Control system components topic actuators and valve positioner
This presentation is the property of its rightful owner.
Sponsored Links
1 / 43

Control System Components Topic: Actuators and Valve Positioner PowerPoint PPT Presentation


  • 71 Views
  • Uploaded on
  • Presentation posted in: General

Control System Components Topic: Actuators and Valve Positioner. Prepared by : Prof. Rajesh Zadfiya Instrumentation & Control Engg . Institute of Technology Nirma University. A ctuators.

Download Presentation

Control System Components Topic: Actuators and Valve Positioner

An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -

Presentation Transcript


Control system components topic actuators and valve positioner

Control System ComponentsTopic: Actuators and Valve Positioner

Prepared by :Prof. Rajesh Zadfiya

Instrumentation & Control Engg.

Institute of Technology

Nirma University


A ctuators

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

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

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.


Contd1

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).


Contd2

Contd..


Flapper nozzle amplifier

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.


Contd3

Contd..


Characteristics of a flapper nozzle amplifier

Characteristics of a flapper nozzle amplifier


Limitations of flapper nozzle amp

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.


Solenoid

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

It moves a rod by electromagnetic energy


Contd4

Contd..


Applications

Applications


Contd5

Contd..


Contd6

Contd..


Applications combined

Applications (Combined)


Contd7

Contd..


Advantages of pneumatic actuators

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

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

Operators

Manual

General manual

Lever

Push button

Pedal

Pull button

Treadle

Push/pull button

Rotary knob


Operators1

Operators

Mechanical

Plunger

Pressure

Spring normally as a return

Pilot pressure

Roller

Differential pressure

Uni-direction or one way trip

Detent in 3 positions


Operators2

Operators

Electrical

Solenoid direct

Solenoid pilot

with manual override and external pilot supply

Solenoid pilot

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

Solenoid pilot

with manual override

and integral pilot supply


Pneumatic rotary actuators

Pneumatic Rotary Actuators


Pneumatic valve positioner

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

Direct acting type valve positioner


Contd8

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.


Contd9

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.


F eedback type valve positioner

Feedback type valve positioner


Contd10

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.


Contd11

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 cylinders

Hydraulic actuators: cylinders

Double acting piston:

Single acting:

work can be done only in one direction

Work is done in both directions

Plunger

Cylinder types:

Piston rod on both sides

Piston

Tandem

Fast moving

Telescopic

Telescopic

Fast moving


Hydraulic cylinders

Hydraulic cylinders

  • 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.

Properties:


Hydraulic cylinders1

A2

A1

vB

p2

p1

v0

Q

Hydraulic cylinders

maximum load

friction forces

inertial forces

Calculation of cylinders

slow motion, can be often neglected

Outward:

Backward:


Hydraulic cylinders2

Ff

1

2

3

ηc

outwards

inwards

v

Δp

Hydraulic cylinders

  • Stick-slip

  • Transition

  • Normal behaviour

Calculation 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

at higher pressures


Hydraulic cylinders3

Hydraulic cylinders

Maximum permissible force:

Checking 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


Hydraulic cylinders4

Hydraulic 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.

Cushioning of cylinders:


Rotary hydraulic actuators

Rotary hydraulic actuators

Parallel piston rotary actuator

Limited angle in both directions

Maximum angle always smaller than 360°

The same torque in both directions

Párhuzamdugattyús lengőhajtás

Swivel vane rotary actuator:

Limited angle rotary actuator

Limited angle rotary actuator

Piston rotary actuator:

With rack and gear coupling

Here maximum angle may be larger than 360°


  • Login