Using diagnostic functions to improve system safety

Some diagnostic capabilities are built into smart instruments, while others are designed into a process.

05/31/2017


Effective process automation systems depend on many types of field devices, controllers, and networks to provide basic control functions along with safety-instrumented functions. Unfortunately, all these systems can fail in a variety of ways, allowing problems to develop or escalate if not countered quickly and effectively. Users cannot assume failures simply don't happen, so they must make appropriate plans for how to deal with these possibilities.

Figure 1: Temperature transmitters provide critical signal processing functions for temperature sensors along with diagnostics, and come in many shapes and sizes to satisfy any application. As shown here, the Rosemount 248 temperature transmitter is avail Figure 1: Temperature transmitters provide critical signal processing functions for temperature sensors along with diagnostics, and come in many shapes and sizes to satisfy any application. As shown here, the Rosemount 248 temperature transmitter is availFigure 1: Temperature transmitters provide critical signal processing functions for temperature sensors along with diagnostics, and come in many shapes and sizes to satisfy any application. As shown here, the Rosemount 248 temperature transmitter is avail

Among the best practices and technologies available today are diagnostic functions built into smart field instruments that are capable of identifying covert failures as they happen. This improves 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. 

Making temperature sensors smarter and safer

Many temperature measurement applications suffer from electrical noise, spiking, and signal dropouts. Noise can come from radios, motors, and lightning. Other problems can be caused by wiring problems, mechanical shock, or vibration. Temperature measurements are more susceptible than most other field instruments because the sensors-resistance temperature detectors (RTDs) and thermocouples (TCs)-provide very low-amplitude signals that must then be processed and amplified by the transmitter before being sent to the logic solver. For example, the signal strength of a TC is about 1/400th the strength of the 4-20 mA signal provided by the transmitter. For this reason, best practices suggest locating the transmitter as close to the sensor as possible, minimizing the length of the lead wire (see Figure 1).

Even with close coupling between sensor and transmitter, noise or dropouts still can be problematic in some installations, so most users apply damping to suppress spikes and dropouts. While damping improves stability, it slows down the response of the transmitter to rapid changes in process temperature. Because redundant sensors typically are exposed to the same electrical and physical conditions, most users will set the same damping for all, so this slow response is a common cause.

A better approach is to use a signal validation capability built into a transmitter as part of its signal processing and diagnostic functions. The thermal inertia of a temperature sensor inside a thermowell makes extremely fast temperature changes, such as from 200°C (392°F) to 400°C (752°F) in half a second, physically impossible. Even if the transmitter sees such an instantaneous and unrealistic shift between successive readings, it can reasonably assume the change is a spike (or dropout if the change is moving lower), 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.

Although a sensor can be damaged by a single extreme mechanical shock, most failures are caused by ongoing vibration, loose or corroding connections, or chemical attack. These weaken the sensor and wiring, causing 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. Signal validation digs more deeply into the condition of the sensor itself, which can improve both safety and availability of temperature measurements.

Spotting tricky TC failures

Here's a typical application where validation can predict sensor failure. In hydrocarbon processing applications, TCs are often preferred over RTDs when fast response or high temperatures (greater than 600°C or 1,112°F) are involved. TCs are typically more physically robust than RTDs, but they can fail in a way not readily apparent. The junction at the tip where the dissimilar wires are joined is the measuring point, but if physical shock or vibration breaks down the insulation and the two wires form a contact (short circuit) somewhere else, the new contact point becomes the measuring point, wherever it might be.

Because this new junction is invariably farther from the hot process, in most hydrocarbon applications, a damaged TC will read low, although the opposite is true in cryogenic applications. Most processes are dangerous when they run too hot, so a low reading can create a safety risk. Because one physical shock could damage multiple TCs designed to be redundant, especially when they are installed close to each other or the lead wires are routed in the same bundle, this problem can manifest itself as a common cause.

Modern smart temperature transmitters are configurable to accept either RTD or TC inputs. When configured for a TC, they use their voltage circuitry to determine temperature. But transmitters also can use their resistance measuring circuitry, which would be used with an RTD, to monitor the resistance of the TC. While resistance of the TC cannot be used to determine temperature, it does help to detect and predict failures.

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. 


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