How does the hydraulic valve work?

Valves have a unique ability to control fluid within a hydraulic system.

Directional Control Valves (DCVs) do so by directing the path of hydraulic fluid (flow) through a circuit. They are used to start, stop, slow, accelerate, and change the direction of motion in a cylinder.

The internal control element of a DCV (sliding spool, rotary spool, or ball) is designed for all these applications. 

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One of the most common valve types are Sliding Spool Valves. They have different flow path configurations depending on the sliding spool design.  All these sliding spool flow paths are found within the same body.  Externally they look the same, internally they are different.  

It is this construction that allows them to stop or shift flow suddenly, and are sometimes referred to as either &#;switching&#; or &#;bang-bang&#; valves for the sound they make when doing so.

DCVs are also more formally called discrete valves, referring to how the valves shift from one discrete position to another.

When Are Directional Control Valves Used?

DCVs are essential with any hydraulic circuits to create bi-directional motion for a hydraulic actuator, or anything with a motor is going to benefit from their use, as they can be actuated into a variety of discrete positions for many functions.

Their usefulness derives from this process of actuation, or the movement of the valve from one position to another. Actuation makes DCVs vital instruments in controlling flow within complex hydraulic systems.

They work to actuate using these four methods:

• Manual
• Mechanical
• Solenoid-Operated
• Pilot-Operated

The main function of a DCV is to start, stop, accelerate, decelerate, and change the direction of motion of a hydraulic actuator.

Some other examples of DCV usage include:

• To isolate a branch of the circuit
• To vent the relief valve
• To allow free flow from the pump to the reservoir
• How Does a DCV Work?

Broadly, DCVs have a general order of operations once called into action by hydraulic fluid flow.

The valve will shift to perform the operation and then shift back to the neutral position when completed. This action occurs instantaneously, causing fluid to accelerate and decelerate.

How the operation plays out specifically depends on variations in spool-type, spool positioning, and method of control.

Check valves, for example, are binary directional control valves that allow fluid to flow in one direction while preventing it to flow in the opposite direction.

Spring-loaded check valves work like this:

• The flow enters the input port and the pressure pushes the disc open (A-B)
• The spring closes the disc after enough back pressure is reached (C-D)

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Flow control valves are used to regulate the flow rate and pressure of liquids or gases through a pipeline. Flow control valves are essential for optimizing system performance, relying on a flow passage or port with a variable flow area.

Here&#;s a look at the function of hydraulic flow control valves, the various types and components, and how they work, as well as some important considerations in selecting the appropriate hydraulic flow control valve for a specific application.

Function of Hydraulic Flow Control Valves

The purpose of a flow control valve is to regulate the flow rate in a specific portion of a hydraulic circuit. In hydraulic systems, they&#;re used to control the flow rate to motors and cylinders, thereby regulating the speed of those components.

Hydraulic flow control valves also control the rate of energy transfer at a given pressure. This is based on the physics concept surrounding work, energy, and power:

Actuator force x distance traveled = work done on load

The energy transfer must be equal to the total work done. Because the actuator speed determines the rate of energy transfer, speed is a function of the flow rate. Directional control valves serve a different purpose, directing the energy transfer system to the appropriate location at the appropriate time, although some pressure and flow rate control may be achieved using directional control valves, as they can throttle the flow of fluid.

How Hydraulic Flow Control Valves Work

There are a variety of flow control valve designs, with most intended for specific applications. Therefore, understanding how hydraulic flow control valves work is crucial in selecting the right valve for an application. The most common types of flow control valves include: 

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• Ball
• Diaphragm
• Needle
• Butterfly
• Plug

The simplest flow control valves have an aperture which opens or closes in order to increase or slow down the flow rate. Ball valves are among the simplest options, consisting of a ball attached to a handle. The ball has a hole through the center, and when the handle is turned, the hole is aligned with valve openings in order to permit flow. In order to shut off the flow, the handle is used to turn the hole perpendicular to the valve openings, which obstructs the flow.

Other valve types function in a similar way with some mechanism for either permitting or blocking flow. A butterfly valve, for instance, has an internal metal plate attached to a turning mechanism. The plate opens or closes when the mechanism is turned. Needle valves, which are among the more precise valve options, have an adjustable needle and a valve stem which restricts or allows the flow of liquid. The needle can be adjusted to block the flow of fluid completely, allow the free flow of liquid, or partially obstruct the flow to varying degrees, therefore allowing for greater and more precise control over the flow rate.

When it comes to hydraulic circuits, there are a variety of options for controlling flow, ranging from the simple to the sophisticated, including hybrids that couple hydraulic valve actuation with sophisticated electronic controls. These options include:

• Orifices
• Flow regulators
• Bypass flow regulators
• Demand-compensated flow controls
• Pressure-compensated, variable flow valves
• Pressure- and temperature-compensated, variable flow valves
• Priority valves
• Deceleration valves
• Flow dividers
• Rotary flow dividers
• Proportional flow-control valves
• Pressure-compensated proportional flow-control valves
• Proportional flow-logic valves

Orifices represent the most simplistic hydraulic flow control valve option, in which an orifice is placed in series with a pump, either as a fixed orifice or a calibrated needle. Obstructing the orifice results in a decrease or blocked flow.

The more sophisticated options can sense pressure changes and respond accordingly, or monitor flow rate and respond when the flow rate exceeds a defined threshold. Pressure-compensated variable flow valves have a compensator which automatically adjusts to various load and inlet pressures to maintain a consistent rate of flow (with a typical accuracy within 3% to 5%). Add temperature compensation to the mix to account for variations in the viscosity of hydraulic oil (which is impacted by variations in temperature).

Challenges of Hydraulic Flow Control Valves

Hydraulic flow control valves provide a cost-effective solution to concerns surrounding the flow rate. However, they are not without their problems, leading to a loss of pressure when valves are partially obstructed, which can impact performance. With more basic flow control valves, changes in flow rate may occur even when the control valve is in a static position, due to system pressure, temperature (which alters the viscosity of some fluids), or other variables, leading to reliability concerns.

Selecting the right hydraulic flow control valve can address some of these concerns, although completely mitigating these issues may require a sophisticated control valve, such as a pressure- and temperature-compensated variable flow control valve.

Design Considerations for Hydraulic Flow Control Valves

In a hydraulic circuit, actuators are controlled by the flow control valve. In addition to the control valve, there are other variables that influence the flow rate, including temperature, pump output, and other factors. Designing an appropriate valve for an application requires careful consideration of the various factors that can influence performance, flow, and durability, such as:

• Fluid density
• Maximum and minimum flow rate
• Corrosive property of the liquid
• Required pressure drop across the valve
• Allowable leakage limit when the valve is in a closed position
• Maximum noise tolerance
• Connectivity to the process (screws, welding, etc.)

Fluid density, along with minimum and maximum flow rates, are important to determine proper sizing for the valve, while the corrosive property of the liquid should be considered when selecting materials for the valve.

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