Pneumatic control valve

Pneumatic control valve
Actual Pneumatic Control Valve
Typical Actuator & Valve
introduction to actuator
Actuator power
Actuator Fluids
Diaphragm Actuator
Positioner Indicator
Valve Body
Valve Plugs
Reverse & Direct Actuators
Air-To-Open vs. Air-To-Close
control valve
Controller Tuning
Selection of controller modes
Tuning Rules
Ziegler – Nichols Controller Settings

1. Pneumatic
Control Valve

2. Introduction to Pneumatic Control Valve
•Avalveinwhichtheforceofcompressedair
againstadiaphragmisopposedbytheforceof
aspringtocontroltheareaofopeningfora
fluidstream.
•Itconsistofanactuatorandavalve.
•Theactuatormovesthevalvesstemasthe
pressureonaspringloadeddiaphragm
changes.

3. Schematic diagram

4. Schematic diagram

5. Actual Pneumatic Control Valve

6. Typical Actuator & Valve

7. INTRODUCTION TO ACTUATOR
•Actuatorconvertsthecommandsignalfrom
controllersorhigher-levelcomponentsinto
physicaladjustmentinadjustableprocess
variable.

8. Actuator
•Itconvertcontrolleroutputsignal(4-20mA
or3-15psig)tophysicaladjustmentinthe
processinputvariables.
•Forprocesscontrol,themostcommontype
ofactuatoristhecontrolvalve.
•Othersinclude
–Variablespeedpumps
–Hydraulicactuators

9. Actuator power
•Pneumatic:simple,lowcost,fast,lowtorque
•Electric:motorandgearbox,hightorque,
slow
•Hydraulic:hightorque,fast,expensive

10. Actuator Fluids
•Air
•Oil (mineral and synthetic) which is
Clean & Moisture Free

11. Diaphragm Actuator

12. Positioner Indicator

13. Valve Body

14. Valve Plugs

15. Reverse & Direct Actuators
CloseOpen

16. Reverse & Direct Actuators

17. Air-To-Open vs. Air-To-Close
•Air-to-Open (+ gain)
More air →larger opening No air → Valve closes.
•Air-to-Close (-gain)
More air →smaller opening No air → Valve opens
completely.
•Proper type to use is determined from safety considerations
•Air-to-close: Coolant valve in an exothermic reactor or in a
condenser of a distillation column.
•Air-to-open: Steam valve in a reactor, inlet flow valve to a
tank.

18. Failure Mode

19. CONTROL VALVE
•Valve+Actuator
-Valve opening is adjusted by an actuator
•Pneumatic Control Valve
–Usually 3~15 psig signal is provided.
–I/P transmitter converts 4~20mA signal to 3~15 psig pneumatic
signal via 20psig supply air.

20. Useful definitions
•Cycle time —Also known as duty cycle; the total length of time for
the controller to complete one on/off cycle. Example: with a 20
second cycle time, an on time of 10 seconds and an off time of 10
seconds represents a 50 percent power output. The controller will
cycle on and off while within the proportional band.
•Proportional band —A temperature band expressed in degrees (if
the input is temperature), or counts (if the input is process) from
the set point in which the controllers’ proportioning action takes
place. The wider the proportional band the greater the area around
the set point in which the proportional action takes place. It is
sometimes referred to as gain, which is the reciprocal of
proportional band.

21. •Integral, also known as reset, is a function which
adjusts the proportional bandwidth with respect to the
set point, to compensate for offset (droop) from set
point, that is, it adjusts the controlled temperature to
set point after the system stabilizes.
•Derivative, also known as rate, senses the rate of rise
or fall of system temperature and automatically adjusts
the proportional band to minimize overshoot or
undershoot.

22. Controller Tuning
•Theadjustmentofcontrolparameterstoachieve
satisfactorycontroliscalledTuning.
•Theprocessoftuningcanvaryfromtrial-and-error
toanelaborateoptimizationcalculation.
•Atypicalcriteriaforgoodcontrolisthatthe
responseofthesystemtoastepchangeinsetpoint
orloadshouldhaveminimumovershootandone
quarterdecayratio.

23. Selection of controller modes
•P (Proportional Control)
•PD (Proportional Derivative Control)
•PID (Proportional Integral Derivative Control)

24. Load response of typical control system
using various modes of control

25. Tuning Rules
•Ziegler –Nicholas Rules (Z -N)
•It’s a closed loop tuning system as the controller
remains in the loop as an active controller in
automatic mode.
1.After the process reaches a steady state at normal
level of operation, remove integral and derivative
modes of controller, leaving only proportional
control. On some PID controllers this requires that
the integral time (τ
1) be set to its maximum value
and derivative time (τ
D) to its min. value.

26. Tuning Rules
2. Select a value of proportional gain (K
c), disturb
the system and observe the transient response.
If the response decays select a higher value of K
c
and again observe the response of the system.
Continue increasing the gain in small steps until
the response first exhibits a sustained oscillation.
The value of gain and period of oscillation that
correspond to the sustained oscillation are the
ultimate gain (K
cu) and ultimate period (P
u).
3. From the values of (K
cu) & (P
u). Find controller
settings K
c, τ
1, τ
D

27. •Ku = 1/A ( Over all gain is A at the crossover frequency)
•Pu = 2pi/w
c0 time/cycle (w
c0 Crossover frequency)

28. Ziegler –Nichols Controller Settings
Type of controlG
c(s) K
c τ
1τ
D
Proportional (P)K
c 0.5K
u
Proportional-Integral
(PI)
K
c( 1 + 1/
τ
1s )
0.45K
uP
u
1.2
Proportional-Integral
–Derivative (PID)
K
c( 1 + 1/
τ
1s + τ
Ds )
0.6K
u P
u
2
P
u
8