One of the topics we like to talk about with smart instrumentation is diagnostic capabilities: the ability for a transmitter to tell us how it’s working and what kind of condition it’s in. In fact, this ability is one of the things that makes a smart instrument smart.
In a recent article in Applied Automation, Using Diagnostic Functions to Improve System Safety, Mark Menezes extends the traditional thoughts about smart instrumentation, applying the same concepts to the safety instrumented functions (SIFs) that make up a larger safety system
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.
Let’s unpack what Mark just said. There are three elements to the discussion:
- Diagnostics identify when an individual device may have suffered some sort of failure
- Diagnostics identify when an individual device may be showing early signs of a failure beginning to happen, and
- Safety devices can become the diagnostic elements for the larger process unit.
The article examines all three, so let’s do the same here. For the first one, Mark reminds us that a failure can come from something other than an outright breakdown:
Many temperature measurement applications suffer from electrical noise, spiking, and signal dropouts. Noise can come from radios, motors, and lightning. 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.
Strategic use of a temperature transmitter can make the signal more robust, and therefore more reliable. But it can also add diagnostic functions able to predict future problems. Here’s a good example:
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. 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.
That’s just a single example. Different types of instruments have different ways of failing and predicting failure, so the types of diagnostic approaches are appropriate to the function. Let’s look at Mark’s third point, the idea of instrumentation serving as the diagnostics for the larger process. He talks about adding instrumentation to a pressure-relief valve (PRV) adding diagnostics to a dumb mechanical device.
Although the PRV will close itself after the pressure returns to a safe condition, it is common for dirt in the process fluid to prevent it from fully re-seating, leading to small, ongoing leaks. These leaks are often difficult to detect, yet over time can cause significant process loss and environmental impact. Because PRVs are mechanical devices, there are no electronic elements capable of providing diagnostic functions. New instruments combining acoustic and temperature sensors capable of capturing telltale sounds from malfunctioning valves can identify direct releases as well as ongoing leaks from incomplete valve seating. Such devices can be wired, or can communicate via WirelessHART, in either case, sending data to the BPCS.
So this acoustic device can be added to the valve to determine what it’s doing. Most of the time it should be doing absolutely nothing, but the sensor can spot when it’s open due to an overpressure event, or not fully seated after it has opened and closed. The article has more examples. It is very comprehensive and well worth a read.
You can find more information like this, and meet with other people looking at the same kinds of situations , here, in the Emerson Exchange365 community. It’s a place where you can communicate and exchange information with experts and peers in all sorts of industries around the world. Look for Temperature Group and other specialty areas for suggestions and answers.