How to Read a DCS Cascade Control Loop Diagram: A Complete Guide with Example

What is a Loop Diagram?

A loop diagram is an essential document in industrial process control that graphically shows the wiring, signal flow, instrumentation, and control system hardware connected to a given process control loop. Whereas a process flow diagram or P&ID offers a general process perspective, a loop diagram focusing into one control loop shows how it is physically implemented from sensor to controller to actuator.

Commissioning, field calibration, system testing, and maintenance all use these diagrams extensively. By carefully studying a loop diagram, an engineer can understand the exact role of each instrument, its signal range, wiring path, and how it communicates with the distributed control system (DCS).

When the loop includes two interlinked controllers, such as in a cascade control strategy, the diagram provides insight not just into the wiring but also into the hierarchy and logic of control.
Unlike P&IDs or process flow diagrams, which provide high-level functional overviews, loop diagrams go deeper into:

• Field wiring and terminal blocks
• Analog and digital signal paths
• Device calibration and tag numbers
• Input/output (I/O) assignments in the Distributed Control System (DCS)
• Signal ranges (e.g., 4-20 mA, 3-15 psi)
• Final control element actuation

What is Cascade Control?

A cascade control system is one in which the output of one controller serves as the setpoint for another. Often a level or temperature loop, the slower dynamic loop serves as the master or main controller. The slave or secondary controller is the loop having faster dynamics, say flow or pressure.

In essence, the master loop monitors the main process variable (such as tank level), and when a deviation occurs, it adjusts the setpoint of the slave loop (such as flow rate) to bring the main variable back to setpoint. This structure improves control performance by reacting more quickly to disturbances that affect the secondary variable.

Cascade control is commonly used where the primary process variable changes slowly, and the secondary variable responds faster. A typical example is tank level control using inflow regulation via flow control.
Cascade loops are commonly used in applications such as:

• Maintaining tank level by adjusting inflow (via flow control)
• Controlling reactor temperature by adjusting steam flow
• Managing pressure via control of feed or exhaust gas

Purpose of a DCS Cascade Loop Diagram

While cascade logic is configured inside the DCS, the loop diagram shows how all elements involved in the control loop are connected physically. It covers signal ranges (such 4-20 mA or 3-15 psi), input/output assignments, terminal numbers, cable pathways, power supply, junction boxes, and specifics on the type and tag of every instrument.

This makes it an invaluable source during:

• Instrument loop testing and cold loop checks
• Hot loop and logic verification
• isolation of field faults
• channel allocation validation for DCS I/O
• Instrument calibration and documentation reviews

Example System: Fresh Feed Flow Control to a Process Unit

Our working example will be the above depicted loop diagram “Fresh Feed Flow Control to Unit 3”.

This is a cascade control loop designed to maintain a target level in a tank or vessel by manipulating the fresh feed flow rate.

The loop consists of two process variables:

• Tank Level (controlled by the master loop)
• Fresh Feed Flow (controlled by the slave loop)

The process instruments and tags used in the loop are as follows:

• LT-36: Level Transmitter
• FIC-36: Level Controller (Primary controller)
• FT-69: Flow Transmitter
• FIC-69: Flow Controller (Secondary controller)
• FY-69: I/P Converter (current-to-pressure)
• FCV-69: Flow Control Valve (pneumatic)

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Process Description: Control Strategy and Sequence

This system operates as follows:

1. The level in a downstream tank is measured by LT-36, which sends a 4-20 mA signal to the DCS.
2. In the DCS, FIC-36 compares the measured level to a predefined setpoint and outputs a flow setpoint based on the required correction.
3. This flow setpoint is sent internally in the DCS to FIC-69.
4. FIC-69 compares the setpoint to the actual flow (from FT-69) and sends a 4-20 mA control signal to FY-69.
5. To operate FCV-69, FY-69 turns the current signal into 3-15 psi pneumatic pressure.
6. FCV-69 changes the fresh feed flow rate to put the tank level within the required range.

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Detailed Breakdown of Each Component in the Diagram

Level Transmitter (LT-36)

• Function: Measures the liquid level in the tank.
• Signal Output: Based on calibration, signal output ranges from 4-20 mA, matching 0-10 inches of water column.
• Wiring Path: From LT-36, the signal is routed first to a junction box (JB-5), then via terminal block TB-40, and lastly to analog input channel 07 on Rack 01, Card 01 of the DCS.

For the level controller FIC-36 this signal serves as the process variable.

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Level Controller (FIC-36)

• Function: Receives the tank level signal then matches the setpoint.
• Output: Rather than directly controlling a valve, it sends the slave controller FIC-69 a new setpoint.
• Signal Type: Internal DCS communication. This is a software connection; no wiring is shown in the loop diagram.

FIC-36 dynamically adjusts the flow setpoint based on the tank’s liquid level. If the level falls, the controller increases the flow setpoint, and vice versa.
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Flow Transmitter (FT-69)

• Function: Designed to track fresh feed flow rate.
• Sensing Method: Differential pressure measuring with an integrated orifice plate uses a sensing method.
• Signal Output: 4 to 20 mA signal output, scaled to show 0 to 3.3 gallons per minute.
• Wiring Path: Signal is routed to analog input channel 07 on Rack 01, Card 03 in the DCS from junction box JB-10, terminal block TB-41.

This becomes the process variable for the flow controller FIC-69.

Flow Controller (FIC-69)

• Function: Receives the measured flow from FT-69 and the setpoint from FIC-36.
• Output: A 4-20 mA signal is sent to the I/P converter FY-69.
• Wiring Path: Signal is transmitted via terminal block TB-82 to FY-69. The output is from channel 08 on Rack 01, Card 01 of the DCS analog output module.

FIC-69 performs the closed-loop control of the final element based on flow rate deviations.

I/P Converter (FY-69)

• Function: Converts the DCS 4-20 mA signal to a pneumatic pressure signal.
• Output: 3-15 psi
• Pneumatic Output:  Linked straight to FCV-69’s actuator.

The control valve is pneumatically activated and cannot directly accept electrical data, hence this link is absolutely necessary.

Control Valve (FCV-69)

• Function: Regulates the flow of fresh feed into the unit.
• Type: Pneumatic diaphragm valve, air-to-open (ATO).
• Response: Fully closed at 3 psi, fully open at 15 psi.

Its position is continuously adjusted by the pneumatic pressure received from FY-69 to match the flow setpoint.

Understanding Signal Flow and Terminal Points

All field devices send their signals through specific junction boxes and terminal blocks. For instance:

• JB-5 and TB-40 link LT-36.
• FT-69 connects via TB-41 and JB-10.
• FY-69 gets output via TB-82.
• Shielding and grounding are managed at specific terminal points.

Managed at particular terminal points are shielding and grounding.

Red (RD), black (BK), blue (BL), and white (WT) wire color codes also help the technician find proper connections during troubleshooting.

To guarantee clarity in loop setup in the control system, every analog channel is shown together with the matching rack, card, and channel number in the DCS cabinet.

Importance of the Cascade Relationship

In this example, the primary loop (level control) influences the secondary loop (flow control). The controller FIC-36 does not operate the valve directly; instead, it intelligently adjusts the setpoint of FIC-69 based on actual level conditions. This achieves two goals:

1. Faster correction of disturbances affecting the flow.
2. More stable control of the tank level, especially when inflow disturbances are present.

The inner loop (flow) has faster dynamics and can respond quickly. The outer loop (level) has slower dynamics, and its effect is more stable over time.

Summary of Diagram Insights

From this single diagram, an engineer can identify the following:

• Signal types used: analog 4-20 mA and pneumatic 3-15 psi.
• Input and output channel assignments in the DCS.
• Loop direction: from transmitter to controller to final control element.
• Instrument calibration ranges.
• Manufacturer and model details.
• Proper wiring and cable routing to junction boxes and terminal blocks.
• Logical relationships between master and slave loops.

Refer the below link for Complete Guide to Interpreting Instrument and Electrical Drawings for Engineers

Complete Guide to Interpreting Instrument and Electrical Drawings for Engineers

Internal DCS Communication: AI, AO, and Function Block Flow

Beyond field wiring, it is important to understand how the DCS internally handles signals between these components.

Analog Input (AI) Function Blocks

Each analog input (from a transmitter) is received by an AI block, which:

• Converts the raw current signal (e.g., 4-20 mA) into engineering units using scaling
• Applies filtering and alarm limits
• Sends the result to the assigned controller block

In our example:

• AI_036 receives input from LT-36 and provides the level reading to FIC-36.
• AI_069 receives input from FT-69 and provides flow reading to FIC-69.

PID Control Function Blocks

Controllers (FIC-36 and FIC-69) are function blocks that:

• Compare process variable (PV) to setpoint (SP)
• Calculate output (OUT) using proportional-integral-derivative (PID) logic
• Send the output to the final control element (or to another controller in case of cascade)

Here:

• FIC-36 OUT becomes the setpoint (SP) for FIC-69
• This link is configured within the DCS and does not use any physical I/O
• This makes the cascade control responsive and modular.

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Analog Output (AO) Function Blocks

The controller output (such as from FIC-69) is passed to an AO block, which:

• Converts the controller output (typically 0-100%) into a 4-20 mA signal
• Sends it to the appropriate DCS output card (e.g., Channel 08)
• Drives actuators like FY-69 or other final control devices

Block and Signal Mapping Summary

Block	Source / Function	Destination / Result
AI_036	LT-36 signal (level)	FIC-36 PV input
FIC-36	Controls level	Outputs flow setpoint
FIC-36.OUT	Internal signal	Becomes SP for FIC-69
AI_069	FT-69 signal (flow)	FIC-69 PV input
FIC-69	Controls flow	Output goes to AO_069
AO_069	Sends 4–20 mA	FY-69 → FCV-69

This internal communication is what makes DCS-based cascade control reliable, scalable, and easily modifiable.

Signal Type and Wiring Overview

Component	Signal Type	Connection Description
LT-36	4-20 mA	Wired to AI_036 (Level PV)
FT-69	4-20 mA	Wired to AI_069 (Flow PV)
FIC-36	DCS logic	Outputs flow setpoint to FIC-69
FIC-69	DCS logic	Outputs controller signal to AO_069
AO_069	4-20 mA	Wired to FY-69 (Current-to-pressure converter)
FY-69	3-15 psi output	Pneumatically drives FCV-69
FCV-69	Pneumatic valve	Modulates fresh feed flow to maintain level

Summary of Internal DCS Communication Flow

Here is a logical summary of how data flows inside the DCS:

1. LT-36 (Field)— AI_036 —FIC-36 (Master PID)
2. FIC-36 OUT — internal link — FIC-69.SP (Slave PID)
3. FT-69 (Field) — AI_069 — FIC-69 (PV input)
4. FIC-69 OUT — AO_069 — FY-69 (Field) — FCV-69

This flow highlights that:

• Analog Input modules only provide data to the system.
• Controllers process this data and generate setpoints and outputs.
• Analog Output modules push data from the DCS to the field actuators.
• The cascade link between FIC-36 and FIC-69 happens internally in software, without occupying I/O channels.

Engineering Configuration Example (Typical Parameters)

Function Block	Parameter	Signal Source / Target
AI_036	PV	LT-36 (level transmitter)
FIC-36	PV	From AI_036
FIC-36	OUT	Connected to FIC-69 SP
AI_069	PV	FT-69 (flow transmitter)
FIC-69	PV	From AI_069
FIC-69	OUT	Connected to AO_069
AO_069	Output Channel	FY-69 (current-to-pressure)

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Why is it Important in real operation to comprehend the cascade loop diagram?

Understanding this internal flow is vital for:

• DCS programmers setting the control plan
• Teams verifying logical implementation in commissioning
• Tracing control problems for maintenance technicians
• Operators in control rooms reading the control block behavior in unusual circumstances

It allows quick troubleshoot as well. An engineer can examine, for instance, whether the flow is not rising even with a low level.

• Whether FIC-36 is raising production.
• Whether FIC-69 gets this setpoint
• Whether FIC-69 is changing its output to suit
• Whether the AO block produces the right output for FY-69

Reading a DCS loop diagram calls for both knowledge of process control ideas and close attention to physical wiring details, particularly in a  cascade control loop.The example discussed demonstrates how a properly designed cascade loop improves performance by delegating fast, responsive control to the inner loop while reserving high-level supervision to the master loop.

This integration of field instrumentation, control logic, and wiring architecture is what allows modern process systems to operate safely, efficiently, and automatically.

Engineers engaged in loop inspection, commissioning, maintenance, or control logic creation need to be at ease reading these diagrams to guarantee all components run as expected. Developing this ability not only helps to assist effective project implementation but also provides the basis for more complex automation tasks including ratio control, feedforward, or advanced regulatory techniques.

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FAQ on  cascade control

What is a cascade control loop?

A cascade control system links two feedback loops in such way that the output of the main controller sets the setpoint for the secondary controller to increase response and stability.

What are the two elements of cascade control?

To improve control precision and dynamic reaction, cascade control combines a key element the main variable that is temperature with a secondary element supporting variable that is flow.

What are the principles of cascade system?

Several boilers or controllers in a cascade system are under shared control logic. Every unit runs on its own exhaust system; the controller turns on units depending on real-time need, therefore preserving great efficiency.

What is the difference between cascade control and PID?

PID control control with rapid disruptions while managing one loop at a time. More robust cascade control makes use of two PIDs one to manage slow dynamics and the other for quick reactions.
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Why is cascade control faster?

Because the inner loop works immediately on the faster variable, therefore correcting disturbances before they affect the slower, outer loop, cascade control responds faster to process disruptions.