Split Range Control

Split Range Control

Rationale

Understanding split range control is crucial for troubleshooting, configuring, and tuning processes employing this control strategy.

Outcome

Upon completion of this module, you will be able to describe the principles, applications, and operation of split range control loops.

Objectives

1. Describe the advantages and applications for split range control.

2. Draw a block diagram of a split range control system.

3. Describe the methods for tuning split range control systems.

Introduction

This module elucidates the various applications of split range control and its operational mechanisms, utilizing block diagrams for clarifying control issues and offering recommendations for configuring and tuning split range control strategies.

Objective One: Duplex Control

Definition

Duplex control, also known as split range control, is a control strategy where two independent control elements share a common input signal to operate separate final control elements (FCEs). Both elements influence the controlled variable. This scheme uses two manipulated variables to control one controlled variable.

Example: Heating and Cooling System

In a ventilating duct, a duplex control strategy can manage air temperature by controlling the flow of heating and cooling fluids. The air temperature is the controlled variable, while the heating and cooling fluid flows are the manipulated variables. If the inlet air temperature is too low and cooling is off, the temperature indicating controller (TIC) increases heating fluid flow. Conversely, if the air temperature is too high and heating is off, the TIC increases cooling fluid flow.

Efficiency Considerations

Duplex control should ideally avoid simultaneous heating and cooling for optimal efficiency. Simple systems may require manual switching between heating and cooling, while industrial systems operate automatically.

Duplex Control Strategies

Duplex control is implemented as either:

• Split range control strategy

• Full range control strategy

Split Range Control Strategy

Each FCE utilizes a portion of the control signal. Consider temperature control in a ventilation duct:

• The cooling fluid valve (TV-101A) is fully open at 0% control signal (4 mA) and fully closed at 50% control signal (12 mA). Signals above 12 mA have no effect.

• The heating fluid valve (TV-101B) is fully closed at 50% control signal (12 mA) or less and fully open at 100% control signal (20 mA). Signals below 12 mA have no effect.

• The TIC-101 needs to be a reverse acting controller where high air temperatures result in decreased control signal, calling for less heating or more cooling.

Valve configuration:

• 0% control signal: TV-101A is 100% open, TV-101B is 0% open.

• 50% control signal: TV-101A is 0% open, TV-101B is 0% open.

• 100% control signal: TV-101A is 0% open, TV-101B is 100% open.

Fail-safe operation results in full cooling: TV-101A (cooling) is fail open (FO), and TV-101B (heating) is fail closed (FC).

Full Range Control Strategy

Each manipulated variable uses the full range of the control signal. For example, controlling the temperature of a fluid using an inline heat exchanger:

• The bypass valve (TV-101A) is fully open at 4 mA and fully closed at 20 mA.

• The heating valve (TV-101B) is fully closed at 4 mA and fully open at 20 mA.

• TIC-101 needs to be a reverse acting controller. Higher fluid temperatures result in decreased control signals, calling for more bypass.

Valve Configuration:

• 0% control signal: TV-101A is 100% open, TV-101B is 0% open.

• 50% control signal: TV-101A is 50% open, TV-101B is 50% open.

• 100% control signal: TV-101A is 0% open, TV-101B is 100% open.

Fail-safe operation results in full bypass: TV-101A (bypass) is FO, and TV-101B (heating) is FC.

Duplex Control Advantages

Duplex control simplifies implementation by using a single controller instead of two. A standard control strategy requires two controllers and additional programming for automation.

Standard Control Strategy

Using separate controllers for heating and cooling requires the following:

• When heating is needed, the cooling controller (TIC-101) must be in manual with a 4 mA output to shut down the cooling system.

• The heating controller (TIC-102) must be in automatic and reverse-acting.

• When cooling is needed, the heating controller (TIC-102) must be in manual with a 4 mA output to shut down the heating system.

• The cooling controller (TIC-101) must be in automatic and direct-acting.

Automating the switching requires additional programming, instrumentation, and a temperature transmitter to measure air inlet temperature.

Duplex Control Applications

Duplex control applies to scenarios requiring two manipulated variables or controlling a single variable with two FCEs. Examples include:

• Temperature control

• Energy economizing

• Neutralization

• Dilution control

• Excess gas flaring

Temperature Control

Building heating systems and reactors use duplex control. Reactors employ cascade duplex control strategies.

Cascade Duplex Temperature Control for Batch Reactor

Exothermic reactions require precise temperature control. High temperatures cause rapid reactions and excessive heat, ruining the batch, while low temperatures prolong the process. The control strategy is as follows:

• Heating: TIC-102 (reverse-acting) increases its output, serving as the remote setpoint (RSP) for TIC-101, controlling the temperature of the circulating fluid.

• TIC-101 (direct-acting) increases its output, opening TV-101B to the required amount and keeping TV-101A closed.

• Cooling: As the reaction produces heat, TV-101A opens, and TV-101B closes.

• The system adjusts heating or cooling as needed.

Energy Economizing

In-line heat exchangers and fluid flow systems use duplex control to economize energy.

Energy Economizing Control Strategy

Minimize cooling and reheating by maximizing direct flow from Plant 1 to Plant 2 while maintaining level and flow control:

• Excess Flow: If Plant 1's flow exceeds Plant 2's demand, FIC-101's output is selected by LY-101, opening LV-101C to meet demand, and LV-101B remains closed. LIC-101 controls LV-101A to maintain the level, and excess flow goes to storage.

• Insufficient Flow: If Plant 1's flow is less than Plant 2's demand, LIC-101's output is selected by LY-101, opening LV-101C to keep the level constant, and FV-101A remains closed. FIC-101 controls FV-101B to maintain the required flow rate, using makeup from storage.

Neutralization

Neutralizing waste effluent requires a duplex split range control strategy using acidic and caustic solutions to maintain a pH of 7.

Neutralization of Waste Water

The control system uses a duplex split range control strategy to bring the waste effluent to a pH of approximately five in the first stage. The second stage uses a single loop control strategy to finish the neutralization by controlling the pH at seven.

• Low pH: AIC-101 throttles AV-101B to increase pH by controlling caustic flow, while AV-101A remains closed.

• High pH: AIC-101 throttles AV-101A to decrease pH by controlling acid flow, while AV-101B remains closed.

• The second stage provides finer control with a smaller valve.

Dilution Control

Paper pulp dilution employs a duplex full range control strategy using two valves for consistency control.

Pulp Dilution Control

One valve makes coarse adjustments, and a smaller valve makes fine adjustments. The consistency controller's (NIC-101A) output directly affects NV-101A and serves as the process variable (PV) for valve controller NIC-101B. The setpoint (SP) for NIC-101B, is the optimal valve position for NV-101A, determined by production rate.

• Change in Pulp Consistency: NIC-101A changes its output to restore the pulp consistency to its setpoint, establishing a new temporary steady-state output.

• NIC-101B, tuned to react slowly, gradually adjusts the coarse control valve NV-101B. NIC-101A adjusts accordingly.

• At steady-state, the output of NIC-101A matches the setpoint of NIC-101B.

Excess Gas Flaring

Safe burning of gas using a flare stack.

Gas Flaring

Pressure relief valves and emergency safety valves automatically direct gas to the flare stack during emergencies.

During normal operations, PV-101A controls gas pressure, and PV-101B remains closed. During emergencies or excess gas production, PV-101B controls gas pressure, and PV-101A is fully open.

Objective Two: Duplex Control Block Diagram

Block Diagram Purpose

Block diagrams model control loop elements using transfer functions, providing insight into process variable reactions to controller output changes.

Static Gains

Using static gains models the steady-state response of the control loop and helps to optimize process control.

Duplex Split Range Control Block Diagram

The block diagram shows that each FCE utilizes a portion of the controller's output signal.

Temperature Duplex Split Range Control Strategy

Controller output (CO) affects TV-101A between 0% and 50% and TV-101B between 50% and 100%.

Block Diagram Variables

• $KVA$: Static gain of control valve TV-101A

• $KVB$: Static gain of control valve TV-101B

• $KNA$: Static gain of the cooling process

• $KNB$: Static gain of the heating process

• $KT$: Static gain of the transmitter TT-101

Plant Gain Equation

$Kp = \frac{\Delta PV}{\Delta CO}$

Split Range Equations

• CO between 0% and 50%: $\Delta PV = (\Delta CO)(KVA)(KNA)(KT)$

• CO between 50% and 100%: $\Delta PV = (\Delta CO)(KVB)(KNB)(KT)$

Solving for the static gain of the plant:

• CO between 0% and 50%: $KPA = (KVA)(KNA)(KT)$

• CO between 50% and 100%: $KPB = (KVB)(KNB)(KT)$

Implications of Gain Differences

If $KVA = KVB$, plant static gains are similar, and the process reacts similarly. If $KVA \neq KVB$ or $KNA \neq KNB$, the plant static gains differ, causing different reactions. If the gains are not constant, the plant is nonlinear.

Duplex Full Range Control Block Diagram

Temperature Duplex Full Range Control Strategy

Controller output affects both TV-101A and TV-101B simultaneously.

Block diagram variables:

• $Kv$: Static gain of both control valves

• $KN$: Static gain of the heat exchanger process

• $KT$: Static gain of the transmitter TT-101

Heat Exchanger Control

Control valves change the flow rate through the heat exchanger. A step in the controller's output changes the flow of fluid A through the heat exchanger, but not the total flow of fluid A. If the flow through the heat exchanger increases, then the bypass flow decreases by the same amount, which keeps the total flow the same.

Process Variable Change

$\Delta PV = (\Delta CO)(Kv)(KN)(KT)$

Plant Static Gain

$Kp = (Kv)(KN)(KT)$

Nonlinear static gains can cause instability or slow response at different operating points.

Consistency Duplex Full Range Control Strategy

• (NIC-101A) output directly affects the operation of the control valve NV-101A and indirectly affects the control valve NV-101B.

Block diagram variables:

• $KVA$: Static gain of control valve NV-101A

• $KVB$: Static gain of control valve NV-101B

• $KNA$: Static gain of the consistency process due to NV-101A

• $KNB$: Static gain of the consistency process due to NV-101B

• $KT$: Static gain of the transmitter TT-101

Tuning Considerations

NIC-101B controller needs to be in manual when you tune the NIC-101A.

Objective Three: Tuning Split Range Control Systems

Tuning Complexity

Tuning ranges from simple to complex, with nonlinear systems and systems using multiple controllers requiring extra attention.

Split Range System Transfer Functions

Split range systems have two transfer functions, each describing the open loop response for the lower (0% to 50%) and higher (50% to 100%) control ranges.

Neutralization Process Tuning

Plant transfer function with CO between 0% to 50% is $GPB$, while with CO between 50% to 100% is $GPA$.

Equations for transfer functions are as follows:

$GPA = \frac{KPAe^{-TDA s}}{1 + \tau_A s}$

$GPB = \frac{KPBe^{-TDB s}}{1 + \tau_B s}$

$KPA = (KVA)(KNA)(KT)$

$KPB = (KVB)(KNB)(KT)$

Nonlinearities are addressed with a characterizer within the controller to linearize both control loops (0% to 50% and 50% to 100%).

Tuning Methods

• Similar Transfer Functions: If static gain ($KP$), dead time ($TD$), and time constant ($\tau_1$) are similar, use standard linear control loop tuning methods like Ziegler Nichols.

• Different Transfer Functions: If parameters vary by more than 20%, use adaptive gain control or detuning.

Detuning involves tuning the fastest acting loop and accepting sluggish response from the slower loop, or use an adaptive gain control strategy.

Adaptive Gain Control

Adaptive gain control switches tuning parameters when CO crosses 50%.

Dead Zone Prevention

To prevent dead zone cycling, calibrate control valves to overlap (e.g., one valve operates from 0% to 53%, the other from 47% to 100%).

Full Range Control Systems

Duplex full range control systems have only one transfer function.

Tune this type of control system the same way you would tune a standard feedback control loop.

Consistency Control Systems

Tune the controller that directly affects the controlled variable first, while the other controller is in manual. For this application, use the Ziegler Nichols quarter amplitude decay tuning criteria, as it is appropriate because you want the response to be fast acting.

Tune the controller that indirectly affects the controlled variable secondly with the other controller in automatic. For this application, use the Lamda tuning criteria (non-oscillatory), as it is appropriate because you want the response to be slow acting.

Self-Test

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Self-Test Answers

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