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Design steps and criteria of distribution system

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.

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.

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.

✓ Answer checked by Claude Opus 5.5 (high effort)

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