control system components topic: actuators and valve positioner prepared by :prof. rajesh zadfiya...

Control System ComponentsTopic: Actuators and Valve Positioner

Prepared by :Prof. Rajesh Zadfiya Instrumentation & Control Engg.

Institute of Technology Nirma University

Actuators• A valve actuator is a

device that produces force to open or close the valve utilizing a power source. • This source of power

can be manual (hand, gear, chain-wheel, lever, etc.) or can be electric, hydraulic or pneumatic.

Contd..• Basic actuators turn valves

to either fully opened or fully closed positions. • But modern actuators have

much more advanced capabilities. They not only act as devices for opening and closing valves, but also provide intermediate position with high degree of accuracy.

Type of Actuators

• Two types of actuators are common: pneumatic and electric actuators.• Pneumatic:

• Pneumatic actuators utilize an air signal from an external control device to create a control action. These are commonly available in two main forms: • Diaphragm actuators and • Piston actuators

• Electric:• Electric actuators are motor driven devices that utilize an electrical input signal to

generate a motor shaft rotation. • This rotation is, in turn, translated by the unit’s linkage into a linear motion, which drives

the valve stem and plug assembly for flow modulation. • In case of electric signal failure, these actuators can be specified to fail in the stem-out,

stem-in, or last position. • Commonly used motors for electric actuators include steppers and servos.

Contd..

Diaphragm actuators – Diaphragm actuators have compressed air applied to a flexible

membrane called the diaphragm. These types of actuators are single acting, in that air is only supplied

to one side of the diaphragm, and they can be either direct acting (spring-to-retract) or reverse acting (spring-to-extend).

Contd..

Flapper nozzle amplifier

• A pneumatic control system operates with air. • The signal is transmitted in form of variable air pressure (often in the

range 3-15 psi, i.e. 0.2 to 1.0 bar) that initiates the control action. • One of the basic building blocks of a pneumatic control system is the

flapper nozzle amplifier. • It converts very small displacement signal (in order of microns) to

variation of air pressure. • The basic construction of a flapper nozzle amplifier is shown below.

Contd..

Characteristics of a flapper nozzle amplifier

Limitations of Flapper Nozzle Amp.

• The major limitation of a flapper nozzle amplifier is its limited air handling capacity. The variation of air pressure obtained cannot be used for any useful application, unless the air handling capacity is increased. • Another problem of a flapper nozzle amplifier is its sensitivity

variation.

Air Relay

The principle of operation of an air relay can be explained using the schematic diagram shown here.

It can be seen from Fig. that the air relay is directly connected to the supply line (no orifice in between). The output pressure of the flapper nozzle amplifier (p2) is connected to the lower chamber of the air relay with a diaphragm on its top.

Contd..

The variation of the pressure p2 causes the movement (y) of the diaphragm. There is a double-seated valve fixed on the top of the diaphragm. When the nozzle pressure p2 increases due to decrees in xi, the diaphragm moves up, blocking the air vent line and forming a nozzle between the output pressure line and the supply air pressure line. So more air goes to the output line and the air pressure increases. When p2 decreases, the diaphragm moves downward, thus blocking the air supply line and connecting the output port to the vent. The air pressure will decrease.

Limitations of Air Relay

Problem with of an air relay is its sensitivity variation.

Flapper Nozzle Amplifier with Feedback

Flapper nozzle amplifiers are never used in open loop; it is always used in closed loop. The scheme shown below is a torque balance arrangement.

Contd..

Anticlockwise moment: and

Clockwise moment:

Where AB1 and AB2 are the areas of the two bellows, a and b are the corresponding lengths of the link segments.

Thereby at balance:

solenoid

• Solenoid is an electromagnet which can be used as an actuator. • Electrically operated actuators.• Solenoid valves are used in hydraulic and pneumatic systems.

It moves a rod by electromagnetic energy

Contd..

Applications

Contd..

Contd..

Applications (Combined)

Contd..

Advantages of Pneumatic Actuators

• Weight• Cylinders much lighter than motors

• Simple• Much easier to mount than motors• Much simpler and more durable than other for linear motion

• Fast on/off type tasks • Big forces with elasticity• No leak problems

Disadvantages of Pneumatic Actuators

• All the components are quite expensive• A properly designed system is more complex than an equivalent

electromechanical system (electric motors, power screws and other linear actuators). • All these components take up quite a bit of valuable space (For

example within a robot). • No weight advantage if only one cylinder used (still need compressor,

reservoir, pressure sensors, regulator)

Operators

General manual

Push button

Pull button

Push/pull button

Lever

Pedal

Treadle

Manual

Rotary knob

OperatorsMechanical

Plunger

Spring normally as a return

Roller

Uni-direction or one way trip

Pressure

Pilot pressure

Differential pressure

Detent in 3 positions

Operators

Solenoid direct

Solenoid pilot

Solenoid pilotwith manual overrideand integral pilot supply

Solenoid pilotwith manual override and external pilot supply

Electrical

When no integral or external pilot supply is shown it is assumed to be integral

Pneumatic Rotary Actuators

Pneumatic Valve Positioner • Pneumatic valve positioner is another important component used in

process control.• The control valve should be moved up or down, depending on the air

pressure signal (3-15 psi). • The valve postioner can be of two types, (a) direct acting type and (b)

feedback type.• The direct acting type valve positioner is shown below.

Direct acting type valve positioner

Contd..• Here the control pressure creates a downward pressure on the

diaphragm against the spring, and the stem connected to the diaphragm moves up or down depending on the control pressure p. At equilibrium the displacement of the stem can be expressed as:

pA=Kx ---------------(1)where A is the area of the diaphragm and K is the spring

constant. • But the major shortcoming of this type of positioner is the nonlinear

characteristics. • Though ideally, the stem displacement is proportional to the control

pressure (from (1)), the effective area of the diaphragm changes as it deflates.

Contd..• The spring characteristics is also not totally linear. Moreover, in (1) we

have neglected the upward thrust force exerted by the fluid. • The change in thrust force also causes the change in performance of

the positioner. • Besides the force exerted on the control valve is also not sufficient for

handling valves for controlling large flow. • As a result, the use of direct acting type valve positioner is limited to

low pressure and small diameter pipelines.

Feedback type valve positioner

Contd..• The feedback type valve positioner has a pilot cylinder with which the

diaphragm is attached. • The piston of this pilot cylinder opens or closes the air supply and vent ports to

the main cylinder whose piston is connected to the stem of the control valve (not shown). • There is a mechanical link connected to the stem that adjusts the fixed end of

the spring connected to the diaphragm. This link provides the feedback to the postioner. • As the control pressure increases, the diaphragm moves down, so is the piston

of the pilot cylinder. This causes the lower chamber of the main cylinder to be connected to the 20 psi line and the upper chamber to the vent line. • Compressed air enters the bottom of the main cylinder and the piston moves

up.

Contd..• As the piston moves up, the feedback link compresses the spring

further and this causes the diaphragm to move back to its original position. • The air supply and the vent ports are now closed and the piston of the

main cylinder remains at its previous position. The relationship between the control pressure and movement of the stem in this case is more or less linear. • Moreover due to presence of power cylinder, the scheme is more

suitable to position large control valves.

Hydraulic actuators: cylindersCylinder types:

Single acting: work can be done only in one direction

Piston

Double acting piston:

Piston rod on both sides

Plunger

Work is done in both directions

Telescopic Telescopic Fast moving

Tandem

Fast moving

Hydraulic cylinders

Properties:

The cylinders have to be good quality steel with close tolerances.

There have to be good sealing both at the piston rod and at the cylinder.

With time dirt may come in and damage the surfaces. This has to be possibly reduced.

In this case, the leakage will increase all the time.

Hydraulic cylindersCalculation of cylinders

1maxmaxFFFF FLc FLC FFF

maxmax

22110 pApAFC

1122 pApAFCB

10 A

Qv

2A

QvB

friction forces inertial forcesmaximum loadslow motion, can be often neglected

Backward:

Outward:

Q

A2A1

p1

p2vB

v0

Hydraulic cylindersCalculation of cylinders

Hydraulic cylinders should be possibly operated in the 3rd region for smooth operation.

If the cylinder is new, the leakage losses are negligibly small so that:

ηc = ηmech

Ff1

v

2 3

1. Stick-slip2. Transition3. Normal behaviour

22112211 ApAp

F

QpQp

Fv LLc

92,085,0max

c

ηc

Δp

outwardsinwards at higher pressures

Hydraulic cylindersChecking for buckling

n: safety factor: 1-3,5

lk: buckling length

I1: moment of inertia of the piston rod

E: elasticity modulus of the rod material

1

2

max

1IE

lnF

kL

64

4d

Maximum permissible force:

Hydraulic cylindersCushioning of cylinders:

A hard impact of the piston at the end surfaces has to be inhibited – kinetic energy has to be absorbed.

This is done by increasing the hydraulic resistance at the end of the stroke.

Rotary hydraulic actuators

Swivel vane rotary actuator:

Limited angle in both directions

Maximum angle always smaller than 360°

The same torque in both directions

Piston rotary actuator:

With rack and gear coupling

Here maximum angle may be larger than 360°

Párhuzamdugattyús

lengőhajtás

Limited angle rotary actuator

Limited angle rotary actuator

Parallel piston rotary

actuator