What is a Hydraulic Valve and Why Do You Need It?

A hydraulic valve properly directs the flow of a liquid medium, usually oil, through your hydraulicsystem. The direction of the oil flow is determined by the position of a spool.A hydraulic system canonly function - as per requirements - by using valves. Thus,you should always look for the correcttype of hydraulic valve to serve your intended purpose.

Hydraulic valves are available in a variety of sizes. The size required is determined by the maximumflow of the hydraulic system through the valve and the maximum pressure in the hydraulicsystem. Hydraulic valves are available with different mountings: e.g. mounting in pipe lines,threaded connection as cartridges, subplate mounting, etc.

How DTA Can Help You:

DTA has extensive expertise with hydraulic valves and carries a substantial inventory of valves indifferent sizes and spool configurations. Depending on your requirements,DTA can supply high-quality hydraulic valves, taking into account a wide variety of functional and hydraulic systemrequirements.

Hydraulic valves are available from several major vendors; our preferred hydraulic valve suppliersare Parker Hannifin and Denison Hydraulics. Additionally, we supply hydraulic valve parts andaccessories such as sub plates, seal kits, O-rings, solenoids and hydraulic valve control electronics.

Different Types of Hydraulic Valves

Hydraulic valves are sometimes referred to as hydraulic components. These are subdivided intothree main categories: directional control valves, pressure control valves and flow controlvalves. We have added proportional and servo control hydraulic valves as a separate categorybelow:

Directional control valves:

Check valves

Directional spool valves

Directional poppet valves

Pressure control valves:

Control task: variable throttle valves

Switching task: fixed throttle valves

Flow control valves:

Throttle valves: flow Δp dependent

Control valves: flow Δp independent

Electro-hydraulic valves

Servo hydraulic valves

Proportional hydraulic valves

Different valves function in different ways. Check valves permit free flow in one direction andblock flow in the opposite direction. The directional control valve is used to pass on the pressuremedium (i.e. flow) in an orderly fashion to a particular direction. Pressure control valves switch(or control) at a certain pressure; the switching pressure may be adjusted on the valve. Flowcontrol valves regulate the flow this is done by adjusting the size of the bores (orifices).

Note that proportional hydraulic valves and servo control valves may be used to perform adirectional control, pressure control or flow control function.

Directional control valves can control the start, stop and change in direction of flow of a pressuremedium (i.e. hydraulic oil). For this reason, they are also referred to as switching valves.Directional control valves are available as spool valves, poppet valves or rotary slide valves.Rotary slide valves, however, are no longer commonly used due to low operating pressure (up to70 bar).

The performance of a directional control valve depends on the following:

Dynamic power limit (as a function of maximum flow and pressure) Static power limit (adhesiveforce may be produced between the spool and housing due to standstill) Resistance to flow(internal resistance, i.e. pressure drop) Leakage (spool valves only) Switching time (time betweenthe actuating force and the completion of the stroke of the control element)

The function of a directional control valve is determined by the number of working ports(excluding control ports) and the number of spool positions. A directional control valve comprisesat least two spool (switching) positions and two working ports. The most common directionalcontrol valve is a 4/3-way valve, which means the valve has four working ports and three spoolpositions, in accordance with DIN ISO 1219.

Check Valves

A check valve is the simplest type of directional control valve used in hydraulic systems. Checkvalves stop the flow of fluid in one direction and allow free flow in the opposite direction. They arealso known as non-return valves. Check valves may be used as:

Prefill valves (anti-cavitation)

Bypass valves (e.g. throttling points or return-line filters)

Stops for flowin one direction Pre-tensioning by creating a certain backpressure Protection ofhydraulic components against pressure surgesstion

Most check valves are spring-loaded and use a ball or plate to seal the flow in one direction. Checkvalves are designed with seats and thus are able to isolate circuits with no leakage. Balls, plates,poppets or poppets with soft seals are used as isolating elements.

There is a special type of check valve that prevents pistons or cylinder plungers from coming downand causing accidents. This is called a line rupture valve. When the line ruptures, the flow throughthe line rupture valve increases substantially, causing an increased pressure drop. This in turncreates a stronger force on the ball, which will close immediately.

Other special types of check valves are pilot-operated check valves and shuttle valves.

A pilot-operated check valve allows flow in either direction by application of an external

pilot pressure signal. A shuttle valve permits free flow at the highest operating

pressure.

Directional Spool Valves

Directional spool valves comprise a moving spool situated in the valve housing. When an actuatingforce moves the control spool, the annular channels in the housing are connected or separated.Directional spool valves have several unique features, such as:

Low cost due to simple design

Low actuating force (due to good pressure compensation)

High switching power

Low losses (even though oil leakage flows continuously from the high pressure to the low pressureside)

Wide variety of control functions

Directional spool valves may be direct-operated or pilot-operated. A direct-operated

spool valve is either electrically controlled with solenoids, mechanically (e.g. manually)

controlled with levers or rollers, or controlled with hydraulics or pneumatics. Whether a

directional spool valve is direct- or pilot-operated depends on the actuating force

needed to move the spool. Thus, this is dependent on the flow, i.e. nominal size of the

directional spool valve.

When valves are operated at higher hydraulic system operating pressures, leakage

losses around the spool and the housing should be taken into account, especially at

system pressures over 350 bar. The leakage loss is determined by the size of the gap

between the spool and the housing, the viscosity of the hydraulic fluid and the

hydraulic system pressure.

Typical parameters of spool valves are:

Nominal sizes: NG6 and NG10 (up to

NG120 for pilot-operated valves)

Maximum flow: up to 150 l/min (up to

7,000 l/min for pilot-operated valves)

Maximum pressure: up to 350 bar

Directional Poppet Valves

Directional poppet valves can operate at a maximum operating pressure of up to 1,000

bar. Directional poppet valves are directional control valves that are fitted into housing

bores with a threaded connection. This is why they are often referred to as hydraulic

cartridge valves. The moveable seating elements may consist of balls, poppets or

plates.

Key features of hydraulic cartridge valves:

Pressure Control Valves

No leakage

Long product life

High maximum and continuous

operating pressures due to zero

leakage losses

Very good sealing characteristics

without additional isolating elements

The design permits the valve to become

more tightly sealed when operating pressure increases. The maximum flow of

cartridge valves is limited compared to spool valves. Larger flows would require larger

cartridges to fit into housing bores; the threaded connection will not hold.

Typical parameters of poppet valves are:

Nominal size: NG6 and NG10, SAE,

and ¼" - 2" BSP

Maximum flow: up to 60 l/min (up to

400 l/min for pilot-operated valves)

Maximum pressure: up to 1,000 bar

Back to top - Directional Control Valves

Every pressure control valve switches (or controls) at a predetermined pressure

setting. The switching pressure is generally a variable setting on the valve itself. A

change in position of the pressure control valve (i.e. open or closed) occurs either

gradually (by control) or suddenly (by switch). Pressure control valves can be

subdivided into control and switching task:

Control task (variable throttle)

Pressure relief valves

Pressure reducing valves

Pressure drop valves

Pressure difference valves

Pressure valves with additional

electric switch-off

Switching task (fixed throttle)

Pressure shut-off valves

Pressure sequence valves

Sequence valves are used to direct the flow to a secondary circuit. When the primary

hydraulic circuit reaches a preset pressure, flow is permitted to the secondary circuit.

Unloading valves are remotely-piloted pressure shut-off valves that direct flow to the

tank when the preset pressure in a particular location of the hydraulic circuit has been

reached.

The most common types of pressure control valves are the pressure relief valve and

the pressure reducing valve. Pressure relief valves control the system pressure by

relieving part, or all, of the flow to tank. Pressure reducing valves reduce the pressure

supplied to a sub-system of a hydraulic system.

Pressure control valves may be directly or pilot operated. Larger flows require larger

spools or poppets, which increases the area of the spring diameter: the spring force

increases proportionally. Pilot-operated pressure control valves require only a small

spring, are suitable for compact space requirements and have maximum flows of up to

650 l/min.

Back to top - Hydraulic Valves

Pressure Relief Valves

There are two pressure control valves: one

is a pressure relief valve, and the other is a

pressure reducing valve. Pressure relief

valves have higher flow capacities than

pressure reducing valves because

pressure relief valves have to be capable

of directing all flow to the tank.

Pressure relief valves are used in hydraulic systems to limit the system pressure to a

specific set level. If this set level is reached, the pressure relief valve responds and

feeds the excess flow from the system back to the tank.

Note: the pressure relief valve is a normally closed pressure control valve.

Back to top - Pressure Control Valves

Pressure Reducing Valves

In contrast to pressure relief valves, which

affect the level of input pressure (i.e. pump

pressure) fed into the hydraulic system,

pressure reducing valves are used to

influence the output pressure of the

hydraulic system to a subsystem (e.g.

actuator pressure).

Reducing the input pressure (primary pressure) or maintaining a particular output

pressure (secondary pressure) is achieved at a set value which is below the varying

pressure occurring in the main hydraulic circuit. It is thus possible to reduce the

pressure in one part of the circuit to a lower level than the system pressure.

Note: the pressure control valve is normally open.

Back to top - Pressure Control Valves

Flow Control Valves

Flow control valves manage the flow by decreasing or increasing the opening at the

throttling point. This helps to determine speed of movement for the actuators. The

simplest design for a flow control valve is a needle or longitudinal slot mounted in the

pipeline and connected to a screw that adjusts the opening at the throttling point.

These are called throttle valves and they are regularly used in combination with a

check valve, i.e. the throttle check valve for speed control in one direction of flow. A

disadvantage of throttle valves is that at varying loads a change in pressure drop will

change the flow; thus, the speed of the moving actuator will also be affected. Flow

control valves are divided into two types:

Throttle valves (flow depends on Δp)

Viscosity dependent

Viscosity independent

Flow control valves (independent of Δp)

2-way flow control valves

3-way flow control valves

The difference in pressure before and after the throttling point, i.e. the pressure drop

(Δp), determines the rate of flow through the throttle valve at a particular setting. If the

pressure drop over the throttling point remains the same, the flow of oil that passes the

throttling point remains the same as well. This allows operation at constant speeds,

regardless of the load.

Flow control valves are used to influence the speed of movement of actuators by

changing the opening for the flow (decreasing or increasing it) at the throttling point. In

fact, these are two flow control valves placed in series and built together. The throttling

point operates at a fixed setting, but the throttle opening before the throttling point

varies with pilot pressure of the load.

Note that flow dividers have a certain special standing: they divide an oncoming flow

into two or more flows. Usually it is used to distribute the flow of a single pump to two

or more sub-systems that have different operating pressures.

Back to top - Hydraulic Valves

Throttle Valves

Throttle valves come in piston and needle-valve structures, but we also know them as

turn plugs with axial or radial grooves. The smothering of grooves is more accurate but

provides more variability than those with piston structures. The needle throttle has one

disadvantage: a great deal of power is required to move the needle only a slight

amount.

The pressure difference (Δp) determines the amount of flow that passes the throttle. In

systems with varying loads (and therefore varying pressures), the flow passing the

throttle will always be different.

You can find non-adjustable (fixed) smothering in situations where it is needed to

gauge against excessive pressure surges. Also, one can find non-adjustable throttles

in control systems.

Back to top - Flow Control Valves

Flow Control Valves

A pressure-compensated flow control valve has a fixed throttling flow at all pressures.

The two-way pressure-compensated flow control valve is also called a series valve.

The pressure reducing valve and the smothering of this valve are placed in series with

each other. At a three-way pressure control valve, the pressure relief valve and the

throttle in this valve are parallel to each other.

The two-way and three-way pressure compensated flow control valves are quite

similar. The only difference is that in the latter type, the excess oil flow is not drained

through the pumps safety valve but through the flow control valve itself. The pump

pressure for a three-way pressure compensated flow control valve is always equal to

the pressure exerted by the actuator plus the spring pressure.

Note that using three-way flow control valves increases efficiency because the system

pressure will never be higher than as required for moving the load. With two-way flow

control valves, this is not the case because the pump keeps feeding the system with

oil until the preset pressure of the pump's safety - pressure relief - valve has been

reached.

Back to top - Flow Control Valves

Electro-hydraulic Valves

When your hydraulic application requires a very accurate control of the flow, hydraulic

valves may be equipped with advanced control electronics. This allows the use of

inductive path measuring devices that monitor the position of the spool continuously to

ensure optimum position of the spool.

Proportional hydraulic valves are able to control the opening to flow proportionally

instead of gradually, as is the case for most standard hydraulic valves. The simplest

type of proportional hydraulic valve is a variable throttle controlled by a manual lever,

as illustrated below:

Proportional and servo hydraulic valves are usually classified as high-performance

valves. This distinction gives an expected indication of performance, which tends to

generalise the true differences between various types of servo and proportional

hydraulic valves. Each type has its own unique performance characteristics either in

controlling pressure or controlling flow.

Servo hydraulic valves use closed-loop control to monitor and feed back the

main-stage spool position to a pilot stage (hydraulic/pneumatic) or driver (electronic).

Proportional hydraulic valves, on the other hand, move the main-stage spool in direct

proportion to an external piloted control signal and do not have any means of feedback

within the valve.

Back to top - Hydraulic Valves

Servo Valves

The main stage spool of a servo hydraulic valve is equipped with a path measuring

system for accurate adjustment. This system measures the exact position of the

main-stage spool and feeds its position directly to the electronic control unit. If sworn

position is not consistent, the control signal corrects the position of the main-stage

spool.

Servo hydraulic valves are widely used in aerospace and industrial applications.

Back to top - Electro-hydraulic Valves

Proportional Valves

The most common proportional hydraulic valves are directional control valves,

pressure relief valves, flow controllers and adjustable throttling. Proportional hydraulic

valves convert an incoming mechanical or electrical signal directly proportional to a

shear mode. The movement follows a continuously incoming signal. Different types of

available proportional hydraulic valves are:

Various directional control valves

Flow control valves

Pressure relief valves

Pressure reducing valves

Counter balance valves

Typical applications of proportional hydraulic valves include cranes and industrial

applications such as injection moulding.