Understanding Lift Stations: A Walkthrough of Force Main and Pump Sizing Calculations

Lift station pump sizing comes down to four checks, run in order: force main velocity, total dynamic head (TDH), the pump's operating point and wet well cycling. Then size storage for the time the station cannot pump. Each step feeds the next, so a wrong diameter or friction value early shows up later as a clogged line, a pump running off its curve or a motor that starts too often.

What goes into a lift station sizing calculation?

Design flows, the force main route and profile, the pipe's actual inside diameter, the wet well levels and the pump maker's curves.

The numbers below follow one hypothetical duplex station: two equal pumps, 150 gpm peak flow, 60 gpm average (a peaking factor of 2.5), 1,500 feet of 4-inch force main and 35 feet of static lift. They show the method, not a design.

How do you size the force main?

Pick the diameter that keeps velocity above your state's minimum at design flow, figured on the pipe's actual inside diameter, not its nominal size.

A 4-inch Schedule 80 pipe has a 4.500-inch outside diameter and a 0.337-inch wall, so its bore is about 3.83 inches. At 150 gpm that runs 4.2 feet per second.

TCEQ's force main rule, 30 TAC 217.67, sets a 4-inch minimum diameter unless grinder pumps are used and, at a two-pump station, 3.0 feet per second with one pump running. The Recommended Standards for Wastewater Facilities, 2014 edition (the Ten States Standards, since replaced by a 2026 edition) ask for at least 2 feet per second and recommend no more than 8 (Section 49.1). Colorado's CDPHE design criteria, WPC-DR-1, require 2 feet per second at firm capacity plus a periodic 3.5 feet per second flush (Section 4.4.1).

In the same bore, 90 gpm runs about 2.5 feet per second: enough for the Ten States and Colorado 2 feet per second, short of the Texas 3.0 (108 gpm) and Colorado's 3.5 flush (125 gpm). At small flows, velocity rather than inflow can set the pump size in Texas pump and lift station design.

How do you calculate total dynamic head?

Add static head, friction loss and minor losses, plus any pressure the discharge needs, which is usually zero at a gravity manhole.

Static head runs from the wet well level to the highest point the flow must reach. It changes as the well draws down, so carry both pump levels: 35.0 feet at pump-off and 33.2 at pump-on.

Friction is usually figured with the Hazen-Williams formula and its coefficient, C. TCEQ's pump rule, 30 TAC 217.61(d), requires the report to state C for the pipe material and to figure pipe, valve and fitting losses with the Hydraulic Institute standards. The Ten States Standards use C = 100 for unlined iron or steel and allow up to 120 for PVC, polyethylene or lined ductile iron (Section 49.61).

Minor losses are each fitting's loss coefficient, K, summed and multiplied by the velocity head, V²/2g. Figure each pump's discharge piping up to the common header separately.

At 150 gpm with C = 120, friction is about 34.4 feet and a total K of 6 adds 1.6 feet. At pump-off, TDH = 35.0 + 34.4 + 1.6 = 71.0 feet.

Why build the system curve at more than one C value?

New pipe is smoother than the design C, so the pump runs farther out on its curve, moving more water at more power.

The Ten States Standards say to weigh the higher C of new pipe when checking maximum power and duty cycle (Section 49.62). TCEQ requires system curves at the force main's C values for stations with more than two pumps, a force main over half a mile, or firm capacity of 100 gpm or more. Bound it with two curves: old pipe at pump-off and new pipe at pump-on.

Flow (gpm)	TDH, C = 120, pump-off (ft)	TDH, C = 140, pump-on (ft)
50	39.7	36.8
100	51.9	46.1
150	71.0	60.7
200	96.5	80.1

The pump must make 150 gpm on the upper curve, and the motor must handle where the pump curve crosses the lower one.

How do you pick the pump?

Choose the pump whose curve crosses your system curves near its best efficiency point (BEP) across the range, then confirm on the maker's data.

The DOE and Hydraulic Institute pumping system sourcebook explains why. At the BEP efficiency is highest and radial bearing loads lowest. Cavitation usually occurs far right of it, where a radial pump also draws the most power. Far left, low flow brings internal recirculation, and oversized pumps running there tend to wear bearings and seals and start and stop often.

TCEQ requires at least two pumps, each shown alone and with the others, and firm capacity (largest pump out of service) that handles peak flow.

How big should the wet well be?

Big enough that pumps do not start too often, small enough that sewage does not sit and go septic.

For one pump of capacity Qp, the cycle is fill time plus pump-down time. It is shortest when inflow is half the pump rate, which gives active volume V = T × Qp / 4, where T is the minimum cycle time in minutes. For a 10-minute cycle, V = 10 × 150 / 4 = 375 gallons, or 1.77 feet between pump levels in a 6-foot-diameter wet well.

TCEQ's wet well rule, 30 TAC 217.60(b)(7) and (8), sets a minimum cycle time at peak flow and a volume formula, both in attached figures. The Ten States Standards use the pump maker's duty cycle and cap fill time at 30 minutes at design average flow (Section 42.62). They also call for automatic alternation, which halves each pump's starts.

Check the slow end too: at 60 gpm the example fills in 6.25 minutes, at a first-phase 10 gpm in 37.5, past the cap. Colorado limits detention to one hour at average flow (Section 4.2.1). Stations sized for build-out face this, as with a lift station sized for three phases of growth.

How much emergency storage does a lift station need?

Enough to hold peak inflow until pumping resumes, and each state defines that time differently.

In Texas, 30 TAC 217.63(g) to (i) requires the systems that prevent a discharge (generators, bypass pumps, storage or a mix) to last at least the longest power outage in the electric utility's last 60 months of records, or 20 minutes, whichever is longer, designed on peak flows, inflow and infiltration. The wet well is assumed full to pump-on at the start, and collection system storage cannot stand alone.

Colorado's WPC-DR-1 defaults to 60 minutes at projected peak hour flow (Section 4.1.1), less with added resiliency. Here that is 9,000 gallons, about 61 feet of 60-inch pipe.

Do you have what you need to size the pumps?

Six yeses mean the numbers can be run. Any no is the first input to get.

1. Do you have average and peak flows for build-out and the first phase?
2. Do you have the force main route and profile, including its highest point?
3. Do you know the pipe material and class, and so its inside diameter?
4. Do you know which rules set velocity, cycle time and storage?
5. Do you have the pump maker's curves and starts-per-hour limit?
6. Do you have outage records (Texas) or the operator's response and repair time (Colorado)?

Replacing a station? Utilities and districts can start from measured flows, as the Ten States Standards prefer (Section 11.242). Ask your engineer: which C values did you use, and what is the fill time at first-phase flow?

A note on limits: this is general education from 30 TAC Chapter 217 and CDPHE's WPC-DR-1 as of October 2026 and the archived 2014 Ten States Standards (we did not review the 2026 edition). The example is hypothetical; your state, your utility's standards and the pump maker's data govern.

Frequently asked questions

What is the minimum force main velocity in Texas?

3.0 feet per second with one pump running at a two-pump station. With three or more, 2.0 feet per second on the smallest pump plus a 5.0 feet per second flush at least twice a day.

Hazen-Williams or Darcy-Weisbach for force main friction?

Hazen-Williams uses a C value and is the method TCEQ and the Ten States Standards name. Darcy-Weisbach uses a friction factor, never a C value.

I'm developing a 200-lot subdivision in North Texas that needs a lift station. What should I give my engineer before pump sizing starts?

Flows by phase, the route to the receiving manhole, the utility's standards and the electric utility's written outage records for 60 months, which TCEQ requires in the engineering report.

I manage a small sanitation district in Colorado, and one lift station pump seems to start every few minutes at the morning peak. What should our engineer check?

The volume between pump levels against the maker's cycle limit, whether alternation works, and whether the pump is oversized. The DOE sourcebook lists frequent on-off operation as one sign of oversizing.

Related resources

• Lift Stations and Collection Systems: What the Plant Needs Upstream to Function Properly
• How to Read a Hydraulic Grade Line
• Lift Station Design in Houston’s High Groundwater Zones: Why Standard Specs Fail in Clay-Heavy Soils
• When Lift Station Force Mains Fail: Your Colorado Community’s CIPP Solution

Sizing pumps for a new or replacement lift station?

We run the force main, system curve, wet well and storage checks against your state's rules and your utility's standards before a pump is picked.

Modern Engineering Solutions, McKinney, Texas.

Talk to an engineer about your lift station or call (214) 833-6748.