In the third article in a series on diagnostics, Emerson’s Kelly Albano looks at ways advanced diagnostics can help detect a range of process problems.
Picture this situation in the control room of a typical process plant: Output from the main distillation column has started dropping off, and it’s getting worse. The least-senior operator gets dispatched to climb the ladder and start looking in the viewports to see what’s gone wrong. After a few minutes, the message comes over the walkie-talkie that “We’re flooded at level five.” Operators quickly turn down the reboiler and throttle back on reflux to clear the column. They call production and warn that a big slug of off-spec product is coming through. What seemed like a minor increase to reboiler heat 30 minutes ago has turned into a process upset.
Unfortunately, process problem situations like these are pretty common, but they don’t have to be. What’s more, preventing them might have a very modest cost by taking advantage of process diagnostic capabilities built into advanced instruments, like Emerson’s Rosemount 3051S Pressure Transmitter. How all this works is covered in the third installment of my series on diagnostics in the August issue of Process Instrumentation, Process Diagnostics Support Process Improvements and Optimization.
So how is this possible? What does a 3051S do? We call it Process Intelligence, and it looks for process “noise.”
Noise in this context is the differences in readings when measured at very short intervals. For example, instruments based on Emerson’s Rosemount 3051S pressure transmitter take readings 22 times per second. Its transmitter electronics can process data that quickly and perform statistical process modeling in real time to provide its Process Intelligence diagnostic. This is what we mean by noise. The basic process variable, say a simple pressure measurement, may be steady as a rock when viewed on a control room monitor, but noise level details may be telling a deeper story.
The transmitter performs real-time statistical analysis, calculating the mean, standard deviation, and coefficient of variation. The most common way of using this capability is to detect plugged impulse lines. This alone is a big deal because plugs can throw off pressure and flow measurements. But this can also uncover other problems, even something as complex as distillation column flooding. All you need is a
3051S providing a DP reading between the top and bottom of the column, something you may already have.
The interaction between vapor and liquid creates specific noise patterns detectable by the DP transmitter when it has Process Intelligence capabilities. When liquid begins to build up and blocks vapor flow, the standard deviation begins to climb rapidly, ahead of the change in DP and far ahead of changes in temperature. Even if an operator is watching through a sight port, Process Intelligence is faster because it will warn of incipient flooding as soon as the standard deviation line turns upward.
Process Intelligence can sense when the problem is just starting so adjustments can be made before it escalates and causes a bigger disruption or shutdown. This is an excellent reason to evaluate upgrading some of your instrumentation, or to take advantage of capabilities in existing instruments.
When considering the cost of a process upset due to a clogged impulse line, or a major drop in distillation efficiency due to flooding, the money and engineering effort required to implement these capabilities can be earned back in a very short time. There are many more opportunities to take advantage of advanced diagnostics, and Emerson can support end users as they investigate new and unique ways Process Intelligence can deliver overall process improvements.
Read the blog post for the First Article in the series.
Visit the Field Device Management pages at Emerson.com. You can also connect and interact with other engineers in the Downstream Hydrocarbon and Chemical Processing Groups at the Emerson Exchange 365 community.
