Duplex Lift Station Loses a Pump Ahead of a Storm
A duplex submersible lift station serves 900 homes through a 6-inch force main 2,200 ft long, with 22 ft of static lift and a normal total dynamic head of about 48 ft. Each pump is rated 300 gpm. The lead pump just tripped on a VFD overload fault and will not reset; the lag pump is running but delivering only about 300 gpm. The wet well is 8 ft by 8 ft in plan, currently at 5.2 ft, with the high-level alarm at 6.0 ft and overflow at 7.5 ft. Inflow is 330 gpm and climbing. A 1.5–2 inch rain is forecast tonight, and this catchment has a history of steep infiltration and inflow response. A creek runs 150 ft downgrade of the station, and an earlier overflow two years ago reached it. A portable 6-inch diesel bypass pump rated 400 gpm at 60 ft TDH is in the yard. It is 14:00 on a Monday.
1. Calculate your decision window and give the sequence of actions with approximate times.
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Run the clock first - the arithmetic decides everything else.
- Wet well plan area = 8 ft × 8 ft = 64 ft², so each foot of depth holds 64 ft³ ≈ 479 gal
- Remaining storage before overflow = (7.5 − 5.2) ft × 479 gal/ft = 1,100 gal
- Net accumulation = 330 gpm inflow − 300 gpm pumped = 30 gpm
- Time to overflow = 1,100 ÷ 30 = about 37 minutes
That is the whole decision window, and the forecast rain will shorten it.
Sequence, in parallel not in series:
- Notify the supervisor and dispatch the vacuum truck now. A 3,000-gallon truck on site in 20 minutes buys roughly 100 extra minutes even if it only keeps pace with the 30 gpm net gain.
- Order the bypass pump at the same moment - rigging suction and discharge takes 45–60 minutes, so it will not be running before the wet well tops out.
- Set containment now: sandbags and absorbent along the path to the creek, 150 ft downgrade.
- Have electrical staff determine whether the faulted pump can be run in hand or its starter bypassed. That is an electrician's call, not a guess at the panel.
2. How would you size, place and authorize the bypass pump? What is your backup plan?
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Size it against the real operating point, not the nameplate. The portable unit is rated 400 gpm at 60 ft TDH. The station's static lift is 22 ft and the force main adds friction on top, so confirm where the pump actually sits on its curve at 300–330 gpm - you want it comfortably left of the runout point so it is not cavitating or shaking itself apart.
Placement:
- Suction into the wet well with a strainer kept well off the floor so you are not pulling grit and rags.
- Discharge downstream of the station, to a gravity manhole where added flow will not surcharge the upstream collection system or back water into basements.
- Fuel, spill containment and a floodlight at the pump; someone assigned to monitor it through the night.
Authorization: bypass discharge is a discharge to waters of the state. Call the state clean water program, confirm the permit or emergency authorization route, and record volumes and run times. Do not just start pumping.
Plan B: a second pump on standby, the vacuum truck as backup storage, and a written trigger for when to stop pumping rather than cause a worse problem downstream.
3. Assume the station overflows 4,000 gallons to the creek before the bypass is running. What are your notification, documentation and follow-up obligations?
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If sewage reaches the creek, you have a spill, and reporting is not optional.
Notify the state environmental agency's water program as soon as practicable - many jurisdictions require an immediate verbal report followed by a written report within days - and verify the exact timelines in the collection system or NPDES permit rather than relying on memory. Also notify the local health department, any downstream property owners, and the recreational or shellfish advisory program if the discharge can reach coastal waters. In Hawaii that means the shoreline, any fishpond, and any nearby shellfish harvest area.
Document as you go: start and stop times, estimated volume (pump rate × duration plus wet well drawdown), cause, corrective actions, sampling results, photos, and who was notified when. Sample the creek if there is any surface water impact.
Then do the after-action work that prevents the next one: VFD spares on the shelf, a preventive maintenance schedule for both pumps, SCADA alarm to the on-call phone rather than a local horn, a portable generator connection, and an infiltration and inflow review before the next storm season. The exam answer is the response; the real answer is the reliability program.