A new instrument tech is facing a major challenge: “Why is the pressure transmitter I installed yesterday giving an obviously incorrect value today? The control room operators are not happy because the reading is thwarting close-loop control. Where do I start looking?” A few years ago, solving such a problem could have been a major undertaking requiring lots of crawling around with a hand-held HART communicator, and maybe tracing wires. In many plants, that is still the situation.
But in other facilities, instruments with advanced diagnostics are able to help sort through the possibilities so the number of potential causes can be narrowed down quickly, without the need to set foot into the field. How this works is the topic of my article in the September issue of Process Instrumentation, Installation and Maintenance Diagnostics Help Speed Commissioning and Simplify Ongoing Support. This is the fourth article in a series on instrument diagnostics, and it begins to pull together elements from the previous three. Back to our hypothetical problem:
There is a possible problem in one or more of these areas:
-
-
- A process issue is interfering with the reading, such as a clogged impulse line.
- The instrument itself is malfunctioning or is out of calibration and providing an incorrect value.
- Something has gone wrong with the host system, possibly the I/O card.
- A problem has developed in the wiring carrying the 4-20 mA signal.
-
The article goes into more depth, looking at different ways diagnostics work in combination, but this chapter focuses primarily on loop integrity.
An instrument depends on flawless communication with the host system to provide an accurate process variable to control and optimize operation. With most plants using 4-20 mA with HART® loops, this can get complicated since an instrument may depend on hundreds of feet of cabling through multiple marshalling cabinets. There can easily be a dozen terminations between the instrument and I/O card of the host system. Much can go wrong in between with moisture, corrosion and inadequate maintenance interfering with proper operation.
Diagnostics built into instruments such as Emerson’s Rosemount™ 3051S Pressure Transmitter can detect these kinds of wiring problems or when a power supply is unstable. They can also point to potential calibration errors, and even to measurement distortions due to process causes. At the same time, mechanisms to interact with diagnostics are getting easier, such as adding Bluetooth® technology to instrument transmitters, making it easier for technicians to access the data without a physical connection.
The ability to use diagnostic capabilities to confirm correct installation and troubleshoot problems can save much time wasted on guesswork. Fixing problems quickly and decisively depends on eliminating unrelated causes, thereby isolating the cause positively, so the right corrective actions can be taken, returning the plant to full production. The technologies discussed in this and prior articles provide the tools needed to realize these and other benefits.
Catch up on the entire series here:
Read the blog post for the First Article in the series.
Read the blog post for the Second Article in the series.
Read the blog post for the Third Article in the series.
