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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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Remote Control of Valves RTU – The Glue
Remote Control of Valves • SCADA & Communications • Local PID Control • Types of Processes • Scale Factor Calculation • Valves
Supervisory Control And Data Acquisition = SCADA Host Location Field Location
Remote Terminal Units (the glue) • Local Valve Control • Custom logic for special cases • PID Control
Commonly Controlled Process • Run Switching • Flow Control • Pressure Regulation • Pressure Control • Emergency Shutdown • Line Break/Leak Detection • Leak Detection • ON/Off Control
PID – Type of Process OV • Fast Acting • Flow Control Time PV Time OV • Slow Acting • Pressure • Level Loop Time PV Time
PID in Simple Terms. • SETPOINT • PROCESS VARIABLE • OUTPUT
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
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
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?
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
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
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
PID Tuning – Valve Control Enter Calculated Scale Factor Adjust Gain for smooth increase with minimum overshoot Increase Reset for best performance
General Control Valve Selection • ANSI Class • Pipe Size • Material • Capacity • Rangeability • Functionality (control valve vs. regulators)
Specifics for Remote Stations • Pneumatic or MOV • Compressed air or Process Media • Communication and Control • Fail mode/safety
When Using Process Media • Pressure reduction to pneumatic actuators • Fisher 1301 or 1305 • Low Bleed Controllers • Fisher FIELDVUE™ DVC
Final Assembly • Instrument Air or reduced media Control Signal Feedback/Diagnostics