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How Do Pumps Lose Prime?
Pumps lose prime when air, vapor, or insufficient suction head interrupts the liquid's continuity at the inlet. Designing systems that maintain prime requires controlling suction conditions, accounting for vapor pressure and selecting pump technology based on how it behaves during air ingestion and recovery.
Contributors
This blog was developed using insights from PSG® subject matter experts with extensive experience in troubleshooting pump failures and designing reliable pumping systems across industrial, chemical, terminal and marine environments.
Loss of prime is routinely treated as a pump defect, even though it is most often a system failure. The pump is simply the first component to reveal the symptoms. Once liquid continuity at the suction is interrupted, many pump technologies cannot maintain the conditions required to restart flow without intervention.
This pattern appears repeatedly in oil and gas transfer, wastewater handling, chemical unloading and shipyard service, where suction conditions are unstable, fluids are unpredictable, and operators are not continuously monitoring the pump.
If the design assumes perfect flooded suction or constant tank levels, prime loss becomes inevitable.
From a technical standpoint, prime loss occurs when the pump inlet is no longer filled with liquid at a pressure high enough to prevent flashing or air accumulation. The failure mechanism typically falls into one of three categories: air ingestion, vapor formation or insufficient net positive suction head available.
Air Ingestion: The Most Common Root Cause
Air ingestion occurs when air enters the suction line and displaces liquid. This can happen even in systems that appear flooded, as small leaks or poor suction geometry can allow air to accumulate, particularly during start-stop cycles.
Air commonly enters systems through low tank levels that uncover suction nozzles, vortexing at tank outlets, leaking gaskets or threaded joints, poorly sloped suction piping that traps air pockets and fittings that create unintended high points.
For example, in marine and shipbuilding service, vessel motion and changing bilge levels make intermittent suction a normal operating condition.
When air occupies the suction, centrifugal pumps can lose the ability to impart energy to the fluid. The impeller spins but does not develop a usable head. Positive displacement pumps can move some air, but sustained air handling increases internal temperature, accelerates wear and often prevents full performance recovery. With the exception of reciprocating pumps, particularly AODDs, which can run dry without seal failure.
Designing against air ingestion requires treating suction piping as both a fluid path and an air path. If air can enter, it will. If air can be collected, it will.
Vapor Formation and Vapor Lock
Vapor formation occurs when suction pressure drops below the fluid's vapor pressure at operating temperature. Unlike air ingestion, this failure mode involves the liquid changing phase within the suction region.
This is common in hydrocarbon services, warm chemical applications and systems with long suction runs or high friction losses. The condition becomes more severe at the minimum tank level when the static head is lowest, while suction losses remain unchanged.
Vapor formation disrupts liquid continuity and can lead to vapor lock, where the pump cannot recover because vapor occupies the inlet region.
In centrifugal pumps, vapor at the impeller eye triggers cavitation, vibration, seal distress and rapid performance collapse. Even if the pump continues to rotate, the delivered flow may drop to near zero.
Vapor-related prime loss may frequently be misdiagnosed because systems can operate reliably during commissioning and fail later as temperature increases, product composition changes or fouling raises suction losses.
NPSHa Problems: Why Calculations Fail in the Field
Net positive suction head available (NPSHa) is often calculated once using average conditions and then treated as a fixed value. In real operation, NPSHa is dynamic and changes continuously.
NPSHa decreases with lower tank levels, higher fluid temperatures, increased suction losses from fouling or additional fittings, higher-than-expected flow rates during upset conditions and changes in fluid vapor pressure.
A pump that appears properly sized on paper can lose prime under worst-case conditions that occur routinely in daily operation.
Prime loss typically occurs at the conditions many calculations discount: minimum liquid level, maximum temperature and maximum suction losses. If the design does not maintain margin at these points, intermittent cavitation often precedes chronic prime loss.
Loss of prime often accelerates wear on valves, seals and diaphragms long before a full failure occurs. Using genuine replacement components helps restore original performance and ensures repairs address the root cause rather than masking suction or sealing issues. Learn more about available options on our genuine parts page.
How Pump Type Changes Prime Loss Behavior
Prime loss does not manifest the same way across all pump technologies. Each design responds differently to suction disruption and recovers differently once liquid returns. For example:
Centrifugal pumps require liquid at the impeller to function. Once prime is lost, they rarely self-recover without external priming. Dry operation rapidly damages mechanical seals due to heat and insufficient lubrication. Products from manufacturers such as Griswold® are often applied successfully when suction conditions are stable and controlled.
Sliding vane and gear pumps, including Blackmer® designs, are positive-displacement that can self-prime to a degree. They can move mixtures of air and liquid, but sustained air handling increases wear and temperature, while insufficient NPSHa can still drive cavitation and internal damage.
Air-operated double diaphragm pumps tolerate air ingestion by design. Air does not interrupt the pumping mechanism, and the pump resumes normal delivery automatically when liquid returns to the suction. This behavior explains why AODD technology from manufacturers such as Wilden® and All-Flo™ are common in shipyards, bilge service, chemical unloading and other applications where suction conditions are unpredictable.
Selecting a pump for a system with unstable suction should always include recovery behavior as a selection criterion, not just normal flow performance.
System Design Choices That Almost Guarantee Prime Loss
Prime loss is rarely caused by a single catastrophic error. It is usually the result of multiple small design decisions that are compounded over time.
Common contributors include long horizontal suction runs that trap air, suction piping that rises toward the pump creating high points, oversized suction lines without slope control, excessive suction fittings that increase friction losses, ignoring minimum tank level, relying on operators to avoid deadheading and treating priming accessories as permanent fixes.
Foot valves, priming pots and check valves can provide temporary assistance but cannot correct poor suction geometry. When these devices foul, leak or stick, the system reverts to the underlying failure condition.
How to Design Systems That Maintain Prime
Systems that consistently maintain a prime share in a common engineering mindset: they treat worst-case suction conditions as the true design point.
Effective designs minimize suction lift, reduce friction loss, maintain a continuous downward slope toward the pump, eliminate unnecessary fittings at the suction and account for the maximum operating temperature at the minimum tank level. They accept that air ingestion and vapor breakout will occur and select pump technology and controls that tolerate these conditions.
In practice, the most reliable systems choose pump technologies that align with the reality of the suction environment rather than expecting the environment to behave ideally.
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Contributors
Rob Jack
Rob Jack is a long-tenured technical authority in air-operated double diaphragm pumps with extensive experience diagnosing system-level failures. His expertise includes troubleshooting prime loss, diaphragm wear and chemical compatibility issues through failure evidence and operating context.
Steve Cox
Steve Cox has decades of experience across diaphragm, vane and centrifugal pump technologies in industrial markets, including marine-adjacent service. His background includes field troubleshooting and system design guidance with a focus on reliability, serviceability and real-world operating behavior.
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