Choosing the right hydraulic reservoir capacity is one of the most important decisions in hydraulic system design. A reservoir that is too small can cause overheating, aeration, and pump damage. One that is too large increases cost, footprint, and oil consumption.
So how do you size it correctly?
In this guide, we’ll walk through a practical step-by-step approach to selecting hydraulic reservoir capacity based on flow, heat load, fluid dynamics, and real operating conditions.
Why Reservoir Capacity Matters
A hydraulic reservoir does much more than store oil. It also helps:
1） dissipate heat
2） release entrained air
3） settle contaminants
4）stabilize system volume during actuator movement
5）ensure reliable pump suction
That means reservoir sizing directly affects system efficiency, reliability, and service life.
How to Select Hydraulic Reservoir Capacity
Step 1: Use a Rule-of-Thumb for Initial Sizing
A common starting point is:
Reservoir volume = 3 to 5 times pump flow (L/min)
or roughly 0.8 to 1.5 times pump flow (GPM) in gallons.
Example
If pump flow is 80 L/min, initial tank estimate:
80×(3 to 5)=240 to 400 L
This gives a preliminary range—but final sizing must be validated.
Step 2: Check Fluid Residence Time
The oil should remain in the reservoir long enough for:
1. air bubbles to separate,
2. contaminants to settle (partially),
3. heat to transfer to tank walls.
A typical target is 2–5 minutes residence time.
V=Q×t
Where:
- V = effective oil volume
- Q= return flow
- t = residence time
If your calculated residence time is too short, increase effective reservoir volume or improve internal baffling.
Step 3: Evaluate Thermal Load (Critical)
In many systems, heat rejection is the limiting factor—not storage volume.
Hydraulic inefficiencies convert power into heat. If the reservoir cannot dissipate this heat, oil temperature rises, viscosity drops, seals degrade, and component wear accelerates.
Thermal check questions
- What is the system’s average and peak heat generation (kW)?
- What is the allowable oil temperature range?
- Can tank surface area reject enough heat at your ambient temperature?
- Do you need an oil cooler?
For high-duty systems, always combine reservoir sizing with a heat balance calculation.
Step 4: Account for Working Volume Changes
Reservoir capacity must cover dynamic oil volume changes caused by:
1. cylinder rod-side/cap-side volume differences,
2. accumulator charging/discharging,
3. thermal expansion of oil,
4. drain-back from lines when the system stops.
Also ensure the pump suction line remains submerged at minimum oil level to prevent vortexing and cavitation.
Step 5: Design the Internal Layout Correctly
A well-sized reservoir can still perform poorly if internal design is wrong.
Best practices:
- keep return and suction ports separated,
- use baffles to force flow path across the tank,
- place return outlet below minimum fluid level,
- avoid high return velocity and turbulence,
- maintain freeboard above maximum level for foaming control.
In short: capacity and layout must be designed together.
Step 6: Validate Filtration and Breathing Strategy
Reservoir volume helps dilute contamination temporarily, but it is not a substitute for cleanliness control.
Make sure your design includes:
- return-line filtration,
- proper breather filtration,
- clean fill port,
- optional offline (kidney-loop) filtration for critical systems.
Common Sizing Mistakes to Avoid
- Using only rule-of-thumb values without thermal or drawdown checks.
- Ignoring duty cycle, especially in intermittent high-load systems.
- Undersizing for mobile systems where space is tight but heat is high.
- Oversizing excessively, causing unnecessary cost and slow warm-up.
- Poor baffle/port design, leading to aeration despite “large enough” volume.
Quick Sizing Workflow
- Gather data: flow, pressure, duty cycle, actuator volumes, ambient temperature.
- Estimate reservoir volume with 3–5× flow rule.
- Add drawdown and expansion allowances.
- Verify 2–5 min residence time.
- Run thermal balance check.
- Finalize min/max level, freeboard, and internal baffle arrangement.
- Validate with commissioning temperature and level data.
