You cannot vent an operating pump. The rotating impeller acts as a centrifuge where the heavier fluid is expelled outwards while the lighter air remains at the center where it also blocks the impeller suction eye. When you vent the casing on an operating pump, you may get some air out and then fluid. But if there is air at the eye, it will likely remain there. Top centerline discharge designs like American National Standards Institute (ANSI) pumps are designed to be self-venting. Fluids that are directed to the supply source should not be allowed to free fall into the tank. If this is the case, then the area should be as far as possible from the pump suction. Weirs and/or divider barriers should be installed.
Testing for Air Entrainment
Not sure if you have air entrainment? You can conduct some simple tests that are covered by the Hydraulic Institute (HI). In general, you capture samples and analyze them. You can also calculate if you have proper submergence. Submergence charts are another handy tool, but you need to have one foot of submergence for every foot of suction side fluid velocity. You can look up your flow rate and pipe size to determine the fluid velocity in the “Cameron Hydraulic Data Book” or “Cranes Technical Publication 410.”
In recent years, there is a financially driven trend to reduce the size of suction supply tanks. On closed systems and systems with returns or air entrained supplies, the tanks can be designed and built to present a “torturous path” for the fluid. A system of weirs, barriers and baffles can be engineered to present the maximum amount of hold time/ transient time for air bubble elimination/mitigation.
For most simple systems using Newtonion fluids (no slurries) I suggest four to five minutes transient time as a respectable design-versus-cost compromise.
Unpreventable Air Entrainment
In general, for every type of centrifugal pump and impeller design, there are different results when pumping fluids with entrained air. It is a difficult task to give a simple answer of what will or will not work in determining inconvenient yet accurate results. There is no easy magic formula app at this time. The specific speed (NS) and suction specific speed (NSS) are both relevant factors. Typically, impellers of NS higher than 3,000 will perform better than impellers below 3,000. Also, the actual impeller size, suction eye diameter, speed of the pump and the number of vanes will be factors.
The number of volutes in the casing will also play a major role. Typically, a double volute will work better than a single volute pump. Note that some manufacturers produce pumps with more than two volutes. A diffuser pump (think volutes of a high multiple) will work better than a simple volute style pump.
One method to deal with entrainment is to open the clearances of the pump beyond normal recommendations. Many operators have success with doubling the clearance. The pump will be less efficient, but it will operate.
In flooded suction situations, if the pump does become air bound, you can stop the pump and allow the bubble to pass through and escape the pump and then restart the pump.
Some operators have incremental success using a larger impeller than the hydraulic conditions call for (based on zero air entrainment).
A less frequent and perhaps more desperate approach is to add an inducer (attached to the impeller); this route often requires outside technical or engineering assistance.
Disc friction pumps have great success handling air entrainment (reportedly) of up to 70 percent, with the disadvantage of high cost. Vortex style and recessed impeller pumps also have good success, usually up to 20 to 22 percent air entrainment. I have witnessed a few recessed impeller pumps handle up to 24 percent air. It is also common to use a self-primer pump in applications above 18 percent air entrainment.
Minimum flow rates will also play a major role when managing air entrainment issues. The higher the air entrainment percentage the more flow that will be required to push or sweep the air bubbles pass the impeller eye (see Figure 1). So, operating the pump to the left side of the curve can lead to air binding blockages.
Air Entrainment Disguised
Often times, operators believe that a pump problem is cavitation when the real culprit is air entrainment. Like diagnosing some diseases in the medical field, the process of eliminating other reasons that could be causing the symptoms is how the conclusion is formed. If you look at the components that make up the formula for NPSH available, you can determine action steps to potentially reduce the effects. If you raise the static suction head or cool the liquid, it should reduce the effects. Or if you slow the pump down or throttle back on the discharge, the symptoms should reduce.
Air entrainment issues are often ignored or mistaken for cavitation and will create performance and mechanical issues with a centrifugal pump. Reduce or eliminate the air to prevent problems with shaft failures and premature bearing and mechanical seal failures.