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RTU – The Glue

Remote Control of Valves. RTU – The Glue. Remote Control of Valves. SCADA & Communications Local PID Control Types of Processes Scale Factor Calculation Valves. S upervisory C ontrol A nd D ata A cquisition. = SCADA. Host Location. Field Location.

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RTU – The Glue

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  1. Remote Control of Valves RTU – The Glue

  2. Remote Control of Valves • SCADA & Communications • Local PID Control • Types of Processes • Scale Factor Calculation • Valves

  3. Supervisory Control And Data Acquisition = SCADA Host Location Field Location

  4. Remote Terminal Units (the glue) • Local Valve Control • Custom logic for special cases • PID Control

  5. Commonly Controlled Process • Run Switching • Flow Control • Pressure Regulation • Pressure Control • Emergency Shutdown • Line Break/Leak Detection • Leak Detection • ON/Off Control

  6. PID – Type of Process OV • Fast Acting • Flow Control Time PV Time OV • Slow Acting • Pressure • Level Loop Time PV Time

  7. PID in Simple Terms. • SETPOINT • PROCESS VARIABLE • OUTPUT

  8. What is the P, I, & D • P is Proportional Gain or Just Gain • I is integral Gain or Reset • D is Derivative Gain or Rate Remote Automation Solutions uses a Scale Factor (SF) that establishes the relationship between the Output and the Process Variable. SF * P represents total Gain Loop Time, the time between start of each PID Calculation

  9. PID Scale Factor – Why is it so important? • It is the factor in correctly determining the direction of the control action • Major factor in correctly calculating the magnitude of the control action • Reduces the possible range of the proportional gain from -∞ – ∞ to 0.1 – 2.0 • If the PID scale factor is not close, tuning a PID control loop becomes a MAJOR trial and error exercise

  10. PID Scale Factor – How is it Calculated? SF = -1 * ΔOV / ΔPV where: SF = PID Scale factor, in units of OV / units of PV ΔOV = Change in Output Value, inEngineering Units ΔPV = Change in Process Variable, in Engineering Units How are ΔOV and ΔPV determined?

  11. How are ΔOV and ΔPV determined?? Good – Have the instrument folks tell you the range of the PV that corresponds to the full range of the OV. Be careful about the units and direction. Better – Have operators tell you from their experience what ΔPV is for a given ΔOV. Be careful about the units and direction. Best – With loop in manual mode, adjust the Output Value and observe the value of the Process Variable. ΔOV = OV2 – OV1, ΔPV = PV2 – PV1

  12. Scale Factor – Lets Determine It for a Flow Control Loop Best Way – With loop in manual mode, adjust the Output Value and observe the value of the Process Variable. ΔOV = OV2 – OV1, ΔPV = PV2 – PV1 SF = -1 * ΔOV / ΔPV OV Time PV Time

  13. Scale Factor –Level or Pressure Control Best Way – With loop in manual mode, adjust the Output Value and observe the rate of change of the Process Variable. ΔOV = OV2 – OV1 ΔPV = (PV2 – PV1) / (T2 – T1) * Loop Period where: (T2 – T1) = Time difference between reading PV2 and PV1, in seconds. Loop Period = Time between loop execution, in seconds. SF = -1 * ΔOV / ΔPV OV Time PV Time

  14. PID Tuning – Valve Control Enter Calculated Scale Factor Adjust Gain for smooth increase with minimum overshoot Increase Reset for best performance

  15. Control Valve Selection for Remote Stations

  16. General Control Valve Selection • ANSI Class • Pipe Size • Material • Capacity • Rangeability • Functionality (control valve vs. regulators)

  17. Specifics for Remote Stations • Pneumatic or MOV • Compressed air or Process Media • Communication and Control • Fail mode/safety

  18. When Using Process Media • Pressure reduction to pneumatic actuators • Fisher 1301 or 1305 • Low Bleed Controllers • Fisher FIELDVUE™ DVC

  19. Final Assembly • Instrument Air or reduced media Control Signal Feedback/Diagnostics

  20. Questions

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