actuators process control process control

process control

1. UNIT II FINAL CONTROL ELEMENT

2. Actuators https://www.youtube.com/watch?v=LHn7O6PUaoY

3. Introduction An actuator is a mechanism that converts some type of energy into motion in order to do work (move a force over a distance) An actuator uses some type of energy to provide motion or to apply a force. The input energy of actuators can be hydraulic or pneumatic (e.g., pistons and valves), thermal (e.g., bimetallic switches or levers), and electric (e.g., motors). For example, an electric motor uses electrical energy to create a rotational movement or to turn on object, or to move an object. In short, an actuator converts some type of energy into motion Actuators include motors, gears, pumps, pistons, valves, and switches.

4. Types of Actuators Electric actuators Pneumatic actuators Hydraulic actuators Thermal actuators

5. Electric actuators Electric actuators use electricity or electrical energy to create motion. An electric motor is a type of an electric actuator. An electric motor is a transducer AND an actuator because it converts electrical energy to magnetic energy to mechanical energy or motion. Eg. . DC Motor, AC Motor, Stepper Motor

6. Pneumatic Actuators Pneumatic actuator converts pressurized air to mechanical energy or motion. The motion can be rotary or linear, depending on the type of actuator. Eg. . Control valve

7. Control valve Control valves are valves used to control conditions such as flow, pressure, temperature, and liquid level by fully or partially opening or closing in response to signals received from controllers that compare a "setpoint" to a "process variable" whose value is provided by sensors that monitor changes in such conditions. The opening or closing of control valves is usually done automatically by electrical, hydraulic or pneumatic actuators. https://www.youtube.com/watch?v=Vn298-ZXZfk

8. Cont …

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10. Valve Body

11. Bonnet

12. Trim

13. Trim Components

14. Parts of control valve

15. Control valve - classification

16. Types of Control Valves based on Action Control valves operated through pneumatic actuators can be either ( i ) air to open. (ii) air to close.

17. Types of Control Valves based on Number of plugs Control valves can also be characterized in terms of the number of plugs present, as ( i ) single-seated valve (ii) double-seated valve

18. Flow Characteristics It describes how the flow rate changes with the movement or lift of the stem. The shape of the plug primarily decides the flow characteristics The three most common types of flow characteristics are: Linear Equal percentage Quick opening

19. Control valve characteristics

20. Applications of quick opening, linear and equal percentage General applications of  of quick opening, linear and equal percentage valves are : i ) Quick opening valve: a) Frequent on-off service. b) Used for systems where ‘instant’ large flow is needed (safety or cooling water systems). ii) Linear valve: a) Liquid level and flow control loops. b) Used in systems where the pressure drop across the valve is expected to remain fairly constant. iii) Equal percentage valve (most commonly used in valves): a) Temperature and pressure control loops. b) Used in systems where large changes in pressure drop across the valve are expected

21. Classification of Control Valves

22. Cont..

23. Gate valve

24. Plug valve

25. Diaphragm valve

26. Ball valve

27. Globe valve

28. Butterfly valve

29. Valve sizing What is Valve Sizing? It is a procedure by which the dynamics of a process system are matched to the performance characteristics of a valve. It is used to  calculate  pressure drop, flow rate, or the flow coefficient . This is to provide a control valve that will best meet the needs of managing flow within that process system. To  size  the  valve  such that it operates between 20 to 80% open at maximum required flow rate. 

30. Control valve flow coefficient (C V ) The valve flow coefficient, C V is the flow rate of water in gallons per minute of water at 60 degrees F which will pass through a given flow restriction with a pressure drop of 1 psi. where

31. Basic requirements for effective valve sizing For the system: Pressure before and after the control valve, Δ P Flow rate, quantity and units, Q Process temperature with units, T Properties of the media, (viscous, fiber suspension, gaseous its vapor pressure, sometimes molecular weight) For the control valve: Flow capacity ( Cv ), inherent throttling curve, Kc (Cavitation Index), FL² (Critical Flow factor)

32. Valve positioner Pneumatic control valves can be equipped with a valve positioner’ It is a type of mechanical or digital feedback controller that senses the actual stem position, compares it to the desired position, and adjusts the air pressure to the valve accordingly. A  Control Valve Positioner  is a device used to increase or decrease pressure driving the actuator until the valve’s stem reaches a “POSITION” balanced to the output SIGNAL from the process variable instrument controller.

33. The input pressure from the controller to the  bellows  falls. The flapper moves away from the nozzle and the pressure after the orifice falls. The pressure to the diaphragm falls and the valve begins to close. The feed back arm moves up rotating the cam clockwise. This raises the beam increasing back pressure in the nozzle until equilibrium is again achieved. The change over cock allows the signal from the controller to be placed directly on the diaphragm

34. I TO P CONVERTER

35. CONT… Flapper Nozzle arrangement: It is type of pressure controlling device. It controls the input pressure by the moving the Flapper away or towards the nozzle, if we need high pressure, then nozzle will be closed by the Flapper and if we need less pressure, then nozzle will be opened by moving flapper away . If the current is more, then the power of magnet will get increased. The Flapper of the  Flapper-Nozzle  instrument is connected to Pivot so that it can move up and down and a magnetic material was attached to other end of flapper and it is kept near the electromagnet . As the magnet gets activated. the flapper moves towards the electromagnet and the nozzle gets closed to some extent. So the some part of 20 P.S.I supplied will escape through nozzle and remaining pressure will come as output. If the current signal is high, then power of the magnet will increase, then flapper will move closer to the nozzle, so less pressure will escape through nozzle and output pressure increases . For the input current of 4 – 20 mA we can get the output pressure of 3 – 15 P.S.I

36. P TO I CONVERTER

37. CONT… First Input pressure is supplied to the Flapper- Nozzle arrangement.  Then it will supplied through a pipe and that pressure is given as input to the bellows. These bellows are connected to the Core of LVDT. When pressure is applied to bellows, they will expand thus core displaces and the voltage is induced on the secondary coils of LVDT. As voltage is induced, current will flow through the coil. That current is proportional to the input pressure applied. Thus Pressure is converted into equivalent current. When compared with standard values we can covert 3-15 P.S.I of pressure can be converted to 4-20 mA of current

38. Cavitation and Flashing

39. P 1 <P V Bubbles formed at the downstream Flashing P 2 <P V Liquid flashes into vapour as it enters into downstream pipe and results in erosion

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41. CONT..

42. Integral wind-up : Integral windup is  the process of accumulating the integral component beyond the saturation limits of final control element . If the set point raises suddenly then there will be large error between the set-point and actual value This error causes the integral component to accumulate with the time period to bring back the actual value near to the set-point. As a result the controller output may exceeds the saturation limits of final control element. Anti-reset windup  speeds up the departure from the saturation limit by stopping the build up of the integral during saturation .