Loop checking is a critical activity carried out during the commissioning phase of an industrial plant to verify that every instrument loop functions correctly from field device to control system and back. It ensures that signals are properly transmitted, displayed, controlled, and responded to as per the design intent. Loop checking confirms that instruments, wiring, logic, and control actions work together seamlessly before plant startup.
In simple terms, loop checking proves that the entire control loop—from sensor to final control element—is complete, accurate, and operational.
What is Loop Checking?
A Loop Checking is a systematic operational verification of all components in a control loop, including the field devices, controllers, and HMI elements, to ensure the proper functioning and integrity of the loop. The goal is to confirm that signals, responses, and control actions are accurate and aligned with engineering specifications.
Key aspects verified during a loop check include:
- Field Sensing Device Installation and Calibration
- The field sensing device is installed according to engineering specifications.
- It is calibrated accurately to measure process variables within the designed range.
- Communication and Signal Verification
- The device is properly connected to the DCS controller through the correct communication path (e.g., current loop, multi-device bus protocol, or other systems).
- The device generates and transmits an appropriate analog or digital signal in response to a simulated or actual process variable.
- Controller Logic and HMI Verification
- The DCS controller logic is configured to interpret the sensor signal correctly, with proper scaling in engineering units.
- The HMI displays the real-time response accurately, and alarm points are programmed and logged correctly as thresholds are reached.
- Control Functionality Check
- If control action is required, the signal from the sensing device is received by the control algorithm, generating a corrective output signal.
- Field control devices, such as control valves or variable speed drives, are installed and calibrated according to specifications.
- The control device responds appropriately to the controller’s output signal, producing the desired change in the process variable.
- Feedback from the control device, where applicable, is accurately transmitted to the DCS and displayed on the HMI.
In essence, a loop check ensures that all components in the control loop—from field sensing to controller logic to field control devices—function correctly and interact seamlessly, providing operators with accurate, reliable information and proper control of the process.
Procedure:
The loop checking procedure follows a structured approach to ensure accuracy and traceability.
A master list of instrumentation loops is first compiled, with each loop clearly identified by its unique tag number.
To improve efficiency during execution, loops are then organized into logical groups based on criteria such as physical location, system, equipment, availability, or process area. Among these methods, grouping loops by physical location generally provides the most efficient loop-checking workflow.
When loops are arranged geographically, the field team can move in a logical path from one device to the next nearby instrument, minimizing delays caused by accessing different floors, buildings, or plant sections. While loop availability constraints may sometimes limit this approach, planning availability to support locational grouping significantly improves productivity.
Each individual loop is assigned its own unique Loop Folder to ensure traceability and proper documentation.
Every Loop Folder is assembled with a complete Loop Sheet and/or supporting documentation, which typically includes the following details:
- Field device tag number
- Field device operating parameters
- Calibrated zero process value and its corresponding analog signal (commonly 4 mA)
- Calibrated full-scale process value and its corresponding analog signal (commonly 20 mA)
This structured documentation approach ensures accurate loop verification, efficient execution, and clear records for commissioning and future maintenance.
Action
Each Loop Folder contains detailed technical information required to perform accurate and traceable loop checking. This includes a complete list of field device accessories, such as transmitter three-valve manifolds, pneumatic valve latches, positioners, solenoids, and other associated components.
The Loop Folder also identifies the physical location of the field device, along with any specific manufacturer instructions or handling guidelines relevant to testing and operation.
Detailed field wiring information is provided, including terminal numbers, polarity, and cable identification. Wiring routing details—such as junction boxes, terminal strips, and intermediate connection points—are clearly documented to support troubleshooting and verification.
On the control system side, the Loop Folder includes DCS I/O marshalling details, covering cabinet numbers, terminal assignments, and polarity, as well as the associated I/O card or module information. Relevant DCS parameters, including measurement ranges and alarm setpoints, are also defined to ensure correct system configuration.
During loop checking, specific actions are carried out based on the instrument type. These actions may include injecting current or voltage signals, varying actual process conditions such as pressure or temperature, manually operating switches, or stroking control valves through their operating range. Each applied input is expected to produce a response that matches the approved design and control philosophy.
The Console Engineer observes the system response via the PLC or DCS, confirming that displayed values, alarms, and device actions align with expectations. Any deviation from the intended behavior is documented as a punch point and recorded for corrective action before loop acceptance.
DCS HMI Graphics assignments
At the start of the scheduled activity, the Console Engineer and Field Technician select the required Loop Folders from the Pending Loops File for testing during the defined time period.
They jointly review each Loop Folder in the selected group to ensure all documentation is complete, accurate, and consistent. This includes verifying tag numbers, signal ranges, alarm settings, and I/O assignments to confirm readiness for loop checking.
Once the review is complete, an agreed sequence for loop checking is established to maintain an efficient and systematic workflow.
The Console Engineer then prepares the loop tracking and record-keeping tools, organizing the loops in the agreed sequence. As part of this preparation, special attention is given to DCS HMI graphics verification. During loop checking, correct tag assignments are confirmed, live process values are validated for accuracy, alarm limits are tested, and trend displays and faceplate functions are checked. This step ensures operators receive accurate, real-time information during plant operation.
Meanwhile, the Field Technician inspects and prepares the necessary test equipment, gathers the required tools, and proceeds to the first field device scheduled for checking.
After reaching the instrument, the Field Technician communicates the device tag number to the Console Engineer. The Console Engineer then opens the corresponding I/O channel and displays the appropriate HMI graphic or faceplate to monitor loop behavior throughout the test.
Transmitters
The Field Technician begins by inspecting the transmitter installation and its associated process connections to confirm compliance with project specifications, approved drawings, and accepted installation practices.
As part of loop checking of transmitters, the Field Technician uses a signal simulator, manual “pump-up” method, or a communicator device to drive the transmitter output across its full operating range. This process verifies accurate measurement and reliable signal transmission. Particular attention is given to zero and span verification, ensuring that 4 mA corresponds to the minimum range value and 20 mA represents the maximum range value, with proper scaling confirmed in the PLC or DCS.
During signal simulation, the output is slowed at predefined alarm setpoints to allow the Console Engineer to verify correct alarm activation and system response. While older pneumatic and electromechanical instruments often required checks at 0%, 25%, 50%, 75%, and 100% to confirm linearity, this practice is now generally optional for modern smart transmitters and is performed based on project or client preference.
Once the signal impression is complete, any wiring, tubing, or process piping temporarily disturbed during testing is restored to its original and correct condition.
Finally, the Field Technician confirms that the process connections are properly aligned for normal operation. For example, when placing a differential pressure flow transmitter into service, the three-valve manifold is configured with the equalizing valve closed and the process isolation valves open. Pressure, temperature, flow, and level transmitters must demonstrate correct response throughout the entire range before the loop is accepted.
Control Loops
While the transmitter signal is being simulated or impressed, the Console Engineer opens the control algorithm faceplate and places the PID controller in Auto mode to verify proper closed-loop operation. During this step, the Engineer confirms that the process variable (PV), setpoint (SP), and output (OP) are correctly mapped and responding as designed.
The controller’s control action—direct or reverse—is checked to ensure the output changes appropriately in response to the input signal. The overall loop response is observed for stability, confirming that the controller is ready for tuning and does not exhibit abnormal oscillations or delays.
A control loop that passes these loop-checking steps demonstrates correct functionality, minimizes startup delays, and contributes to stable and reliable process operation during commissioning and normal plant running.
Control Valves
The Field Technician begins by inspecting the control valve installation to ensure it complies with project specifications, approved drawings, and accepted installation practices.
For pneumatic control valves, the instrument air supply is opened and checked to confirm correct pressure settings and acceptable air quality. For electrically actuated valves, the appropriate power breaker is energized to supply the actuator.
During control valve loop checking, the Console Engineer gradually varies the output signal from the PLC or DCS while the Field Technician observes the physical response of the valve in the field. The valve is stroked at multiple output percentages to confirm smooth and accurate movement. Both engineers jointly verify that the valve opens and closes as commanded and that the direction of travel corresponds correctly with the control signal and the intended control algorithm.
Special attention is given to the valve’s fail-safe position—fail open (FO), fail closed (FC), or fail last (FL)—to ensure it operates correctly under loss of signal or air/power conditions. Where valve position feedback is provided, the Console Engineer confirms that the feedback signal displayed on the control system accurately reflects the actual valve position reported by the Field Technician.
In addition, all valve accessories such as positioners, solenoid valves, air regulators, and limit switches are checked for proper operation. The valve and associated tubing and wiring are also inspected to confirm there is no air leakage, signal loss, or abnormal behavior.
Switches
The Field Technician begins by inspecting the instrument installation to ensure it complies with project specifications, approved drawings, and accepted installation practices. This verification confirms that the loop is mechanically and electrically ready for testing.
For loop checking, the Field Technician intentionally creates a change in the field device condition using signal simulation, manual actuation, or temporary conductor manipulation. In the case of switches, this involves physically operating the device to confirm reliable ON/OFF status indication. Typical examples include pressure switches, level switches, temperature switches, and limit switches.
From the control room, the Console Engineer observes the corresponding response on the PLC or DCS and verifies that the indicated status, value, or alarm accurately reflects the physical state applied at the field. Any change in switch position must be instantly and correctly displayed, with proper logic execution and alarm activation.
Throughout the loop check, the Console Engineer documents all results, including signal behavior, alarm set points, measurement ranges, control actions, and control valve responses where applicable.
If the loop operates as intended and no deficiencies are identified, the loop documentation is transferred to the Completed Loops File, and the loop is formally accepted and signed off by the client.
If any discrepancies are observed by either the Field Technician or the Console Engineer, each issue is recorded on a uniquely numbered deficiency report, commonly referred to as a “Kick Back.” This ensures traceability and accountability for all identified problems.
All Kick Back forms are entered into a centralized Master Kick Back Log, maintained by the Console Engineer, using sequential numbers for effective tracking and closure.
Once corrective actions have been implemented, the completed Kick Back record includes:
- Date of resolution
- Responsible party who performed the corrective work
- A clear and detailed description of the corrective actions taken
This structured approach ensures that all instrument loops—including switches—are fully verified, properly documented, and ready for safe and reliable commissioning.
Conclusion
Loop checking is an essential step in plant commissioning that ensures every instrument loop operates exactly as designed. From transmitters and switches to control valves and DCS graphics, loop checking validates the complete signal path and functional behavior. Performing thorough loop checking minimizes startup risks, prevents operational failures, and ensures safe, reliable plant performance.
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