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What is control Valve?

A control valve is a device capable of modulating flow at varying degrees between minimal flow and fullcapacity in response to a signal from an external control device. The control valve often referred to as thefinal control element, is a critical part of any control loop, as it performs the physical work and is theelement that directly affects the process.

Flow-control valves

The purpose of flow control in a hydraulic system is to regulate speed. All the devices discussed here control

the speed of an actuator by regulating the flow rate. Flow rate also determines rate of energy transfer at any

given pressure. The two are related in that the actuator force multiplied by the distance through which it

moves (stroke) equals the work done on the load. The energy transferred must also equal the work done.

Actuator speed determines the rate of energy transfer (i.e., horsepower), and speed is thus a function of

flow rate.

Directional control, on the other hand, does not deal primarily with energy control, but rather with directingthe energy transfer system to the proper place in the system at the proper time. Directional control valves

can be thought of as fluid switches that make the desired "contacts." That is, they direct the high-energy

input stream to the actuator inlet and provide a return path for the lower-energy oil.

It is of little consequence to control the energy transfer of the system through pressure and flow controls if

the flow stream does not arrive at the right place at the right time. Thus, a secondary function of directional

control devices might be defined as the timing of cycle events. Because fluid flow often can be throttled in

directional-control valves, some measure of flow rate or pressure control can also be achieved with them.

Different types of flow measurement

Controlling flow of a fluid-power system does not necessarily mean regulating volume per unit of time from a

valve. Flow rate can be specified three different ways, so it is important to be aware of how flow is to be

specified or measured:

Volumetric flow rate , Q v, expressed in units of in.3/sec or min - or cc/sec or cc/min in SI metric measure - is

used to calculate the linear speeds of piston rods or rotational speeds of motor shafts.

Weight flow rate , Q w , expressed in units of lb/sec or lb/min, is used to calculate power using English units ofmeasure.

Mass flow rate , Q g, expressed in units of slugs/sec or slugs/min for English measure - or kg/sec or kg/min in

SI metric measure - is used to calculate inertia forces during periods of acceleration and deceleration.

Because they control the quantity of fluid that flows through the valve per unit of time, the same control

valves are used for all three types of flow rates.

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Control of flow rate with valves

There are eight types of flow-control valves:

Orifices - A simple orifice in the line, Figure 1(a), is the most elementary method for controlling flow. (Note

that this is also a basic pressure control device.) When used to control flow, the orifice is placed in series with

the pump. An orifice can be a drilled hole in a fitting, in which case it is fixed; or it may be a calibrated needlevalve, in which case it functions as a variable orifice, Figure 1(b). Both types are non-compensated flow-

control devices.

Flow regulators - This device, Figure 2, which is slightly more sophisticated than a fixed orifice, consists of an

orifice that senses flow rate as a pressure drop across the orifice; a compensating piston adjusts to variations

in inlet and outlet pressures. This compensating ability provides closer control of flow rate under varying

pressure conditions. Control accuracy may be 5%, possibly less with specially calibrated valves that operate

around a given flow-rate point.

Bypass flow regulators - In this flow regulator, flow in excess of set flow rate returns to reservoir through a

bypass port, Figure 3. Flow rate is controlled by throttling fluid across a variable orifice regulated by the

compensator piston. The bypass flow regulator is more efficient than a standard flow regulator.

Demand-compensated flow controls - Flow controls can also bypass excess system flow to a secondary

circuit, Figure 4. Fluid is routed at a controlled flow rate to the primary circuit, and bypass fluid can be used

for work functions in secondary circuits without affecting the primary one. There must be flow to the

primary circuit for this type of valve to function - if the primary circuit is blocked, the valve will cut off flow to

the secondary circuit.

Pressure-compensated, variable flow valves - This flow control is equipped with an adjustable variable

orifice placed in series with a compensator. The compensator automatically adjusts to varying inlet and load

pressures, maintaining an essentially constant flow rate under these operating conditions to accuracies of 3%

to 5%, Figure 5. Pressure-compensated, variable flow-control valves are available with integral reverse-flow

check valves (which allow fluid to flow unrestricted in the opposite direction) and integral overload relief

valves (which route fluid to tank when a maximum pressure is exceeded).

Pressure- and temperature-compensated , variable flow valves - Because the viscosity of hydraulic oil varies

with temperature (as do the clearances between a valve's moving parts), output of a flow-control valve may

tend to drift with temperature changes. To offset the effects of such temperature variations, temperature

compensators adjust the control orifice openings to correct the effects of viscosity changes caused by

temperature fluctuations of the fluid, Figure 6. This is done in combination with adjustments the control

orifice for pressure changes as well.

Priority valves - A priority valve, Figure 7, is essentially a flow-control valve that supplies fluid at a set flow

rate to the primary circuit, thus functioning as a pressure-compensated flow-control valve. Flow in excess of

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that required by the primary circuit bypasses to a secondary circuit at a pressure somewhat below that of the

primary circuit. Should inlet or load pressure (or both) vary, the pr imary circuit has priority over the

secondary - as far as supplying the design flow rate is concerned.

Deceleration valves - A deceleration valve, Figure 8, is a modified 2-way, spring-offset, cam actuated valve

used for decelerating a load driven by a cylinder. A cam attached to the cylinder rod or load closes the valve

gradually. This provides a variable orifice that gradually increases backpressure in the cylinder as the valvecloses. Some deceleration valves are pressure-compensated.

What are Valves?

Valves are mechanical devices that control the flow and pressure within a system or process. They areessential components of a piping system that conveys liquids, gases, vapors, slurries etc.Different types of Valves are available: gate, globe, plug, ball, butterfly, check, diaphragm, pinch, pressurerelief, and control Valves. Each of these types has a number of models, each with different features andfunctional capabilities. Some Valves are self-operated while others manually or with an actuator orpneumatic or hydraulic is operated.Functions from Valves are:

Stopping and starting flow Reduce or increase a flow Controlling the direction of flow Regulating a flow or process pressure Relieve a pipe system of a certain pressure

There are many Valve designs, types and models, with a wide range of industrial applications. All satisfy oneor more of the functions identified above. Valves are expensive items, and it is important that a correct Valveis specified for the function, and must be constructed of the correct material for the process liquid.

Classification of Valves

The following are some of the commonly used Valve classifications, based on mechanical motion: Linear Motion Valves. The Valves in which the closure member, as in gate, globe, diaphragm, pinch, and

lift Check Valves, moves in a straight line to allow, stop, or throttle the flow. Rotary Motion Valves. When the Valve-closure member travels along an angular or circular path, as in

butterfly, ball, plug, eccentric- and Swing Check Valves, the Valves are called rotary motion Valves.

Quarter Turn Valves. Some rotary motion Valves require approximately a quarter turn, 0 through 90,motion of the stem to go to fully open from a fully closed position or vice versa.

Common Types of Pneumatic Valves

Pneumatic valves are one of an array of components responsible for controlling the pressure, rate, andamount of air as it moves through a pneumatic system. Pneumatic systems, which depend on the force ofcompressed air to transmit power, can be found in countless industrial applications, from pneumaticpressure power tools to diesel engines. Based on other components within a given application and the typeof pneumatic system used, one of several types of pneumatic valves may be found at the heart of the device.

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Functional directional control valves, those that control the direction of air flow or inhibit flow all together,are a large class of pneumatic valves that houses multiple variants.

Functional Directional Control Valves Many functional directional pneumatic control valves are classified based on the number of entry and exitports they possess, the number of flow paths they create, and the mechanism by which ports are openedand closed.

Two-Way Directional Valve A two-way directional valve passes air in two directions, through two ports which can be open or closed. Ifthe valve ports are closed no air can flow through the valve. If the ports are open, air may move from thefirst port through the valve and through the second port or in the opposite direction.

Three-Way Directional Valve A three-way directional valve has three ports, each of which serves a different purpose. The first port is usedto connect the valve to an actuator or another device. The second port is connected to an air-flow. The thirdport is used as an exhaust exit. When the first and second ports are open and the third is closed, air movesthrough the valve to the device. When the first and third ports are open and the second port is closed, theactuator can vent exhaust. Three-way valves are often connected to actuators in cylinders, or used in pairsand connected to double-acting cylinders.

Four-Way Directional Valves A four-way directional valve has four distinct ports, two of which connect to actuators, one that connects toa pressurized air-flow, and one that serves as an exhaust pathway. They are among the most common typesof valves found in pneumatic systems because the four distinct paths allow the valve to effectively reversethe motion of a motor or basic cylinder. An additional port is sometimes added to a four-way valve, making ita five-ported four-way valve. A four-way valve with an additional port is often used to provide dual pressure,meaning the valve can apply one of two kinds of pressure and alternate between the two depending on whatthe application requires. Alternatively, the valve can use the other port as a secondary exhaust port.

Spring Offset This type of pneumatic valve classification refers to the manner in which air-flow direction is switched. For

example, in a two-way directional valve, the valve is either open (air-flow is enabled) or closed (air-flow isprevented). In order for each port to assume an open or close position, an actuator moves a valve spool intoposition. To release the valve spool and return the pneumatic valve to its previous position, a spring releasesthe spool. A two-way directional valve that functions in this manner is also called a spring offset valve.

Resting state: Open v. Closed In two-way directional spring offset valves, there are two positions they can assume when the connectedactuator isnt active: open or closed. In devices where an open resting position is standard, air moves freelythrough the valve. In a closed resting state, the air-flow is blocked. In three-way valves, one port is alwaysopen. In such cases, a closed resting state usually results in blocking the air- flow port, so pressure isntmoving unless the device is turned on.

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