An eductor is a simple version of a pump used to move a liquid form of a fluid out of a certain place. However, unlike more complex versions of pumps, they do not have any mechanized components or moving parts that create an external force to transport a certain quantity of water. Instead, they rely on the simple theory of Bernoulli’s principle and a specific case of this, the Venturi effect.
About Bernoulli’s Principle
As we know from the classical theory of fluid mechanics, Bernoulli’s principle states that a change in fluid velocity is marked within any system due to the pressure differential of the incompressible fluid or any change in the potential energy.
A Venturi effect is a specific case of this principle wherein there is a reduction or drop in fluid pressure and a consequential gain in the fluid velocity (from the fundamentals of fluid continuity) when it passes through a reduced cross-sectional area or a partial obstruction in its flow path.
In other words, any change in speed that a volume of fluid (and a resultant increase in kinetic energy) may attain while passing through changing cross-sectional areas in its path amounts to a proportional drop in static pressure head.
A venturi meter is a demonstrative representation of this effect. In it, a fluid is made to pass through a certain tube that is tapered midway, resulting in a decreased cross-sectional area. This changing area increases the fluid’s velocity and kinetic energy, accompanied by a sudden decrease in static pressure reflected in the water columns, as shown in the standard arrangement below.
Eductors use this principle, making them known as jet pumps. They essentially use the suction effect that arises due to the pressure drop stemming from Venturi action and, in turn, help strip off or transfer fluid from another source into another location.
Understanding The Working Principle
The working of an eductor is pretty simple. A driving or motive incompressible fluid is made to enter through a tapering inlet nozzle. This fluid is drawn from another pump or a flow source.
When the fluid exits this nozzle, owing to the decreased cross-section and consequential rise in flow velocity, there is a decrease in pressure. This low-pressure, high-velocity fluid enters into another chamber. Now, in the wake of the low-pressure zone created by the fluid, the subject or other body of fluid is drawn in from another inlet opening into the system. For the suction fluid to be drawn in, the pressure created in the wake of the driving fluid has to be lower than this.
Energy transfer occurs as these two fluids get mixed up in the throat section. The suction fluid gets entrained into the driving fluid and gains kinetic energy. The mixture gets transferred to the diffuser section, where the area increases again. The resultant velocity decreases, and the pressure energy increases again. The resultant fluid now gets ejected at a definite outlet.
For all practical purposes, the suction and driver fluid in the eductor’s areas of application are mostly the same: water.
However, for all practical purposes, the valves and stoppers in the suction line are kept mostly open, as even when the suction fluid has reduced inflow velocity, it goes back to its source. It can be re-drawn back again after necessary pressure adjustments to the eductor. But if the backflow of this line is stopped during high-flow rate operations, there is a chance of excess pressure build-up and the eductor collapsing or bursting.
For all practical purposes, the drained contents from the eductor during tank stripping operations are collected into sludge or slop tanks, where they are discharged at very high rates through electrically driven pumps. During operations like deck de-flooding, the eductor outlet is connected directly to a discharge line from where it is drained into the sea.
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Source: MarineInsight
