Vapor lock is one of those hidden problems that can bring a pumping system to a standstill. Unlike cavitation, which is violent and noisy, vapor lock is silent. The pump simply loses its ability to move liquid because vapor has displaced it in the suction line or inside the casing.
The root cause is simple: when the pressure in the suction line drops below the liquid’s vapor pressure at the operating temperature, the liquid flashes into vapor. Once that vapor pocket forms, the impeller or pumping chamber can no longer fill, and the pump starves.
Some fluids are far more prone to this problem than others. Knowing which liquids are likely to flash gives you the opportunity to design the system correctly — and prevent costly downtime.
Fluids Most Likely to Cause Vapor Lock
1. Low-Boiling Hydrocarbons
- Examples: butane, pentane, propane condensates, light naphtha.
- These liquids have very low boiling points and high vapor pressures, meaning they can flash even with minor suction lift or a small temperature increase.
2. Light Fuels and Solvents
- Examples: gasoline, kerosene, alcohols (ethanol, methanol), acetone.
- Their volatility makes them extremely sensitive to suction restrictions, warm pipework, or inadequate venting. Vapor lock in these services is common if tanks are above ground in hot climates.
3. Hot Water and Condensate
- Water close to its boiling point (for example, boiler feedwater or hot condensate return) often flashes when pressure drops in the suction line.
- Even a short length of undersized suction piping can be enough to create vapor pockets.
4. Refrigerants and Cryogenic Liquids
- Examples: ammonia, CO₂, Freon types.
- At ambient conditions these fluids have very high vapor pressures, so they require carefully designed sealed systems. Any pressure loss or heat gain on the suction side can result in vapor lock.
5. High-Temperature Process Liquids
- Examples: thermal oils operated near their boiling point.
- Even if the boiling point is high, running close to it with limited suction head is asking for vapor formation.
System Conditions That Contribute
It’s not only about the liquid — the installation makes a big difference:
- High fluid temperature relative to its boiling point.
- Insufficient suction head or excessive suction lift.
- Undersized or long suction lines causing pressure drop.
- Blocked strainers or partially closed valves creating extra restriction.
- Ambient heating of tanks or piping in outdoor installations.
Preventing Vapor Lock
- Keep suction piping short and properly sized. Large diameters reduce velocity and pressure drop.
- Maintain adequate Net Positive Suction Head (NPSH) margin. Compare pump NPSHr to available NPSHa with the fluid at its actual temperature.
- Use flooded suction where possible. Placing the pump below liquid level reduces the chance of vapor pockets.
- Control temperature. Insulate lines, cool return flows, or use heat exchangers to keep liquid below its flashing point.
- Vent air and vapors. Install vent connections at high points to prevent vapor pockets forming.
Key Takeaway
Fluids with high volatility or those operated close to their boiling point are the ones most likely to cause vapor lock — light hydrocarbons, solvents, hot water, refrigerants, and hot process streams. The solution lies in careful piping design, suction head management, and temperature control. Addressing these factors early ensures the pump handles liquid, not vapor, and the system runs reliably.
