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What Is Vapor Lock? | Process Engineering Glossary
What Is Vapor Lock?
Vapor lock is a condition in which vapor bubbles form within a liquid-filled pump suction line or piping system, disrupting the intended liquid flow and, in severe cases, causing a pump to lose its prime entirely and stop delivering flow. Vapor lock typically occurs when local pressure at some point in the suction system drops below the liquid’s vapor pressure at the prevailing temperature, causing the liquid to partially flash into vapor.
Vapor lock shares underlying physics with cavitation, though vapor lock specifically refers to vapor accumulation disrupting flow, often in suction piping upstream of the pump, while cavitation more specifically describes vapor bubble formation and collapse occurring within the pump itself.
Applications of Vapor Lock
Pump Suction System Design
Pump suction piping is designed to minimize pressure drop and avoid conditions that could allow local pressure to fall below the fluid’s vapor pressure, directly addressing vapor lock risk.
Volatile Fluid Handling
Vapor lock risk is particularly relevant when handling volatile fluids with vapor pressures close to atmospheric or suction conditions, requiring extra design margin in suction system layout.
Elevated Temperature Service
Fluids handled near their boiling point are especially prone to vapor lock, since even small pressure drops or temperature increases can be enough to trigger local vaporization.
Benefits of Knowing Vapor Lock
Explains otherwise puzzling pump performance loss. Understanding vapor lock helps diagnose situations where a pump suddenly loses flow or prime without an obvious mechanical cause.
Informs suction system design margin. Recognizing vapor lock risk supports designing suction piping with adequate margin against local vapor pressure conditions.
Supports safer handling of volatile or hot fluids. Understanding vapor lock risk is particularly important for safely designing systems handling volatile or near-boiling-point fluids.
Limitations to Consider
Can be difficult to predict precisely. Predicting exactly where and when vapor lock will occur requires accurate fluid property and system pressure data, and small errors can significantly affect the prediction.
Recovery can require manual intervention. Once a pump has vapor locked and lost prime, restoring normal operation sometimes requires manual re-priming rather than simply waiting for conditions to normalize.
Shares causes with cavitation, complicating diagnosis. Since vapor lock and cavitation share underlying physics, distinguishing between the two during troubleshooting requires careful investigation.
Vapor Lock FAQ
How does vapor lock relate to a can pump’s suction conditions?
A can pump, like any centrifugal pump, is vulnerable to vapor lock if suction conditions allow local pressure to drop below the fluid’s vapor pressure, an important consideration alongside proper line sizing of the suction piping.
Why is vapor pressure directly relevant to vapor lock risk?
Vapor lock occurs specifically when local pressure falls below the fluid’s vapor pressure at the prevailing temperature, making accurate vapor pressure data essential to predicting risk.
How does vapor lock relate to pump priming and dry running?
Severe vapor lock can cause a pump to lose its prime entirely, requiring the same pump priming procedures used for initial startup, and prolonged vapor lock can contribute to dry running damage, an important consideration alongside pump affinity laws, deadheading risk, and overall low-low level trip protection.
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