Bypass Pumping TDH Calculator (Hazen-Williams)

Bypass Pumping TDH Calculator

Hazen-Williams friction loss and total dynamic head for temporary bypass pumping and forcemain sizing. Runs entirely offline; inputs persist in this browser via localStorage.

Use limits: This is a field arithmetic aid for sizing temporary pumps and discharge lines, not a substitute for the approved bypass plan or the pump supplier's system analysis. Hazen-Williams applies to full-pipe, turbulent water flow at ordinary temperatures; it does not model sludge, high-solids flow, or air entrainment.

Flow & Discharge Piping

Design flow — total (GPM)

Parallel discharge lines

Flow is split equally per line; friction is computed per line.

Pipe inside diameter (in)

Use actual ID, not nominal — for HDPE the DR controls (see notes below).

Line length, one line (ft)

Hazen-Williams C

Minor loss allowance (% of friction)

Lump allowance for fittings, valves, and the discharge connection. 10–15% is typical for a reasonably direct temporary line; increase for lots of bends.

Static Head

Suction lift (ft)

Vertical distance from water surface down in the wet well / manhole up to the pump. Enter 0 for submersible or flooded-suction pumps.

Discharge static head (ft)

Elevation of discharge point minus elevation of the pump. Can be negative for a downhill discharge.

Results at Design Flow

Flow per line	—
Total flow	—
Velocity in line	—
Friction slope	—
Friction loss (one line)	—
Minor losses	—
Total static head	—
Total dynamic head (TDH)	—
TDH in psi	—

System Curve Points

Overlay these on the rental pump's published curve. The operating point is where the curves cross — if the pump curve is above the system curve at design flow, the pump will deliver at least the design flow.

% Design	Total GPM	Velocity (ft/s)	TDH (ft)

How the Numbers Are Derived

Hazen-Williams friction loss

Friction slope in feet of head per 100 ft of pipe, US customary GPM/inch form:

h_f = 0.2083 × (100/C)^1.85 × Q^1.85 / d^4.8655

where Q is flow in GPM in the individual line and d is actual inside diameter in inches. Total friction is the slope times line length divided by 100. Minor losses are applied as a percentage allowance on top of pipe friction.

Velocity and TDH

Velocity: V = 0.4085 × Q / d² (ft/s, Q in GPM, d in inches).

Total dynamic head: TDH = suction lift + discharge static head + friction + minor losses. Conversion to pressure: psi = ft × 0.4331.

HDPE actual inside diameters (common bypass sizes, IPS)

Nominal size is the OD basis for IPS HDPE; the wall (set by DR) eats into the bore. Approximate IDs in inches:

Nominal (IPS)	OD	DR 17	DR 11
4 in	4.500	3.94	3.63
6 in	6.625	5.80	5.35
8 in	8.625	7.55	6.96
10 in	10.750	9.41	8.68
12 in	12.750	11.16	10.29

IDs computed as OD − 2 × (OD/DR), ignoring the small manufacturing tolerance. Verify against the pipe supplier's submittal for the actual material on site.

Velocity guidance for sewage bypass

Minimum: keep velocity at or above roughly 2 ft/s (many specs say 2.5–3.5 ft/s) at the flow that actually runs most of the day, or solids settle in the temporary line and you get grease/rag accumulation and slug discharges.
Maximum: above roughly 8–10 ft/s, friction losses climb steeply, fused joints and restrained fittings see high thrust, and hose whip at the discharge becomes a hazard. It is usually cheaper to go up a pipe size than to buy the head.
Bypass systems sized for peak flow often run at a fraction of peak most of the time — check the velocity at average flow, not just peak.

References

Hazen-Williams empirical formula — standard form per Ten-State Standards and common hydraulics references (e.g. Lindeburg, Civil Engineering Reference Manual).

Plastics Pipe Institute, Handbook of Polyethylene Pipe — HDPE dimensions and C-factor guidance. https://plasticpipe.org/

Manufacturer pump curves — always verify the operating point against the published curve for the specific rental unit.