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Control Valves

Control Valves. Purpose Terminologies Construction Selection criteria Sizing calculation Issues on control valve. Cheer up. It is not as difficult as you thought!. Purpose

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Control Valves

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  1. Control Valves • Purpose • Terminologies • Construction • Selection criteria • Sizing calculation • Issues on control valve Cheer up. It is not as difficult as you thought!

  2. Purpose to regulate flow of any phase (liquid, gas, fluid and mixed phases) that could pass through it. At the moment, solid phase (such as sand, flour, sugar) is not possible. Chemical process applications >80% of final control element found in chem. proc. ind. Symbols Introduction =

  3. Construction Actuator Body Plug Seat

  4. Selection Criteria Valve action: ATO vs. ATC  Air-to-closed (ATC) or Fail open (FO). Use to allow flow of fluid during loss of pneumatic air supply. This action may save overheating of temperature related processes such as catalyst heating or heating of polimerization process.  Air-to-open (ATO) or Fail closed (FC). Use to prevent flow when no pneumatic air supply. This action may prevent hazardous chemical or flammable fluid from endangering lives. compare compare compare compare

  5. Selection Criteria Valve action: ATO vs. ATC The selection of control valve action would depend upon safety of personnel, action time and process hazard upon loss of pneumatic signal. Prevent pressure build-up Preventing reaction upstream. Prevent material from over-heating. (Smith & Corripio, 3rd. Ed., p.158)

  6. P in psi Selection Criteria Capacity and sizing Fluid S.G. Flowrate In gpm Example Calculate the Cv of a control valve when a 12.5 gpm fluid having a SG of 0.8 pass through it. The pressure drop across the control valve is 14 psi.

  7. Selection Criteria Valve characteristics: Linear, equal percentage and quick opening.

  8. Example 8 gpm 6 gpm 4 gpm 2 gpm 0 gpm Selection Criteria Valve characteristics: Linear, equal percentage and quick opening. What will be the flowrate when the indicator shows 0.75? Quick opening Flow coef. = 0.96 Flowrate = 0.96 x 8 = 7.7 gpm Linear Flow coef. = 0.75 Flowrate = 0.75 x 8 = 6 gpm Equal percentage Flow coef. = 0.3 Flowrate = 0.3 x 8 = 2.4 gpm

  9. Flow condition Flowrate, gpm Pressure drop, psi Valve lift, % 10 80 120 Maximum 15 50 42 Normal 20 20 13 Minimum Maximum 120 10 80 Normal 42 15 50 Maximum 13 20 20 Selection Criteria Control valve size selection Select an equal percentage control valve for the following condition.   Cv actual Flow condition Flowrate, gpm Pressure drop, psi Valve lift, % Cv calculated 37.9 37.3 10.8 10.8 2.9 2.93 Contiunue on next page for valve rangeability calculation…

  10. Selection Criteria Control valve rangeablity Check for valve rangeability Small difference. Selection 2” okay to use.

  11. Issues on Control Valves • What make a control valve becomes linear, equal percentage or quick opening control valve? • Why flashing and cavitation are serious problem in control valve? • What causing flashing and cavitation in a control valve? • What is the current solution to flashing and cavitation in control valve?

  12. What make a control valve becomes linear, equal percentage or quick opening control valve? The amount of flow produced with respect to steady increment in stem travel.

  13. Why flashing and cavitation are serious problem in control valve? Erosion

  14. What causing flashing and cavitation in a control valve? Hear sound of cavitation and flashing.

  15. What is the current solution to flashing and cavitation in control valve? Read p.135 of Fisher Control Valve Handbook

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