Control Valve : The Final Control Element. Nimish Shah

Transcription

1 Control Valve : The Final Control Element Nimish Shah

2 VALVES

3 How to select Control Valve??? selection parameters for the valve : size, material of construction, pressure and temperature ratings, and End connections.

4 How to select Control Valve??? In addition, if the valve is for control purposes one must define additional parameters.

5 How to select Control Valve??? These parameters include : Method of operation, maximum and minimum flow capacity requirements, pressure drop during normal flowing conditions, pressure drop at shutoff, and Maximum and minimum inlet pressure at the valve.

6 How to select Control Valve??? Meeting these parameters happens through and by selecting body styles, material of construction, seats, packing, end connections, valve operators actuators and supports.

7 Cross-section of a Globe Valve

8 Control Valve Flow Characteristics The relationship between valve capacity and valve travel is known as the flow characteristic of the valve.

9 Control Valve Flow Characteristics There are main three types of Flow characteristics: Quick Opening Linear flow Equal Percentage

10 Control Valve Flow Characteristics

11 Control Valve Flow Characteristics Quick Opening- used for safety by-pass applications where quick opening is desired

12 Control Valve Flow Characteristics Equal Percentage- used for about 90% of control valve applications since it results in the most linear installed characteristics

13 Control Valve Flow Characteristics Linear- used when a relatively constant pressure drop is maintained across the valve

14 Control Valve Flow Characteristics

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16 Inherrent Flow Characteristics For Common Valve Trim Designs

17 Control Valve Characteristics Quick opening Most of the flow variation takes place in the lower third of the valve (stem) travel Not ideal for regulating flow Appropriate for relief valves and onoff control (safety; avoid overpressuring vessels)

18 Control Valve Characteristics Linear Produces flow proportional to valve position At 50% valve position, flow through valve is 50% of maximum flow

19 Control Valve Characteristics (cont.) Equal Percentage Equal increments in valve position result in equal relative or percentage increments in flow When valve position increases 1% going from 20% to 21%, flow increases by 1% from value at 20% When valve position increases 1% going from 60% to 61%, flow increases by 1% from value at 60% Percentage change is equal, but the flowrate at 60% is greater than the flowrate at 20%

20 What happened if power failed?

21 What happened if power failed? It is a power-operated device Well, what happens if the power is cut off? "fail-safe mode" it is the key to troubleshooting it. In most applications (about 80%), it is desirable for valves to fail closed. In other applications, you might want a valve to fail open or fail in place. Safety concerns and process requirements will mandate the fail mode of the valve.

22 Control Valve "Fail-Safe" Positions Cause of Fail-Safe Condition: Loss of Air Pressure

23 Control Valve "Fail-Safe" Positions Cause of Fail-Safe Condition: Loss of Air Pressure

24 Fail Safe Mode in Rotary Valve

25 How is works? When a valve is not sitting in its fail position, it is being told how and when to move by some external signal. If a Control Valve is observed in an unstable condition or appears to not be responding correctly to an input signal, remember that something is telling the valve to behave that way. Two Components: Actuator Positioner

26 Actuators The actuator receives the conditioned signal and changes it to some form of mechanical energy or motion. Typical devices used as actuators include solenoids, pneumatic valve positioners, AC and DC motors, stepper motors, hydraulic motors, and hydraulic pistons. Many control valves include a pneumatic valve positioner.

27 Actuators

28 Actuators

29 Types of Actuators Spring and Diaphragm Pneumatic Piston Electric Motor Electro hydraulic

30 Valve Actuator Selection Choose an air-to-open for applications for which it is desired to have the valve fail closed. Choose an air-to-close for applications for which it is desired to have the valve fail open.

31 Cross-section of a Globe Valve

32 Optional Equipment Valve positioner- a controller that adjusts the instrument air in order to maintain the stem position at the specified position. Positioners are almost always used on valves serviced by a DCS.

33 Photo of a Valve Positioner

34 Valve Positioners This device increase or decrease the air pressure operate the actuator until the valve stem reaches the position. generally mounted on the side or top of the actuator. Connected mechanically to the valve stem.

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36 Valve Positioners

37 Pneumatic Control Valves

38 Thank You