In a Plant Engineering article, Using diagnostic functions to improve system safety, Emerson’s Mark Menezes shares examples of how these diagnostics are used to self-diagnose instrumentation problems as well as those in the surrounding production process.
…safety, and also can predict failures before they happen, improving availability. In other cases, a plant may design its own diagnostic, adding devices such as pressure relief valves, rupture disks, and corrosion/erosion monitors in critical places to watch for larger things going wrong. Let’s consider all three approaches.
Mark shares temperature measurement devices for his first example. Measurement values from temperature measurement sensors—resistance temperature detectors (RTDs) and thermocouples (TCs), are:
…very low-amplitude signals that must then be processed and amplified by the transmitter before being sent to the logic solver.
The measurement value can be affected by electromagnetic inference, signal spikes and dropouts. Signal processing in the transmitter can help assess these non-temperature related changes:
…and simply repeat the last good measurement. This approach provides stability without damping or slow response, but it should not be applied where the measurement can legitimately see fast full-scale excursions.
From a self-diagnosing standpoint, issues such as excessive vibration, corroding electrical connections or chemical attack may cause:
…the frequency of spikes and dropouts to increase over time. The transmitter can detect and trend this increasing frequency and predict impending failure, alerting maintenance early enough to take action and prevent total signal loss.
Mark also describes how the change in resistance in thermocouple wires can be used in diagnosing the sensor.
Changes in TC circuit resistance can suggest several things. If resistance goes to infinity, the circuit is open. If resistance decreases from its normal level, there is probably a short circuit. If resistance increases, the wire or termination is probably corroding. These changes may be immediate, but more often they’re gradual, so measuring and trending resistance changes can be used to predict failure and improve availability.
Read the article for other examples of how wired or wireless acoustic sensors can be used to monitor pressure relief valve (PRV) and rupture disc pressure relief events and leaks. He also describes how to monitor online and continuously for corrosion and erosion problems in pipelines and vessels. Finally, he shares how these diagnostics from HART smart instrumentation can be used to augment the traditional 4-20mA analog signal used in safety instrumented systems to provide diagnostic health information about the sensors.