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Hydraulic symbols ISO 1219 complete

The international standard ISO 1219 establishes graphic symbols for hydraulic and pneumatic circuits, allowing a unified representation of components, actuators, and controls. It consists of two parts: ISO 1219-1:2012 (basic and functional symbols) and ISO 1219-2:2012 (rules for application and drawing of diagrams). This reference collects the most commonly used symbols, organized by functional families, and is complemented by the ISA instrumentation codes used in associated control loops.

Lines and connections

Section titled “Lines and connections”
Designation Graphic representation
Working line (pressure) Thick continuous line
Return line Medium continuous line
Pilot / control line Short dashed line
Drain / leakage line Long dashed line
Flexible line (hose) Continuous line with arc
Line junction Filled circle 1 mm / 0.04 in diameter
Crossing without connection No junction point, optional arc
Connection point (flange) Rectangle enclosing the junction
Pressure tap / bleed Equilateral triangle connected to line
Continuous bleed line Dashed line with triangle

Reservoirs, filters, and accumulators

Section titled “Reservoirs, filters, and accumulators”
Designation Graphic representation
Reservoir open to atmosphere Rectangle open at the top
Pressurized reservoir Closed rectangle with compressed air line
Filter Rhombus with optional bleed line and filter element symbol
Oil cooler Rectangle with coil or fins and flow arrows
Heater Rectangle with coil and heating lines
Bladder accumulator Circle with fill line and precharge triangle
Piston accumulator Circle with piston and spring
Membrane accumulator Divided circle with flexible membrane
Pressure gauge Circle with needle and numerical scale
Thermometer Circle with bulb and graduated scale

Pumps and hydraulic motors

Section titled “Pumps and hydraulic motors”
Designation Graphic representation
Fixed displacement unidirectional pump Circle with solid black triangle pointing outward
Fixed displacement bidirectional pump Circle with black triangle in both directions
Variable displacement pump Circle with diagonal arrow at 45°, black triangle
Fixed displacement unidirectional motor Circle with black triangle pointing inward
Variable displacement bidirectional motor Circle with triangle in both directions and diagonal arrow
Reversible pump-motor Circle with double triangle and adjustment arrow
Pressure compensator Straight arrow next to pump circle
Stroke compensation Control points on pump symbol

Directional control valves

Section titled “Directional control valves”
Designation Number of ways/positions Graphic representation
2/2 directional valve 2 ways, 2 positions Two adjacent squares with arrows and exhausts
3/2 directional valve 3 ways, 2 positions Two squares with three connections
4/2 directional valve 4 ways, 2 positions Two squares with four ports
4/3 directional valve, closed center 4 ways, 3 positions Three squares, center with all lines blocked
4/3 directional valve, tandem center 4 ways, 3 positions Center with P and T ports connected, A and B blocked
4/3 directional valve, open center 4 ways, 3 positions Center with P, A, B, and T connected
Lever actuation — Lever symbol at one end of the valve
Roller actuation — Circle with spring on the valve
Solenoid actuation — Inclined rectangle with electrical line
Hydraulic pilot actuation — Rectangle with pilot triangle

Check valves and pilot-operated valves

Section titled “Check valves and pilot-operated valves”
Designation Graphic representation
Non-return valve (simple check) Seat with ball and spring, unidirectional flow line
Pilot-operated check valve Check valve with external pilot line for opening
Double pilot-operated check valve (dual) Two check valves in one body, cross pilot
Sequence valve Relief valve symbol with set spring and pilot line
Counterbalance valve Pilot-operated check valve with load control
Brake valve Directional control valve with brake holding

Flow control valves

Section titled “Flow control valves”
Designation Graphic representation
Fixed restrictor Two converging lines with restriction indication
Adjustable throttle valve Restrictor with adjustment arrow
Flow control valve with check valve Adjustable restrictor with check valve bypass
Compensated flow control valve Restrictor symbol with pressure compensator line
Flow divider Block with two outlets and calibrated orifices
Flow synchronizer Flow divider with integrated position sensors

Cylinders and actuators

Section titled “Cylinders and actuators”
Designation Graphic representation
Single-acting cylinder, spring return Piston with single port and spring (multiple coils)
Double-acting cylinder Piston with two ports and rod on both sides
Differential cylinder Piston with rod on one side only
Single-acting telescopic cylinder Multiple concentric sleeves, single port
Double-acting telescopic cylinder Concentric sleeves, two ports
Rack and pinion rotary actuator Rectangle with gear and rack
Pneumatic motor Circle with pneumatic triangle and exhaust ports
Pneumatic gripper Parallelogram with open jaws

Electrical and mechanical control

Section titled “Electrical and mechanical control”
Designation Graphic representation
Proportional solenoid Inclined rectangle with double control line
Servovalve Circle with solenoid and hydraulic feedback
LVDT position transducer Electronic symbol with coils and movable core
Mechanical limit switch Switch with normally open/closed contact and cam actuation
Pressure switch Electrical symbol with pressure port
Thermostat Electrical symbol with thermal bulb
Timed relay Coil with adjustable delay and switched contact

Basic control functions

Section titled “Basic control functions”
Designation Graphic representation
Pressure relief valve Square with inclined arrow, spring, and pilot line
Pressure reducing valve Square with horizontal arrow and compensating spring
Pilot-operated relief valve Simple relief with pilot circuit and external vent
Backpressure brake valve Block with exhaust control and set spring
Regulator block with sensors Integration of pressure and flow valves with transducers

ISA codes for process instrumentation

Section titled “ISA codes for process instrumentation”

In addition to ISO 1219 functional symbols, hydraulic control diagrams often use ISA (Instrument Society of America) code letters to identify variables and the function of each instrument.

Process variable ISA symbol Instrument type ISA symbol
Analysis / quality A Alarm A
Burner / combustion B Free choice B
Combustion B Controller C
Free choice C Control valve CV
Free choice D Trap CV
Voltage V Sensor (primary element) E
Flow rate F Rupture disk E
Free choice G Sight glass / level glass G
Electrical current I Monitor G
Power J Indicator I
Time K Control station K
Level L Pilot light L
Free choice M Free choice N
Free choice N Restriction orifice O
Free choice O Test point / sample P
Pressure / vacuum P Recorder R
Radiation R Switch S
Speed / frequency S Transmitter T
Temperature T Multifunction U
Multivariable U Valve / damper V
Vibration V Thermowell W
Weight / force W Unclassified X
Unclassified X Relay Y
Event Y Actuator / driver Z
Position / dimension Z Actuator Z

Example: a level indicator is designated as LI, and a pressure transmitter with alarm is called PTA.

Frequently Asked Questions (FAQ)

Section titled “Frequently Asked Questions (FAQ)”

What is the standard design pressure for a hydraulic system according to ISO 4413?

Section titled “What is the standard design pressure for a hydraulic system according to ISO 4413?”

The most common design pressure in industrial applications is 210 bar / 3046 psi, although there are standardized high-pressure lines at 350 bar / 5076 psi.

What minimum flow rate can a fixed restrictor symbol represent?

Section titled “What minimum flow rate can a fixed restrictor symbol represent?”

The symbol does not define a flow rate, but commercial micro-restrictors handle flows from 0.05 l/min / 0.013 gpm to 2 l/min / 0.53 gpm, with calibrated orifices of 0.2 mm / 0.008 in.

Section titled “What is the recommended viscosity for an ISO VG 46 hydraulic oil at 40 °C?”

The typical kinematic viscosity is 46 mm²/s / 46 cSt at 40 °C, with an optimal operating range between 20 and 80 cSt / 20‑80 mm²/s.

How many threaded connections does a CETOP 3 4/3 directional valve have?

Section titled “How many threaded connections does a CETOP 3 4/3 directional valve have?”

The CETOP 3 subplate has 5 threaded holes, commonly G 1/4” (major diameter 13.157 mm / 0.518 in), with a recommended tightening torque of 55 Nm / 41 lb‑ft.

What line thickness does ISO 1219‑2 recommend for different circuits?

Section titled “What line thickness does ISO 1219‑2 recommend for different circuits?”

The main pressure line is drawn with a line width of 0.5 mm / 0.020 in, while the return line and the pilot line use 0.35 mm / 0.014 in.

What is the standard size of a bladder accumulator according to the PED directive?

Section titled “What is the standard size of a bladder accumulator according to the PED directive?”

Standard accumulators have a volume from 0.075 l / 2.54 US fl oz to 50 l / 13.2 US gal, with N₂ precharge pressures from 10 bar / 145 psi to 210 bar / 3046 psi.

References

Section titled “References”