Process control and safety systems depend on field devices, and there can be thousands in production units. Keeping those calibrated and in top condition requires consistent maintenance following well developed schedules guided by diagnostic information.
Safety devices are particularly vulnerable to maintenance issues since many of them have little or nothing to do when the plant is operating normally. Without regular checking or diagnostic evaluation, they can slip into an inoperable state without anyone noticing and then fail to do their job when called upon during an incident.