Reservoir Functions

The hydraulic reservoir serves multiple critical functions beyond simply storing oil:

• Heat dissipation: Tank walls radiate heat to ambient air
• Air separation: Allows entrained air bubbles to rise and escape
• Contaminant settling: Particles settle to bottom before reaching pump
• Thermal expansion: Accommodates oil volume changes with temperature
• Cylinder displacement: Stores oil displaced by differential cylinders

Sizing Rules of Thumb

Traditional sizing guidelines based on pump flow rate:

Mobile/Compact: 2-3 x pump GPM

Standard Industrial: 3-4 x pump GPM

Severe Duty/Precision: 4-5 x pump GPM

These multipliers assume the oil circulates through the reservoir 15-20 times per hour minimum for adequate heat transfer and air release.

Heat Dissipation Capacity

Reservoir walls dissipate heat through natural convection:

Q = h x A x dT

Where h is approximately 1-2 BTU/hr-ft2-F for still air. A painted steel tank with 50F temperature rise above ambient can dissipate roughly 60-120 BTU/hr per square foot of surface area.

For systems with significant heat generation, additional cooling is typically required beyond reservoir dissipation alone.

Air Release Time

Entrained air bubbles need time to rise and escape. Minimum dwell time in the reservoir should be 1-3 minutes. Air rise velocity in hydraulic oil is approximately:

V_rise = 0.1 to 0.5 in/sec (depending on bubble size and oil viscosity)

Larger reservoirs provide more dwell time for complete air separation, critical for pump longevity and system performance.

Baffle Design

Baffles serve to:

• Separate return flow from pump inlet
• Maximize oil path length for cooling and air release
• Reduce turbulence that re-entrains air
• Help settle contaminants away from suction

Baffle height should be 2/3 of oil level. Return line should discharge below oil surface and away from pump inlet.