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How to Use Process Diagnostic Capabilities in Pressure Transmitters to Improve Plant Performance

 Imagine this: your boss calls you into the office and says, “The company is hiring 12 new instrumentation techs. They’ll be doing calibrations, loop checks, configuration, and whatever.” A dream come true, but too far-fetched to happen? Maybe you could use something just as helpful.

The advanced diagnostic capabilities available today as part of instrumentation transmitters can perform many of the same tasks as technicians, and do so automatically and continuously, freeing up skilled techs to perform higher value tasks. That’s the main topic of my article in the October 2019 issue of Processing, How to Use Process Diagnostic Capabilities in Pressure Transmitters to Improve Plant Performance.

Diagnostic capabilities have become more sophisticated and are able to reach out beyond simple internal component monitoring functions. They can look at what’s happening with the loop itself and even the process. For the article, we concentrated on pressure transmitters, like Emerson’s Rosemount 3051S Coplanar Pressure Transmitter.

Pressure loops take all sorts of forms and may involve level and flow measurements. As a result, pressure transmitters send their primary process data to the distributed control system (DCS). This primary reading is the most important task a transmitter performs. Providing data to the DCS depends on a reliable connection to the process so it can get a reading that reflects the true reality and send a signal to the DCS without distortion capable of making the value inaccurate. These requirements are external to the transmitter itself, but it can still evaluate its situation and report when conditions are not correct.

So, the transmitter can monitor itself to make sure the reading is correct, but it also looks at the larger loop to see if the measurement data is getting to the automation system. This begins with something as basic as the wired connection.

In a typical plant environment where a pressure transmitter is communicating with the DCS using a 4-20 mA plus HART wired analog current loop, there can be a long cable — potentially several hundred feet or more — between the transmitter and the DCS I/O connection. A long cable could easily have a dozen termination points from one end to the other as the cable passes through trays and marshalling cabinets. Screw terminals that have worked loose or have become corroded can cause a signal to be lost completely. A short circuit can also cut off data entirely. Tracking such problems down can be tedious, but operators will hopefully recognize the reading is obviously incorrect or gone entirely and will take appropriate action to create a workaround while technicians locate and fix the fault.

There’s the technician, but do we have to depend on her or him to figure out if there is current leakage or an increased electrical load on the loop?

Both these conditions can be detected through manual loop checks by a qualified technician, but few facilities have the resources to do these with sufficient regularity. New advanced transmitters have loop integrity diagnostics that can perform these tests automatically, monitoring the available voltage on a continuous basis. If voltage deviates from baseline conditions, operators and maintenance technicians can be notified immediately to avoid potential process upsets or even safety incidents. Thresholds can be programmed in the configurations to correspond to the criticality of a given loop.

You can watch a video on how the loop integrity diagnostic works. It represents the connection side of the transmitter, but what about the process end? Are there things the transmitter can do to tell if it’s seeing the actual process pressure? The answer is yes, and you can learn more by reading the whole article, or looking at some other materials Emerson has for the Rosemount 3051S Advanced Diagnostics. There’s even a downloadable eBook.

You can find more information like this and meet with other people looking at the same kinds of situations in the Emerson Exchange 365 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 the Pressure Group and other specialty areas for suggestions and answers.