How to Run a Wet-Well Pump-Down Test
A pump-down test is the field measurement that tells you what a lift-station pump is actually moving today, not what its curve promised when it was new. By timing how fast a pump lowers the wet-well level through a known volume, you get real delivered capacity in gallons per minute, and by timing how fast the level rises with the pump off you get the incoming flow. This procedure is for the operator or technician verifying pump condition, calibrating a flow estimate, or troubleshooting a station that seems to be falling behind.
Wet-Well Pump-Down Test in one line: To run a wet-well pump-down test, measure the wet-well plan area, mark two level references a known distance apart, then time how long one pump takes to draw the level down between them with inflow blocked or accounted for. Pumped volume divided by that time gives the delivered gallons per minute. Timing the level rise with the pump off, over the same known volume, gives the current inflow rate.
Gather the Geometry and Isolate Inflow If You Can
The test rests on one piece of data: the volume of the wet well per unit of depth. For a rectangular well that is length times width; for a round well it is pi times the radius squared. Multiply that plan area by a chosen vertical drawdown to get the volume you will pump out, converted to gallons. Get these dimensions from the drawing and confirm them against a tape measure at the hatch, because a well that was modified or that has a benched bottom will not match its original print near the floor.
Ideally you block inflow during the drawdown so the pump works against a static well, which gives the cleanest capacity number. In a real collection system you usually cannot stop inflow, so instead you measure inflow separately and correct for it. The honest approach is to run both halves of the test back to back: a fill test with pumps off to measure inflow, then a drawdown with one pump running, and add the measured inflow rate back to the apparent pumping rate to recover true pump capacity.
Time the Fill to Measure Inflow
With both pumps stopped and locked out from auto-start, let the well fill and time how long the level takes to rise through your marked drawdown distance. Volume divided by time is the current influent flow in gallons per minute. Run this while the level is well below the lowest incoming invert and comfortably below the high-level alarm so you never risk an overflow during the measurement, and keep an operator at the panel ready to restore the pumps instantly.
This inflow figure is valuable on its own. It is the ground truth against which a station's calculated flow estimate is checked, and it is the number that tells you whether the station is sized for present-day flow. If your measured inflow already approaches the delivered capacity of a single pump, the station has little margin and depends on both pumps, which is exactly the condition that a good level band and a healthy lag pump exist to handle. The same drawdown geometry underlies the station's continuous influent flow estimate.
Time the Drawdown and Compute Delivered Capacity
Start one pump and time how long it takes to lower the level through the same marked distance, running the pump only within its safe submergence so it never breaks suction or runs dry. Divide the pumped volume by that time to get the apparent pumping rate. Because inflow was still arriving during the drawdown, the pump actually moved more than that: add the inflow rate you measured in the fill test to the apparent rate, and the sum is the pump's true delivered capacity at the station's operating head.
Compare that delivered capacity against the pump curve at the station's total dynamic head. A pump delivering well below its curve is worn, partially clogged, or fighting a restriction in the discharge, and a pump delivering above expectation usually means the head is lower than assumed or the geometry used in the math is off. Repeat the drawdown for each pump individually so you can tell a weak pump from a shared discharge problem, and note that if a pump struggles to hold prime or draws down erratically you may be looking at a suction or prime fault rather than a capacity loss, which the guide on a flooded-suction pump losing prime addresses.
Verifying the Result and Common Mistakes
A trustworthy pump-down test produces numbers that reconcile: the two pumps tested individually should sum to roughly the two-pump drawdown rate minus overlap losses, and the delivered capacity should sit sensibly on the pump curve at the measured head. File the delivered gallons per minute and the inflow rate with the date, because these become the baseline that later tests compare against to catch gradual pump wear. On a monitoring platform the same level trace during the test is recorded, so the drawdown slope can be re-derived from history and cross-checked against your stopwatch.
The most common mistake is ignoring inflow and reporting the apparent drawdown rate as pump capacity, which understates a healthy pump on any live station. The second is using the drawing's plan area near the floor where benching or a sloped bottom makes the real volume per foot smaller than the print suggests, which skews every number. The third is drawing the level so low the pump loses submergence mid-test, which corrupts the timing and risks damaging the pump; always stop the drawdown at a safe minimum level above the pump intake.
Frequently Asked Questions
How do you calculate pump capacity from a drawdown test?
Multiply the wet-well plan area by the vertical drawdown distance to get the volume moved, convert to gallons, and divide by the time the pump took to lower the level that far. Because inflow keeps arriving during the drawdown on a live station, add the separately measured inflow rate to that apparent rate to recover the pump's true delivered gallons per minute at the operating head.
How do you measure inflow with a pump-down test?
Stop and lock out both pumps, then time how long the level takes to rise through a known vertical distance. The wet-well volume for that distance divided by the rise time is the current influent flow. Keep the level below the lowest incoming invert and well below the high-level alarm throughout, with an operator ready to restore the pumps immediately so the well never approaches an overflow.
Why test each pump separately?
Testing pumps one at a time separates a single weak pump from a problem shared by both, such as a restricted common discharge. If one pump delivers on its curve and the other falls short at the same head, the shortfall is that pump: worn, clogged, or losing prime. If both fall short equally, suspect the discharge piping, a partially closed valve, or a head higher than the design assumed.
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