Fluid pumps are specifically designed to optimize the flow of fluids through the systems they are connected to with as little non-condensable gases as possible. Air trapped in the fluids can cause cavitation at higher flow rates that damages pump internals as well as pipe seals and joints. For these reasons the proper depth to submerge pumps is pivotal to prevent as much air from flowing through a pump.
As the velocity of the fluid flowing through a pipe increases the depth required for a pump pushing the same rate of flow increases. Because velocity increases as pipe diameter decreases and flow rate remains static, if the same pump is connected to a smaller pipe, the input source for the pump needs to be deeper to prevent vortices from forming. If the source tank is too shallow to allow for this minimum submergence other methods like suction bells and barriers can be introduced to lengthen the travel distance of the fluid to the suction inlet of the pump.
As a general guideline, 2 percent free air will reduce pump flow by 10 percent, and 4 percent air will reduce the flow by more than 40 percent. The average pump will likely fail to operate at 14 percent air entrainment. Some pump designs that use vortex impellers, recessed impellers, separation chambers or air escapes can handle air entrainment up to 24 percent.
The recommended suction velocity for pumps is between 5 and 8 feet per second, with a suggested maximum of 6 feet per second unless suspended solids or fluid characteristics demand faster flow rates.
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