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Lecture 3 Control valves

Lecture 3 Control valves. Elements of the final control. Automatic Valve actuator. I/P transducer. Valve body. Current Signal 4-20 ma. Flapper. Nozzle. Back pressure. Pneumatic Signal 3-15 psi. Diaphragm control valves. Globe valve. Rotary valve. Rotary valve. Globe valve.

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Lecture 3 Control valves

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  1. Lecture 3 Control valves

  2. Elements of the final control Automatic Valve actuator I/P transducer Valve body

  3. Current Signal 4-20 ma Flapper Nozzle Back pressure Pneumatic Signal 3-15 psi

  4. Diaphragm control valves

  5. Globe valve

  6. Rotary valve

  7. Rotary valve

  8. Globe valve Most common control valve style Can be single- or double- seated

  9. Single-seated valves • Usually are employed when • Tight shut-off is required, • In sizes of 1 inch or smaller where unbalance forces acting on the valve stem is unimportant. • Usually have a top guided construction

  10. Double-seated valves • Usually is top and bottom guided. • Practical leakage approaches 0.5% of the rated CV. • Advantage lies in reduction of required actuator forces. • Have upper and lower ports of different diameters---allow to withdraw smaller plug through the larger port.

  11. Angle valve • Single seated valves with special body configuration to suit specific piping or flow measurements. • May be used in case where the piping layout does not allow installing a globe valve.

  12. Three way valves • A design extension of a typical double-seated valve. • Can be used for diverting service and for mixing service.

  13. Actuators for control valves: • Pneumatically operated diaphragm actuators; • Piston actuators; • Electro-hydraulic actuators; • Electro-mechanical actuators; More than 90% in use are pneumatically operated piston or diaphragm type

  14. Safety consideration • Air-to-open (AO) Failure close • Air-to-close (AC) Failure open

  15. Loss

  16. Defined at maximum Valve opening Related to valve inherent characteristics

  17. Over sized plug to provide Additional Cv Special seat machined Into the body

  18. Linear valves: A (X) =X= x/xo Equal percentage valves: A (X) = e -k(x/xo-1) = a-(X-1) Rangeability

  19. Distortion after installation

  20. Valve hysteresis

  21. Valve positioner

  22. What accomplishment a positioner can have? • Provide precise positioning of the valve • Provide adequate power on high-pressure applications • Increase control valve speed of response • Reverse valve action • Provide split range operation

  23. Typical positioner performance • Pneumatic signal ranges: 3-9, 3-15, 9-15, 3-27, 6-30 psig • Air supply pressure: 20 to 100 psig • Repeatability: within 0.1% of stroke • Hysteresis: within 0.3% of stroke • Linearity: 0.5% of stroke

  24. Split range control valves

  25. Control valve sizing Given expected pressure conditions, select throttling control valve to pass the required flow rate. It is a key step In ensuring that the process can be properly Controlled. Basic sizing practices have been standardized Upon (e.g., ISA S75.01) and are implemented as PC-based program by manufactures.

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