Sizing a wastewater force main to Ten States Standards

Why "force main" sizing isn't just "pick a pipe big enough"

A wastewater force main carries pressurized sewage from a lift station to a downstream gravity sewer or treatment plant. Two competing pressures shape every design decision:

• Don't go too slow. Below ~2 ft/s, solids settle out, biofilm accumulates, and the pipe slowly accretes a layer of grit you can't easily flush. Cycle pumps long enough at low velocity and you'll be replacing the force main on a 15-year cycle instead of a 50-year one.
• Don't go too fast. Above ~8 ft/s, the same solids that were causing settling problems become abrasive — they erode pipe walls, especially at fittings and bends. Upper-bound erosion damage is a very real failure mode in long-lived municipal force mains.

The Ten States Standards (formally: *Recommended Standards for Wastewater Facilities* from the Great Lakes — Upper Mississippi River Board) codify these limits as design rules. If you're working in any of the ten member states, this is the design code you cite.

The Ten States envelope

For force mains specifically (Section 44, current edition):

• §44.4 Velocity. Force main velocity shall be at least 2 ft/s at design average flow to prevent solids deposition. There is no hard maximum, but velocities above 8 ft/s are discouraged due to erosion + water hammer concerns.
• §44.5 Diameter. Force mains shall be at least 4 inches in diameter. Smaller pipes clog readily.
• §44.6 Air relief. Air-and-vacuum release valves at high points; combination valves where required.
• §44.7 Identification. Cleanout / inspection access at intervals.
• §44.8 Termination. Terminate to a manhole upstream of any treatment process.

The sizing problem becomes: pick a pipe diameter that delivers ≥2 ft/s at design average flow without exceeding 8 ft/s at peak flow.

A worked example

Lift station design conditions:

• Average dry-weather flow: 250 GPM.
• Peak hour flow: 700 GPM (peaking factor of 2.8 — typical for a small system).
• Force main length: 4,500 ft.
• Static lift: 28 ft.
• Material: ductile iron, C = 120 (aged conservative — Ten States §47.2 recommends design with aged values).

Step 1 — diameter candidates that satisfy minimum scour

Velocity at average flow must be ≥ 2 ft/s:

v = 0.4085 · Q / d²

Solve for d at v = 2 ft/s, Q = 250 GPM:

d² = 0.4085 · 250 / 2 = 51.06 → d ≤ 7.15"

So any nominal pipe ≤ 7" inside-diameter passes scour at average flow. Translating to nominal pipe sizes:

| Nominal | Inside Dia (DI) | v at Q=250 (avg) | v at Q=700 (peak) | |---------|-----------------|------------------:|-------------------:| | 4" | 4.10" | 6.07 ft/s | 17.0 ft/s ❌ | | 6" | 6.31" | 2.56 ft/s ✅ | 7.18 ft/s ✅ | | 8" | 8.51" | 1.41 ft/s ❌ | 3.95 ft/s |

4" pipe blows past the upper limit at peak. 8" pipe undershoots scour at average. 6" is the only nominal size that satisfies Ten States §44.4 at both average + peak.

Step 2 — verify TDH is reasonable

For 6" DI at 250 GPM (average):

h_f (Hazen-Williams, C=120):
  = 0.2083 · (100/120)^1.852 · 250^1.852 / 6.31^4.8655 · (4500/100)
  ≈ 0.2083 · 0.717 · 27,625 / 7,775 · 45
  ≈ 23.9 ft

Add static lift (28 ft) + minor losses (~2 ft for entrance/exit/bend/check):

TDH at average flow = 28 + 23.9 + 2 ≈ 54 ft

For 6" DI at 700 GPM (peak):

h_f ≈ 0.2083 · 0.717 · 700^1.852 / 6.31^4.8655 · 45
    ≈ 0.2083 · 0.717 · 187,000 / 7,775 · 45
    ≈ 161 ft

Plus 28 + ~5 minor losses = ~194 ft TDH at peak.

That's a problem — pumping at peak requires nearly 4× the TDH of average flow. A single fixed-speed pump optimized for one duty point can't reasonably cover both. Realistic options:

1. Two pumps in parallel — one runs at average, both at peak. Each sized for ~120 ft TDH. 2. VFD-controlled pump — variable speed tracks the system curve. 3. Equalization storage — wet well + bypass that buffers peak inflow so average forced pumping flow is more uniform.

The first two are common; the third is more common at larger plants. The system curve calc tells you which approach makes sense.

Build a force main system curve →

When 2-8 ft/s isn't quite right

The Ten States envelope is a default. Specific situations push you outside:

• Industrial waste with high solids loading — bump the minimum to 2.5 ft/s for reliable scour.
• Long force mains (>10,000 ft) — water hammer concerns argue for the lower end of the velocity range. 2.5-5 ft/s instead of 2-8.
• Very short force mains (<500 ft) — the velocity-induced solids-deposition concern is somewhat reduced because cycle frequency keeps fluid moving; still aim for 2 ft/s but don't compromise structural design to hit it.
• Cold-climate burial below frost line — temperature-driven viscosity changes friction; design with the colder viscosity if Darcy-Weisbach.

Surge analysis is not optional

Per current Ten States edition §44 commentary, force mains over 1,000 ft should have a surge analysis. Pump trip events (loss of power, motor fault) cause column separation + transient pressures that can exceed the static pressure rating of the pipe by factors of 2-3.

A back-of-envelope test: maximum surge pressure rise from instantaneous pump trip is approximately

ΔP (psi) = (a/g) · ΔV · ρ_w / 144

Where a is the wave speed in the pipe (~3,000-4,500 ft/s for water in DI/PVC), ΔV is velocity change (≈ design velocity if full pump trip), g = 32.2 ft/s². For 6" DI at 5 ft/s with a ≈ 4,000 ft/s:

ΔP ≈ (4000/32.2) · 5 · 62.4 / 144 ≈ 270 psi

That's a ~620 ft head spike. If your pipe is rated for 350 psi, you've got 80 psi of margin — uncomfortable but survivable for a pipe-trip event. If it's 250 psi class, you don't. Surge mitigation (slow-closing check valves, surge tanks, air chambers, bypass-to-suction) becomes mandatory.

Pipe material decision

Ten States §47 lists acceptable materials. The shortlist by typical use:

• Ductile iron (DI) — workhorse for force mains in the 4-24" range. Cement-lined for water service; bare DI is acceptable for sewage but watch for H₂S corrosion in the headspace.
• PVC C900 — lower cost than DI for diameters ≤ 16". Lower hydraulic friction (C ≈ 150 vs 120 aged DI). Adequate for most municipal applications.
• HDPE (DR 11/17) — when trenchless install or directional drilling is required. Slightly higher hydraulic friction than PVC. Resistance to corrosive sewage is excellent.
• Concrete pressure pipe — large-diameter (>24") raw water transmission; rare for sewage force mains.

The hydraulic calc + the procurement context jointly drive material choice. PVC's lower C-factor gives a slightly smaller TDH for the same flow + diameter — sometimes enough to drop a pump size.

What to do next

1. Compute average + peak design flows; apply Ten States minimum scour at average. 2. Iterate diameter candidates until 2 ≤ v_avg ≤ 8 ft/s at peak. 3. Compute TDH at both average + peak; if the spread is large, plan multi-pump or VFD config. 4. Run surge analysis for runs over 1,000 ft — confirm pipe pressure class survives the worst trip event. 5. Specify air-release valves at all high points (Ten States §44.6). 6. Document everything in the pump schedule + engineer's report.

Compute force main TDH →

References

• Great Lakes — Upper Mississippi River Board of State and Provincial Public Health and Environmental Managers. *Recommended Standards for Wastewater Facilities (Ten States Standards).* Latest edition.
• AWWA M55, *PE Pipe — Design and Installation.*
• ASCE Manual of Practice 79, *Steel Penstocks* — for surge calc methodology.
• Metcalf & Eddy. *Wastewater Engineering: Treatment and Resource Recovery.* 5th ed. — pumping system chapter.