Automation systems have now reached such a high degree of reliability that many personnel at plants now take them for granted. That is, of course, until people think the readings don’t match their intuition or “gut feel” — then, right away, the instrument is wrong.
The good news is that modern control systems, when linked to asset management tools, have the capability to make things even better. So how can a modern control system make you more competitive, increase the reliability and integrity of your piping and valves, improve maintenance, and help you handle your materials more safely? The answer is by taking advantage of the digital communications possible.
Modern control systems have the advantage over their predecessors of supporting digital communications from the widely used HART protocol, on through the various fieldbuses such as Foundation Fieldbus, Profibus, DeviceNet, LONworks, and AS-I, to name a few. Once a system supports and has installed digital field devices, it is no longer constrained by the single Process Variable (PV) of the analog signal and the need to rely on algorithms in the host Distributed Control System (DCS) to monitor the degree of variability to confirm that the signal is still valid rather than “frozen” at a single point due to some mechanical or electronic malfunction.
Ironically, one of the ways that has been used to increase system reliability is to install “redundant” control systems. Unfortunately, these systems aren’t nearly as redundant as many people believe, because the redundancy is only from the Input/Output (I/O) cards inward and doesn’t include the field devices.Yet, the I/O and CPU are the most reliable portions of a control system, while the field devices are most prone to failures (Figure 1). Of course, this makes perfect sense since it is the field devices that are exposed to the process and atmosphere, and are physically changing or moving to respond to or control the process. Also as expected, the level of the failures attributed to actuators exceeds their relative proportion among total installed field-control equipment.
Figure 1. Sensors and actuators cause more than three-quarters of control system problems.
Fortunately, fieldbus systems help overcome this challenge in multiple ways — not the least is that, in the case of Foundation Fieldbus with its Link Active Scheduler (LAS) capability, system redundancy now can extend down to the field device level. The LAS provides single loop integrity back to control systems. As a result, it is possible to continue to operate a facility or alternatively determine how it will “gracefully” shut down in the event of loss of communications with the host system.
Fieldbus and the LAS offer a capability common in a pneumatic control loop. As long as the analog input and analog output are on the same physical wire (pneumatic supply in the old days) and one of the devices is LAS capable (local pneumatic controller), it is possible for Foundation Fieldbus to control the loop at a single set point. Of course, should the set point need to be changed, some means is required to do that and normally this is the responsibility of the host system.
All fieldbus systems, including HART, are able to report on the status of the field devices on a regular basis. Most HART installations obtain this information by polling the devices for the data, while true fieldbus systems provide a status bit with every update of the process variable itself. Therefore, the operator will always know if the devices in the control loop are functioning properly.
Of course, the application engineers also should be thinking about how to use this status information to improve the control of the facility through something as simple as verifying that the signals being used for the control algorithms, output signals and multivariate optimization routines are valid.
Once it is discovered that a device is operating in a less-than-ideal condition, the next logical step is to use this information to assist with a facility’s maintenance planning activities. Fortunately, almost every host/DCS supplier also offers asset management software tools to effectively and efficiently access not only the status information in the field devices but also an entire range of diagnostic readings. For some devices, nearly 100 different parameters are monitored and available to be reported by the field devices. The limitation is often the power (energy and CPU) available to the device for these analyses. These asset management tools normally reside on a separate dedicated server so that they do not overload the control system with communication and calculation requests that may adversely impact facility operations. While these tools are capable of managing the entire instrument maintenance function, they are normally linked through appropriate security measures such as firewalls to the facility maintenance planning system. This allows for coordination of the work planning for all maintenance disciplines. (It isn’t too often that a valve or field device can be removed without assistance from the pipe fitters or perhaps, in some cases, the electricians.)
A relatively recent survey indicates that almost two-thirds of instrument maintenance is unnecessary (Figure 2). Integrated digital instruments make it possible to eliminate the “Routine Check” and “No Problem” maintenance requests by either displaying the device status directly on the operator console or alternatively through a mouse click to either a status screen or pop-up with the device’s operating status.