For FF the device "pushes" the alert to the intelligent device management software almost instantly. This quick delivery mechanism in the FF protocol enables operators to respond faster to a critical device failure. There is time to take action before the process is affected, for instance operating the loop in manual. Because Foundation devices have an internal real-time clock kept synchronized with the system, the alerts are time-stamped in the device as they are detected, and thus more accurate as compared to traditional time-stamping in the controller after status was polled. This makes it easier to correlate to external events to determine the root cause and eliminate the source. Knowing exactly when abnormal situations occur can help technicians troubleshoot.
Understandable = Provide a clear message that is easily understood, not a cryptic code Diagnostic = Helps with the identification of the problem Advisory = Provides guidance towards the correct action Focusing = Directs attention to the important aspects
Self-diagnostics in transmitters with 4-20 mA output flag sensor failure etc. by driving the signal to a failsafe state: either high > 20 mA or low < 4 mA. This in turn drives the PID controller action to one extreme, fully opening or fully closing the control valve (depending on the transmitter high/low configuration) as if there was a process problem. Similarly, high alarms would trip on the high signal and vice-versa, as if there was a process problem. That is, most control systems can't tell this 20 mA or 4 mA current indicate a sensor failure. The operator cannot tell the difference between a sensor failure and a process problem, because they look the same. However, for FOUNDATION fieldbus, device diagnostics is also reflected in the status communicated together with real-time values between devices. Among other things, this status indicates the quality of the value as a simple Good, Bad, or Uncertain. If there is a sensor failure, the status flags this as Bad. Therefore, both the operators and the control strategy can clearly tell the difference between a device problem and a genuine process problem. If the sensor fails, the status turns Bad. The operator knows that the process itself is most likely OK and in many cases need not be shut down. The control strategy sheds to manual mode. The operator can take action to handle the process, minimizing downtime. However, if the process is critical the loop is configured to initiate its fault state on Bad status. That is, an FF system is able to make use of diagnostics to distinguish between a process problem and a device problem, thus avoiding unnecessary shutdowns increasing plant availability.
This slide merely points out that after the fix, you might be able to get additional information about what led to the problem or what was done to the device at an earlier time. Audit windows can be very helpful in locating exactly when something might have been impacted by an “external” action. For example, one can locate when a range might have been changed in an AI block and from what computer, etc.
More versatile statistical process monitoring statistical process monitoring is available for both the FF and 4-20 mA (HART) versions of pressure transmitters to detect process anomalies. However, the FF version is more versatile since it has a more powerful function block which can extend ASP applications.
Statistical process monitoring The FF version has statistical process monitoring for abnormal situation prevention. statistical process monitoring is able to detect abnormal process behavior before operation constraints are reached, providing an early warning. statistical process monitoring alerts provide process engineers with a better view of what is going on in the process. It may be used to detect hydrate formation in natural gas lines, scaling formation, and thermowells coating etc. Predictive Thermocouple Degradation Diagnostics Both the 4-20 mA (HART) and FF versions detect if sensor wiring is open or shorted or if a sensor breaks and fails completely, but the FF version has predictive thermocouple degradation diagnostics that alerts before the thermocouple fails. Such failure prediction helps reduce process downtime and decrease energy costs Thermocouple Degradation Diagnostic can help detect a failing thermocouple and allow preventive maintenance to be scheduled. Scheduled maintenance avoids the removal of a thermocouple from a process while in operation, which is not always possible. The automatic testing provided by the Thermocouple Degradation Diagnostic feature also eliminates the need for periodic manual resistance testing, which may lower maintenance costs and cause less process disruption.
Continuous Performance Diagnostics The performance diagnostics run continuously inside the positioner known as the "PD inside" feature, and any abnormalities are reported through alerts (in the 4-20 mA HART positioners the diagnostics is done on-demand from software using. Common Diagnostics Partial Stroke Testing (PST), travel histogram, step-response, valve signature, pressure sensor failure, abnormal drive current, travel deviation, reversal count alert, accumulated travel alert, low supply pressure, high supply pressure, position feedback sensor failure, etc.
Similarly, diagnostics such as travel time, stuck valve, and possibly partial stroking for on/off valves is possible with FF valve couplers.
Mount the gas chromatograph near the process to eliminate long sampling lines to the analyzer shelter. Analyser shelter is not required. Use fieldbus to bring the data and diagnostics back. Tank gauging has always used digital communication for accuracy – never 4-20 mA. The difference is how and when Fieldbus makes the digital data available. Convenient and timely.
Examples of a few testers on the market today. More are in the works, so check with the vendors beforehand to see if there is a new one that needs to be mentioned. Note that these are hand-held units. On-board software-based diagnostics will be covered in a later slide. Each tester has features (and limits) to what is does.
Specialized windows place the data in locations where the user can more easily absorb the information and visualize what is occurring. Also, even when specialized windows are not available, some devices contain tattle-tale lights which at least focus the user to a particular reason a diagnostic alert may be occurring.
Note the quantity of available parameters Note the difference among different manufacturer offerings. One make/model only has 29 parameters, while another has 230! Ask what does this mean to the user? The problem this brings to the new user is the amount of data one may have to deal with. Which data points are critical? Which do not make much difference? Which are there only for the manufacture's use