CHAPTER 7

Water Transmission — Raw and Treated Water Mains

Q: What is the test pressure?

1.5 × working pressure for 2 hours hydrostatic test. Example: 8 bar working → 12 bar test. Leakage rate < 0.02 L/mm/km/hour/bar. Failed sections replaced and retested.

Water Transmission Mains

Specifies design of transmission mains (raw water from source to treatment plant, clear water from plant to reservoir or distribution zone). Covers pipe sizing, velocity limits, hydraulic grade line, pipe materials, laying, thrust blocks, air valves, scour valves.

🔀 Transmission MainsManual on Water Supply and Treatment3rd Edition (1999) with 2024 revision updates

Key formulas

Hazen-Williams equation (turbulent flow in circular pipes): V (m/s) = 0.849 × C × R^0.63 × S^0.54, where C = Hazen-Williams roughness coefficient, R = hydraulic radius (m), S = hydraulic gradient.
Hazen-Williams rearranged for Q: Q (m³/s) = 0.278 × C × D^2.63 × S^0.54, where D = internal diameter (m).
Darcy-Weisbach: h_f = f × (L/D) × (V²/2g), where f = friction factor (0.015-0.025 for typical pipe), L = length, D = diameter, V = velocity.
Economic diameter (Lischer formula): D (m) = 0.78 × Q^0.5 × (cost factor)^0.125 — balances pipe cost vs pumping energy cost.
Thrust at bend: T = 2 × P × A × sin(θ/2), where P = water pressure, A = pipe cross-section area, θ = bend angle.

Key values & thresholds

transmission velocity min mps: 0.6
transmission velocity max mps: 2.5
transmission velocity typical mps: 1.0 - 2.0
hazen williams C new cast iron: 130
hazen williams C aged cast iron: 100
hazen williams C ductile iron new: 140
hazen williams C steel new: 140
hazen williams C HDPE: 150
hazen williams C PVC: 150
hazen williams C concrete: 130
air valve spacing m: 500 - 1000 (at high points)
scour valve spacing m: at low points, each 2-3 km segment
isolating valve spacing m: 1500 - 3000
operating pressure max mwc: 100 (1 MPa)
test pressure factor: 1.5 × working pressure

Clause-level requirements

Velocity range 0.6-2.5 m/s: below 0.6 m/s silt deposits and stagnation; above 2.5 m/s erosion of pipe lining, water hammer severity, excessive pumping energy.
Hazen-Williams equation is standard for Indian water main design; C value depends on pipe material and age.
Pipe material selection: DI (ductile iron) — durable, most common for 100-1500 mm urban mains; MS (mild steel cement-lined) — large diameters and rising mains; HDPE — corrosion-resistant, for coastal/saline; PVC — small diameter; GI — service connections.
Air valves at all high points (every 500-1000 m or at local crests); scour valves at low points for flushing sediment; isolating valves every 1500-3000 m to allow maintenance sectioning.
Pipe laying: minimum 900 mm cover (urban roads), 750 mm (rural); uniform gradient preferred; no air pockets in profile; thrust blocks at bends, tees, valves.
Pressure testing: hydrostatic test at 1.5 × working pressure for 2 hours; leakage rate < 0.02 L/mm diameter/km/hour/bar. Failed sections replaced and retested.

Practitioner notes — what goes wrong in the field

Velocity 1.0-1.5 m/s is economic sweet spot — balances pipe cost (smaller diameter = cheaper) vs pumping energy (lower velocity = lower friction loss).
Hazen-Williams C decreases with age: new DI C=140 → aged 20-30 years C=100-110 → very old cast iron C=60-80. Always check existing main C before adding load.
DI is most common pipe for Indian water supply — reliable 80+ year life with cathodic protection. Costs ₹2,500-8,000 per meter depending on diameter.
MS cement-lined: used for very large diameters (> 1500 mm) — cheaper than DI per meter. Mumbai Bhandup-Powai main, Delhi Sonia Vihar main use MS.
HDPE: growing use for corrosion resistance and flexibility. Jointing via butt fusion or electrofusion. Cost 20-30% less than DI. Upper diameter limit practical 1000 mm.
Thrust at bend: 200 mm pipe at 10 bar pressure with 90° bend = 2 × 10 × 0.03 × sin(45°) = 0.4 kN thrust; small bend OK with soil resistance. 1200 mm pipe at 10 bar with 90° bend = 14 kN — requires thrust block.
Air valves: single-orifice (small air release during operation), double-orifice (large air release during fill/drain). Must be on all high points; if missed, water hammer + air-locked sections.
Water hammer: sudden valve closure causes pressure surge = ΔP = ρ × a × ΔV, where ρ = density, a = wave speed (≈1200 m/s in water), ΔV = velocity change. Mitigation: slow closure, surge tank, air chamber, pressure relief valve.
Pipe joints: DI mechanical (push-fit rubber gasket), MS welded, HDPE butt-fused, PVC solvent cement. Joint failures most common at DI mechanical joints — often caused by differential settlement.
Cathodic protection: sacrificial anodes (magnesium, zinc) or impressed current for buried metallic pipes in aggressive soil. Extends life 30-50 years.
Pipe bedding: sand/murram bed below pipe for uniform support; prevents point loading which causes cracks. Specify in contract.
Transmission main failures: air valves missing (52% of NRW audit issues), silt sediment at low points (reducing capacity), valve corrosion (non-operational when needed).

FAQs

What velocity in transmission main?

0.6-2.5 m/s range; 1.0-1.5 m/s economic optimum. Below 0.6 m/s silt deposits; above 2.5 m/s pipe erosion and water hammer. Most Indian transmission mains designed at 1.0-2.0 m/s.

What is Hazen-Williams C for ductile iron?

New DI pipe: C = 140; aged 20-30 years: C = 100-110. Used in Hazen-Williams equation for head loss calculation. Verify existing main's actual C before adding flow.

What air valve spacing?

Every 500-1000 m, at all local high points. Without air valves, trapped air causes flow reduction and water hammer. Double-orifice at pumping station start/stop points; single-orifice at intermediate high points.

How to calculate thrust at pipe bend?

T = 2 × P × A × sin(θ/2), where P = internal pressure, A = cross-section area, θ = bend angle. Example: 600 mm pipe, 8 bar pressure, 90° bend: T = 2 × 800 × π(0.3)² × sin(45°) = 320 kN. Requires engineered thrust block.

What pipe material is best?

No single best — depends on diameter and conditions: DI (100-1500 mm general use), MS cement-lined (> 1500 mm large mains), HDPE (corrosive/saline soil), PVC (< 300 mm minor), GI (service connections).

What is the test pressure?

1.5 × working pressure for 2 hours hydrostatic test. Example: 8 bar working → 12 bar test. Leakage rate < 0.02 L/mm/km/hour/bar. Failed sections replaced and retested.

Calculator

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Hazen-Williams Pipe Sizing & Head Loss

↓ Download Excel

Compute velocity and head loss for a given pipe using the Hazen-Williams equation. Standard Indian design basis for water supply transmission mains.

Inputs

Design flow Qm³/hr

500 m³/hr ≈ 12 MLD continuous

Internal diametermm

Pipe lengthm

Hazen-Williams C

DI new 130-140 · DI aged 100-110 · HDPE/PVC 150 · MS new 140

Outputs

Velocity

1.105m/s

V = Q / A = (Q/3600) / (π×D²/4)

↪ Target 0.6–2.5 m/s (1.0–1.5 economic optimum)

Hydraulic gradient

0.00291m/m

S = (V / (0.849 × C × R^0.63))^(1/0.54)

Head loss over length

2.911m

h_f = S × L

Head loss per km

2.91m/km

CPHEEO Reference Values

Velocity range0.6 – 2.5 m/s

Economic velocity1.0 – 1.5 m/s

C — DI new130 – 140

C — HDPE / PVC150

C — Cast iron aged60 – 80

Download the Excel version to keep a local copy with live formulas — change inputs in the sheet and outputs recompute automatically.

Cross-references

CPHEEO WS Chapter 9 DistributionIS 1172IS 3114 DI pipe laying