Water Supply Engineering – Sizing of water distribution systems For municipal water networks, the design diameter of distribution mains is typically chosen to satisfy which combination of demand conditions?

Introduction / Context:
Water distribution systems are sized so that pressures and velocities remain within permissible limits while meeting critical demand scenarios. Designers consider diurnal variations, peak factors, and contingency flows such as firefighting. This question asks which demand combination is commonly used as a governing condition for sizing distribution mains in urban networks.

Given Data / Assumptions:

• Public water supply system with storage and pumping arranged to meet variable demand.
• Fire demand may occur concurrently with normal consumption.
• Hydraulic design must respect minimum residual pressure at hydrants and consumer taps.

Concept / Approach:

Distribution mains convey water from service reservoirs to consumers and hydrants. The “maximum daily demand” represents the higher end of routine consumption over a day. Fire demand (a short-duration but high-rate withdrawal) can coincide with peak-day consumption. Sizing for “maximum daily demand plus fire demand” ensures that, even during a fire event on a peak day, adequate residual pressures and flows are available without unacceptable headloss or velocities. Transmission mains and storage are sometimes checked for maximum hourly demand, but distribution main diameters are usually governed by the fire-plus-peak-day criterion in many design manuals and exam standards.

Step-by-Step Solution:

Verification / Alternative check:

Many standard texts specify checking both “maximum hourly demand” and “maximum daily + fire.” In distribution design, the latter commonly governs because fire flows are large and local headlosses are significant in reticulation pipes.

Why Other Options Are Wrong:

(a) Ignores simultaneous fire flow; (b) is too low for design; (d) uses average day instead of maximum day; (e) suggests the smaller of two peaks, which is unsafe—design should envelope the more severe condition.

Common Pitfalls:

Using peak-hour factors without superimposing fire flow, neglecting residual pressure criteria at hydrants, or overlooking looping benefits that reduce headloss.

Final Answer:

Maximum daily demand plus simultaneous fire demand