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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.

Types of Hydraulic 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, and 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:

hydraulics iconHydraulic Pressure Control Valves
hydraulics iconHydraulic Flow Control Valves
hydraulics iconHydraulic Directional Control Valves
hydraulics iconHydraulic Pressure Control Valves
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
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:

hydraulics iconBall
hydraulics iconButterfly
hydraulics iconDiaphragm
hydraulics iconNeedle
hydraulics iconPlug

 

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:

Volumetric flow rate – measured in units of volume per unit time, such as in3/sec or cc/min.
Weight flow rate – measured in units of a weight per unit time, such as lb/sec.
Mass flow rate – measured in mass per unit time, such as slugs/sec or kg/min.
Some of the common hydraulic flow control valves are:

hydraulics iconPressure-compensated, variable flow valves
hydraulics iconPressure- and temperature-compensated, variable flow valves
hydraulics iconPriority valves
hydraulics iconDeceleration valves
hydraulics iconPressure-compensated proportional flow-control valves
hydraulics iconProportional flow-control logic valves
hydraulics iconHydraulic Directional Control Valves

 

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:

hydraulics iconNeutral – all valve ports are blocked, and no fluid flow is permitted
hydraulics iconExtend – the valve routes fluid from a hydraulic pump to the cap end of the cylinder, causing the cylinder to extend
hydraulics iconRetract – the valve routes fluid from a hydraulic pump to the rod end of the cylinder, causing the cylinder to retract

 

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 Valve Specifications
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.

Valve type – refers to the specific hydraulic valve type needed, which may reflect the physical style (ball, check, needle, etc.) or may refer to the control being sought (flow control, pressure control, or directional control).
Valve actuation mechanism – reflects the means by which the valve position is changed or by the way in which the valve is operated, such as pilot, solenoid, or mechanical.
Valve configuration – reflects the number of ports, the number of switching states or positions, and the defined rest state for the valve, e.g. 3/2 normally closed (NC).
Body material – identifies the material from which the valve body is produced, which may be aluminum, brass, bronze, stainless steel, or engineered plastic, to name a few possible options.
Media type – identifies the nature of the specific fluid (liquid or gas) for which the valve is capable of handling without experiencing any deleterious effects. Examples of media types include fuel, oil, and water.
Port size – reflects the dimensional size of the inlet and outlet ports of the valve, represented in either imperial units such as inches or metric units such as millimeters.

 


Port type (or mounting type) – identifies the desired port style or mounting/interface for the valve, such as flanged, manifold, threaded, etc.
Operating voltage – for electrically actuated valves, indicates both the magnitude and type of electrical control signal that is used to energize the valve solenoid. Solenoid valves are available with a wide range of AC and DC operating voltages that may be used to satisfy different application conditions.
Operating frequency – for electrically actuated valves that are powered with AC voltages, frequency is the number of cycles per second of the alternating current that is applied to the solenoid, shown usually in Hertz (for example 60 Hz).

 


Flow coefficient – the flow coefficient, or Cv of the valve, measures the valve’s ability to allow the flow of liquids or gases through it. The standard definition of flow coefficient is that it represents the volume of water (in U.S. gallons) that will flow through the valve at a temperature of 60oF in a one-minute interval of time when there is a 1 psi pressure drop across the valve (outlet-inlet differential pressure). Larger values of flow coefficient reflect a greater amount of flow.

 


Flow rate – in lieu of flow coefficient, valve suppliers may specify the flow rate of the valve in units such as gallons or liters per minute, for example.
Maximum rated pressure – is the maximum pressure value that the valve can handle when installed in the hydraulic circuit or system.
Minimum operating pressure – reflects the lowest pressure that must exist in the system for the valve to function effectively. While many direct operated valves can function at 0 bar pressure, indirect operated valves may require a minimum pressure to exist that can be used to assist with valve actuation. Some valves are specified using a pressure range.

 


Operating temperature or temperature range – indicates the recommended range of temperatures over which the valve has been designed to function.
Application – indicates the intended use or market for the valve, examples being chemical, elevators, or aircraft for example. Having a definition around the intended industry or the use case can prove beneficial when selecting a valve as understanding that industry may help to bring to the surface additional requirements or specifications necessitated by these operating conditions.


Hydraulics is mechanical function that operates through the force of liquid pressure. In hydraulics-based systems, mechanical movement is produced by contained, pumped liquid, typically through cylinders moving pistons. Hydraulics is a component mechatronics, which combines mechanical, electronics and software engineering in the designing and manufacturing of products and processes.

Simple hydraulic systems include aqueducts and irrigation systems that deliver water, using gravity to create water pressure. These systems essentially use water’s own properties to make it deliver itself. More complex hydraulics use a pump to pressurize liquids (typically oils), moving a piston through a cylinder as well as valves to control the flow of oil.

A log splitter is a single-piston hydraulic machine that uses a valve at either end of the cylinder that allows the pistons to be moved by the pressurized liquid, driving a wedge to force wood into smaller pieces and return to a home position. Force multiplication can be created by using a cylinder with a smaller diameter to push a larger piston in a larger cylinder. Often, there will be a number of pistons. Industrial equipment such as backhoes often use a number of cylinders to move different parts. Electronic controls are generally used for these more complicated setups on large, powerful equipment.

Hydraulics are similar to pneumatic systems in function. Both systems use fluids but, unlike pneumatics, hydraulics use liquids rather than gasses. Hydraulics systems are capable of greater pressures: up to 10000 pounds per square inch (psi) vs about 100 psi in pneumatics systems. This pressure is due to the incompressibility of liquids which enables greater power transfer with increased efficiency as energy is not lost to compression, except in the case where air gets into hydraulic lines. Fluids used in hydraulics may lubricate, cool and transmit power as well. Pneumatics, being less multifaceted, require oil lubrication separately, which can be messy with air pressure. Pneumatics are simpler in design and to control, safer (with less risk of fire) and more reliable, partially as the compressibility of the gas-absorbing shock can protect the mechanism.

The external gear pump belongs to the class of rotating positive displacement pumps. They can manage low viscosity liquids like alcohols, solvents or liquid gases as well as medium and high viscosity liquids like polymer melts, gum base or rubber. Liquids containing solids are not really suitable for gear pumps unless the particles are really small.
A gear pump comprises a housing with two covers. The powered gear wheel and driven gear wheel are borne in four friction bearings. The protruding drive shaft is sealed. The meshing gear wheels are enclosed by the housing. The amount of clearances between the tips of the teeth and the housing is extremely small and precisely defined There are ports in the housing. One port is at the suction side (inlet) and the other at the pressure side of the pump (discharge). One gear wheel is normally driven by a motor connected to the drive shaft, which protrudes from the housing. When the gear wheels are rotating, a chamber, which is filled with the medium being pumped, is formed between two teeth and the housing. The medium is thus conveyed from the suction side to the pressure side of the pump. The medium being pumped is displaced from between the gap between the teeth at the point at which the teeth mesh. For this reason, the gear pump is known as a displacement pump.
 

How to choose the right gear pump
 
External gear pumps are self-priming and can dry-lift although their priming characteristics improve if the gears are wetted. The gears need to be lubricated by the pumped fluid and should not be run dry for prolonged periods. Some gear pump designs can be run in either direction so the same pump can be used to load and unload a vessel, for example.
The close tolerances between the gears and casing mean that these types of pump are susceptible to wear particularly when used with abrasive fluids or feeds containing entrained solids. External gear pumps have four bearings in the pumped medium, and tight tolerances, so are less suited to handling abrasive fluids. For these applications, internal gear pumps are more robust having only one bearing (sometimes two) running in the fluid. A gear pump should always have a strainer installed on the suction side to protect it from large, potentially damaging, solids.
Generally, if the pump is expected to handle abrasive solids it is advisable to select a pump with a higher capacity so it can be operated at lower speeds to reduce wear. However, it should be borne in mind that the volumetric efficiency of a gear pump is reduced at lower speeds and flow rates. A gear pump should not be operated too far from its recommended speed.
For high temperature applications, it is important to ensure that the operating temperature range is compatible with the pump specification. Thermal expansion of the casing and gears reduces clearances within a pump and this can also lead to increased wear, and in extreme cases, pump failure.
Despite the best precautions, gear pumps generally succumb to wear of the gears, casing and bearings over time. As clearances increase, there is a gradual reduction in efficiency and increase in flow slip: leakage of the pumped fluid from the discharge back to the suction side. Flow slip is proportional to the cube of the clearances between the cog teeth and casing so, in practice, wear has a small effect until a critical point is reached, from which performance degrades rapidly.
Gear pumps continue to pump against a back pressure and, if subjected to a downstream blockage will continue to pressurize the system until the pump, pipework or other equipment fails. Although most gear pumps are equipped with relief valves for this reason, it is always advisable to fit relief valves elsewhere in the system to protect downstream equipment.
The high speeds and tight clearances of external gear pumps make them unsuitable for shear-sensitive liquids such as foodstuffs, paint and soaps. Internal gear pumps, operating at lower speed, are generally preferred for these applications.
 
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