Before a new process control system can go live, every loop must be tested, verified, and documented—a process known as loop checking. While often treated as a final checklist item, loop checks are essential for ensuring safety, reliability, and proper communication between field instruments and control systems.
Whether you're commissioning a new plant, expanding an existing unit, or performing a major turnaround, loop checks are where design meets reality. And in high-stakes industries like oil and gas, that reality better be flawless.
What Is a Loop Check?
A loop check is the end-to-end testing of a control loop—from the sensor in the field, through wiring, marshalling panels, input/output (I/O) cards, the control logic, and finally to the control room indicators or actuators. The goal is to confirm that the signal is accurate, correctly scaled, and flowing to the right place—and that it can drive intended action.
Types of loops include:
- AI (Analog Input): e.g., temperature transmitters
- AO (Analog Output): e.g., control valves
- DI (Discrete Input): e.g., limit switches
- DO (Discrete Output): e.g., pump start/stop relays
Loop Check Best Practices
1. Start With Documentation
Ensure you have:
- Instrument loop drawings (ILDs)
- I/O lists and ranges
- P&IDs
- Control narratives
- Wiring schematics
Verify that the instrument tag matches all documents and that the signal range, type, and location are correct.
2. Use the Right Tools
- Calibrators: To simulate or read current, voltage, resistance, or RTD signals.
- Communicators: For smart device configuration (e.g., HART or FF).
- Multimeters: For basic electrical verification.
- Loop testers or signal generators: To test analog outputs.
- Mobile devices or laptops: If using app-based diagnostics.
3. Verify Device Configuration
Before testing signal flow:
- Ensure zero/span is set properly.
- Confirm correct device type and units.
- Test local diagnostics.
- Check alarm thresholds and fail-safe settings.
4. Perform Signal Verification
Inject a known signal at the instrument and verify it appears correctly at the DCS or PLC. Check for:
- Signal integrity (no noise or drop)
- Accurate scaling (e.g., 4–20 mA = 0–100 psi)
- Correct response time
- Functional alarming, if configured
5. Test the Entire Loop, Not Just the Instrument
It’s not a loop check unless you test the entire signal path, including field wiring, termination, I/O cards, and logic configuration.
6. Document Everything
Good loop check documentation includes:
- Date/time
- Who performed the check
- Device tag
- Range/units
- Signal check results
- Observations and corrections
Use checklists and cloud-based commissioning software for traceability.
Common Mistakes to Avoid
- Skipping loop checks due to time pressure
- Only checking signals at the marshalling panel
- Assuming a powered device is a functional one
- Failing to reset smart devices to operational mode after simulation
- Not involving control room operators in the loop check
Loop Checks in the Digital Age
With today’s smart transmitters and app-based communicators, loop checks can be faster and more insightful than ever. Features like:
- Live diagnostics
- Automatic loop documentation
- Remote verification via wireless HART
- Mobile loop check apps (e.g., Emerson AMS Trex, ProComSol DevCom)
These technologies help teams detect wiring errors, configuration mismatches, or non-responsive devices before startup.
Commissioning vs. Startup: Don’t Confuse the Two
Loop checks are part of commissioning—not the startup. The goal is to verify readiness, not to control the process yet. Only once all loops are checked, documented, and signed off should the system be handed over to operations.
Final Thoughts
Loop checks aren’t just a formality—they’re a frontline defense against process risk. Done right, they save plants from costly downtime, prevent dangerous incidents, and ensure the investment in automation delivers its full value.
At IVAC, we encourage instrumentation professionals to treat loop checking as both a science and a discipline. Whether you’re working on a $10 million upgrade or a small facility expansion, the integrity of your loops is the integrity of your system.