Hydraulic Valves and Pressure Relief Valves Guide

Hydraulic Valves

Designed to maximize system performance, hydraulic valves control the flow, pressure and direction of fluid through the circuit of an actuator. More specifically, these mechanical devices are used to direct the flow to a specific area of a hydraulic system, divert pressurized fluid or close a line altogether. This flow of liquid is used to accelerate or decelerate the speed of an actuator.

Controlled manually, via electric solenoid or automatically through hydraulic pressure, circuit valves must be sturdy enough to handle massive amounts of pressure — around 3,000 psi or more. As such, they are usually constructed from strong and heavy materials such as iron or steel. 

Types of Hydraulic Valve

Types of Hydraulic Valves

There are three main categories of hydraulic valves.

  • Hydraulic Directional Control Valves
  • Hydraulic Flow Control Valves
  • Hydraulic Pressure Control Valves


Hydraulic Directional Control Valves
An important part of hydraulic systems, directional control valves manage the direction of fluid flow through circuits. Sometimes called switching valves (when they lack proportional control), directional control valves are used to start, stop, pause and redirect the flow of a liquid medium through a system.

Hydraulic directional control valves feature a standardised numbering system where the first digit indicates the number of the valve’s working ports and the second the number of possible valve — or spool — positions. The most popular hydraulic directional control valve is the 4/3 valve, which features four working ports and three valve positions. These positions include: 

  • Neutral Position: All valve ports are closed.
  • Extended Position: The valve channels fluid from a pump to the cap end of a cylinder making it extend 
  • Retracted Position: The valve channels fluid from a pump to the rod end of a cylinder making it retract. 

Hydraulic Flow Control Valves
Fitted with an adjustable port, flow control valves regulate liquid flow in hydraulic systems. An increase in the flow rate (this can be achieved by enlarging the opening at the throttle point) increases the speed of a hydraulic actuator. Conversely, a decrease in the flow rate decreases the speed of an actuator.

There are different types of hydraulic flow control valves, including:

  • Adjustable flow control valves
  • Fixed flow control valves
  • Pressure compensated flow control valves
  • Throttling flow control valves

Depending on their mechanical design and complexity, hydraulic flow control valves feature different valve styles for opening or blocking flow, including: 

  • Butterfly
  • Ball
  • Needle
  • Diaphragm
  • Plug

Hydraulic Pressure Control Valves
Hydraulic pressure control valves are designed to adjust fluid pressure in hydraulic systems. They are crucial to preventing rises in pressure that could lead to mishaps ranging from fluid leaks to burst pipes and tubing. Pressure control valves change pressure position (open or close) at a predetermined setting. This can either take place gradually in the case of a variable throttle (control) or suddenly in the case of a fixed throttle (switch).

There are different types of pressure control valves, including: 

  • Reducing valves 
  • Relief valves 
  • Counterbalance valves
  • Sequence Valves 
  • Unloading Valves

Things to Consider Before Buying a New Hydraulic Valve

There are a few things to take into account when investing in a new circuit valve. They include valve type, flow capacity and size.

  • Type: What kind of control does the valve feature (directional, flow or pressure)? 
  • Actuation mechanism: In what way is the valve operated (pilot, mechanical, solenoid)?
  • Configuration: How many working ports and positions does the valve have? What is its resting state? 
  • Material: What is the valve made from?  
  • Port size: What are the dimensions of the valve’s inlet and outlet ports? 
  • Industry and media type: What industry and materials has the valve been designed for (oil, water, gas, etc.)?
  • Operating voltage: What is the voltage that activates the valve solenoid? 
  • Flow coefficient or flow rate: How much liquid can pass through the valve in a specific amount of time? 
  • Maximum pressure rate: What is the maximum pressure that the valve can handle?
  • Operating temperature range: What is the temperature range at which the valve has been designed to function? 

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At their most basic, hydraulic valves are devices that open and close to regulate the flow, pressure and direction of a fluid in a hydraulic circuit. This flow, in turn, is used to control the speed of actuators. The movement of the flow through a system is determined by the position of spools, plunges or poppets. 

The three most common variants of hydraulic valves are directional control valves, pressure control valves and flow control valves. All are tasked with performing different functions. In addition, shuttle valves can be used to let in fluids from alternative sources — however, this kind of valve is most commonly used in pneumatic systems. While some hydraulic systems require a single valve, others utilize a combination of different valve types. 

Unusually manufactured from iron or steel, hydraulic valves have to be strong enough to withstand at least 3,000 psi of fluid pressure — all while maintaining smooth and accurate function. They can be electric solenoid-operated or activated manually with buttons, knobs, handles or levers. Alternatively, some circuit valves — such as relief valves — self-activate when the hydraulic pressure reaches a certain level.

Available as poppet or spool valves, directional control valves start, stop and redirect the flow of a liquid or gas medium through a circuit. Directional control valves usually feature proportional control — those that do not are called switching valves.

The most basic form of this type of valve is a binary valve, which has an inlet port and an outlet port that let in or cut off fluid. Meanwhile, more complex hydraulic directional control valves come with multiple ports and valve positions. 

Utilized to maintain a steady level of pressure in a hydraulic circuit, pressure control valves are an important safety feature of a hydraulic system. This is because unchecked pressure can cause leakages and even lead to disasters such as burst pipes or tubes. 

Pressure control valves ensure that a predetermined pressure level is maintained through a hydraulic system by gradually regulating it through a variable throttle or suddenly with a fixed throttle. For example, reducing valves and relief pressure control valves feature variable throttles whereas sequence valves come with fixed throttles. Most hydraulic pressure control valves are normally closed, only opening to release excess pressure. 

While most hydraulic valves increase flow gradually, proportional control valves open the flow proportionally. The most basic type of proportional control valve features a variable throttle, which is controlled manually with a lever. On the more complex end of the spectrum, proportional control valves are managed electronically and feature integrated feedback devices. Just like other hydraulic circuit valves, proportional control valves come in three different types. These include proportional directional control valves, proportional pressure control valves and proportional flow control valves.

Hydraulic flow control valves regulate the speed of the actuator by increasing and decreasing the rate of flow of fluid through a circuit. This, in turn, is done by increasing or decreasing the opening at the throttle point.

Running the gamut of simple to complex, hydraulic flow control valves come in many different types. Some of these include priority valves; pressure compensated variable flow valves; pressure and temperature compensated variable flow valves; pressure compensated proportional flow control valves; decelerated valves and proportional flow control logic valves.

Hydraulic flow control valves measure flow rate in different ways. Some of these include mass flow rate in mass per time units, weight flow rate in weight per time units and volumetric flow rate in volume per time units.

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