I didn’t plan on writing about air entrainment again, but recent field issues pushed me to revisit the subject. In recent months, I have been involved in four cases of misunderstood pump performance where air entrainment was determined to be the main culprit.
Air entrainment may sound like and be confused with cavitation in an operating pump. The difference is that cavitation is the formation and subsequent collapse of vapor bubbles, while air entrainment is simply air bubbles that were in the process stream before the pump and will remain there after the pump. The bubbles may change in size but typically do not collapse when running through the pump.
Air entrainment in very small amounts can actually be a good thing. Many users will inject up to 0.9 percent air to cushion the effects of cavitation. However, air entrainment even in amounts of 1 to 2 percent will cause pump performance to drop dramatically. Poor pump performance is often confused and erroneously blamed on a multitude of symptoms other than air entrainment. A thorough root cause analysis in addition to education regarding the matter will show air entrainment is a much more common pump ailment than initially believed. This issue is a rising trend and has become more common for several reasons. Pulp and paper companies inject air in stock/slurry mixes, especially as an increase in recycling is occurring. Oil and gas industries are working to pump more dual phase fluids at the wellhead and downstream, and wastewater treatment plants are using more dissolved air flotation (DAF) systems.
Additionally, the industry is seeing an increased use of closed loop systems and a drive for less expensive, but consequently more shallow cooling tower basins. Further, the chemical process industry (CPI) and general industrial plants have renewed and are increasing financial pressure to design systems with smaller “footprints.” Filed under the heading of “unintended consequences” is the result of shorter and smaller storage tanks and process vessels. The shorter tanks equate to lower submergence values, which then have a higher probability of creating vortices and the smaller vessels yield reduced volume and consequent reductions in transient times for air bubble mitigation.
Why Are We Concerned About Air Entrainment?
Increases in air entrainment more than 1.5 to 2 percent will have immediate and deleterious effects on the pump from both an immediate performance aspect and a mechanical aspect in a protracted way.
As air bubbles become trapped at the pump suction, they block the fluid flow, and the pump performance will drop off. The flow rate will decrease, the developed head will drop off, and the efficiency will decline. Air entrainment even at values as low as 2 to 4 percent will cause increased pump vibration, which leads directly to premature bearing failure. Vibration is frequently caused by the unbalanced hydraulic loads on the impeller due to partial air blockage.
The unvented air also collects in the seal chamber (standard stuffing box configurations) to create air pockets that cause dry running of the mechanical seals. Dry/non-lubricated seal face operation contributes to shortened life and ultimate failure. If you hear a squeal when the pump starts, it is usually the seal faces running dry.
Air entrainment is one of the major contributors to broken pump shafts because of the hydraulic surging and axial shuttling that occurs from a pump that is stalled one second (no load) and pumping the next (full load) in an endless and unintended “go to” loop of stress fatigue cycling.
Air entrainment also introduces unwanted free oxygen into the system, which is the major component in both general and chloride stress corrosion formulas, just to name two types.
Air entrainment at 2 percent will reduce pump performance by up to 12 percent. At 4 percent, it will reduce pump performance by 40 percent and at 10 percent, it will likely stall the pump altogether.
Reducing or Eliminating Air Entrainment in the System
Submergence is an often overlooked parameter. The proper level of submergence is critical, and vortexing is the most common source of undesired air introduced into pump systems. Improper submergence creates the conditions for the vortexing phenomena, which introduces large quantities of air leading to pump damage in a short period of time. If the submergence is insufficient, you can add vortex barriers, raise the height of the liquid level or increase the size of the suction line opening effectively reducing the fluid velocity. Just because you fail to see vortexing does not mean that it is not occurring.
Properly fill and vent the system initially. I have witnessed many occurrences where the operator did not properly vent the system or vent the high points, casings or stuffing boxes. Sometimes it is prudent to “jog” the pump a few times for a short period (10 to 15 seconds +/-) to “sweep” the air bubbles away and downstream, away from the pump.
No high point vents are installed, or if manual vents are present, they are not exercised. Automatic air release valves are not maintained.