How to Choose the Right Reservoir Size for a Hydraulic Power Unit

Choose a hydraulic power unit reservoir by matching the system's oil-volume changes to the reservoir's permitted operating levels, then checking orientation, inlet conditions, air release, cooling, and service access. The nominal tank size is only one part of that decision. It does not tell you how much oil can safely leave the reservoir during a machine cycle.

A useful selection starts with a defined machine state and a volume balance. From there, the supplier can confirm whether a particular reservoir, pickup, return arrangement, and set of options support the application. This guide focuses on selecting an HPU reservoir package and uses an illustrative comparison to show why two quoted tank capacities may provide very different usable operating ranges.

Sommaire

1. Define which reservoir volume is being quoted
2. Describe the oil demand across operating states
3. Match the demand to a permitted fill range
4. Verify orientation and inlet conditions
5. Assess air release and return-flow management
6. Check thermal duty independently
7. Compare the complete reservoir package
8. Specify and verify the selected configuration
9. Foire aux questions

Define which reservoir volume is being quoted

Ask the supplier to define every capacity term. Gross internal capacity, recommended fill quantity, maximum permitted oil quantity, minimum operating quantity, and usable exchange volume can all describe different things.

Gross volume includes space that may need to remain unfilled. The minimum operating level preserves required conditions around the pump inlet and other components. The range between approved minimum and maximum liquid levels is the available operating swing for the stated configuration and conditions.

Rexroth's power-unit information separately identifies tank capacity and pendulum volume for some products. That distinction is a useful reason to request explicit definitions rather than assume all quoted liters can be sent to actuators. Rexroth power-unit information.

State whether quantities are measured in the reservoir alone or across the entire filled system. Oil in hoses, cylinders, valves, coolers, and filters contributes to total fluid inventory but is not necessarily available as reservoir operating swing.

Also check units. Liters and gallons need an explicit conversion basis, including which gallon is meant. Compare supplier options using the same definitions and installed orientation. The wider selection framework is covered in hydraulic power unit selection.

Describe the oil demand across operating states

Choose a reference state, such as the specified initial fill condition with actuators in named positions and relevant circuits filled. Record oil leaving and returning to the reservoir relative to that state.

Include the combinations that can actually occur. Single-acting cylinders, double-acting cylinders, accumulators, remote lines that can drain back, and auxiliary circuits may affect the level differently. Cylinder drawings and circuit information establish the relevant geometry and connections; cylinder drawings and specification sheets explains how to read those inputs.

Do not multiply pump flow by the whole operating time and call the result the required tank capacity. Much of that oil circulates back to the tank. The important inventory question is how much is temporarily held outside the reservoir at each state.

Record maximum net withdrawal and maximum net return separately. A shutdown state can have a higher level than a running state. A hot condition may require extra liquid-volume allowance, while the supplier's maximum level must still preserve the required headspace.

Use a state table to distinguish simultaneous events from mutually exclusive ones. If the available information is incomplete, identify the missing amount and its owner. An attractive reservoir option cannot be approved using an incomplete inventory.

Match the demand to a permitted fill range

The following numbers are invented for a selection example. They are not manufacturer ratings, standard margins, or recommendations for a particular machine.

Assume the system can withdraw 5.5 L from a reference reservoir quantity V0. In another reviewed state, it can return a net additional 3 L relative to V0. The latter allowance already includes the relevant return and thermal-volume effects for this example. These are alternative states, not two sequential additions.

The reservoir must satisfy both:

V0 − 5.5 L ≥ minimum permitted quantity

V0 + 3 L ≤ maximum permitted quantity

Hypothetical option	Gross capacity	Permitted liquid range	Required initial quantity V0	Volume result
A	18 L	6–13 L	At least 11.5 L and at most 10 L	No feasible initial quantity
B	24 L	8–18 L	13.5–15 L	A feasible interval exists

For B, selecting an illustrative V0 of 14 L gives a low-state quantity of 8.5 L and a high-state quantity of 17 L. These remain inside the assumed permitted range. The operating swing is 8.5 L, while B provides a 10 L permitted liquid range.

This arithmetic only passes the inventory screen. It does not approve inlet performance, return-flow behavior, thermal duty, sensor settings, or manufacturing tolerances. Any extra uncertainty allowance must be identified and checked rather than silently assumed to fit.

Option A cannot be rescued by simply topping it up: enough oil for its low state would exceed its high-state limit. Likewise, filling B to its nominal gross capacity would defeat the defined operating limits. Pump and motor capability remains a separate check in motor and pump sizing.

Verify orientation and inlet conditions

Permitted liquid quantities depend on geometry. A level reservoir may keep the pickup submerged while the same quantity in a tilted reservoir may not. Check the exact horizontal or vertical mounting arrangement and any allowable slopes or vehicle motion.

Return to option B. Suppose a different installation orientation raises its required minimum liquid quantity from 8 L to 10 L while its maximum remains 18 L. The initial quantity must now be at least 15.5 L and at most 15 L. No feasible interval remains. The same nominal tank has become unsuitable under these assumed conditions.

Actual orientation limits must come from the configured design and verification. Do not infer them from a drawing that merely shows the unit rotated for illustration.

The inlet assessment also considers fluid viscosity, temperature, inlet-path losses, pump position, and transient conditions. A sufficient static oil level does not guarantee acceptable pump inlet pressure. Conversely, adding tank volume does not remove a restricted inlet path.

Specify what happens during the most demanding approved movement, including rapid changes in level. Where low-level detection is required, confirm its location, response, and control action. Investigating symptoms is a separate task; see hydraulic power unit noise.

Assess air release and return-flow management

The return stream should be managed so that oil reaching the pump has acceptable condition. Tank geometry, separation between return and pickup, internal flow paths, fluid properties, and air-entry sources all affect that outcome.

A simple volume divided by flow calculation gives a nominal turnover time, not proof that every parcel of oil remains in the tank for that duration. A direct path from return to suction can reduce effective separation even in a comparatively large tank.

Rexroth's myCRO approach uses flow evaluation and application-dependent degassing arrangements when reducing tank size. It demonstrates that compactness can depend on engineered fluid management. It does not establish a general reduction percentage for another design. myCRO technical overview.

HYDAC's OXiStop is another specific architecture combining membrane separation with active fluid treatment. Its sizing statements apply to that solution; they should not be generalized into a claim that pump flow is irrelevant to every reservoir. OXiStop description.

Ask what evidence supports the selected tank at the expected return rates and air-entry conditions. A smaller validated package may be suitable, while an unvalidated reduction of a conventional tank may not be. The test purpose should be defined clearly, as discussed in types of hydraulic test benches.

Check thermal duty independently

A reservoir stores heat as its oil and structure warm. It also rejects heat through its surroundings, but storage and continuous heat rejection are different functions. More oil can slow a temperature rise without solving a long-term excess of heat input over heat removal.

Estimate heat generation from the actual operating cycle, including throttling, leakage, drive-related losses entering the fluid, and other relevant sources. Do not assume the motor's full nameplate power becomes oil heat during every stage.

Review the allowed fluid and component temperatures, ambient conditions, cooling arrangement, and duty duration. A unit installed inside an enclosure may have different heat-rejection conditions from the same unit in open air.

HYDAC's cooling tool asks for variables including liquid flow, fluid and ambient temperatures, power, and tank volume. This supports treating thermal assessment as a separate set of inputs. No cooler has been selected or simulation run for the example in this article. HYDAC Cool-IT inputs.

Check cold conditions as well as hot ones. The selected fluid and temperature affect viscosity and inlet behavior. Heating, cooling, and control requirements must be coordinated with the complete HPU. The duty and power discussion in motor and pump sizing provides related context.

Compare the complete reservoir package

A reservoir option is more than a container. Review the pickup, return entry, drain, filler, breather or sealed arrangement, level indication, filter locations, temperature measurement, and service openings that apply to the design.

Selection item	Supplier evidence to request
Capacity and levels	Defined volumes and the conditions for minimum and maximum limits
Inlet arrangement	Approved orientation and relevant pump inlet requirements
Return management	Permitted return conditions and fluid-management provisions
Thermal package	Duty assumptions, cooler or heater options, and verification basis
Maintenance	Access, drainability, cleaning provisions and replacement clearances
Documentation	Configuration drawing, instructions and identification of included options

Parker's power-unit catalog includes different filter and heat-exchanger options and directs users to confirm detailed installation information. Such option-dependent layouts are why comparisons should use the final configuration, not a generic series illustration. Parker HPU catalog.

Consider how oil will be filled, identified, sampled, and serviced without introducing avoidable contamination. Access to a filter element or breather may determine the practical installation envelope even if the bare tank fits.

Compare options using the same duty and maintenance assumptions. A smaller purchase volume may create additional cooling or fluid-treatment requirements. A larger package may introduce weight, space, or warm-up consequences. These tradeoffs belong in the overall hydraulic power unit selection.

Specify and verify the selected configuration

Provide the supplier with the circuit revision, state-volume table, fluid specification, temperature range, flow profile, installation orientation, expected motion, duty cycle, and service constraints. Include the required initial fill state and the definition of every level limit.

Ask for a written response identifying the selected reservoir, usable operating range, excluded conditions, and any options required to achieve that range. Record unresolved assumptions as open items.

The verification plan should cover the low and high inventory states, approved orientations, relevant cold and hot conditions, and the most demanding return event. It should also check inlet behavior, fluid condition, thermal response, and any level-control function under an approved procedure.

Separate measurements from assumptions in the final record. If a state cannot be demonstrated directly, explain the calculation or other evidence used and the remaining limitation. The Liste de contrôle des caractéristiques techniques des cylindres can help assemble actuator inputs, but the complete HPU assessment requires the wider circuit.

The filling instruction should identify the machine state, fluid temperature condition, and whether remote components have already been filled. Otherwise, the same sight-glass mark may be interpreted differently during commissioning and routine service. State who checks the final level after the approved commissioning sequence, and keep the recorded fill quantity separate from replacement oil supplied for future maintenance.

Changing an actuator, accumulator operating range, remote line, or machine sequence may alter reservoir demand. Preserve the calculation and configuration revision so that later modifications trigger a meaningful review.

Foire aux questions

Can reservoir size be chosen from a fixed multiple of pump flow? A multiplier can be a preliminary convention for a particular design practice, but it cannot replace inventory, inlet, air-release, thermal, and installation checks.

Is usable oil volume the same as nominal capacity? No. Ask how the supplier defines each quantity and under which orientation and operating conditions.

Will a larger tank cure overheating? It may change heat storage and surface area, but the system still needs adequate heat rejection for its actual duty.

Can the same tank be installed horizontally or vertically? Only if the configured unit supports the orientation and its pickup, return, level limits, and service features are verified accordingly.

What is the next calculation after identifying candidate sizes? Confirm the full system's state-by-state oil balance and map it to the candidate's permitted liquid levels. Then complete the inlet, fluid-management, thermal, and application verification before release.