DCS Scaling and Alarm Error Found During a Level Loop Check
A new tank farm has 22 guided-wave radar level loops wired to a DCS. The loop-check procedure requires injecting 4, 12, and 20 mA at the transmitter terminals and verifying the DCS indication and alarms. On loop LT-104, the engineering range is 0–5.0 m. Injecting 12.00 mA gives a DCS display of 2.40 m, and injecting 20.00 mA gives 4.80 m. The high-high level alarm is configured at 4.50 m, but when the simulated level is raised to 4.70 m the alarm does not annunciate; it annunciates at 4.70 m only after the input is increased further to the equivalent of 4.70 m. The instrument technician says the error is within 2% and wants to accept the loop because startup is two days away. The DCS configuration printout shows an analog input range of 0–4.8 m and a high-high setpoint of 4.70 m. The radar transmitter itself is ranged 0–5.0 m.
1. Calculate the expected DCS indications for 4, 12, and 20 mA with a 0–5.0 m range, and determine whether the loop check passes.
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With a 0–5.0 m range, 4 mA = 0 m (0%), 12 mA = 2.5 m (50%), and 20 mA = 5.0 m (100%). The observed 12 mA indication is 2.40 m, which is 48% and 0.10 m low; the 20 mA indication is 4.80 m, which is 96% and 0.20 m low. This is a span error, not a small fixed offset, and it grows with level. The high-high alarm is also wrong: it is set at 4.70 m instead of 4.50 m, so the tank would be filled 0.20 m above the intended alarm point before annunciation. The loop does not pass. A 2% midscale error might be acceptable for some process indications, but this is a configuration error in a safety-relevant level alarm, and the DCS scaling should match the transmitter and engineering documents exactly.
2. What configuration errors are likely, and what would you correct and retest?
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The DCS analog input range is configured as 0–4.8 m instead of 0–5.0 m, probably from a copied template or a transposed span. The high-high alarm is set at 4.70 m instead of 4.50 m. Correct both in the DCS under change control, then retest at 0, 25, 50, 75, and 100% of range. Verify the alarm at 4.50 m and confirm that it clears at the correct reset point. Check the transmitter range, units, and damping, and confirm the radar's 4–20 mA output is linear with level. Also test the high-high output to the annunciator or interlock, not just the DCS screen. After correction, inject 12.00 mA again and confirm 2.50 m ± the allowed DCS indication tolerance. Record the before-and-after configuration printouts.
3. What documentation and turnover evidence should be complete before startup?
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The loop folder should contain a signed loop-check sheet with injected values, DCS indications, alarm results, and test-equipment identification. Attach the as-found and as-left DCS configuration printouts, the alarm setpoint list, the instrument index, the loop drawing, and the P&ID reference. Raise a change record for the DCS range and alarm corrections and record who approved and performed them. Confirm the punch-list items are closed and that no temporary jumpers or forced values remain. The turnover package should also include the radar vendor manual, calibration certificate, and operations acceptance signature. Startup should not proceed on a verbal assurance that the loop is close enough; the configuration error affected both indication and alarm action, so the signed retest is the acceptance evidence.