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Hydraulic valves are mechanical devices that are used to regulate the flow of fluid within a hydraulic circuit or system. They can be used to completely close a line, to redirect pressurized fluid or to control the level of flow to a certain area. Designed in a wide range of styles, these valves can be controlled manually or automatically, by physical, mechanical, pneumatic, hydraulic, or electrical activation. Hydraulic valves must be capable of withstanding large amounts of fluid pressure, as the nature of many hydraulic systems will entail high pressures upwards of 3,000 psi or more. For this reason, they are often constructed of steel, iron, or other metals that have enough strength to withstand continuous operation under pressurized conditions.
This article will present information about hydraulic valves, including the different types, their construction, and pertinent specifications. To learn more about other types of valves, see our related guide Understanding Valves.
Hydraulic valves are available in a wide variety of styles including many that are common to other types of valves, such as ball, butterfly, bypass relief, check, needle, diverter, regulating, pilot-operated, proportional and directional. From a broad perspective, these hydraulic valves may be characterized as being of three primary types which are:
Hydraulic pressure control valves are used to regulate the fluid pressure that is passing through hydraulic systems to maintain that pressure at desired levels as determined by the system operator. Fluid systems are typically designed for operation at a set range of pressures. These types of valves serve a key role to prevent rises in pressure that may result in leaks of hydraulic fluid or the bursting of pipes and tubing. They are also used to maintain a set pressure in a part of a hydraulic circuit.
The various types of pressure control valves used in hydraulic systems include relief valves, reducing valves, sequence valves, counterbalance valves, and unloading valves.
Hydraulic flow control valves are used to adjust the flow rate of hydraulic fluid in a hydraulic system. These valves have a port that is able to be adjusted so that the flow area may be changed to provide an alteration in the flow rate through the valve. An example of how this type of hydraulic valve would be used is in control circuits for devices such as cylinders, motors, or actuators. The speed of motion of these devices is a direct function of flow rate – reducing the flow rate reduces the speed of their operation and vice versa.
The different types of hydraulic flow control valves include fixed flow control valves, adjustable flow control valves, throttling flow control valves, and pressure compensated flow control valves. The mechanism for flow control within these valves will vary based on the mechanical design of the valve, which usually is one of the familiar valve styles common to other valves, namely:
Flow rate can be measured in several different ways, which are not equivalent, so the selection of a flow control valve necessitates understanding what is meant by flow rate. The three common measures of flow rate include:
Some of the common hydraulic flow control valves are:
Hydraulic directional control valves are used to route hydraulic fluid in a circuit or system to various devices as needed. They shift between discrete positions such as extend, retract, or neutral position for controlling a hydraulic cylinder, for example. They are also capable of shifting into intermediate states wherein they can be used to control the speed, direction, or acceleration of an actuator.
A simple form of discrete hydraulic directional control valve is a binary valve, which either blocks or passes fluid flow. Check valves are an example and use a plunger, ball, or poppet to seal against a seat when fluid attempts to pass in the opposite direction of flow from what is desired.
More complex hydraulic directional control valves may have multiple ports as by their nature they shift fluid between these different valve ports to feed hydraulic devices. As a result, they are characterized by a standardized numbering system that consists of two numerical values such as 2/2 or 4/3. The first number in this system identifies the number of fluid ports that the valve contains, and the second number indicates the number of valves states or positions that the valve can achieve. (Note - in the U.S., the number of ports is sometimes also known as the number of ways.) So with this convention, 2/2 represents a two-port valve that has two positions, and 4/3 represents a four-port valve that has three positions. In the latter example of a 4/3 valve that might be used to control a hydraulic cylinder, the three positions would represent:
Many hydraulic directional control valves make use of spools that slide between passages allowing fluid to flow through open ports, depending on the position of the spool in the valve body. Valves may use single or multiple spools to accomplish the desired port control. Other flow control elements in these valves may be plungers or poppets.
The valve component that moves these flow control elements is known as the valve operator or actuator. These devices provide for the proper sequencing and timing of the valve position changes that are needed to control the hydraulic circuit or systems. Options for the type of actuator mechanism include mechanical actuation, pilot actuation, or electrical/electronic actuation.
Mechanical actuation can include manual valve controls such as levers, push buttons, or pedals, but more often refers to automated mechanical devices such as cams, rollers, levers, springs, and the like.
Pilot actuation refers to the use of pressurized fluid to assist with moving the valve flow control elements. This style of operator is also useful in explosive environments where the use of electrical/electronic devices may not be recommended due to the potential risk of sparks causing an explosion.
Electrical/electronic actuation involves the use of solenoids that convert electrical signals in the form of a current supplied to the solenoid coil into the mechanical movement of a plunger that can generate either linear or rotary displacement. Electrical solenoids are limited as to the amount of force that can be generated, and so switching high-pressure hydraulic circuits by direct action is not possible. Combining solenoid use with pilot actuation allows the solenoid to switch lower pressure pilot circuits that can then be used to control higher pressure ports. More on this concept is available in our related guide on solenoid valves.
Hydraulic valves are specified using several parameters that relate to their size, flow capacity, connections, and actuation mechanism. The typical specifications for these valves are outlined below but recognize that there can be variations in these parameters among different valve manufacturers and suppliers, and so differences in representation may exist from supplier to supplier. The data presented below should serve as a general indicator of what needs to be considered when looking to specify a hydraulic valve.
This article presented a summary of information about hydraulic valves, including what they are, the different types, and key specifications. For information on other topics, consult our additional guides or visit the Thomas Supplier Discovery Platform where you can locate potential sources of supply for over 70,000 different product and service categories.