Step-by-Step Logic Check Procedure for Pre-Commissioning Activities in Process Plant

Pre-commissioning is a vital phase in plant commissioning that guarantees all systems, components, and control logics operate as per the design requirements before they begin use of process fluids. Logic checks which confirm safety, control, and interlock sequences specified in the Cause & Effect Matrix, Control Narrative, Piping and Instrumentation Diagrams (P&IDs), and the most recent Alarm and Trip Schedule are among the main responsibilities during this phase.

This article outlines the essential steps, checks, and best practices required to conduct comprehensive logic checks during plant pre-commissioning, with an emphasis on field verification, system response accuracy, and functional testing.

Step 1. Understand and Validate the Cause & Effect (C&E) Matrix

• Initially, it is recommended to do a thorough analysis of the Cause and Effect Matrix. Should errors, out-of-date data, or missing logic be found, record them right away.
• Make sure the most recent version of the C&E Matrix. DCS/ESD graphics often still represent older versions. Examine and match these with the revised matrix to identify mistakes connected to tag numbers, reset logic, and final effects.
• Keep a list of all Cause Tags and Effect Tags; before starting any logic check, get consent from pertinent discipline in-charges (Operations, Electrical, Instrumentation).

Begin by collecting and examining all current documentation work, which includes:

• Cause & Effect Matrix (C&E)
• Control Narrative
• Piping & Instrumentation Diagram (P&ID)
• Latest Alarm & Trip Schedule
• PLC and DCS Logic Diagrams
• SIS Functional Specification
  

Check for:

• Tag mismatch or outdated references
• Logic sequencing issues
• Reset conditions
• Undocumented bypass or override features
  

Before starting inspections, work with engineering, operations, and maintenance to address any inconsistencies.

Refer the below link to understand more about the Piping & Instrumentation Diagram 

Piping & Instrumentation Diagram (P&ID)

Step 2. Coordination and Safety Before Initiating Logic Checks

• Logic checks have to be done with the Field and Control Room working closely together.
• Designate an operations team member or field operator to be physically present close to the equipment under test.
• Actual field start-up is best for rotating equipment (e.g., pumps, compressors). Should it not be feasible, run logic by putting the equipment into Test Mode and specifically note this on the Punch List.
• When permission is granted, at least one or two real starts should be performed to confirm real-world logical functioning.

Step 3. Simulating Causes and Verifying Effects

• To find the proper simulation values for starting causes, consult the most recent Alarm and Trip Schedule.
• Before applying the cause that should activate control valves or SDVs (Shut Down Valves), make sure they are in suitable states (e.g., open). Verify they don’t run early and only react once the cause condition is fulfilled.
• Alarms and status changes should be verified in:
  • DCS Alarm Window
  • ESD Graphic Displays
  • Faceplates

Step 4. ESD System Logic Checks

Safety automation in process facilities is fundamentally based on the Emergency Shutdown (ESD) system.

ESD Logic Check Guidelines:

• Check that each beginning cause (e.g., high pressure, manual trip) causes the proper shutdown sequence.
• Verify that Control Valves and SDVs (Shutdown Valves) actuate to their fail-safe position as predicted.
• Check reset logic: ESD trips must not be reset on their own.. Make that Field Reset and Control Room Reset are functioning.
• Review SOV (Solenoid Valve) command output, valve position feedback (open/close), and alerts in the DCS faceplate, Alarm Window, and Graphics.
• For ESD trip of motors, check and confirm:
  • Trip feedback signal
  • Motor de-energization
  • Trip alarm indication in faceplate and alarm window

Document ESD trip signal simulation at the logic solver or relay interface in the punch list for re-verification if field testing is not feasible.

Emergency Block Valve Explained: What is an Emergency Block valve and How does it work

Step 5. SIS Logic Verification and Safety Function Testing

Defined in the risk assessment, the Safety Instrumented System (SIS) offers protective layers for process hazards and must fulfill the SIL (Safety Integrity Level).

Key SIS Logic Checkpoints:

• Validate 2oo3, 1oo2, or custom voting logic with various transmitter combinations. For a 2oo3 logic (A, B, C),
  • Simulate AB, BC, and CA combinations independently.
  • Verify that the shutdown mechanism only runs when needed votes are satisfied.
• According to SIL loop settings in the SIS logic, simulate out of range process values for e.g., high-high pressure or temperature.
• Check bypass overrides, trip limits, and interlock timers.
• Make sure the SIS flags diagnostics including signal loss and input disparity.
• Check final element actions (e.g., valve closing, pump trip) both in the field and using PLC diagnostics or test panels.

During testing, monitor inputs/outputs and safety instrument reactions using the SIS Engineering Station.

OIl and Gas Interview preparation: Safety Instrumented System(SIS) Interview Questions and Answers

Step 6. Physical Field Verification of Logic Effects

• Always check impacts physically in the field and connect them to C&E logic. Don’t assume DCS signals indicate completed logical verification.
• For SDVs, ensure:
  
  • The SOV command is initiated.
  • Open/Close feedback alarms are received.
  • Reset functionality (both from Control Room and Field) operates correctly.
• For motors, check:
  • Trip alarms show up in the Faceplate and Alarm Window.
  • Graphic depiction indicates proper status e.g., blinking for tripped.

Voting Logic in SIS: Voting Logic in Safety Instrumented System

Step 7. PLC Logic Checks for Process Control and Interlocks

Many plants operate batch logic, permissive checks, startup sequences, and process interlocks using PLC systems.

Important PLC Logic Verification Steps:

• Align PLC logic with the C&E matrix and Control Narrative.
• Only with appropriate permission and under restricted circumstances should forcing or simulation mode be used.
• Check ladder logic sequences, such as:
  • Start-up permissives
  • Alarm escalation
  • Interlock resets
• Check the behavior of the PID control loop:
  • Set a fixed Set Point
  • Vary the Measured Variable
  • Observe the controller output
    
• Make sure the process design matches the controller action (direct or reverse). Such as:
  • In direct acting, when PV rises above SP, the control valve should open more.
  • In reverse acting, when PV rises above SP, the valve should close more.
• Confirm analog signal scaling, deadband handling, and hysteresis logic.
  

Observe real-time logic changes using the PLC programming software or SCADA interface; troubleshoot as needed.

Step 8. Simulating Causes and Monitoring Effects

Refer to the Alarm and Trip Schedule for the appropriate analog values or digital states to simulate each cause.

Simulation Tips:

• Keep SDVs or control valves in known positions (e.g., open) before simulating causes.
• Monitor the real movement of valves and verify status feedback using:
  • Field inspection
  • Faceplate feedback
  • Alarm logs
• Check the logic path, power supply, or signal routing if a cause is simulated but the anticipated field action does not take place.
  

Always record incomplete inspections in the punch list and C&E Matrix sign-off sheet.

Step 9. Highlighting and Documentation During Checks

• Maintain a marked-up copy of the C&E Matrix for real-time tracking.
• Highlight just items that were field validated completely. Do not point out if just the ESD signal or alarm was verified. Rather, record it in the punch list for rechecking.
• Deviations such as wrong tag references, effect mismatches, or missing alarms have to be:
  • Recorded on the C&E sign-off page
  • Recorded in the Daily Progress Report (DPR)

You must know: Signals for Emergency Valve Shutdown in Critical Processes

Step 10. 2oo3 (Two out of Three) Voting Logic Verification

Fully validate the 2oo3 logic for safety-critical systems including HIPPS (High Integrity Pressure Protection Systems):

• Simulate every three voting combination: AB, BC, CA.
• Verify at least one combination’s actual field impact.
• For others, verify signal processing using the Engineering Station.
• Validate differential response and system activity by inducing pressure imbalance in transmitters.

Must try for: Designing 2 out of 3 Voting Logic in Control Systems: A Step-by-Step PLC Ladder Diagram Tutorial with Video

Step 11. Field Verification and Manpower Coordination

Before and during logic checks, make sure appropriate field coordination.

Have a qualified O&M field technician verify impacts physically.

• Assign a qualified O&M field technician verify impacts physically.
• Field verification is essential for:
  • Valve actuation
  • Motor trip
  • Hooter and beacon activation
  • Reset switch response
    
• Keep a properly marked-up copy of the C&E Matrix showing:
  • What was tested successfully in the field
  • What was simulated or pending
  • Any deviations or discrepancies

HIPPS System Functionality Overview: How does the HIPPS system work in the Oil and gas Industry?

Step 12. Handling Unavailable Equipment During Pre-Commissioning

• Should motors and other equipment be unavailable for testing:
  • Simulate causes and check only the ESD signal and alert in graphics/faceplate.
  • Note in C&E matrix that physical check is outstanding.
  • Include a punch point calling for post-availability re-verification.

Step 13. Control Narrative Validation

• Compare anticipated vs actual logical behaviour using the revised Control Narrative document.
• For instance, check that a control valve actually opens in the situation if it is meant to open to keep a process setpoint.
• Testing controller logic by:
  • Keep the Set Point constant.
  • Vary the Measured Variable and observe controller output.
  • Confirm the controller response as per Direct Acting or Reverse Acting settings.
• Cross-verify from faceplate, DCS graphics, and field instrument response.
  

Refer the below link to know more about Control Narrative 

Control Narrative

Step 14. Maintenance and Startup Override Checks

• After logical verification, make sure Override Functions are examined:
  • Maintenance Override: Confirms that logic is bypassed during scheduled maintenance.
  • Startup Override: Guarantees no false trips at system startup.
• These have to be obviously recorded as part of last logic testing.

Step 15. Fire & Gas (F&G) System Logic Checks

• F&G system simulations must come from the field, not technical tools.
• Example for a 2oo8 logic: randomly simulate two detectors of different types and verify:
  • Hooter activation
  • Zone-specific response
• Verify Tag Numbers and positions of detectors/manual call points using Layout Drawings.
• Check for:
  • Hooter response to gas detection and Gas Beacon (amber).
  • Fire Beacon (red) and Fire Hooter activation on manual call point or flame detection.
  • Gas and fire alarms have various sound tones.
• Check every alarm and indication in:
  • FGS Graphics
  • FGS Alarm Window
  • FGS Cause & Effect Matrix

Click here to learn: Designing 2 out of 4 Voting Logic in Control Systems: A Step-by-Step PLC Ladder Diagram Tutorial with Video

Step 16. Signal Integrity and Redundancy Checks

• Examine fail-safe setup and redundancy.
• Simulate out-of-range, disconnection-related signal failures and verify alarm/failure response.
• Check primary/secondary switching for redundant transmitters.

Step 17. Loopbacks and Simulation Verification

• In loopback mode, verify internal signal processing.
• Use loop test features in PLCs/DCS to test output without energizing field devices.
• Confirm simulation modes are deactivated after testing.

Step 18. Communication and Interface Checks

• Verify SCADA, DCS, SIS, and PLC communication.
• Review protocol health (Modbus, Profibus, Profinet, OPC).
• Check tag changes throughout several platforms and backup failover.

Step 19. Data Logging and Historian Checks

• Data Logging and Historian Inspections, Step 19
• Check that the historian is recording all vital data—alarms, travels, actions.
• Verify sequence of events (SOE) time stamp correctness.
• Ensure system-wide time synchronization.

Step 20. Final Sign-off and Mark-Up

• Update the Master Copy of the C&E Matrix with highlights for field-verified causes and effects.
• Effects or logic not tested should be clearly marked as pending and added to punch lists.
• Include notes regarding hysteresis behavior, reset mechanisms, and any temporary logic changes.
• Maintain real-time documentation of each test.
• Update and mark the master copy of C&E Matrix.
• Highlight only those logics verified in the field.
• Record any deviations in the DPR(Daily Progress Report) and C&E sign-off sheet.

Complete logic checks done during plant pre-commissioning ensure that all safety and control systems operate consistently before startup. The plant’s readiness is guaranteed by methodically testing PLC logic, ESD sequences, and SIS safety functions and validating all causes and effects with real field activities. It also helps to find configuration mismatches, programming mistakes, and defective devices early and ensures compliance with safety requirements such IEC 61511.

Test Your Expertise in Safety Instrumented Systems (SIS

Refer the below link to test your Expertise in Safety Instrumented Systems (SIS) –  Knowledge Quiz

Test Your Expertise in Safety Instrumented Systems (SIS): Knowledge Quiz