All tools

Hazen-Williams Head Loss — 6-inch PVC Water Main Worked Example

A distribution lateral capacity and pressure-loss check on a new 6-inch C900 PVC water main serving a small commercial pad. The booster pump delivers 65 psi at the pump discharge; we need the residual pressure and velocity at the end of an 850-ft run at the design flow of 250 gpm. Hazen-Williams handles the distributed friction loss; the velocity-head method adds localized losses at fittings. Sources: AWWA M11 (steel pipe design, Hazen-Williams application and velocity guidance), AWWA M22 (PVC pipe), and Crane TP-410 for minor-loss K values.

The lateral

An 850-ft run of 6-inch DR-18 C900 PVC ties a new 4-lot commercial cluster into an existing 8-inch county main. The pump curve and static head give 65 psi at the pump discharge flange at design flow. Field layout includes two 90° bends and one fully open gate valve in the run. Design drawings give:

Pipe	6-inch C900 PVC, nominal OD 6.90 in; use D = 6.0 in for Hazen-Williams (ID ≈ 5.9 in — within typical tolerance)
Length L	850 ft (centerline, including fittings)
Hazen-Williams C	150 (new PVC; see C reference)
Design flow Q	250 gpm
Pump discharge pressure	65 psi at design Q
Fittings	2 × 90° long-radius flanged elbows; 1 × gate valve, fully open
Velocity target	3–6 ft/s per AWWA M11 transmission/distribution guidance

Step 1 · Unit conversions

Flow and diameter in Hazen-Williams form

The PE-Calc Hazen-Williams equation uses US customary units with Q in gpm, D in inches, and L in feet. Convert to cfs only for the velocity check.

Given: Q = 250 gpm, D = 6.0 in, L = 850 ft, C = 150

Q (cfs) = 250 / 448.831 = 0.557 cfs
D (ft) = 6.0 / 12 = 0.500 ft
A = π(D/2)² = π(0.25)² = 0.196 ft²

Step 2 · Friction head loss (Hazen-Williams)

Distributed loss along the barrel

AWWA M11 and most municipal hydraulic manuals use the Hazen-Williams formula in US units (valid for water at typical temperatures, turbulent flow, and pipe diameters above ~2 in):

h_f = 4.52 · L · Q^1.852 / (C^1.852 · D^4.87)

Exponents (evaluate once):
  Q^1.852 = 250^1.852 = 41,842
  C^1.852 = 150^1.852 = 16,847
  D^4.87 = 6^4.87 = 5,992

Friction loss:
  h_f = 4.52 · 850 · 41,842 / (16,847 · 5,992)
  h_f = 160,629,164 / 100,907,624 = 1.61 ft

Confirm in the Hazen-Williams tool with L = 850 ft, D = 6 in, Q = 250 gpm, C = 150 → h_f ≈ 1.6 ft.

Step 3 · Minor losses at fittings

Velocity-head method: h_m = ΣK · V² / (2g)

First compute mean velocity (same value used for every fitting in this run):

V = Q / A = 0.557 / 0.196 = 2.84 ft/s

K values (from minor-loss reference, flanged water-main fittings):
  2 × 90° LR elbow, flanged: K = 0.20 each → 0.40
  1 × gate valve, fully open: K = 0.15
  ΣK = 0.55

h_m = 0.55 · (2.84)² / (2 · 32.2)
  = 0.55 · 8.07 / 64.4 = 0.09 ft

Minor losses are small here because velocity is low. If this were a 4-inch lateral at the same Q, V would exceed 6 ft/s and the fitting losses would dominate the friction budget.

Step 4 · Total head loss and residual pressure

Feet of head to psi at the end of the line

Total head loss (friction + minor):
  h_total = 1.61 + 0.09 = 1.70 ft

Pressure conversion (fresh water, ≈ 0.433 psi/ft at 60°F):
  ΔP = 1.70 · 0.433 = 0.74 psi

Residual at end of lateral (pump discharge minus line loss, neglecting elevation):
  P_end = 65.0 − 0.74 = 64.3 psi ✓

Loss component	Head (ft)	Pressure (psi)
Pipe friction (Hazen-Williams)	1.61	0.70
Minor losses (ΣK = 0.55)	0.09	0.04
Total	1.70	0.74

Pressure is not the constraint on this reach — the 850-ft run loses less than 1 psi. Fire-flow or peak-day scenarios at higher Q are where Hazen-Williams head loss actually matters on a 6-inch lateral.

Step 5 · Velocity check (AWWA M11)

Is V in the 3–6 ft/s range?

AWWA M11 recommends keeping distribution velocities generally between 3 and 6 ft/s to limit surge risk, water-age concerns in dead ends, and transient pressures — while staying below values that accelerate tuberculation or erosion in other materials. Check at design Q:

V = Q / A = 0.557 / 0.196 = 2.84 ft/s

AWWA M11 target: 3–6 ft/s
Result: 2.84 ft/s < 3 ft/s — marginally below the lower bound

Flow needed for V = 3.0 ft/s in 6-inch:
Q = 3.0 · 0.196 · 448.831 = 264 gpm

Low velocity is not automatically a pass. At 250 gpm the lateral sits just under the 3 ft/s AWWA lower guideline. For a short commercial lateral with modest demand, many authorities accept 2.5–3 ft/s if water quality (chlorine residual, stagnation) is addressed. If the owner expects future build-out to 400+ gpm, upsize to 8-inch now or accept higher velocity and head loss at peak. Do not upsize based on pressure loss alone — this pipe has plenty of pressure margin.

Step 6 · Gotchas: aged C and when to leave Hazen-Williams

Design-life friction and small-pipe limits

Aged pipe check (assume C = 130 after 20–30 yr biofilm/sediment, per AWWA M22 aged-PVC guidance):
  h_f = 4.52 · 850 · 41,842 / (130^1.852 · 5,992)
  h_f = 2.09 ft (ΔP ≈ 0.91 psi) — still minor at 250 gpm

At fire-flow Q = 500 gpm (illustrative, C = 150):
  h_f ≈ 5.5 ft (1.4 psi); V ≈ 5.7 ft/s — inside AWWA range

New C vs. aged C. Plans often size on C = 150 for new PVC, but the utility’s C-factor model may require C = 130–140 for head-loss allocation over the system’s design life. At 250 gpm on 6-inch, the difference is only ~0.2 psi — negligible. At 1,000 gpm on the same pipe, aged-C friction is roughly 30% higher and may fail a fire-flow test.

When to switch to Darcy-Weisbach. Hazen-Williams is empirical for water in turbulent flow and breaks down for: (1) pipe diameters under ~2 inches, where f and Reynolds number matter; (2) non-water fluids or extreme temperatures; (3) HDPE or insert-rehab liners where equivalent roughness ε is better documented than C; and (4) surge/transient analysis, which needs wave speed and needs Darcy-Weisbach or full fluid transient software. For service laterals and mains above 4 inches carrying water at ordinary temperatures, Hazen-Williams remains standard practice per AWWA M11.

Step 7 · What we did NOT check, and when you would

Beyond a single-pipe Hazen-Williams run

No elevation profile. This example assumes level grade. A 40-ft rise adds 17.3 psi static loss not counted here.
No pump curve intersection. We took 65 psi at 250 gpm as given. A full design plots the system head curve (static + friction + minor + PRV settings) against the pump curve to find the actual operating point.
No water-hammer / surge. Closing a hydrant or power failure at the booster can produce transient pressures well above steady-state. AWWA M11 surge chapters and software such as KYPIPE or Bentley HAMMER apply.
No equivalent-length shortcut audit. Some spreadsheets replace fittings with extra pipe length (e.g., 25 ft per elbow). We used explicit K values; equivalent-length methods should reconcile within ~10% for the same V.

Tools used in this example

Reproduce every step in the live PE-Calc tools: Hazen-Williams equation (friction head loss). The Hazen-Williams C = 150 came from the Hazen-Williams C reference card — “PVC, HDPE (new)”. Fitting K values came from the minor loss coefficients card. For comparison on the same diameter at higher roughness, run Darcy-Weisbach with PVC equivalent roughness.

Buying the lot? Check the water service before you buy.

Available pressure at the tap, main size, and distance to the county line all affect whether a 6-inch lateral is even feasible without a booster. Easements, floodplain limits, and utility conflicts show up in the feasibility review long before Hazen-Williams. SitePrior screens FEMA, NWI, NRCS soils, and USGS context for $29 in about 60 seconds. Run it before the pre-application meeting, not after the easement is recorded.