Hydraulic Reservoir Symbol

Definition: The Hydraulic Reservoir symbol represents the vented tank that stores the system's fluid and performs conditioning duties — heat dissipation, air release, and contaminant settling — drawn per ISO 1219-1 as an open-top rectangle (three-sided box) with connection lines for Supply (to the pump suction) and Return (from the system), where line ends drawn to the bottom of the box indicate connections below fluid level.

Also known as: hydraulic tank, oil reservoir, sump, oil tank, fluid reservoir, vented reservoir, hydraulic oil tank.

What the Hydraulic Reservoir symbol means

The Hydraulic Reservoir symbol denotes far more than a fluid bucket: the tank is the circuit's thermal mass, deaeration chamber, and settling basin. On ISO 1219-1 schematics the open-top rectangle means a vented (atmospheric) reservoir — the free surface breathes through a filler-breather — while a closed rectangle or capsule marks a pressurised reservoir (aircraft and some mobile systems). Every line that terminates at a tank symbol is a return to this store of fluid, and drawing convention carries information: a line drawn down to touch the bottom edge of the box connects below fluid level (correct for pump suction and main returns, preventing aeration), while a line stopping short of the bottom connects above fluid level (acceptable only for drains that must not siphon).

Schematics often scatter many small tank symbols across the page rather than routing every return line to one drawing location — they all represent the same physical reservoir. Around the main reservoir symbol you will typically find its accessory family: strainer on the Supply (suction) line, return-line filter, filler-breather, fluid-level gauge with thermometer, level and temperature switches, and sometimes a heater or cooler — each drawn with its own ISO 1219-1 symbol attached to the tank.

How to identify the Hydraulic Reservoir symbol

The vented reservoir is a rectangle open along its top edge — just three sides — usually with a horizontal fluid-level line inside. Return and suction lines drop into it; check where each line ends: touching the bottom = submerged connection, ending above the level line = above-fluid connection. The ubiquitous shorthand is the small 'comb' tank symbol terminating return and drain lines all over the schematic — every one of them is this same reservoir. A closed (four-sided) rectangle means a pressurised reservoir, and should not be confused with an accumulator, which is drawn as a capsule/oval with a gas precharge marking.

As with all fluid power symbols, ISO 1219-1 governs internationally and legacy ANSI Y32.10 used the same forms, so there is no separate American symbol. Do not confuse the hydraulic tank symbol with the electrical ground/earth symbol — functionally they play a similar 'common return' role on their respective diagrams, which is a useful reading analogy but a drafting distinction that matters.

Function in a circuit

The reservoir's engineering duties are set by residence time — the minutes each litre spends in the tank between trips around the circuit. Heat dissipation: the tank's walls reject heat to ambient, and in many industrial systems the reservoir provides the majority of cooling without a dedicated heat exchanger. Deaeration: entrained air bubbles rise and burst at the free surface — helped by internal baffles that force returning fluid on a long path to the suction side and kill turbulence. Settling: particles and water sink to the bottom, away from the (slightly elevated) suction strainer, to be removed via the drain plug at the tank's low point.

The classic industrial sizing rule makes residence time concrete: reservoir volume of 2–3 times the pump's per-minute flow (a 60 L/min system gets a 120–180 L tank), relaxed to roughly 1× or less on mobile equipment where space and weight rule and coolers compensate. Baffles separate return from suction zones; returns discharge below fluid level to avoid churning in air; the breather filters incoming air as the level rises and falls; and level/temperature switches wired to the control system guard the two failure modes the tank sees — running dry and running hot.

Standards: IEC vs ANSI

Terminals / pins

Typical values

Industrial sizing rule: 2–3 × pump flow per minute (e.g. 40 L/min pump → 80–120 L tank); mobile equipment runs 0.5–1× with auxiliary coolers. Normal operating oil temperature is 40–60 °C, with alarms typically at 65–70 °C and viscosity-driven cold-start limits around 10–15 °C for standard ISO VG 32–68 fluids. Fluid level is kept at roughly 75–85% of tank volume to leave an air cushion and thermal expansion space. Breathers filter to 3–10 µm on quality units; suction strainers are commonly 74–149 µm (100–200 mesh) sized for under 0.1 bar pressure drop. Steel tanks dissipate very roughly 20–30 W/m²·°C of surface area to still air — the number behind the 2–3× rule.

Where the Hydraulic Reservoir symbol is used

• Hydraulic power units for presses, machine tools, and injection moulders — the rectangular tank the pump and motor sit on
• Mobile equipment (excavators, cranes, refuse trucks) as compact side- or frame-mounted tanks with coolers making up the reduced size
• Steel mills and marine steering-gear systems with large baffled reservoirs and redundant level/temperature instrumentation
• Log splitters and small power packs where the tank doubles as the machine's structural base
• Test stands and laboratories, where generous reservoirs stabilise fluid temperature for repeatable results
• Aircraft hydraulic systems using closed, pressurised reservoirs (drawn as the closed-rectangle variant) to guarantee pump feed at altitude

Example

In a power-unit schematic drawn to ISO 1219-1, the Hydraulic Reservoir symbol's Supply pin feeds the pump suction through a 125 µm strainer, with the line drawn touching the tank bottom to show a flooded, below-level connection; the Return pin receives system flow through a 10 µm return filter, also terminating below fluid level on the far side of a baffle. Sized at 2.5× the pump's 40 L/min (a 100 L tank filled to about 80%), the reservoir gives each litre over two minutes of residence per pass to shed heat, release entrained air, and drop contamination before the fluid is drawn back into the Supply line.

Key facts

• ISO 1219-1 draws a vented reservoir as an open-top (three-sided) rectangle; a closed rectangle means a pressurised reservoir — and an accumulator is a different symbol entirely (pre-charged capsule).
• Line-ending convention carries meaning: a line drawn to the bottom of the tank symbol connects below fluid level; a line stopping above the level line connects above it — suction and main returns should be submerged.
• The many small tank symbols scattered across a schematic all represent the same physical reservoir — a drafting shorthand to avoid routing every return across the page.
• The reservoir does three jobs beyond storage: heat dissipation through its walls, deaeration at the free surface, and settling of particles and water to the drain point.
• Industrial sizing rule: tank volume = 2–3 × pump flow per minute; mobile systems accept ~1× or less and add coolers.
• Baffles force returning fluid on a long path to the suction side, preventing the pump from re-ingesting hot, aerated fluid.
• Normal oil temperature is 40–60 °C; sustained operation above ~70 °C roughly halves oil life for every additional 10 °C and cooks seals.
• The filler-breather is a filter, not just a cap — the tank inhales ambient air every time the fluid level drops, and unfiltered breathing is a major contamination ingress route.

Frequently asked questions

What does the open rectangle tank symbol mean in a hydraulic schematic?

It is the ISO 1219-1 vented (atmospheric) reservoir — open along the top edge to show the free fluid surface breathing to atmosphere through a filler-breather. A fully closed rectangle would mean a pressurised reservoir. Every return, drain, and suction line ending at one of these little tank symbols connects to the same physical reservoir, no matter how many times the symbol repeats across the drawing.

Why do some lines touch the bottom of the tank symbol and others stop short?

It encodes the connection depth. A line drawn down to touch the tank's bottom edge terminates below fluid level — required for pump suctions (flooded inlet) and desirable for returns (no splashing air into the oil). A line ending above the drawn fluid level indicates an above-level connection, used for certain drains (e.g. motor case drains that must not siphon). It is a small drafting detail with real aeration and priming consequences.

How big should a hydraulic reservoir be?

The classic industrial rule is 2–3 times the pump's per-minute flow: a 60 L/min pump gets a 120–180 L tank. This buys residence time for cooling, air release, and settling. Mobile equipment cannot afford the size or weight, so it runs 0.5–1× and compensates with oil coolers and better filtration. Fill to about 75–85% — the air space above the fluid absorbs thermal expansion and lets bubbles burst.

What do the baffles inside a hydraulic tank do?

A baffle plate divides the return zone from the suction zone, forcing returning oil to travel a long path before it can be drawn back into the pump. This gives entrained air time to rise, heat time to reach the tank walls, and particles time to settle — and it stops the pump short-circuiting on hot, foamy, just-returned fluid. Baffles typically stand about two-thirds of fluid height with flow openings at alternating ends.

Is the hydraulic tank symbol the same as an electrical ground symbol?

They look loosely similar and play analogous roles — both are the 'common return' their diagrams route everything back to — but they are different symbols in different languages. The hydraulic tank is an open rectangle per ISO 1219-1 on fluid power drawings; electrical earth/ground symbols (IEC 60617) are the descending-bars or rake shapes on wiring diagrams. The analogy is a good learning aid for electricians reading their first hydraulic schematic, and that is where it should stop.

Why does my hydraulic oil look milky, and what does the reservoir have to do with it?

Milky oil is finely entrained air or emulsified water — and the reservoir is where both should be leaving the system. Common tank-related causes: fluid level too low (vortexing at the suction), return line discharging above the fluid level (beating air in), missing or damaged baffle, undersized tank giving no deaeration time, or a failed breather letting moist air breathe unfiltered. Check level, breather, and return-line submergence before blaming the pump.

Related symbols

• Check Valve symbol
• Float Switch symbol
• Flow Control Valve symbol
• Pressure Switch symbol
• Pump (Motor-driven) symbol
• Temperature Switch symbol

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