Air ingestion cavitation

AIR INGESTION A013

A centrifugal pump can handle aboit0.5% air by volume.

The effect of air sucked into the pump is twofold.

  • It mixes with the liquid to form a two-phase mixture, which then has a lower specific gravity resulting in a lower discharge pressure from the pump. Experience has shown that as much as six percent of the liquid pumped can be handled before the pump stops pumping liquid. This limit assumes that the pump is operating near the best efficiency flow.
  • The air binding of the pump. The impeller also acts like a centrifuge, causing the air to move to the center of the impeller eye. If the flow is high enough, the velocity will carry the mixture through the impeller. As the flow is reduced, the air will accumulate in the impeller eye, blocking any further flow. If the pump is operating below the BEP, the percent of air that can be handled will be decreased.

Air gets into a piping system several ways that include:

      • Air injected in a fermentation process,
      • Crude oil containing natural gas
      • Through the pump stuffing box. This problem occurs in any packed pump that lifts liquid or pumps from a condenser, evaporator or any piece of equipment that runs in vacuum.
      • Some pumps are equipped with a repeller that will lower the pressure in the stuffing box
      • Through valves above the water line.
      • Through leaking flanges.
      • Any vortexing fluid.
      • A pump discharge bypass line that has been installed too close to the pump suction.
      • The suction inlet pipe is out of fluid. This can occur when the tank level gets too low or there is a false reading on the gauge because the float is stuck on a corroded rod.

Air coming into the pump creates a couple of problems.

      • It mixes with the liquid to form a two-phase mixture, which then has a lower specific gravity resulting in a lower discharge pressure from the pump.
        • As much as six percent (6%) of the liquid pumped can be handled before the pump stops pumping liquid.
        • This limit assumes that the pump is operating near the best efficiency flow.
      • Air binding of the pump.
        • The impeller acts like a centrifuge throwing out the heavier liquid causing the air to gravitate to the center of the impeller eye.
        • If the flow is high enough, the velocity will carry the mixture through the impeller.
        • As the flow is reduced, the air will accumulate in the impeller eye, blocking any further flow.
        • If the pump is operating below the BEP, the percent of air that can be handled will be decreased

The air bubbles collapse as they pass from the eye of the pump to the higher-pressure side of the impeller. When the bubbles collapse, as a result of air ingestion, they do very little damage to the impeller and casing walls. The main effect of air ingestion is loss of capacity.

Although air ingestion and vaporization look alike they have separate solutions. The obvious solution for air ingestion is to stop air from coming into the system by correcting the above problems. Fortunately air ingestion is not as severe as vaporization because the bubbles are not imploding, they just shrink to a smaller diameter.

To improve the operation of the pump, the following steps can be taken to reduce air ingestion:

      • Increase the size of the suction pipe to reduce the suction pipe losses, thereby increasing the absolute suction pressure.
      • Vent the air from the suction pipe back to the liquid source by a continuous slope in the suction pipe or a separate vent pipe from the top of the suction pipe close to the pump.
      • Check all pipe joints, valve stems, and other sources of air leakage.
      • Change the impeller configuration to put discharge pressure on the shaft seal, thereby reducing air leakage into the pump at that point.
      • Use a self-priming pump design which can handle more air. Use an inducer on the pump suction, which will increase the air handling capability of the pump.

In a boiler feed pump CO(carbon dioxide) is another gas that can be ingested (all mammals exhale CO2). The CO2 combines with the boiler water (H2O) to form H2CO3 (carbonic acid) that will reduce the boiler pH causing an increase in the need for boiler water treatment and more frequent expensive “blow downs”.

See: Cavitation

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Posted

  • On February 13, 2018