Hydraulic and Pneumatic P&ID Diagrams and Schematics

Hydraulic and Pneumatic P&ID Diagrams and Schematics

Because fluid power diagrams and schematics use a distinct set of symbols and norms, they require an independent assessment.

Fluid Power Diagrams and Schematics

When working with systems that utilize fluid power, a different symbology is used. The motive media for fluid power are either gas (such as air) or hydraulic (such as water or oil). Some of the symbols used in fluid power systems are close to or identical to those previously mentioned, but many are completely different.

Fluid power systems are divided into five basic parts:

  • Pumps,
  • Reservoirs,
  • Actuators,
  • Valves, and
  • Lines.


Pump symbols are divided into two types in the fluid power industry, based on the motive media (i.e., hydraulic or pneumatic). The pump’s basic symbol is a circle with one or more arrow heads denoting flow direction(s), with the arrows’ points in contact with the circle.

Solid arrow heads indicate hydraulic pumps. The hollow arrow heads symbolize pneumatic compressors. In fluid power diagrams, the symbols for pumps (hydraulic) and compressors (pneumatic) are shown in Figure 19.



Any component in a fluid power system that transfers hydraulic or pneumatic pressure into mechanical work is known as an actuator. Linear actuators and rotary actuators are the two types of actuators.

A piston device is used in linear actuators. Figure 21 depicts a variety of linear actuators and their corresponding drawing symbols.

Rotary actuators are commonly referred to as motors, and they can be fixed or variable. Figure 22 depicts a few of the more frequent rotary symbols. The rotary motor symbols in Figure 22 and the pump symbols in Figure 19 are very similar.

The distinction is that in a pump, the point of the arrow contacts the circle, whereas in a motor, the tail of the arrow touches the circle.


In a fluid power system, the main purpose of piping is to transfer the working media under pressure from one place to another. Figure 23 depicts the symbols for the various lines and termination sites.


Reservoirs give a location for the motive media to be stored (hydraulic fluid or compressed gas). Despite the fact that the symbols used to symbolize reservoirs vary, some conventions are utilized to show how a reservoir handles fluid. Pneumatic reservoirs are typically basic tanks with a design that resembles the cylinder depicted in Figure 20.

In terms of how fluid is admitted to and extracted from hydraulic reservoirs, they can be far more complicated. Symbology conventions have been created to transmit this information. Figure 20 depicts these symbols. In fluid power systems, valves are the most intricate symbols. Valves give the necessary control to ensure that the motive medium is channelled to the correct location when it is required.

Due to the sophisticated valving utilized in fluid power systems, fluid power system diagrams require far more complex valve symbols than normal P&IDs. A valve opens, closes, or throttles the process fluid in a normal P&ID, but it is rarely required to route the process fluid in any complex fashion (three- and four-way valves being the common exceptions).

A valve with three to eight pipes linked to the valve body is typical in fluid power systems, with the valve being capable of routing the fluid, or many different fluids, in any number of ways. Fluid power valve symbols must carry far more information than normal P&ID valve symbols. For fluid power P&IDs, the valve symbology shown in the accompanying figures was designed to satisfy this demand. Figure 24 shows the internal complexity of a simple fluid power type valve in a cutaway view.

A four-way/three-position valve is shown in Figure 24. how it works to change the flow of a fluid The operator of the valve is not stated in Figure 24, although it might be a diaphragm, motor, hydraulic, solenoid, or manual operator, just like a conventional process fluid valve. When a solenoid is used to operate a fluid power valve, it is drawn in the de-energized position.

The valve will slam shut if the solenoid is energised. If the valve is not powered by a solenoid or is a multiport valve, the information needed to figure out how it works will be provided on each drawing or on the legend print that goes with it.

Reading Fluid Power Diagrams

A fluid power diagram may now be read using the previously discussed symbology. But first, let’s take a look at a simple hydraulic system and see how it may be converted into a fluid power diagram.

The left portion of Figure 28 uses the drawing in Figure 27 to list each part and associated fluid power symbol. The fluid power diagram that represents the drawing is shown on the right side of Figure 28.

Any diagram may be deciphered if you grasp the principles involved in reading fluid power diagrams. Figure 29 depicts the type of diagram that is commonly used in the engineering profession.

A step-by-step explanation of what is happening in the system will be offered to read this diagram.

The first stage is to gain a broad picture of what’s going on. The arrows in the lower right-hand corner of the illustration indicate that the system is designed to press or clamp a part between two machine sections. Hydraulic systems are frequently utilised in press operations and other applications that require the work piece to be held in position.


An open reservoir with a strainer is represented by this symbol. Before the oil enters the system, it is filtered through the strainer.


Pump with a fixed displacement that is powered by electricity. The system’s hydraulic pressure is supplied by this pump.


A relief valve with a separate pressure gauge is represented by this symbol. The spring-loaded relief valve protects the system against over-pressurization. When the cylinder is not in use, it also serves as an unloader valve to alleviate pressure. When the hydraulic fluid pressure in the system surpasses the setpoint, the valve opens and returns it to the reservoir. The gauge measures the amount of pressure in the system.


A four-way, two-position valve is represented by a composite symbol. The valve is activated by pressing the PB-1 pushbutton, which activates the S-1 solenoid (note the valve is shown in the de-energized position). The high-pressure hydraulic fluid is channelled from Port 1 to Port 3, then to the piston’s bottom chamber, as indicated. The piston in local area #5 is driven and held in the retracted position by this.

When it comes to pistols, The 1-2 ports are aligned and the 3-4 ports are aligned when PB-1 is pushed and S-1 is powered. This permits hydraulic fluid to enter the piston’s top chamber and push it down. The fluid in the bottom chamber drains back into the reservoir through the 3-4 openings. The piston will continue to descend until PB-1 is released or full travel is attained, at which point the unloader (relief) valve will be activated.


Cylinder and piston that are being actuated. Fluid can be received in either the upper or lower chambers of the cylinder. When pressure is applied to the top chamber, the bottom chamber is oriented to drain back to the reservoir, according to the system’s design. The top chamber is oriented so that it drains back to the reservoir when pressure is applied to the bottom chamber.

Types of Fluid Power Diagrams

To demonstrate how systems work, a variety of diagrams can be utilized. A reader will be able to interpret all of the diagrams that follow if they know how to interpret Figure 29.

The physical arrangement of the elements of a system is depicted in a graphical diagram. The components are outline drawings that depict each item’s outward shape. The internal function of the device is not shown in pictorial drawings.

The physical arrangement as well as the operation of the various components are depicted in a cutaway diagram. It’s commonly used for educational purposes because it shows how the system works while also displaying how it’s set up. Because they take up so much room, these diagrams are rarely utilized for complex systems.

Figure 31 is a cutaway diagram of the system depicted in Figure 30 that demonstrates the similarities and differences between the two types of diagrams.

The elements of a system are represented by symbols in a schematic diagram. The purpose of schematics is to provide the system’s functioning information. They don’t adequately depict the components’ relative positions. Understanding schematics is a vital part of troubleshooting since they are useful in maintenance work.

The mechanism depicted in Figures 30 and 31 is schematically depicted in Figure 32.


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