One of the factors that separates process plants operating reasonably well from those achieving a high level of excellence is how a plant uses information. Best-performing plants achieve their status by reducing production outages through proactive maintenance. This increases revenue generation and lowers maintenance costs to drive increasing profitability.
Some companies enjoy this state of affairs as a way of life, while others ask, “How do they do it?” One key difference is the ability to use instrument diagnostic information effectively, but how is such a concept put into practice?
Plant personnel have dashboards showing how various units within a plant are performing; they can see how a reactor, heat exchanger, boiler or some other major piece of equipment is performing at any time. For each of those units to perform optimally, control loops supporting them must perform as designed, which depends on instruments operating reliably to provide accurate data.
Smart instruments—including transmitters, sensors and actuators—are capable of monitoring their own performance, and they can tell you a great deal about what’s going on with a process well beyond the specific variable they measure or function they control. Just as the control system depends on these smart instruments, so can your production and maintenance teams.
Some smart instruments are able to detect deterioration in a key component or realize sensor drift. Others look at the process and might spot changes. For example, a differential pressure sensor can measure process noise and determine the normal level (Figure 1). When the noise level begins to fall or differences emerge between high and low readings, it may signal a problem with impulse line plugging. Such plugging can mask the true pressure differential and result in an unreliable reading. Today’s smart instrument can send a warning when they begin to notice such changes.
Working up the performance chain
Operational excellence depends on many things, but one critical element is hardware performance (Figure 2). This begins at Level 1, the lowest level with individual field devices. No matter how sophisticated the control system, a flow loop will never work properly if the flowmeter is not reliable or if the control valve is not acting properly. Conversely, the right kinds of process instrumentation and final control elements working together ensure tight control.
Level 2 comprises all the small groups of individual loops working together to provide basic functions. Those functions work properly when all the individual loops are performing per design. With enough information from smart instrument, plants can establish performance levels for those groups of loops and subunits.
Level 3 is the operational level. These are the asset diagnostics that operators and plant management watch on the main dashboards, such as heat exchangers, reactors, mixers and the like. These assets are dumb in and of themselves, so the only way to determine how they’re performing is to go back down the chain and see what’s happening at Level 1. Smart instruments are the eyes and legs of this process, and information they provide is the basis for everything higher up the chain.
You can’t do unit diagnostics without loop diagnostics, and you can’t do loop diagnostics without device diagnostics. Operational effectiveness always finds its way back to individual field instruments, sensors and actuators.
Solving pain points
Most plants have trouble areas which continually cause problems. These bad-actors cause an inordinately high number of outages and soak up more than their share of maintenance dollars. Those two factors taken together rob the plant of output and profitability. Going through maintenance orders and reports can normally make them easy to spot and categorize.
Once these trouble spots are identified, the next step is determining the root cause of the problem. Does the same pump drive motor burn out again and again? Does the same valve stick open? Is the energy efficiency of one heat exchanger far worse than others like it? The information available from smart instruments can help you identify these root causes so they can be fixed. Level 1 device diagnostics provide visibility into assets so you can see the problem clearly.
Once the bottleneck in an asset has been removed, it can perform with a higher level of reliability and efficiency, which will improve the whole plant. Once one bad actor problem is solved, the next step is to attack the next asset that comes to the top of the list. One by one, they can be identified and eliminated by using a well-thought-out strategy of analysis based on diagnostic information.
Having accurate performance information and eliminating bad actors is certainly valuable, but it provides only part of the picture. Most companies find even greater payoff from using diagnostics as the basis for a comprehensive predictive maintenance program.