12.2: Pulling Suction of a Fluid
- Page ID
- 117050
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One common misconception in hydraulics is that a pump "sucks" oil out of the reservoir. In reality, a pump doesn’t pull fluid in —it relies on atmospheric pressure to push the oil into its inlet.
Let’s break that down.
The Role of Pressure Differential
Fluid always moves from areas of higher pressure to lower pressure. A hydraulic pump takes advantage of this by creating a low-pressure area at its inlet during rotation. When the pump begins to turn, the internal components (such as gears or pistons) create space at the inlet, increasing its internal cavity space and reducing the pressure below atmospheric pressure.
At the same time, the surface of the reservoir is exposed to the atmosphere. Since atmospheric pressure is now higher than the pressure at the pump inlet, it pushes the oil through the suction line into the pump.
This flow process only works if there’s a clear, unrestricted path for the oil to follow.
Issues That Affect Suction Flow
If anything restricts the inlet flow, the pressure differential may not be enough to move the oil fast enough. This can result in cavitation or pump starvation, which are damaging to the pump.
Here are a few things that can cause this:
- Clogged inlet filters
- Too small or too long of a suction line
- Poor reservoir design or low oil levels
Even a simple restriction can prevent oil from reaching the pump in time, which may lead to internal scoring, noise, loss of flow, or even total pump failure.
Effect of Altitude
Another factor to consider is elevation. At higher altitudes, atmospheric pressure decreases, which means there’s less force available to push the oil into the pump inlet.
For example, a system designed to run at sea level in Seattle, Washington, might not operate correctly at high elevation in Denver, Colorado. The atmospheric pressure in Denver is lower, so the effective suction pressure drops. This could cause performance issues—or worse, pump damage—if the system isn't adjusted for the new conditions.
Designers must account for this when sizing suction lines and selecting pumps, especially for systems expected to operate in a wide range of altitudes.