I/O: Process control's weakest link
We are all familiar with the phrase "garbage in, garbage out," but how many of us realize that this statement is also true for our control systems?
Garbage in - If you are operating with a failed or malfunctioning sensor, how do you expect to control what you cannot measure or even be able to tell that you are in control? Failure to be in control while assuming that everything is operating within tolerances could have catastrophic results.
The same is true for the output end of a control loop - typically a valve/final control element. If the final control element fails, then the control system has no way to alter the process, meaning that all forms of control - regulatory and advanced - will not work. The result is "garbage out."
In addition, if the infrastructure to connect these sensors to the control system is not working properly, the system is likely operating off of incorrect data, which could, once again, have catastrophic results.
Years ago, I worked at a facility where a steam stripper was used to recover light hydrocarbons from a water stream. The operators were always pleased with how steady this unit operated. However, during one outage when we were upgrading the dp cell associated with the orifice plate to measure the steam flow to the stripper, we discovered that the taps were so badly plugged that they could not be cleaned. We had to install new taps. The primary variable to control the stripper operation is the feed/steam ratio, which basically indicates the amount of energy required to enter the vessel and cause the hydrocarbons to evaporate so that they can then be condensed in the overhead coolers and recovered/returned to the process. Because of the nature of this service, the unit was only shutdown and accessible once every 18 to 24 months. With the new smart transmitter installed, the operators started complaining that they could not control the process as well as they had in the past. After a few weeks, the pressure transmitter sent a maintenance alert indicating that the signal was deteriorating. When the maintenance technician went to check the transmitter, he discovered that the taps were already 50 percent plugged. It is, therefore, likely that without this warning, within three months the orifice taps would be totally plugged - all the while either using far more steam than required (the plant was often steam restricted) or emitting more hydrocarbons than estimated. This could potentially violate one of the conditions of the operating licence; all because the dp flow transmitter was not working properly. This example demonstrates the importance of using diagnostics to monitor field devices. The unfortunate part of this particular example, however, is that the facility operator elected to disable the notification feature rather than implement a system to use this information to improve operations.
The majority of today's instruments contain diagnostics that are able to tell you when they are failing. Unfortunately, though, most instrument technicians use these digital communications capabilities to perform maintenance activities. Only 10 to 15 percent of installations gather and use this information to continuously monitor the status of their field devices, according to the HART Communications Foundation. Part of the reason that this happens is because it takes work to install and maintain instrument asset management software. The challenges continue to build when you consider that there are few people who understand how to install these sorts of systems and integrate them into existing work practices and associated maintenance planning packages.
Fortunately, there is some work being discussed to help in this regard. The following purpose and scope are being considered for submittal to ISA as a possible new series of standards on instrument asset management.
• Purpose: Provide a vocabulary, process set, best practices guidelines and infrastructure definition for the management of automation system assets from the field device level to enterprise asset management systems. Create work products that enhance the probability that intelligent instrumentation diagnostics are used to enhance performance and reliability of the process.
• Scope: With an emphasis on automation systems performance and reliability - including instrumentation, control systems and enterprise asset management systems - the yet-to-be-named series will create standards and practices for asset management globally for industrial and building automation.
If you are interested in participating or helping with this activity, please reach out to the author at the contact information below.
Remember, your control system is only as good as its eyes, ears and hands - the sensors and actuators you use to measure and control your process. Even though instrumentation may not seem as glamorous as all of the applications that use them, without functioning I/O, you may as well give the applications the heave ho.
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