Entrained air is typically measured by volume at atmospheric pressure and not by weight. While not a totally inclusive list, the following are the most common reasons for air entrainment in pump systems:
- Submergence (think fluid static height above the pump suction inlet) is inadequate, which will result in vortices. Vortexing is likely the most common source of entrained air in pump systems. (For more details, see my Pumps & Systems column on this subject from April 2016. Read it here.)
- Initial fill: The pipe and tanks were full of air to begin with and not properly filled and/or vented in the startup process.
- Liquids being directed to the suction supply tank/sump are “free falling” onto the fluid surface. This action drags air into the process.
- Suction systems are frequently operated at pressures below atmospheric (vacuum), which can cause air to leak in at several places, but will not leak fluid out when in a static condition. Examples: packed pumps, loose or corroded joints and/or improper gaskets can allow air into the system along with any opening, however small, on a self-primer pump on suction lift service.
- Mixers and agitators.
- Air or non-condensable gases are purposely injected into the fluid as part of the process.
- The process itself creates air or non-condensable gases (chemical reaction).
- Water naturally has some air entrainment, a fact that fish depend on to breathe.
- Improper design of suction pipe geometry/arrangement with unvented high points that will not dissipate or reduce the air and gases present.
- Additives, such as surfactants and dyes.
- Temperature changes that affect the solubility of air and gases in the fluid. See “Henry’s Law.” Note: in this column I have frequently interchanged the terms dissolved air and entrained air. There is a technical difference that I am ignoring for the sake of brevity.
- Pressure at the suction of the pump is the lowest pressure in the system that the fluid will experience. The air bubbles in the solution will increase in size proportionally to the pressure drop ratio. That is, the pressure drop from upstream of the pump to the suction eye.
Read the rest of the rest of Jim Elsey's December 2017 column on "How to Reduce or Eliminate Air Entrainment" here.