How to Control Pump Cavitation From Entrained Air

What the symptoms are telling you

Unstable pump output, slow actuators, abnormal noise, or recurring internal wear can point to air in the fluid, but symptoms alone do not quantify it. This guide explains how pump cavitation and entrained air are connected, where air enters or leaves solution, and how to preserve a representative sample. It then shows how direct fluid-compressibility testing measures air and how to use the result to correct ingress, improve deaeration, and verify the change.

How entrained air causes pump cavitation and hardware wear

We separate dissolved air from entrained air because they behave differently. Dissolved air is molecularly dispersed in the liquid. It does not occupy the fluid as separate bubbles and normally has little effect on the mixture’s mechanical properties while it remains dissolved.

The risk appears when pressure falls locally. Dissolved air leaves solution and forms free bubbles, adding to any air already entrained in the fluid. This can happen at a pump inlet, through a restriction, or anywhere local velocity creates a sharp pressure drop. The bubbles then travel into a higher-pressure zone, where they compress or collapse. This gaseous cavitation can coexist with vapor cavitation and contributes to pitting and erosion.

Entrained air also makes the fluid more compressible. Part of each pump stroke or impeller action is spent compressing bubbles rather than moving liquid. Pump volumetric capacity falls, pressure builds more slowly, and actuators can respond late or inconsistently. A study available through ResearchGate found that just 1% entrained air by volume can reduce effective bulk modulus by more than 50% at low to moderate pressures.

Bubble collapse can also create intense local heating. In oils, repeated compression and collapse may contribute to thermal degradation and carbonaceous deposits. Those products can restrict fine passages and interfere with valve operation. For further technical context, see our guide to preventing hydraulic cavitation from entrained air.

Common ingress points and desorption mechanisms

Start the inspection on the suction side. A pump may draw air through a worn shaft seal, loose fitting, porous hose, cracked connection, or poorly sealed flange without showing an obvious external liquid leak. Low inlet pressure makes these faults especially significant.

Next, inspect the reservoir and return arrangement. Low fluid depth above the suction inlet can create a vortex with an air core. A return line that discharges above the liquid surface can make the fluid cascade and carry atmospheric air into the reservoir. Agitation, undersized tanks, and short residence time can prevent bubbles from reaching the surface before the fluid recirculates.

Air already dissolved in the liquid can desorb even when there is no external leak. Typical triggers include a clogged suction strainer, excessive suction-line length, restrictive bends, throttling valves, and sudden changes in flow area. Higher fluid temperature can further reduce the amount of gas the liquid retains. According to Power & Motion, dissolved gas follows Henry’s law, so its concentration is related to the gas pressure above the liquid.

Record operating conditions while inspecting these points. Suction pressure, strainer differential pressure, fluid temperature, tank level, pump flow, and discharge pressure help you connect the air result to the condition that produced it. A reading without this context may not represent startup, peak demand, or other damaging operating states.

Why fluid compressibility outperforms indirect measurement methods

At PAPEC, we recommend measuring fluid compressibility because it responds directly to the gas volume in the sample. PAPEC brings over 40 years of expertise in controlling entrained air, dissolved air, and foam. Gas is much more compressible than liquid. By applying a controlled pressure change and tracking fluid displacement, a purpose-built instrument can calculate the air volume rather than infer it from a secondary bulk property.

Density methods depend on the difference between mixture density and an assumed air-free baseline. A change in fluid formulation, temperature, solids loading, or contamination can shift that baseline. Optical methods can be affected by fluid color, opacity, suspended solids, and bubble-size distribution. Acoustic response is highly sensitive to free gas, but it can also vary with fluid composition and operating conditions. These techniques may help identify a change, but they do not by themselves provide the most direct quantification of air content.

Compressibility testing avoids subtracting two large bulk-property measurements to find a small air fraction. It measures the property responsible for capacity loss and sluggish response. For dissolved-air measurement, controlled expansion first releases gas from solution. Subsequent compression and thermodynamic gas-law calculations determine the released volume.

Our air measurement instruments support different testing needs. The QuickAir_OC is a manually operated entrained-air tester that can sample a process stream in place and provide an immediate digital reading. The D_GR series supports automated dissolved-air monitoring, while the Squeezer and Squeezer DualAir provide manual options for entrained-air or combined measurements. Instrument selection should follow the fluid, pressure cycle, required measurement, and speed of process change—not a fixed rule about portable versus continuous testing.

Field sampling protocols to preserve air content accuracy

A poor sample can lose bubbles, release dissolved gas prematurely, or pick up atmospheric air. The resulting number may describe the sampling method rather than the process. Plan the extraction point and handling method before comparing equipment, operating states, or corrective actions.

1. Choose a representative process point. Avoid dead legs, stagnant pockets, wall boundaries, and locations where separated air accumulates unless that location is the subject of the investigation.
2. Flush the sampling path so old fluid does not dilute or distort the sample. Use controlled flow and avoid violent throttling that could shear bubbles or create a new pressure drop.
3. Use closed-circuit, isobaric extraction where pressure loss could cause dissolved air to break out. Mechatest identifies closed sampling and suitable sample cylinders as ways to maintain representative pressurized-fluid samples.
4. Do not dip, pour, shake, or leave the sample open to the atmosphere. These actions can remove existing air or introduce new air.
5. Measure promptly and record sample pressure, fluid temperature, pump load, reservoir level, and valve state. Use the same procedure for baseline and follow-up tests.

If a sample must be transported, use a suitable pressure-maintaining cylinder and control its temperature. For rapidly changing systems, in-place measurement may be more representative than carrying a sample to a remote test location.

Implementing a measurement and mitigation strategy

We recommend beginning with a measured baseline under defined operating conditions. Test entrained air directly and include dissolved-air testing when the system has major pressure drops, changing temperatures, or tight dynamic-response requirements. Noise, vibration, foaming, and pressure fluctuation are useful diagnostic signals, but they do not quantify air contamination on their own.

1. Measure at normal load and at the operating state where symptoms are most severe. Log the associated process conditions.
2. Repair likely ingress points, including degraded seals, damaged suction hoses, loose fittings, and leaking flange connections.
3. Correct reservoir causes by maintaining adequate liquid depth, placing returns below the surface, controlling turbulence, and allowing bubbles time to separate.
4. Reduce avoidable suction restrictions. Check strainers, line sizing, valve position, sharp bends, and inlet conditions before changing the pump.
5. Where passive release is insufficient, evaluate vacuum deaeration or another suitable gas-removal method. Then repeat the same compressibility test to verify the result.

Do not assume an air-removal device is working because it is running. Safe limits depend on the fluid, equipment, duty cycle, pressure profile, and required control accuracy. Establish an operating threshold from baseline data and equipment needs, then trend direct measurements against pump capacity, actuator response, and wear indicators.

PAPEC can help select a portable, combined, or automated measurement approach and interpret compressibility results. Our fluid compressibility and air management consulting can also support sampling plans, process troubleshooting, and verification after mechanical or deaeration changes.

Discuss your air measurement application

To request technical information or discuss specialized testing support, contact PAPEC, call 253-335-3665, or email steve.dougherty@papec.com. PAPEC is located in Eagle, ID, USA. Bring your fluid type, operating pressures, temperatures, sampling constraints, and current symptoms so we can discuss the appropriate measurement approach.