Hydraulic circuit symbols

Solenoid valve

Hydraulic pump

Relief valve

Ball valves and Isolators

Directional control valve

Double triangles are used to indicate ball valves and isolator valves. When the triangles are drawn in black the valve is normally closed and a clear triangle indicates the valve is open. The circular symbol with a two lines at a 90 degree angle is a 3-way ball valve. The symbol shows three port connections and the two lines connected are shown in the normal position. Different connection layouts can be shown in the middle of the circle to indicate layout and connection variations.

Shuttle Valve
Load sensing systems commonly use shuttle valves. Their design ensures that the highest pressure is always fed to the top connection. In the symbol, there is ball valve and when it encounters two different pressures, the ball will move either way to let the maximum pressure flow towards the top connection.

Check Valves and Shut Off Valves

Check valves allow flow in only one direction. The symbol example here shows that flow goes from the top to the bottom only when the pressure is higher than the rating of the spring. A common good practice is to write the spring pressure next to the check valve

Pilot Operated Check Valves

flow control valve

In the symbol example here, the dashed line is used to indicate the pilot line. The pilot line is used to open the check valve and let the flow back through the valve. The bottom symbol in the image above is a common format for a double pilot operated check sandwich plate valve. They are often used under CETOP directional valves. When pressure is applied to one side, the free flow is allowed in both directions. But when the directional valve is closed and there is no pressure applied to the valve, both check valves are closed and hold the load in place.

Valve Activation Types

There are multiple ways to activate a hydraulic valve. The symbols above are the different ways a valve can be activated: electrical solenoid, spring, electrical with a hydraulic pilot, hand emergency, proportional solenoid, manual lever, and a foot operation.
The above image is an example of a pilot operated directional valve. This is a large high flow hydraulically operated bottom valve. It has a small electrically operated pilot valve. The solenoid valves show the hydraulic pilot, the external pilot pressure (X), and the external pilot pressure drain (Y). The symbol indicates the open center pressure (P) to return (T) spool valve.

Hydraulic Filters, Water Coolers, and Accumulators

accumulator circuit

The hydraulic filter above shows that the flow will come from the top due to the bypass check valve shown on the side. The check valve protects the circuit from over-pressurization if the filter becomes clog. The hydraulic water cooler intersects the hydraulic pipe above. The water flow paths are not depicted in the symbol but can be included.
The accumulator images in the image above indicate different stages. The first is a gas-pressurized accumulator and a diaphragm to separate the media. The second is a gas-pressurized accumulator with a bladder to separate the media. The third is a gas-pressurized accumulator with a piston. Lastly, the fourth accumulator is a backup bottle.

Logic Operators

Logic valves are available in several different options. The image above highlights four common cases. The symbols above are all of a directional control cartridge poppet valve. The top left image has a 1:1 ratio. This is indicated by the straight nature of the spool/poppet body. The top right symbol has an area ratio equal or less than 0.7. The bottom left has an area ratio of greater than 0.7. The bottom right has an area ratio equal or less than 0.7 with a damped/throttle nose.

Energy Supply, Service Units, Control Valves, and Actuators

pressure control valve

The remaining symbols below are common depictions of energy supply (i.e. pumps, compressors, and air receivers), service units (i.e. dryers, regulators, lubricators), control valves (i.e., flow and pressure), and actuators (i.e. single acting and double acting). These components are the driving force for hydraulic circuits. They regulate pressure, control flow, and also the direction of the hydraulic flow.

Milan Tomic

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