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.

Live editor identity: Live-library identity: `hydraulic-reservoir`, 60 × 40 canvas units, 2 logical terminals. Match this exact SVG and terminal list when identifying the placeable editor symbol; do not identify a physical package from the schematic outline alone.

Function and usage boundary

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.

Usage boundary: Use the Hydraulic Reservoir abstraction only when the intended circuit can be expressed through these logical ports: `supply` (Supply), `return` (Return). Select a versioned physical profile and exact manufacturer datasheet before assigning package pins, ratings, or fabrication dimensions.

Standards: IEC vs ANSI

The displayed SVG is the live editor implementation, not a licensed reproduction or certification of an IEC or IEEE database glyph. Verify the exact official entry and project edition before issuing work.

Variant availability

• Editor implementation (available): This is the exact SVG and terminal geometry placed by the CircuitDiagramMaker editor.
• IEC 60617 (reference only): IEC 60617 is the official international source. This page does not present the editor SVG as a licensed or certified IEC reproduction.
• ANSI/IEEE 315 (reference only): IEEE 315 is the North American reference listed by IEEE as inactive-reserved; use the edition required by the project.

Logical terminals

The live editor exposes `supply` (Supply), `return` (Return). These are stable logical terminal IDs for diagram connectivity, not a promise that they equal physical package pin numbers.

Polarity and direction

This editor symbol does not encode a universal positive/negative orientation. Do not infer physical polarity or package pin numbering from left/right placement alone.

Reference designator

Project-defined: No single reference letter is asserted for this broad symbol. Apply the project standard and equipment-classification rules consistently.

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

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.

Common interpretation mistakes

• Treating the Hydraulic Reservoir drawing (60 × 40 canvas units) as a physical footprint. The schematic outline expresses electrical intent; package and panel dimensions come from the selected physical profile.
• Copying `supply`, `return` directly to a wire marker or manufacturer package pin number. These are logical Hydraulic Reservoir port IDs and require an exact ordered-part terminal map.
• Assuming every unlabelled or generic Hydraulic Reservoir terminal has the same role across manufacturers and variants.

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.

Sources and verification

• IEC 60617:2026 DB — Graphical symbols for diagrams (International Electrotechnical Commission) — The current official IEC graphical-symbol database and the electrotechnical areas it covers. Exact symbol identity data requires IEC access.; claim scope: standards
• IEEE/ANSI 315-1975 — Graphic Symbols for Electrical and Electronics Diagrams (IEEE Standards Association) — The official catalogue record for North American diagram symbols and reference-designation letters; IEEE lists the standard as inactive-reserved.; claim scope: standards, reference_designator
• IEC 61666:2010+AMD1:2021 — Identification of terminals within a system (International Electrotechnical Commission) — General principles for identifying terminals within systems; it does not replace the exact ordered-device datasheet.
• IEC 81346-2:2019 — Object classes and reference-designation codes (International Electrotechnical Commission) — The official classification scheme used to form reference designations across technical disciplines.; claim scope: reference_designator
• IEC 61082-1:2014 — Preparation of documents used in electrotechnology (International Electrotechnical Commission) — General presentation rules for electrotechnical diagrams, drawings, and tables.; claim scope: standards, usage

Geometry fingerprint: aef94da6053943d9. A changed SVG, canvas size, or terminal definition invalidates the recorded review.

Related symbols

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

Physical breadboard, PCB, and panel mapping

No universal physical profile is published for this symbol yet. Create a versioned custom part from the exact manufacturer dimensions and terminal map before fabrication.

Place the Hydraulic Reservoir symbol directly on a wiring diagram or schematic in the free online editor — no download required.