Blanket Calibration Interval Extension

A blanket extension is not defensible. The overall out-of-tolerance rate of 5.8% is already above a typical 5% target, and the fouling-service flow DP transmitters are at 11%, which indicates a process-dependent failure mode that “smart” electronics do not eliminate. Extending those to 24 months would likely increase drift-related errors and trips. The 1.2% rate for pressure transmitters suggests that some low-risk groups may support a longer interval, but that must be demonstrated group by group.

The SIS transmitters require separate treatment. A SIL 2 proof-test interval cannot be extended by a maintenance budget decision; it must be supported by reliability data and a verified PFD calculation under the plant’s functional safety lifecycle. The two nuisance trips traced to drifted level transmitters are evidence that the current interval is not uniformly conservative. The defensible approach is risk-based stratification, not a single interval for all 420 devices.

I would classify the transmitters by criticality, service, technology, and safety function. SIS devices stay on their proof-test interval unless a formal reliability analysis supports a change. Fouling-service flow DPs should stay at 12 months or be shortened, with impulse-line flushing or remote-seal diagnostics considered. Low-risk pressure transmitters could enter a pilot extension to 18 months, with a 10% sample checked at 6 and 12 months to verify that the out-of-tolerance rate does not rise.

I would use the three-year as-found data to estimate reliability by group, set acceptance criteria before the pilot, and document the basis in the maintenance management system. The program should include a periodic review, a trigger to revert if the observed rate exceeds the target, and a record of every interval change. This captures most of the savings from the low-risk population without gambling on the high-risk and safety-related devices. The final proposal should be a phased plan with measured results, not an immediate 24-month blanket change.

I would present a one-page comparison: the proposed blanket saving, the risk by device group, the SIS compliance issue, and the phased alternative with expected savings and review dates. I would agree with the operations manager that smart transmitters have useful diagnostics, but point out that the plant has no diagnostics server, so those benefits are not being captured. The two nuisance trips are concrete evidence that reliability varies by installation and service.

The message should be that maintenance is not refusing to save money; it is refusing to trade a known safety and reliability risk for an unverified saving. I would ask management to approve the pilot and the SIS-specific review, and to fund the diagnostics server if they want condition-based calibration. I would also make clear what would change my recommendation: a statistically valid as-found dataset for each group, a completed proof-test review for the SIS, and a monitoring plan with defined acceptance criteria. That gives management a defensible decision rather than a budget-driven one.