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Process Industries
Valve diagnostics for safety related controls
Karsten Fischer, International Sales & Marketing Manager for
Positioners, Invensys - Foxboro Eckardt GmbH
Abilities of HART-Communication for predictive maintenance, valve
diagnostic and testing of ESD-functionality, exemplary shown with the
FDT-based ValCare–software for the Intelligent Positioner SRD991/SRD960
by Foxboro Eckardt.
The 1990s were marked by the introduction of microprocessors and
HART-communication in the process automation arena. This allowed new,
until then unknown, opportunities to transfer information from the field
into control systems. Until today the HART-technology is unique, because
the signal-processing can either be done by the traditional analog
signal with 4 to 20 mA or by using the digital frequency modulation.
Technological milestone
The plant owners expected that the integration of a device should be
done as easily as using a printer driver for your PC. A result from this
demand was the Field Device Tool (FDT) technology with the
device-drivers named Device Type Managers (DTM). It supplemented the
DDL-technology and offered a unified integration-technology for all
devices in a plant.
This technological milestone has contributed to the process automation
industry and many automation companies now offer device-specific
valve-diagnostic software (such as - in the case of Foxboro Eckardt -
ValCare, available as DTM. This software can easily be
integrated/installed in every FDT-compliant system environment or PC.
Besides the display of measurement and actuating signals, this software
also offers applications for the processing of collected data retrieved
in the field while the positioner was in operation. As shown in
illustration 1, ValCare includes data to: create histograms for the
valve position history or control response; measure the stem friction;
view alarm condition of exceeding inadmissible upper and lower values;
and display the hours in operation for the Service Management. The
histograms indicate the last 15 minutes, 24 hours, 30 days or 60 months
and offer the plant owner valuable information of the actual valve
condition and overall plant performance.
By storing all data locally in the field device, an evaluation can take
place “online” via the HART communication in the FDT-environment.
Additionally, if the device in the plant is only controlled by an analog
4 to 20 mA signal, data can be retrieved at a later time.
Higher demands on safety
In the past years the demand for field-devices used in safety
instrumented systems (SIS) increased. These devices have to meet the
requirements for safety applications according to SIL (Safety Integrity
Level) based on IEC 61508 / IEC 61511-1. The SRD, for instance, fulfills
the requirements for safety application up to SIL 3 and is used as
Emergency Shutdown Device (ESD).
As shown in illustration 2, the power-supply for the positioner on an
ESD valve can either come from an analog 4-20 mA signal supplied by a
safety system (such as TRICONEX in the case of Foxboro Eckardt) or in
combination with a control system. In both cases the HART communication
can be superimposed to the analog signal. The solenoid is powered by a
separate signal.
The HART communication therefore can also be established in primarily
safety related applications (without a control system for the
communication) via the power-signal from the AO-Modules (AO=Analog
Output). For such applications modern HART-Multiplexers (e.g. by Pepperl
+ Fuchs) provide the possibility to communicate to several valves from
one operating station. The FDT-technology offers the additional
advantage that the DTMs of the Multiplexer and the positioner can be
added into one software package, to create a kind of small control
system. This setup allows the safety system from TRICONEX to communicate
with positioners today.
No matter how the HART-communication is established, it is important
that in all cases the status- and diagnostic messages from the
positioners are transmitted to the operator and a functional test of the
valve can be executed.
Actuators health test
This test on ESD-valves is also known as a Partial Stroke Test (PST)
which is activated and evaluated via the FDT software. This allows an
examination if final control elements / actuators in ESD-applications
function properly on demand. It cannot be excluded that a valve in a
continuous steady state is not blocked by the influence of corrosion.
The result of this PST will indicate, if this valve is fully operational
or if maintenance is required.
These applications clearly show how flexible the FDT system environment
can be to combine and integrate interface components and field devices
in different applications such as control systems or PCs.
Unified displays
Compared to other software tools, the FDT-technology offers the
increased advantage that the device-specific display of the DTMs for the
configuration and diagnosis is always identical, independent of the
communication protocol. Today, some systems still require several
configuration tools by different vendors that run on one operating
station, creating a colourful mosaic of icons. However, this is
unnecessary with FDT. Here the different communication and device
drivers have been united into one “Frame Application” (PACTware).
Foxboro Eckardt offers DTMs for all common protocols such as HART,
PROFIBUS, FOUNDATION Fieldbus and FoxCom. An update of the device
drivers can be arranged very easily, because the DTMs can be
individually replaced.
Unified self-surveillance
Within the phase of converting to FDT, Foxboro Eckardt now offers a
self-surveillance and diagnosis following the Namur Recommendation
NE107. This recommendation defines unified status-messages for field
devices, providing the user with information about the state of the
field instrument. The available information shall indicate clearly what
device alarm was activated, where the alarm initiated from, possible
reason for the alarm and what corrective actions need to be initiated to
restore a normal operating state.
Illustration 3 depicts the realization by Foxboro Eckardt. All alarms
are generated in the positioner and can be uploaded at any time. The
columns show the displayed status messages, e.g. control deviation, air
supply failure, high friction alarm, current or historical message, a
full text description explaining the possible reason for the status
message, and the actions for maintenance.
Status messages are distinguished by messages and color-coding like with
a traffic light.
Green indicates that no status messages are present; gray that a status
message is present but no maintenance is required; yellow that
maintenance is required but an operation is still possible; red
indicates a device failure that requires an immediate service. Comparing
the historical and current alarm supports the operator if all messages
have been eliminated.
These applications clearly show how flexible the FDT-system environment
can be to combine and integrate interface-components and field-devices
in different applications such as control systems or PCs. <<
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