rtu – the glue
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Remote Automations Solutions
RTU – The GlueRemote Control of Valves
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Remote Control of Valves SCADA & Communications Local PID Control
• Types of Processes• Scale Factor Calculation
Valves
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Supervisory Control And Data Acquisition
= SCADA
Host Location Field Location
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Remote Terminal Units (the glue)
Local Valve Control• Custom logic for special cases• PID Control
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Commonly Controlled Process
Run Switching• Flow Control
Pressure Regulation• Pressure Control
Emergency Shutdown• Line Break/Leak Detection• Leak Detection• ON/Off Control
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PID – Type of Process
OV
Time
Time
PV
Fast Acting• Flow Control
Slow Acting• Pressure• Level Loop
OV
Time
Time
PV
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PID in Simple Terms.
SETPOINT PROCESS VARIABLE OUTPUT
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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
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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
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PID Scale Factor – How is it Calculated?
SF = -1 * ΔOV / ΔPVwhere:
SF = PID Scale factor, in units of OV / units of PVΔOV = Change in Output Value,
in Engineering Units
ΔPV = Change in Process Variable, in Engineering Units
How are ΔOV and ΔPV determined?
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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
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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 / ΔPVOV
Time
Time
PV
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Scale Factor –Level or Pressure ControlBest 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
Time
PV
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PID Tuning – Valve Control
Enter Calculated Scale Factor
Adjust Gain for smooth increase with minimum overshoot
Increase Reset for best performance
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Control Valve Selection for Remote Stations
16
Caltrol General Control Valve Selection
• ANSI Class • Pipe Size• Material• Capacity• Rangeability• Functionality (control
valve vs. regulators)
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• Pneumatic or MOV • Compressed air or Process Media• Communication and Control• Fail mode/safety
Specifics for Remote Stations
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• Pressure reduction to pneumatic actuators
• Fisher 1301 or 1305
Low Bleed Controllers• Fisher FIELDVUE™ DVC
When Using Process Media
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Instrument Air or reduced media
Final Assembly
Control SignalFeedback/Diagnostics
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Questions
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