Pump Priming Problems: Why Some Pumps Lose Prime and Others Don't Need It
A pump runs normally all shift, sits overnight, and won’t move liquid the next morning. Someone refills the casing, production restarts, and the same problem returns tomorrow.
An industrial pump losing prime usually points to liquid draining away, air entering the suction side, or gas accumulating faster than the pump can clear it. The cause may be a leaking foot valve, inadequate tank level, trapped air, or changing process conditions—not necessarily a worn-out pump.
Some pumps can evacuate air and lift liquid into themselves. Others need their casing and suction piping filled before startup. That distinction matters, but self-priming does not automatically mean safe to run dry.
What Priming Actually Does
Priming establishes the liquid-filled condition needed for a pump to develop flow and pressure. A conventional centrifugal pump transfers energy through an impeller, but an impeller spinning in air cannot develop enough pressure difference to lift liquid through an empty suction line.
In a suction-lift installation, the pump sits above the source liquid level. The pump must reduce suction pressure so pressure acting on the source surface can push liquid upward. An air leak interrupts that process.
With flooded suction, the source liquid level is above the pump inlet. Liquid can fill the pump by gravity, provided valves are open and trapped gas has an approved escape path. The pump hasn’t become self-priming; the installation supplies its prime.
Which Pumps Need Priming—and Which Can Establish Their Own?
Conventional centrifugal pumps
Most conventional centrifugal pumps need a liquid-filled casing and suction path before operation. A foot valve may retain that liquid during shutdown. Other installations use an external priming system or flooded suction.
Submersible pumps generally avoid a separate suction-lift priming operation because the hydraulic inlet is submerged. They still need adequate submergence and can ingest air if the level falls or a vortex reaches the inlet.
Self-priming centrifugal pumps
Many self-priming centrifugal designs retain liquid in a chamber, recirculate it with incoming air, separate the air, and discharge that air while lifting liquid through the suction piping.
They normally require an initial liquid charge. If the retained liquid drains away, the pump may no longer prime. Air also needs somewhere to go: discharge backpressure or a check valve can prevent successful priming unless the arrangement includes the manufacturer’s required air-release provisions.
Allowable suction lift and priming time depend on the model, speed, piping volume, temperature, and fluid. Don’t apply another pump’s limits.
Positive displacement pumps
Many positive displacement pumps can move air well enough to establish suction, but their limits differ:
Air-operated double-diaphragm pumps: Often self-prime from a dry condition. Dry suction-lift capability differs from wet capability and depends on check-valve sealing, fluid properties, and operating conditions.
Peristaltic pumps: Hose or tube compression can evacuate air without a liquid-filled casing. Suction performance depends on hose recovery, speed, viscosity, and inlet resistance.
Rotary lobe and progressive cavity pumps: May offer self-priming capability, but internal clearances, speed, seals, and wetted components impose limits. Many progressive cavity pumps can quickly damage their stators when operated dry.
Check current manufacturer data for dry versus wet priming, allowable duration, and dry-running restrictions. “Positive displacement” alone isn’t a priming specification.
Use the Timing of Prime Loss to Narrow the Search
Loses prime while stopped
Start with drainback. A foot valve held open by debris, a leaking check valve in a prime-retention arrangement, or leakage from the casing can let the liquid column disappear. An air-entry path elsewhere may allow that drainage to continue.
A pump that restarts after a short stop but not after a weekend suggests gradual leakage. Repeatedly adding water treats the symptom and may contaminate the process if that liquid isn’t compatible.
Loses prime during operation
Look for conditions that introduce air or interrupt supply:
Low source level, surface vortexing, or an inlet exposed during tank drawdown.
Leaking suction flange gaskets, threaded joints, instrument connections, or mechanical seals exposed to vacuum.
Foaming liquid, entrained process gas, or a return stream discharging near the suction opening.
A restricted strainer, partially closed valve, or collapsing suction hose.
Higher liquid temperature or changed fluid composition that promotes vapor formation.
A suction leak may pull air inward without showing a visible liquid leak. A dry floor doesn’t clear the suction piping.
Never primes after installation or maintenance
Check the initial fill requirement, valve lineup, rotation verification records, suction piping geometry, and any priming-system connections. High points can trap air. Suction-lift piping generally needs a continuous rise toward the pump without pockets, with reducers arranged to avoid trapping gas.
On self-priming centrifugal pumps, excessive suction lift, excessive air volume, insufficient priming speed, or a blocked air-discharge path can prevent startup even when the pump is mechanically sound.
Don’t Confuse Lost Prime with Cavitation or Low Flow
A motor running with little discharge doesn’t prove the pump has lost prime. Incorrect rotation, impeller damage, excessive discharge resistance, or unsuitable speed can also reduce output while the casing remains full.
Cavitation is different from air entering through a leak. It occurs when local liquid pressure falls below vapor pressure, producing vapor bubbles that can collapse as pressure recovers. It may cause noise, vibration, unstable flow, and damage without completely emptying the pump.
Net positive suction head available, or NPSHA, describes the suction-pressure margin above the liquid’s vapor pressure. Source pressure, liquid level, suction losses, and temperature all affect it. Compare calculated NPSHA with the pump’s published NPSH requirement and an appropriate operating margin; merely matching the published value isn’t a guarantee of cavitation-free service.
Consider a hypothetical West Tennessee transfer system that works during cooler months but becomes unstable as an outdoor tank warms in summer. If the tank is also drawn lower during higher production, hotter liquid and reduced static head both reduce suction margin. Replacing the seal won’t correct those conditions.
A Practical Troubleshooting Sequence
Start with observations, not disassembly. Follow facility safety procedures and manufacturer instructions. Opening fill plugs, cleaning strainers, or checking valves internally requires proper shutdown, isolation, lockout/tagout, depressurization, and chemical precautions.
Record when it happens. Note shutdown duration, tank level, temperature, product, speed, and whether the problem follows maintenance or a process change.
Review installed instruments. Compare suction pressure or vacuum, discharge pressure, and actual flow with stable operation. Stronger suction vacuum with falling flow suggests increased inlet resistance or lift, but isn’t conclusive by itself.
Observe the source safely. Check for vortexing, foam, changing levels, and return-flow disturbance from designated observation points.
Trace the suction route. Review valve positions, hose condition, high points, recent piping changes, and strainer differential pressure where measured.
Check prime retention under an approved procedure. Qualified personnel can investigate drainback, casing leakage, and foot-valve seating.
Escalate unresolved problems. Controlled leak testing, pressure trending, priming-system testing, or internal pump inspection may be needed. Use component-rated test methods—not improvised pressure tests.
Avoid repeated dry starts or prolonged unsuccessful priming attempts. Liquid trapped in a centrifugal casing can heat during unsuccessful operation, damaging seals and creating a hazardous condition.
Repair the Cause Before Buying Another Pump
For an industrial pump losing prime repeatedly, a larger motor or replacement pump may leave the underlying fault untouched. More flow can increase suction losses and make an already marginal inlet condition worse.
If the process routinely empties tanks or handles entrained gas, review whether the pump technology and controls suit that duty. Low-level shutdowns, approved dry-run protection, revised piping, or a different pump arrangement may be appropriate. Positive displacement pumps also need suitable discharge overpressure protection.
For a repair or selection review, bring the pump model, fluid properties, operating temperature, required flow and pressure, minimum source level, suction elevation, piping dimensions, and failure history. That information is more useful than horsepower alone.
Bottom Line
Prime loss during shutdown calls for a different investigation than prime loss halfway through a batch. Establish which condition you have, then evaluate liquid retention, air entry, suction margin, and the pump’s actual priming capability.
Process & Power can help facilities throughout Tennessee, Arkansas, and Mississippi evaluate pump troubleshooting and repair needs alongside the surrounding piping and process conditions.
Call Process & Power, Inc. at 901-362-5500 or visit us at 1721 Corporate Avenue, Memphis, TN 38132 for help with industrial air compressors, pumps, blowers, vacuum systems, and compressed air equipment and service throughout Memphis and the surrounding area.
