the service reservoir (balancing + fire + emergency storage). 6. Divide the area into zones and find the discharge in each pipe for the peak demand. 7. Size the pipes with Hazen-Williams (or Darcy-Weisbach); for looped networks use Hardy-Cross or software. 8. Check velocity and pressure everywhere; revise the sizes until the criteria are satisfied. 9. Provide valves, hydrants, air valves, scour valves, break pressure tanks and house connections. 10. Prepare drawings, estimate the cost, and compare the alternatives. #### Design criteria - Velocity: 0.6–3.0 m/s (minimum 0.6 to prevent silting; maximum limited to avoid water hammer and erosion). - Residual head: at least 10–15 m at the tap (about 7–10 m for rural stand posts); maximum static pressure about 60–70 m, otherwise use BPTs or pressure-reducing valves. - Minimum pipe diameter: 100 mm for fire hydrant lines in towns; 15–25 mm for rural branches. - Hydrants every 100–150 m; sectional valves at every junction. - Pipes laid with at least 0.75–1 m cover. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. For design, which demand is used to find the discharge in each pipe? A. Average daily demand B. Peak hour demand (about 3 x average) C. Minimum night demand D. Fire demand only Answer: B. Pipes are designed for the peak hour demand, about 3 times average or 2-3 times the maximum day. (Key point: Peak demand factors) 2. How are pipes sized in a looped network? A. Hazen-Williams with Hardy-Cross or software B. Manning formula only C. By choosing the smallest standard size D. By Stokes law Answer: A. Head loss is by Hazen-Williams (or Darcy-Weisbach); loops are balanced by Hardy-Cross or software. (Key point: Pipe sizing method) 3. Why is a minimum velocity of 0.6 m/s specified? A. To prevent water hammer B. To prevent silting in the pipe C. To reduce pressure D. To save pipe cost Answer: B. Minimum 0.6 m/s stops silt settling; the maximum (3 m/s) limits water hammer and erosion. (Key point: Velocity limits) 4. What is done if the maximum static pressure exceeds about 60-70 m? A. Reduce the pipe diameter B. Use break pressure tanks or pressure-reducing valves C. Increase the velocity D. Remove the sectional valves Answer: B. BPTs or PRVs are used to cut excessive pressure. 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