A Primer on Basic on Basic Hydraulic and Pneumatic Symbols
If you look at the plan of a building or a house, you will likely encounter a hydraulic or fluid circuit. This looks similar to an electrical circuit diagram but is composed of hydraulic or pneumatic elements like pipes, pumps, and pressure gauges.
As with an electrical circuit, understanding a fluid circuit diagram requires an understanding of the symbols and the real-world hardware they correspond to. In this article, we will be going through a list of the most common and basic hydraulic and pneumatic symbols to help you the next time you encounter a fluid diagram.
Where did all these symbols come from?
The use of symbols in fluid diagrams allows them to be understood by anyone, regardless of the language they speak or understand. As such, these symbols comply with an international set of standards as created by the International Standards Organization (ISO).
In the US, the standards for hydraulic symbols are maintained by the American National Standards Institute (ANSI). There is only a very slight difference between the ISO and ANSI standards, but many American companies tend to use ISO symbols for seamless interaction with international partners.
The purpose of a fluid diagram is to provide a brief overview of the hydraulic components in a system and how they interact with each other. Details like pipe dimensions or pump capacities are typically not included or emphasized in fluid diagrams. They also do not represent the location of these components in physical space.
Despite there being dozens of standard pneumatic symbols, some facilities may still need to come up with their own symbols to serve their purpose. Some facilities modify these standards, often representing parts made up of combinations of the standard symbols. To clear any ambiguity, these fluid diagrams typically come with a catalog of the symbols that they use.
A list of basic hydraulic and pneumatic symbols
Despite plants or facilities being able to modify these symbols, a fundamental knowledge of the basic hydraulic and pneumatic symbols still goes a long way towards easily understanding fluid diagrams. After all, most of the parts of any fluid circuit will come from a selection of basic symbols – pipes, pumps, orifices, gauges, filters, and the like.
To help you develop a basic level of understanding, the following is a list of the common hydraulic and pneumatic symbols as well as a short description of the real-world parts they represent.
|Piping||The basic unit of a fluid diagram, the main line|
represents pipes through which fluid can flow. The
main line operates at the standard operating pressure
for the facility and is vital to its central operations.
|Pilot line||Pilot lines are very similar to the main line except that|
they operate at a lower pressure. These are often
used as pre-commercial production lines for relatively
|Crossing lines||Crossing lines denote the intersection of two lines that|
are not hydraulically connected. The use of crossing
lines is often necessary for drawing up complex fluid
diagrams. These may or may not represent lines that
cross with each other in the physical space.
|In contrast to crossing lines, connected lines are those|
that have a hydraulic connection. Connected lines can
be denoted with any number of intersecting lines.
|Drain line||As the name implies, this represents a hydraulic line|
that is directed towards the drain.
|Flexible line||These are parts of the hydraulic system that are made|
of flexible material, typically industrial-grade
composite rubber or braided steel.
|A vented reservoir is a container used to hold the fluid|
in the system that is vented to its surroundings. This
can serve several different purposes – as temporary
storage, for pressure equalization, or to allow
|In contrast to the vented reservoir, a pressurized|
reservoir allows for fluid storage at a controlled
pressure. This helps in maintaining fluid pressure and
composition should it contain any volatile
|Check valve||A check valve is a type of valve that allows flow only|
along one direction along the line. The direction of the
check valve symbol also matters – the side of the
circle symbol is the side that does not allow flow.
|Fixed orifice||A fixed orifice is a section of the line with a reduced|
cross-section, thus restricting flow. As the name
implies, a fixed orifice offers no adjustability.
|Variable orifice||A variable orifice, unlike a fixed orifice, has a cross-|
section that can be adjusted. This provides a more
versatile means of flow control.
|Ball valve||A ball valve is a means of restricting fluid flow either|
partially or completely. It can be operated manually or
automated via an actuator. Valves are typically
represented along with their associated actuator
|Level switch||A level switch is typically used to determine if the level|
of fluid in a reservoir has reached a set threshold. This
can result in different outcomes such as the
termination of a pump or closing of a valve.
|Just like a level switch, a temperature switch is used to|
determine if the temperature of the fluid has reached
a threshold value. Temperatures switches are more
versatile and can be installed in reservoirs or along
lines. They are also critical in safe pump operation to
help ensure that pumps do not overheat.
Pressure control and monitoring
|A pressure relief valve is a valve located along the line|
that automatically vents to the environment when a
certain pressure level has been reached. These act as
both pressure control and as safety mechanisms
|Pressure gauge||A pressure gauge is quite intuitive – it is typically a|
standard gauge with pressure readings and an arrow
|A pressure indicator is a less common but more basic|
means of showing pressure. Instead of a dial that
shows pressure values, a pressure indicator only
signifies if a set threshold pressure value has been
Fluid motive devices
|A hydraulic pump converts rotational energy to|
hydraulic pressure. This symbol indicates that the
hydraulic pump delivers a constant displacement
volume and can only rotate in one direction.
|This is a version of the hydraulic pump that allows for|
adjustment of the displacement volume. The diagonal
arrow modifier typically shows up to indicate that the
equipment can be adjusted or customized.
|The hydraulic motor is the opposite of a hydraulic|
pump – it converts hydraulic pressure to physical
movements such as the rotation or lateral motion of a
shaft. Many actuators are based on this principle.
Again, this is a version of the hydraulic motor that
operates at a constant displacement volume.
|The diagonal arrow indicates that this is a version of|
the hydraulic motor that is adjustable in terms of
|The two triangles are used to indicate that this|
hydraulic pump can rotate in either direction. This
means that its inlet and outlet can be interchanged. A
bi-directional hydraulic pump can also have adjustable
|Similarly, a hydraulic motor can also accept hydraulic|
pressure from either direction. This can create either
of two directions of rotation. An adjustable
displacement volume of a bi-directional hydraulic
motor also exists.
|Air pump||This empty triangle symbol is used to differentiate|
pumps or compressors that use air as the main
|The actuator is the mechanism by which a valve is|
opened, closed, or throttled. This symbol merely
indicates that the actuator for this valve is operated
manually but does not indicate the exact control
|This symbol indicates that the valve can be opened or|
closed using a push-button system. Although not a
completely manual mechanism, an operator still has
to be present on-site to push the button.
|Spring actuator||Again, a spring actuator is simply a mechanism that|
allows for the easy operation of a valve. Similar to the
push button actuator, a spring actuator is not
considered a manual operation device but is meant
for manual actuation.
|A direct pneumatic actuator provides remote|
actuation of the valve. Using a pneumatic system, the
valve can be controlled without any human operator
|Filter||A filter is considered a piece of fluid conditioning|
equipment that removes particles of a specific size
from the fluid.
* The diamond symbol is used for fluid conditioning
|Heat exchanger||A heat exchanger allows for heat to be transferred|
from one liquid to another. This is a versatile device
that can be used to either cool or heat the process
|Cooler||A cooler lowers the temperature of the process fluid|
typically via heat exchange with the environment. In
this regard, the arrows in the symbol represent heat
radiating from the process fluid.
|Oval symbols are generally used in hydraulic systems|
to represent an accumulator for hydraulic energy. This
specific symbol with an inverted triangle indicates that
the accumulation medium is an inert gas that is
|If gas is not present, then the spring potential energy|
of a compressed spring can also be used for energy
accumulation. Spring-loaded accumulators are simpler
to operate but cannot be compressed as much as gas
|A weighted accumulator uses a series of weights to|
provide compression to a fluid. A huge advantage of
weighted accumulators is that they can deliver stable
and constant pressure release until the fluid in the
cylinder has been depleted.
|A differential cylinder is a mechanism that uses|
hydraulic pressure to create the linear movement of
a piston. Hydraulic fluid can be introduced to either
side of the piston, allowing its rod to be pushed or
|Ram||A ram puts more emphasis on the girth of the rod,|
representing it with a rectangle instead of a line. The
primary purpose of a rod is to provide a pushing
motion, thus it is designed to have only a single inlet
for hydraulic fluid.
This list is by no means exhaustive of all the possible symbols used in fluid diagrams – there are probably a few dozen standards symbols that we have not covered. Still, you should be able to understand the bulk of any fluid diagram using just the symbols here.
Fluid systems can get very complex, especially in industrial and manufacturing facilities, and even in large commercial buildings. For this reason, it is always a good idea to have a fluid diagram that makes it easy to understand the system or pinpoint the root cause of a problem.
For those with industry experience, a lot of these symbols may already seem very familiar. As those people can attest as well, understanding a fluid diagram goes way beyond just knowing the symbols.