Turning Up The Heat (2)

In some systems in the past, device diagnostics were either nonexistent or only displayed on a separate, usually unattended, maintenance station. This made the diagnostics less accessible and was not beneficial to operations. To make sure device diagnostics are incorporated into daily work processes, they must be displayed to operators at their consoles, integrated in accordance with NAMUR NE. 91 recommendations.

The field diagnostics are prioritized based on the critically of the device to the process and the severity of the fault in the device. This filters device alerts so that only those operation-critical ones reach the operators. It also eliminates alarm flooding in accordance with EEMUA recommendation 191. All device alerts, including predictive diagnostics not yet affecting the process, are reported to the maintenance console so maintenance may be scheduled for a convenient (later) time.

Using EDDL, the temperature transmitter manufacturer designs how the transmitter setup and diagnostics information shall be displayed in the device management software or handheld field communicator. This way, the transmitter manufacturer ensures the setup and interpretation of the temperature sensor diagnostics is as easy as possible for the user.

The multi-pen trend in the screen (overleaf) displays the thermocouple resistance, along with the baseline and trigger value. The trend plotted as a blue line allows any intermittent problems to be visualized, along with displaying the present resistance reading's distance from the trigger level.

Meanwhile, the thermocouple status is indicated by an LED mimic on the right side of the screen. The LED turns red if the trigger level is excedeed, indicating that preventive maintenance should be scheduled to inspect the thermocouple.
Replacing thermocouples during a planned maintenance period is easier, safer, and more cost effective than replacing thermocouples while the plant is running. As a result, downtime due to thermocouple failure and energy cost due to thermocouple degradation can be reduced. This translates into increased profitability, and in some cases, higher product quality.

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.

Process Statistics

Apart from temperature element degradation, another plant challenge is detecting problems within the process. Today, some transmitters have sophisticated diagnostics able to detect abnormal process conditions in a wide variety of applications based on "statistical process monitoring".
Here, the sensor samples process temperature at high frequency and the transmitter internally computes statistical mean of the temperature and standard deviation of the process noise. For a significant change in noise without a change in process temperature, the transmitter alerts the technician to the impending problem. Noise is not just noise anymore, noise contains important process information.

Statistical process monitoring (SPM) is able to detect abnormal process behavior before operation constrains are reached, providing an early warning. SPM alerts provide process engineers with a better view of what is going on in the process.
Abnormal condition detection using statistical process monitoring may be used to detect hydrate formation in natural gas lines, scaling formation, and thermowell coating. Users are discovering and testing many new applications.

The alerts provided by the transmitter, foundation fieldbus, and EDDL allow abnormal situation detection and prevention, and provide notification before a process is affected.
Because a distributed control system (DCS) typically registers input updates once a second or slower, followed by damping to filter out the noise and transients, statistical analysis of noise or capturing intermittent events in system software is not effective.

Boosting Performance


These new diagnostics as described above are in addition to traditional sensor short- and open- circuit detection. Another diagnostic technology used to uncover a drifting or failing sensor is Sensor Drift Alert which detects errors in temperature sensors by comparing readings from two sensors measuring the same point.
Several underlying capabilities, such as time synchronization, time-stamped alerts, and device diagnostics integrated into operator stations, become possible when combining the power of Foundation Fieldbus with the advantages of EDDL.

A digital infrastructure combining these two technologies allows you to take advantage of the latest developments in temperature measurement technology and new innovations that are not possible on an analog platform. It is the basis for a digital plant architecture that uses the power of field intelligence to improve plant performance.