Sole High-Service Pump Cavitating at 0.52 in/s

The pattern is cavitation driven by a restricted suction line, possibly with air entrainment on top of it.

• Suction at 3 psi and fluctuating is the core finding. The pump is being starved, so NPSH available has dropped below NPSH required at the actual flow.
• Rattling, gravel-like noise, discharge needle vibrating through a 15 psi band, and current swinging plus or minus 10 A with a mean below nameplate are the classic signature of a pump repeatedly running off its curve and losing suction. It is not simply overloaded; it is starving.
• A strainer not pulled in a year on a 40-foot river intake is the obvious cause. A partly blocked strainer costs several psi of suction. Check strainer differential, verify the suction valve is fully open (a partly closed valve mimics a plugged strainer exactly), and look for a vortex or air leak at the intake.
• Bearing temperature 168 degrees F and rising, vibration 0.52 in/s (well above the roughly 0.3 in/s alert band) mean damage is already in progress: cavitation erodes the impeller and the resulting vibration pounds bearings and seals.
• Packing drip of 90 drops per minute is roughly double what is normal. A loose packing box on the suction side draws air and makes cavitation worse, so it is a contributor, not just a housekeeping issue.

Do not keep running it as-is. The pump is the only one on line, which is exactly why you cannot gamble: a seal blow or bearing seizure leaves the zone with no high service at 2 a.m.

Decision sequence:

1. Pull and clean the suction strainer now and re-check the readings at the same flow after an hour. If vibration and noise drop and suction pressure steadies, cavitation was the cause and you can run while monitoring.
2. If the strainer is clean and symptoms persist, throttle to the highest flow that holds suction pressure steady and keeps bearing temperature below about 170 degrees F. Reducing flow reduces NPSH required, which is exactly what a starved pump needs.
3. If vibration keeps climbing or bearing temperature exceeds about 180 degrees F, shut down, put the zone on the emergency source or curtailment plan, and notify your chain of command and FDEP if system pressure will drop below the required minimum.
4. Re-check the numbers continuously, not once: suction pressure, discharge pressure, amps, bearing temperature, and vibration every 30 minutes while the pump is running.
5. Log every reading with a time stamp. The trend is what justifies the run-or-shut decision if someone asks later.

If you are forced to run, run with the numbers and the throttle, never wide open on hope.

Inspect, in this order:

• Impeller and wear rings for cavitation erosion (spongy, pitted leading edges), and check clearances against the manufacturer's values.
• Bearing and lubricant condition, shaft runout, coupling alignment, and seal or packing condition. Repack or reseal to the correct drip rate.
• Suction line and strainer after cleaning: measure the actual differential, verify the suction valve opens fully, check the gauge itself with a known reference, and confirm the intake is not drawing air or a vortex.
• NPSH available versus required at the real operating flow, using the actual suction lift and losses, not the design assumptions. If the margin is thin, the pump will cavitate again the next time the strainer loads.
• A pump-curve test after repair to confirm it is back near 1,200 gpm at 190 ft.

Prevention, and this is the part the exam is really asking about:

• Put strainer cleaning on a written schedule with a differential-pressure trigger, not on memory.
• Start a monthly vibration and bearing-temperature route with alert bands, so 0.15 in/s is caught before it becomes 0.52.
• Do not extend a six-day continuous run on a single unit without a documented risk review and a standby plan.
• Keep a critical spare seal and bearing set on the shelf.