rtu – the glue

Remote Automations Solutions

RTU – The GlueRemote Control of Valves


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

Time

Time

PV

Fast Acting• Flow Control

Slow Acting• Pressure• Level Loop

OV

Time

Time

PV


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 / 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?


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 / ΔPVOV

Time

Time

PV


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


PID Tuning – Valve Control

Enter Calculated Scale Factor

Adjust Gain for smooth increase with minimum overshoot

Increase Reset for best performance


Control Valve Selection for Remote Stations

16

Caltrol General Control Valve Selection

• ANSI Class • Pipe Size• Material• Capacity• Rangeability• Functionality (control

valve vs. regulators)


• Pneumatic or MOV • Compressed air or Process Media• Communication and Control• Fail mode/safety

Specifics for Remote Stations


• Pressure reduction to pneumatic actuators

• Fisher 1301 or 1305

Low Bleed Controllers• Fisher FIELDVUE™ DVC

When Using Process Media


Instrument Air or reduced media

Final Assembly

Control SignalFeedback/Diagnostics


Questions

rtu – the glue

Documents

output value

flow control looppid

loop time

pid calculation pid

derivative gain

integral gain

proportional gain

target value