14.2 Sewage Lift Stations & Force Mains

Sewage Lift Stations & Force Mains

Chapter 3 covered building sewage ejectors and sumps under FPC Section 712. This section extends the learning to site lift stations and force mains that move wastewater where gravity conveyance is not feasible. Local exam coverage must be checked in the approved local outline.

Gravity and pumped conveyance are evaluated against terrain, receiving elevations, utility requirements, reliability and lifecycle needs. High groundwater adds excavation and structural demands, but its actual elevation must come from site investigation.

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1. Station Architecture

A duplex submersible station consists of a wet well (usually precast concrete or fiberglass), two pumps with duty/standby or duty/assist capacity defined by the approved design, guide rails and lifting chains, a level control system, a valve vault containing the check valves and isolation valves, a control panel with alternator and alarm, and the force main leaving the site.

Level Control and Alternation

A common control example uses float switches, pressure transducers or ultrasonic sensors to establish four elevations:

1. Pump off (low level) — set for the manufacturer’s required submergence, cooling and operating limits.
2. Lead pump on — sets the working volume between starts.
3. Lag pump on — the second pump joins during peak inflow.
4. High-level alarm — annunciated locally and usually by telemetry.

In this example the controller alternates lead and lag on each cycle to equalize wear. Working volume matters: too small and the pumps short-cycle and burn up motors; too large and sewage sits septic in the well generating hydrogen sulfide.

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2. The Valve Order on a Pumped Discharge

The Florida Plumbing Code fixes the sequence on building sumps and ejectors, and site stations follow the same logic:

FPC Section 712.3.2 — the discharge piping shall have a full open valve located on the discharge side of a backwater or check valve.

Reading it from the pump outward: pump, then check valve, then full open (isolation) valve, then the force main. The reason is maintenance. With the isolation valve downstream of the check, a technician can close the isolation valve, hold the head of the full force main behind it, and pull or service the check valve without draining the entire line back into the well.

FPC Section 712.3.3 governs the discharge pipe size, and Section 712.3.5 the connection of the pump discharge to the gravity drainage system. On a site station, the force main typically discharges into a manhole with an energy-dissipating arrangement so the incoming jet does not scour the invert or splash the bench.

One exception worth remembering: in one- and two-family dwellings, only a check valve is required, located on the discharge piping from the pump or ejector.

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3. Sizing the Force Main

Balance Solids Transport, Friction and Surge

Force-main design balances solids transport with friction, surge, retention time and operating cycles. Low velocity can permit deposition; excessive velocity can increase energy demand and transient loads. Neither water mains nor force mains are designed from just one upper or lower velocity number.

For the supplied design exercise below, the utility criteria set an acceptable operating velocity band of 2–8 ft/s. These are exercise inputs, not universal Florida limits or a guarantee of self-cleansing under every sewage condition. The selected design must also address low-flow periods, retention and the actual solids load.

Total Dynamic Head

The pump is selected on total dynamic head (TDH):

$$\mathbf{TDH = \text{Static Lift} + \text{Friction Loss} + \text{Residual Discharge Head}}$$

• Static lift is the difference between the applicable upstream and downstream hydraulic elevations. It varies with wet-well level and receiving conditions. The example uses the pump-off level as its stated maximum-static-lift case and a free discharge elevation.
• Friction loss is the head lost in the force main and every fitting, computed over the total equivalent length exactly as in water distribution sizing.
• Residual discharge head is any pressure that must remain at the discharge, such as when a force main ties into a pressurized regional main rather than a free-discharging manhole.

The system curve is TDH plotted against flow. The pump's operating point is where the system curve crosses the manufacturer's pump curve, and a competent submittal shows that intersection falling inside the pump's best-efficiency range with the velocity meeting the applicable project criteria over the evaluated operating range.

Worked Example

A duplex submersible station serves a small Florida commercial park at a peak flow of 150 gpm. The selected nominal 4-inch pipe has a supplied internal flow area of 0.0884 ft² and is 900 feet long, with fittings adding 120 feet of equivalent length. The wet well pump-off level is at elevation 4.50 feet; the receiving manhole invert is at elevation 31.00 feet. The supplied hydraulic-data exhibit gives 1.9 feet per 100 feet at 150 gpm for this selected pipe. Neither area nor friction rate is inferred from the nominal diameter or a generic C900 designation. The approved station specification requires either pump alone to meet the duty point; the stated loss data include the modeled system losses for this exercise.

Step 1 — Check the velocity. Use the exhibit’s 0.0884-ft² internal flow area.

$$V = \frac{150\text{ gpm}}{448.8\text{ gpm per cfs}} \div 0.0884\text{ sq ft} = \frac{0.334\text{ cfs}}{0.0884} = \mathbf{3.8\text{ feet per second}}$$

The calculated 3.78 ft/s meets this project’s 2–8 ft/s band. A candidate larger pipe with a supplied area of 0.1963 ft² would have 150/448.8/0.1963 = 1.70 ft/s, below that criterion. This establishes a design mismatch, not proof that sedimentation must occur instantly.

Step 2 — Static lift.

$$31.00 - 4.50 = \mathbf{26.50\text{ feet}}$$

Step 3 — Friction loss over total equivalent length.

$$TEL = 900 + 120 = 1,020\text{ feet}$$ $$h_f = \frac{1,020}{100} \times 1.9 = \mathbf{19.38\text{ feet}}$$

Step 4 — Residual head. The main free-discharges into a gravity manhole, so residual is 0 feet.

Step 5 — Total dynamic head.

$$TDH = 26.50 + 19.38 + 0 = \mathbf{45.88\text{ feet, say 46 feet}}$$

Select a submersible pump delivering 150 gpm at 46 feet TDH, and confirm from the curve that the duty point is near best efficiency. Under this project’s stated duty/standby requirement, each pump alone must make that duty point. “Duplex” alone would not establish that requirement for every other station.

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4. Florida-Specific Design Problems

Evaluate the Empty-Structure Buoyancy Case

For a simplified fully submerged cylindrical exterior 6 feet in outside diameter and 20 feet in submerged height, displaced water volume is π × 3² × 20 = 565.49 ft³. At 62.4 pounds per cubic foot that is about 35,000 pounds of uplift on an empty structure — enough to float a wet well out of the ground during a wet season or a hurricane, shearing the incoming gravity sewer and the force main. Evaluate the empty structure at the design high groundwater level, comparing uplift with permitted resisting dead weight, soil contributions, anchorage and the required safety factor. Add approved resistance where the check fails; buoyancy is not automatically the governing structural load on every site. This is the same buoyancy analysis applied to interceptors in Chapter 3, scaled up.

Hydrogen Sulfide and Corrosion

Warm Florida sewage turns septic quickly. The countermeasures are: keep detention time in the wet well short by right-sizing the working volume, ventilate the wet well, line the receiving manhole with a corrosion-resistant coating, use corrosion-resistant valve vault components, and in chronic cases dose chemically at the station. Station hatches, guide rails and chains should be stainless steel, not galvanized, in a sulfide environment.

Standby Power

The approved reliability plan determines emergency power and storage needs. Options include a permanent standby generator with automatic transfer, or at minimum a quick-connect receptacle and manual transfer switch so a portable generator can be brought in. Storage volume in the wet well between the alarm level and the lowest gravity inlet buys the response time.

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5. Entering a Wet Well Is Confined Space Work

A sewage wet well is a textbook permit-required confined space under OSHA 29 CFR 1926 Subpart AA — it is large enough to enter, has limited means of entry and egress, is not designed for continuous occupancy, and contains a hazardous atmosphere. Hydrogen sulfide is heavier than air, deadens the sense of smell at the concentrations that kill, and pools at the bottom of the well; methane displaces oxygen and is explosive.

Before entry, the atmosphere is tested in this exact order under 29 CFR 1926.1204(e):

1. Oxygen content
2. Flammable gases and vapors
3. Potential toxic air contaminants

The order is not arbitrary: a combustible-gas meter gives a false reading in an oxygen-deficient atmosphere, so oxygen must be confirmed first. Evaluate and implement the applicable permit-space entry program, atmospheric testing/monitoring, isolation, ventilation where needed, attendant and authorized rescue arrangements. Do not treat continuous forced-air ventilation alone as universal authorization to enter, or assume a rescue plan allows an untrained coworker to enter impulsively. Any alternative entry procedure requires its own demonstrated eligibility. Chapter 11 develops the full Subpart AA program; the point here is that no lift station maintenance task is ever a two-minute unattended climb.