Calculators
Pumping & Wet Well
Your wet well is already a flow meter. Work out pump rate from a drawdown test, check cycle time and starts per hour against what the motor can take, and find the horsepower and what it costs to run.
Your numbers, step by step
- Drawdown from the float elevations(−2.50) ft − (−3.25) ft = 0.75 ft
- Wet well area0.785 × 6 ft × 6 ft = 28.3 ft²
- Volume drawn down28.3 ft² × 0.75 ft = 21.2 ft³ × 7.48 = 158.5 gal
- Pump-down time in minutes2 min + 32 sec ÷ 60 = 2.53 min
- Gross pump rate158.5 gal ÷ 2.53 min = 62.6 gpm
- Add the flow that kept coming in62.6 gpm + 49 gpm = 111.6 gpm
- Daily volume at that rate111.6 gpm × 1,440 min = 160,677 gal/day = 0.161 MGD
Why it works
A wet well is a measuring tank you already own. If you know its area and you watch the water drop a known distance, you know exactly how many gallons the pump moved. Time it, and you have the pump rate — no meter needed.
- Pump rate is gallons drawn down divided by the minutes it took. If flow kept coming in during the test, the well emptied more slowly than the pump was actually moving water, so you add the influent rate back on to get the true pump rate.
- Cycle time is the fill plus the pump-down. While the pump runs, the well only drops at the difference between what goes out and what comes in, which is why a station near its capacity takes much longer to draw down than you'd expect.
- Starts per hour is just 60 divided by the cycle length. Motors are rated for a maximum number of starts, because each one heats the windings. Short cycling is the fastest way to kill a pump motor.
- Horsepower starts with the work the water actually needs — flow times head — then divides by each efficiency in turn. The pump loses some, the motor loses some, and what's left is what the power bill sees.
The friction side of total dynamic head comes from the force main. ThePipe Flow calculator works that out with Hazen-Williams. All of these formulas are on theformula sheet.
Running a drawdown test
- Measure the well. Get the inside dimensions, not the outside of the structure. A circular well needs the diameter; a rectangular one needs length and width. Do this once and write it on the panel — you'll use it for years.
- Deal with the incoming flow. The cleanest test is to plug or divert the influent so nothing enters while you time it. If you can't, measure the inflow first: let the pumps stay off, time how long the water takes to rise a known depth, and work out that rate the same way. Then enter it in the influent field.
- Mark your start and stop points. The pump-on and pump-off floats are the obvious choice because they're repeatable. A tape or an ultrasonic reading works too. Just use the same two points every time you repeat the test.
- Time it. Run one pump, not both. Start the watch when the water passes the upper mark and stop it at the lower mark — not when the pump starts and stops, since there's a lag. Record minutes and seconds.
- Do it three times. Average the runs. If one is well off the others, something moved — a check valve, a float, or the influent.
- Test each pump separately. That's the whole point. A lead pump at 110 gpm and a lag pump at 70 gpm tells you which one to pull, and you'd never see it from the station's flow meter.
- Write it down and compare. A drawdown test is only worth something next to the last one. A rate that has quietly fallen 20% is a worn impeller, a partly closed valve or a greased-up force main.
Example: a real 6-foot duplex lift station
These are the published design numbers for the Warwick Park Pump Station (PS-329) in Sussex County, Delaware — a duplex submersible station serving Gull Point and Warwick Cove, from the engineering calculations filed with DNREC permit WR20250183.
- Wet well: 6 ft diameter, circular
- Lead pump on at −2.50 ft, pumps off at −3.25 ft — a 0.75 ft working band
- Average daily flow 70,490 gpd, which is 49 gpm
- Pump operating point 111.5 gpm at 36.5 ft of head; design point 104 gpm at 34.42 ft
- Motor: Flygt NP 3127, 7.5 hp
Work the drawdown out from the geometry: 0.785 × 6 × 6 = 28.3 ft², times 0.75 ft is 21.2 ft³, times 7.48 = 158.5 gallons per cycle. With 49 gpm still coming in, the well drops at 111.5 − 49 = 62.5 gpm, so the pump-down takes about 2 minutes 32 seconds. Run that backwards through the calculator and you land on 62.6 gpm gross, plus 49 gpm influent — 111.6 gpm, within a tenth of the published operating point.
The cycle tab is where it gets interesting. Filling 158.5 gallons at 49 gpm takes 3.24 minutes, the pump-down takes 2.54, so a full cycle is about 5.8 minutes — roughly 10.4 starts per hour. That is right at the usual ceiling for a motor this size, which is exactly the kind of thing worth checking before you narrow a float band.
One caveat on the horsepower tab: the report publishes the flow, head and motor size but not the pump and motor efficiencies. The 65% and 88% loaded below are typical values, not this station's. Use your own pump curve if you want a real answer.
Do the numbers look reasonable?
Motor manufacturers publish a maximum number of starts per hour, because each start heats the windings and the motor needs the run time to cool. These are common guidelines for submersible sewage pumps — they are not a substitute for the curve and data sheet that came with your pump.
| Motor size | Typical maximum starts per hour |
|---|---|
| Up to 5 hp | About 15 |
| 7.5 – 30 hp | About 10 |
| 40 – 100 hp | About 6 |
| Over 100 hp | About 4 |
Check your pump curve. Larger motors take longer to cool and tolerate fewer starts, but the real limit depends on the motor, the starting method and how warm the wet well runs. If the manufacturer's number differs from the table, the manufacturer wins.
| Number | Usually lands near | If it is far off |
|---|---|---|
| Pump rate vs. the pump curve | Within about 10% of the curve at your head | A radius entered as the diameter makes the volume four times too small. Check the units on the drawing too — some are in inches. |
| Cycle time | Rarely under about 5 minutes | Short cycling usually means the floats are too close together or the pump is oversized for the flow. |
| Wire-to-water efficiency | About 45–80% | Under 45% points at a worn impeller, a fouled force main, or a pump running far off its best efficiency point. |
| Total dynamic head | Static lift plus friction, so always more than the lift alone | Forgetting force main friction makes the horsepower read low. Grease buildup raises real head over the design value. |
| Two pumps, two rates | Within a few percent of each other | A big gap between lead and lag is the most useful thing a drawdown test tells you. Test them separately. |
Before you change anything
- Confined space rules apply. A wet well is a permit- required confined space. Measure from the top, use the station drawing, and don't enter for a drawdown test.
- Don't widen a float band to fix short cycling without checking detention. More storage means longer cycles, but sewage sitting in a wet well goes septic and you'll smell it downstream.
- Run one pump at a time. Two pumps in the same force main don't move twice the water, and a test with both running tells you nothing about either.
- Watch the check valve. A valve that doesn't seat lets the force main drain back, which shows up as a pump rate that reads low and a cycle that repeats sooner than it should.
- Baseline it now. A drawdown test on a healthy station is worth more than one on a failing station, because it gives you the number to compare against later.
This is a study and planning aid. Your plant's SOPs, permit and process engineer take priority over any number on this page.