Fire Pump Annual Flow Test: Reading the Curve Right
You are conducting the annual flow test of a 1,500 gpm, 100 psi rated electric fire pump. The pump is a vertical turbine in a wet pit, drawing through a 12 in. suction from a 300,000 gal ground storage tank; the discharge feeds a wet sprinkler system. Today's readings: churn 118 psi discharge, 1,500 gpm at 90 psi, and 2,250 gpm at 61 psi. All pressures are from the permanently installed pump gauges. The suction gauge, which read 0 at churn, reads 2 psi vacuum at 1,500 gpm and 8 psi vacuum at 2,250 gpm. The tank level is 18 in. above the low-water mark. The weekly churn log has gaps: no entries for the last three weeks, and one entry reads 'ran, sounded fine' with no duration. The owner wants the annual test signed off today.
1. Apply the acceptance criteria to these numbers. Does the pump pass, and what does the suction reading do to your calculation?
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Apply the criteria to net head, not to raw gauge readings.
Rated: 1,500 gpm at 100 psi.
| Point | Discharge | Suction | Net head | % of rated |
|---|---|---|---|---|
| Churn | 118 psi | 0 | 118 psi | 118% |
| 100% (1,500 gpm) | 90 psi | −2 psi | 92 psi | 92% |
| 150% (2,250 gpm) | 61 psi | −8 psi | 69 psi | 69% |
- Shutoff: must not exceed 140% of rated head. 118% → OK.
- 150% of rated flow: must be at least 65% of rated head. Corrected, 69% → passes, but only by 4 points. Uncorrected, 61% would look like a failure. This is the trap: with the pump lifting from a pit the suction gauge reads vacuum, and net head is discharge minus suction, 61 − (−8) = 69 psi. Correct for gauge elevation differences as well.
- Rated point: 92% of rated head is low against the certified curve. Confirm gauge calibration and that the suction reading is real before accepting it.
Verdict: passes the two hard limits, but the rated-point shortfall and the trend require a careful retest - not a rubber stamp.
2. What do these readings suggest about the cause, and what would you check in what order?
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Diagnose before you sign.
The signature: churn is healthy (118%), but head falls away faster than the curve as flow rises, while suction vacuum climbs steeply (0 → 2 → 8 psi). That points to a suction-side restriction or air entrainment, not simple impeller wear - wear shows reduced head at churn too.
Check in order:
- Gauge accuracy. Swap in calibrated gauges; a bad pit gauge explains everything cheaply.
- Suction strainer/intake. Fouling produces rising vacuum with falling head - exactly this pattern. Cleaning a wet pit is a confined-space entry: permit, atmospheric testing, attendant, rescue plan.
- Tank level and vortexing. Eighteen inches above low-water is thinner than it sounds at 2,250 gpm; watch for a vortex or air draw during the test.
- Suction piping - an obstruction or partly closed suction valve.
- Pump internals only if suction is clean and the shortfall persists.
Correct the cause, retest, and compare with the certified curve and the last accepted test, recording discharge and suction at every point.
3. The weekly churn log has three missing weeks and one vague entry. How does that affect your report, and what do you tell the owner?
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What the log gap means for the report.
- Weekly no-flow running of the pump (at least 10 minutes per the referenced edition) is the owner's obligation, not the inspector's. Today's flow test proves the pump started and performed today; it cannot backfill eleven weeks of operation.
- Report exactly what the record shows: no entries for three weeks, and one entry documenting neither run time nor result. Call it a records and operational deficiency - a pump not exercised weekly is a pump whose starting reliability is unproven, and starting reliability is the point of a fire pump.
- Do not write, initial, or reconstruct entries you did not observe. State in the report that the missing runs could not be verified.
- Tell the owner what to do: assign a named person; log date, time, duration, discharge pressure, and observations; keep the log with the annual results. Recommend an automatic-start and manual-start check at the next opportunity.
- A weekly churn run does not substitute for the annual flow test, and the reverse is also true. If the adopted edition or AHJ requires written notification of a deficiency affecting pump reliability, send it.