Hydraulic Reservoir Symbols: Types, ISO Symbols & Chart

Hydraulic reservoirs are a fundamental part of almost every hydraulic system. They store hydraulic fluid, supply oil to the pump, receive returning fluid from actuators and valves, and provide space for heat dissipation, air separation, and contamination settling. On hydraulic schematics, these functions are represented using hydraulic reservoir symbols, which allow engineers and technicians to understand how fluid enters, leaves, and circulates through the system.

Although a physical hydraulic reservoir may include breathers, filters, level gauges, drain ports, coolers, and multiple connection points, its schematic representation is usually much simpler. The basic reservoir symbol identifies the fluid storage location, while variations of the symbol can indicate whether the reservoir is vented to atmosphere, sealed, pressurized, or connected above or below the fluid level.

Understanding these differences is important when reading hydraulic circuit diagrams. For example, a pump suction line normally connects below the reservoir fluid level, while certain drain or return lines may terminate above the fluid level. A pressurized reservoir may also be represented differently from a conventional atmospheric tank. These small graphical differences can provide important information about system operation and component installation.

Hydraulic reservoir symbols are commonly interpreted according to established fluid-power graphical conventions such as ISO 1219, which standardizes symbols used in hydraulic and pneumatic diagrams. Using standardized symbols helps engineers, maintenance technicians, designers, and operators read hydraulic schematics consistently regardless of the equipment manufacturer.

This article explains the most common hydraulic reservoir symbols, including basic reservoirs, vented reservoirs, pressurized reservoirs, fluid-level connections, and reservoirs used with filters and other accessories. It also shows how to identify reservoir connections and trace fluid flow through a complete hydraulic circuit.

1. What Is a Hydraulic Reservoir Symbol?

A hydraulic reservoir symbol is a graphical representation used in hydraulic schematics to identify the component that stores and supplies hydraulic fluid to the system. Instead of drawing the actual shape, size, or construction of a hydraulic tank, circuit diagrams use a simplified symbol that communicates the reservoir’s function and its relationship with other hydraulic components.

In a typical hydraulic system, the reservoir serves as the main storage point for hydraulic oil. Fluid is drawn from the reservoir by the hydraulic pump, circulated through control valves and actuators, and eventually returned to the reservoir. Because of this, the reservoir often acts as both the starting and ending point of the hydraulic flow path shown on a schematic.

The basic reservoir symbol usually consists of a rectangular or U-shaped graphical representation with hydraulic lines terminating at or inside the reservoir. The exact appearance can vary depending on whether the reservoir is open to atmospheric pressure, sealed, pressurized, or whether a connection terminates above or below the fluid level.

Purpose of the Reservoir Symbol

The primary purpose of a reservoir symbol is to show where hydraulic fluid is stored and where different system lines connect to the fluid supply.

On a hydraulic schematic, the reservoir symbol can help identify:

• Pump suction lines
• Main return lines
• Case-drain lines
• Leakage or drain lines
• Filter connections
• Cooling circuits
• Fluid-level relationships
• Atmospheric or pressurized tank conditions

For example, tracing the suction line from a hydraulic pump will normally lead back to the reservoir. Similarly, tracing the return line from a directional control valve or actuator will usually lead back to the same reservoir.

Reservoir Symbol vs. Physical Hydraulic Tank

A schematic symbol should not be interpreted as a physical drawing of the reservoir.

An actual hydraulic reservoir can be a relatively complex assembly containing:

• Filler-breather caps
• Air breathers
• Sight level gauges
• Temperature indicators
• Suction strainers
• Return filters
• Baffles
• Drain plugs
• Clean-out covers
• Heat exchangers
• Multiple suction and return ports

The hydraulic schematic normally represents only the information necessary to understand system operation.

For example, a reservoir may physically contain several chambers and internal baffles, but the circuit diagram may show it using only a simple reservoir symbol with several hydraulic connections.

This simplification is one of the main principles of hydraulic schematic drawings: symbols represent functions rather than physical appearance.

Importance of Reservoir Symbols in Hydraulic Schematics

Recognizing the hydraulic reservoir symbol makes it much easier to understand an unfamiliar hydraulic circuit.

A common method for reading a hydraulic schematic is to locate the reservoir first and then follow the flow path through the system:

Reservoir → Pump → Control Components → Actuator → Return Line → Reservoir

From this basic path, additional branches such as relief valves, filters, accumulators, coolers, and drain lines can then be identified.

The reservoir therefore provides an important reference point when interpreting the entire hydraulic circuit.

---

2. Basic Hydraulic Reservoir Symbol

The basic hydraulic reservoir symbol represents the location where hydraulic fluid is stored within the system. It is one of the simplest and most recognizable symbols used in hydraulic circuit diagrams.

In many schematic conventions, an atmospheric reservoir is shown as an open container. Hydraulic lines may extend into the reservoir to indicate where suction, return, or drain connections terminate.

The symbol does not show the actual capacity or physical dimensions of the hydraulic tank. A 10-liter reservoir and a 1,000-liter industrial hydraulic reservoir may use essentially the same basic schematic symbol.

Basic Elements of a Reservoir Symbol

A hydraulic reservoir symbol can contain several graphical elements that provide useful information.

The most important include:

Reservoir boundary

The boundary represents the hydraulic fluid storage container. An open upper boundary commonly indicates that the reservoir is exposed to atmospheric pressure.

Hydraulic connection lines

Lines connected to the reservoir represent suction, return, drain, or other hydraulic connections.

Fluid-level relationship

The way a hydraulic line terminates relative to the reservoir symbol may indicate whether the connection is located above or below the normal hydraulic fluid level.

Understanding these elements is particularly important because two reservoir connections that look similar may have different operating functions.

Open Reservoir Symbol

The standard hydraulic tank used in many industrial systems is an atmospheric reservoir.

Its schematic representation is commonly drawn as an open container, indicating that the hydraulic fluid is effectively referenced to atmospheric pressure.

A simplified representation may look conceptually like this:

        Hydraulic Line
              │
              │
        ┌─────┴─────
        │
        │ Hydraulic
        │   Fluid
        │
        └───────────

The exact graphical form depends on the drawing standard being used, so designers should always refer to the applicable schematic legend when necessary.

Connections Below the Fluid Level

A line extending into the reservoir and terminating below the indicated fluid level represents a submerged connection.

This arrangement is commonly used for:

• Pump suction lines
• Main return lines
• Certain drain lines

A pump suction connection is normally positioned below the fluid level so that the pump receives a continuous supply of oil and the risk of drawing air into the suction line is minimized.

Submerged return lines may also reduce aeration, foaming, and turbulence when returning hydraulic fluid enters the tank.

Connections Above the Fluid Level

Some hydraulic lines may terminate above the reservoir fluid level.

Depending on the circuit and drawing convention, this can be used to represent certain:

• Drain lines
• Leakage lines
• Return connections

The difference between an above-level and below-level connection can be operationally important.

For example, if a component drain line requires minimal backpressure, its routing and termination arrangement can affect component performance. This is particularly important for certain pump and motor case-drain circuits.

How to Identify the Reservoir in a Hydraulic Circuit

The reservoir can usually be identified by looking for the point where the pump suction line begins.

A typical simple hydraulic circuit follows this sequence:

Reservoir → Pump → Directional Valve → Hydraulic Cylinder → Reservoir

If a relief valve is installed, it will normally divert excess pressure flow back toward the reservoir:

Pump → Relief Valve → Reservoir

Other components such as return filters and oil coolers may also appear before returning oil reaches the tank.

For example:

Cylinder → Directional Valve → Return Filter → Oil Cooler → Reservoir

Because many system lines eventually terminate at the reservoir, it is often one of the best starting points for analyzing a hydraulic schematic.

---

3. Vented Hydraulic Reservoir Symbol

A vented hydraulic reservoir is a reservoir that allows the air space above the hydraulic fluid to communicate with the surrounding atmosphere. It is one of the most common reservoir arrangements used in industrial and mobile hydraulic systems.

As hydraulic cylinders extend and retract, the amount of oil contained in the reservoir can change. Temperature variations can also cause the hydraulic fluid and the air above it to expand or contract. A vented reservoir allows air to enter or leave the tank so that the internal pressure remains close to atmospheric pressure.

In practical systems, atmospheric air normally passes through a breather or filler-breather assembly rather than entering directly through an open hole.

How a Vented Reservoir Works

Consider a hydraulic cylinder extending.

Oil is pumped from the reservoir into the cylinder. As the reservoir fluid level decreases, additional air must enter the space above the oil.

When the cylinder retracts, hydraulic fluid returns to the reservoir. As the fluid level rises, air must leave the tank.

The breather allows this exchange of air while helping reduce contamination entering the hydraulic system.

The operating sequence can be summarized as:

Oil leaves reservoir → Fluid level falls → Air enters through breather

and

Oil returns to reservoir → Fluid level rises → Air exits through breather

Without proper venting, pressure or vacuum conditions could develop inside the reservoir.

How the Vented Reservoir Symbol Is Represented

In simplified hydraulic diagrams, an atmospheric reservoir is commonly represented using an open reservoir symbol. The open top indicates that the reservoir is referenced to atmospheric pressure.

If a breather is important to the design or troubleshooting of the system, it may also be shown as an additional component connected to the reservoir.

The schematic therefore communicates the operating principle rather than showing the detailed physical construction of the filler-breather assembly.

Reservoir Breathers

Although the reservoir is described as “vented,” air should ideally not move freely into and out of the tank without filtration.

Hydraulic reservoir breathers are designed to reduce the amount of airborne contamination entering the hydraulic fluid.

A typical breather may contain:

• Filter media
• Filler cap
• Air passage
• Splash protection
• Contamination protection

More advanced systems may use desiccant breathers, which help remove both airborne particles and moisture from incoming air.

This is especially useful in applications where hydraulic oil cleanliness and moisture control are critical.

Why Vented Reservoirs Are Common

Vented reservoirs are widely used because they are simple, economical, and suitable for many conventional hydraulic power units.

Typical applications include:

• Machine tools
• Hydraulic presses
• Injection molding equipment
• Industrial power units
• Agricultural machinery
• Construction equipment
• Material handling systems
• Mobile hydraulic equipment

In these applications, the hydraulic tank does not normally need to operate significantly above atmospheric pressure.

Advantages of a Vented Hydraulic Reservoir

A vented reservoir offers several practical advantages.

Simple construction

The reservoir does not need to be designed as a pressure vessel under normal operating conditions.

Easy filling and maintenance

Fluid can normally be added through a filler-breather assembly.

Pressure equalization

Air can enter or leave the reservoir as the hydraulic fluid level changes.

Lower cost

Atmospheric reservoirs are generally simpler and less expensive than pressurized reservoir systems.

Disadvantages of a Vented Reservoir

The main disadvantage is that the reservoir continually interacts with surrounding air.

If the breather system is poorly designed or maintained, contaminants can enter the hydraulic fluid.

Potential problems include:

• Dust contamination
• Moisture ingress
• Condensation
• Clogged breather elements
• Increased oil contamination

For this reason, reservoir breathers should be treated as important contamination-control components rather than simple air vents.

Vented Reservoir vs. Open Reservoir

The terms vented reservoir and open reservoir are sometimes used interchangeably in hydraulic discussions, but there is a practical distinction.

An open reservoir is conceptually exposed to atmospheric pressure.

A properly designed vented hydraulic reservoir is usually physically enclosed but connected to the atmosphere through a filtered breather.

Therefore, most modern hydraulic systems do not use a completely uncovered tank. Instead, they use a closed physical tank that is vented to atmosphere through a controlled air path.

On a hydraulic schematic, however, this arrangement may still be represented using the conventional atmospheric reservoir symbol.

Understanding this distinction helps avoid confusing the simplified schematic symbol with the actual construction of the hydraulic power unit.

4. Pressurized Hydraulic Reservoir Symbol

A pressurized hydraulic reservoir is a sealed reservoir maintained at a pressure above atmospheric pressure. Unlike a conventional vented tank, it does not freely exchange air with the surrounding environment during normal operation.

Pressurized reservoirs are used when the hydraulic system requires a positive pressure at the pump inlet, improved resistance to cavitation, reduced moisture ingress, or reliable operation under unusual environmental conditions.

On a hydraulic schematic, the reservoir symbol is typically shown as a closed container, distinguishing it from the open-top symbol normally associated with an atmospheric reservoir.

How a Pressurized Reservoir Works

In a pressurized reservoir, the space above the hydraulic fluid is maintained under pressure by a gas source or another pressure-control arrangement.

The gas pressure acts on the hydraulic fluid surface and creates positive pressure throughout the reservoir.

This pressure can help push hydraulic fluid toward the pump inlet.

The basic operating principle is:

Pressurized gas → Acts on hydraulic fluid → Maintains positive pump inlet pressure

This arrangement can be particularly useful when the reservoir is located below the pump or when the suction line is relatively long.

Why Pressurize a Hydraulic Reservoir?

The main purpose of pressurizing the reservoir is to improve hydraulic fluid delivery to the pump.

Hydraulic pumps generally perform best when the inlet pressure is sufficient to prevent cavitation.

Cavitation can occur when pressure at the pump inlet becomes too low, causing vapor bubbles to form in the hydraulic fluid. When these bubbles collapse inside the pump, they can cause:

• Noise
• Vibration
• Reduced pump efficiency
• Surface erosion
• Premature component failure

A pressurized reservoir can increase the available inlet pressure and reduce the risk of these problems.

Typical Applications

Pressurized hydraulic reservoirs are less common than vented reservoirs but are important in specialized systems.

Applications may include:

• Aerospace hydraulic systems
• Aircraft hydraulic systems
• High-performance mobile equipment
• Systems operating at high altitude
• Systems exposed to severe contamination
• Systems requiring positive pump inlet pressure
• Compact hydraulic power units
• Specialized industrial machinery

In these applications, reservoir pressure is part of the hydraulic system design rather than simply a storage condition.

Pressurized Reservoir Symbol vs. Vented Reservoir Symbol

The main graphical difference is usually the reservoir boundary.

A vented or atmospheric reservoir is commonly represented with an open-top symbol.

A pressurized reservoir is normally represented as a closed container.

Conceptually:

Vented reservoir:
Open to atmospheric pressure.

Pressurized reservoir:
Sealed and maintained above atmospheric pressure.

This distinction is important when troubleshooting because the pump inlet conditions can be very different.

Important Design Considerations

A pressurized reservoir should not be treated like a conventional open hydraulic tank.

The design may require:

• Pressure-rated reservoir construction
• Pressure regulator
• Pressure relief device
• Pressure gauge
• Gas supply
• Appropriate seals
• Controlled filling procedures

The reservoir operating pressure must remain within the equipment manufacturer’s specified limits.

Excessive reservoir pressure can damage seals, tank components, filters, or other connected equipment.

---

5. Hydraulic Reservoir Connection Symbols

The way a hydraulic line connects to a reservoir symbol can provide important information about the circuit.

Reservoir connections can indicate whether a line terminates:

• Above the hydraulic fluid level
• Below the hydraulic fluid level
• At the tank boundary
• Through an associated component such as a filter or cooler

These differences are particularly important for pump suction lines, return lines, case-drain lines, and leakage lines.

Pump Suction Connection

The pump suction line carries hydraulic fluid from the reservoir to the pump inlet.

A typical flow path is:

Reservoir → Suction Line → Hydraulic Pump

The suction connection normally terminates below the fluid level so that the pump receives a continuous supply of hydraulic oil.

A properly designed suction arrangement helps reduce:

• Air ingestion
• Cavitation
• Pump noise
• Flow restriction
• Pump damage

In many hydraulic circuits, locating the suction line is one of the easiest ways to identify the reservoir.

Return-Line Connection

The return line carries hydraulic fluid back to the reservoir after it has passed through valves, actuators, or other components.

A typical return path is:

Actuator → Directional Valve → Return Line → Reservoir

The return line may also pass through additional components such as:

Directional Valve → Return Filter → Cooler → Reservoir

In many hydraulic power units, the return line discharges below the normal fluid level to reduce splashing, aeration, and foam formation.

Case-Drain Connection

Many hydraulic pumps and motors have internal leakage that must be returned to the reservoir through a separate case-drain line.

A typical arrangement is:

Pump or Motor Case → Case-Drain Line → Reservoir

Case-drain lines are especially common with:

• Piston pumps
• Piston motors
• Certain vane pumps
• Hydrostatic transmission components

The case-drain line usually needs low backpressure.

Excessive pressure in this line can damage shaft seals or affect component performance.

For this reason, case-drain routing should be carefully interpreted when reading a hydraulic schematic.

Drain and Leakage Lines

Hydraulic valves and other components may also use drain or leakage lines connected directly to the reservoir.

Examples include:

• Pilot-operated valves
• Pressure-control valves
• Servo valves
• Proportional valves
• Certain directional valves

These lines generally carry relatively small flows but can be critical to proper component operation.

In schematic diagrams, drain lines may use a different line style from the main pressure and return lines, depending on the drawing standard.

Connection Below the Fluid Level

A hydraulic line extending into the tank symbol usually indicates that the line terminates below the fluid surface.

This type of connection is commonly used for:

• Pump suction
• Main return flow
• Certain case drains

Submerged connections can help minimize air entrainment and reduce turbulence.

However, the correct arrangement depends on the component and system design.

Connection Above the Fluid Level

A line terminating above the normal oil level represents a connection that discharges into the air space inside the reservoir.

This arrangement may appear with certain:

• Drain lines
• Leakage lines
• Special return circuits

However, returning oil above the fluid level can increase aeration and foaming.

For this reason, major return lines are often designed to terminate below the oil level in actual hydraulic power units.

Why Connection Position Matters

The position of a reservoir connection is not merely a drawing detail.

It can affect:

• Pump inlet conditions
• Oil aeration
• Foam formation
• Drain-line backpressure
• Noise
• Contamination control
• Heat dissipation
• Hydraulic component life

Therefore, engineers and technicians should pay attention to how each line terminates at the reservoir when analyzing a hydraulic schematic.

---

6. Reservoir Symbols with Filters, Breathers, and Accessories

A hydraulic reservoir rarely operates as an isolated component.

In most practical hydraulic power units, the tank works together with filters, breathers, level indicators, coolers, drain connections, and other accessories.

These components are generally represented by separate hydraulic symbols positioned around or connected to the reservoir symbol.

Reservoir with a Breather

A breather allows air to enter and leave an atmospheric hydraulic reservoir as the oil level changes.

The arrangement can be represented conceptually as:

Atmosphere ↔ Breather ↔ Reservoir

The breather helps prevent excessive pressure or vacuum while also reducing the amount of airborne contamination entering the hydraulic system.

Modern hydraulic systems often use:

• Filtered breathers
• Filler-breathers
• Desiccant breathers

The breather symbol may appear near the top of the reservoir when it is relevant to the schematic.

Reservoir with Suction Filter or Strainer

Some hydraulic systems include a suction strainer or filter between the reservoir and the pump.

The flow path may be shown as:

Reservoir → Suction Strainer → Pump

Its purpose is to prevent relatively large contaminants from reaching the pump.

However, excessive restriction in the suction line can create low inlet pressure and increase the risk of cavitation.

For this reason, suction filtration must be selected carefully.

Reservoir with Return-Line Filter

Return-line filtration is very common in industrial hydraulic systems.

The typical flow path is:

Directional Valve → Return Filter → Reservoir

The return filter removes contamination from oil before it re-enters the reservoir.

This helps keep the stored hydraulic fluid cleaner and reduces the circulation of particles through the system.

A return filter may also include:

• Bypass valve
• Clogging indicator
• Differential pressure indicator

These functions may be represented separately depending on the level of schematic detail.

Reservoir with Oil Cooler

A hydraulic oil cooler may be installed in the return line to remove heat before the fluid returns to the reservoir.

A common arrangement is:

Return Line → Oil Cooler → Reservoir

In some systems:

Return Line → Return Filter → Oil Cooler → Reservoir

The exact sequence depends on system design.

Coolers are especially important in systems where a significant portion of the hydraulic input power is converted into heat.

Reservoir with Level Indicator

Hydraulic reservoirs often have a sight gauge or level indicator that allows operators to check the fluid level.

On detailed system drawings, the level indicator may be shown using an associated instrument or equipment symbol.

Low fluid level can cause problems such as:

• Pump cavitation
• Air ingestion
• Poor cooling
• Increased oil degradation

For this reason, maintaining the correct reservoir level is an important part of hydraulic system maintenance.

Reservoir with Temperature Indicator

A temperature gauge, temperature switch, or temperature sensor may also be installed on the hydraulic reservoir.

These devices help monitor oil temperature.

Excessive oil temperature can lead to:

• Reduced viscosity
• Increased internal leakage
• Seal deterioration
• Accelerated oxidation
• Reduced component life

Temperature monitoring is therefore particularly important in high-duty hydraulic systems.

Reservoir Drain Connection

Most hydraulic reservoirs include a drain connection near the lowest point of the tank.

The drain may be used for:

• Oil replacement
• Maintenance
• Removal of settled contamination
• Water drainage
• Reservoir cleaning

The drain connection is typically isolated during normal operation.

Depending on the schematic detail, it may be shown as a separate drain valve or connection at the bottom of the reservoir.

Reservoir with Multiple Accessories

A complete hydraulic power unit may contain several components around the reservoir.

For example:

Breather
↓
Reservoir → Suction Line → Pump

and on the return side:

Valve → Return Filter → Cooler → Reservoir

Additional instruments may include:

• Level gauge
• Temperature gauge
• Pressure indicator
• Clogging indicator
• Level switch
• Temperature switch

The reservoir symbol itself remains relatively simple. The surrounding symbols provide the additional information needed to understand the complete hydraulic power unit.

This is an important principle when reading hydraulic schematics: do not expect every reservoir feature to be incorporated into a single tank symbol. Instead, identify the basic reservoir first and then examine the connected components and accessories around it.

7. Hydraulic Reservoir Symbols According to ISO 1219

ISO 1219 is one of the most widely referenced standards for graphical symbols used in fluid power systems. It provides standardized conventions for representing hydraulic and pneumatic components in circuit diagrams, helping engineers, technicians, manufacturers, and maintenance personnel interpret schematics consistently.

For hydraulic reservoirs, ISO-style symbols focus on function and connection conditions rather than the physical construction of the tank.

Atmospheric Reservoir Representation

A reservoir that is open or vented to atmosphere is generally represented differently from a sealed or pressurized reservoir.

The open-top graphical form indicates that the fluid surface is referenced to atmospheric pressure.

This does not necessarily mean that the physical reservoir is uncovered. In actual hydraulic systems, the tank is usually enclosed and connected to atmosphere through a filtered breather.

The schematic symbol communicates only the pressure condition.

Closed Reservoir Representation

A closed reservoir symbol indicates that the tank is not directly referenced to atmosphere in the same way as a conventional vented reservoir.

This arrangement may be used for:

• Pressurized reservoirs
• Sealed hydraulic systems
• Specialized mobile systems
• Aerospace hydraulic systems
• Systems operating in contaminated environments

Additional symbols may be used to show the source of pressure or other connected equipment.

Above-Level and Below-Level Connections

One of the most useful details in reservoir symbols is the way hydraulic lines terminate relative to the fluid level.

A line extending into the reservoir can indicate that the connection terminates below the normal oil level.

A line terminating at the upper region of the reservoir may indicate an above-level discharge.

This distinction may affect:

• Aeration
• Pump suction conditions
• Drain backpressure
• Return flow behavior
• Noise
• Oil cleanliness

When reading a detailed schematic, the line termination should therefore be treated as meaningful information rather than simply a drafting style.

Symbols Represent Function, Not Construction

ISO 1219 graphical symbols are intentionally simplified.

A reservoir may physically contain:

• Internal baffles
• Suction strainers
• Diffusers
• Level switches
• Temperature sensors
• Clean-out covers
• Magnetic plugs
• Breathers
• Return filters

However, these features are normally represented separately if they are important to the circuit.

The basic reservoir symbol still represents the main function: storing hydraulic fluid and providing a reference point for system flow.

Why Standardized Reservoir Symbols Matter

Standardized graphical symbols make hydraulic documentation easier to understand across different industries and manufacturers.

They are particularly valuable for:

• Machine design
• Equipment commissioning
• Maintenance
• Troubleshooting
• Training
• Hydraulic system modification
• Technical documentation

Without standardized symbols, each manufacturer could represent the same hydraulic function differently, increasing the risk of misinterpretation.

---

8. How to Read Hydraulic Reservoir Symbols in a Circuit

Hydraulic reservoir symbols become much easier to understand when they are interpreted as part of the complete hydraulic circuit rather than as isolated symbols.

A practical approach is to use the reservoir as the starting point and trace the hydraulic fluid through the system.

Step 1: Locate the Reservoir

First, identify the reservoir symbol.

In many schematics, the reservoir appears near the bottom of the drawing, although this is not a strict rule.

Look for:

• An open or closed tank symbol
• Multiple lines entering the same location
• A pump suction line originating from the tank
• Return or drain lines terminating at the tank

Once the reservoir has been located, the rest of the system can often be traced more easily.

Step 2: Identify the Pump Suction Line

Follow the line from the reservoir to the hydraulic pump.

A basic arrangement is:

Reservoir → Pump

The suction line may include additional components such as:

• Suction strainer
• Isolation valve
• Temperature sensor
• Vacuum indicator

The suction side should normally have minimal restriction because excessive pressure loss can increase the risk of pump cavitation.

Step 3: Follow the Pressure Line

From the pump outlet, trace the pressure line through the hydraulic system.

A typical circuit might be:

Reservoir → Pump → Directional Control Valve → Cylinder

The pressure line may also contain:

• Pressure filter
• Check valve
• Pressure gauge
• Accumulator
• Flow-control valve
• Relief valve

This is the main high-pressure flow path of the hydraulic circuit.

Step 4: Locate the Relief Valve Return Path

Most hydraulic systems include a pressure relief valve.

Its basic function is to limit maximum system pressure by diverting excess flow back to the reservoir.

The flow path is approximately:

Pump → Relief Valve → Reservoir

When system pressure exceeds the relief valve setting, the valve opens and directs fluid toward the tank.

Because the relief valve normally connects back to the reservoir, tracing this line is another useful method for finding the tank symbol.

Step 5: Trace the Actuator Return Line

After hydraulic fluid performs work in a cylinder or motor, it normally returns through the control valve.

For example:

Cylinder → Directional Valve → Reservoir

The return circuit may include:

Cylinder → Directional Valve → Return Filter → Reservoir

or:

Cylinder → Directional Valve → Cooler → Return Filter → Reservoir

The exact arrangement depends on the hydraulic power unit design.

Step 6: Identify Case-Drain Lines

Next, look for separate drain lines from hydraulic pumps or motors.

A typical case-drain path is:

Hydraulic Motor → Case Drain → Reservoir

or:

Hydraulic Pump → Case Drain → Reservoir

These lines are usually not part of the main return circuit.

They carry internal leakage and often require low backpressure.

Incorrect routing or blockage of a case-drain line can cause:

• Seal failure
• Excessive housing pressure
• Component overheating
• Reduced component life

Step 7: Identify Filtration and Cooling

Check whether return oil passes through a filter or cooler before reaching the reservoir.

A common industrial arrangement is:

Actuator → Valve → Return Filter → Cooler → Reservoir

Alternatively:

Actuator → Valve → Cooler → Return Filter → Reservoir

These components help control hydraulic fluid cleanliness and temperature.

Step 8: Determine Reservoir Pressure Condition

Finally, determine whether the reservoir is:

• Vented
• Atmospheric
• Sealed
• Pressurized

This information can influence pump inlet pressure, contamination control, and maintenance procedures.

Example Hydraulic Circuit

Consider the following simplified circuit:

Reservoir → Pump → Check Valve → Directional Valve → Cylinder

The return path is:

Cylinder → Directional Valve → Return Filter → Reservoir

A relief path is also provided:

Pump → Pressure Relief Valve → Reservoir

This gives three important reservoir connections:

1. Pump suction
2. Main return
3. Relief valve return

Additional drain lines may also terminate at the reservoir.

By identifying these connections, a technician can understand the main operating flow without needing to examine every symbol at once.

---

9. Hydraulic Reservoir Symbol Chart

The following chart summarizes several common hydraulic reservoir symbol types and their typical meanings.

Reservoir Symbol Type	Meaning	Typical Application
Basic Hydraulic Reservoir	General hydraulic fluid storage	Industrial and mobile hydraulic systems
Vented Reservoir	Reservoir connected to atmospheric pressure	Conventional hydraulic power units
Closed Reservoir	Sealed reservoir	Specialized hydraulic systems
Pressurized Reservoir	Reservoir maintained above atmospheric pressure	Aerospace, mobile, and high-performance systems
Connection Below Fluid Level	Hydraulic line terminates below oil surface	Pump suction and main return lines
Connection Above Fluid Level	Hydraulic line terminates above oil surface	Certain drain or return arrangements
Reservoir with Breather	Tank exchanges air through a filtered breather	Industrial hydraulic systems
Reservoir with Suction Strainer	Suction flow passes through a strainer	Pump protection
Reservoir with Return Filter	Return flow is filtered before entering tank	Contamination control
Reservoir with Cooler	Return flow passes through heat exchanger	Temperature control
Reservoir with Level Indicator	Reservoir fluid level is monitored	Hydraulic power units
Reservoir with Temperature Indicator	Hydraulic oil temperature is monitored	High-duty hydraulic systems

Quick Identification Guide

When looking at a hydraulic schematic, the following observations can help identify the reservoir type quickly.

Open upper boundary:
Usually indicates an atmospheric or vented reservoir.

Closed boundary:
May indicate a sealed or pressurized reservoir.

Line extending into the tank:
Usually indicates a connection terminating below the fluid level.

Separate breather symbol:
Indicates controlled air exchange with atmosphere.

Gas or pressure connection:
May indicate a pressurized reservoir.

Filter in the return line:
Indicates return filtration before fluid enters the reservoir.

Common Mistakes When Reading Reservoir Symbols

One common mistake is assuming that the symbol represents the physical shape of the hydraulic tank.

It does not.

Another mistake is ignoring the way lines terminate at the reservoir. A connection above the oil level can behave differently from a submerged connection.

Technicians should also avoid assuming that every return line has the same function.

A hydraulic schematic may contain:

• Main return lines
• Pilot drain lines
• Case-drain lines
• Leakage lines
• Relief valve return lines

All of these may terminate at the reservoir, but they can have very different flow rates and pressure requirements.

Conclusion

Hydraulic reservoir symbols are among the most important reference symbols in hydraulic schematics. They identify where hydraulic fluid is stored and show how pumps, valves, actuators, filters, coolers, and drain circuits connect back to the system fluid supply.

The most common reservoir is the vented atmospheric reservoir, but hydraulic diagrams may also include closed or pressurized reservoir symbols for specialized applications. The way hydraulic lines terminate at the reservoir can also indicate whether the connection is above or below the normal fluid level.

When reading a hydraulic circuit, a practical approach is to locate the reservoir first, identify the pump suction line, and then trace the pressure, return, relief, and drain paths through the system.

Understanding these symbols according to standardized graphical conventions such as ISO 1219 makes hydraulic schematics easier to interpret, troubleshoot, and maintain across different machines and manufacturers.

Different Types of Pipe Flanges and Their Uses

Pipe Flange Size Chart: ASME, ANSI and EN Dimensions