Using HART Diagnostics in Your Safety Instrumented Systems

Emerson’s Alan Harris has written a new whitepaper, DeltaV SIS HART Capabilities. It describes various HART capabilities that can be used with the DeltaV SIS system as well as HART diagnostics implementation best practices. For those unfamiliar with HART, the HART Communications Foundation describes it:

HART (Highway Addressable Remote Transducer) Protocol is the global standard for sending and receiving digital information across analog wires between smart devices and control or monitoring system.

Alan describes the importance of HART diagnostics in safety applications:

The HART diagnostics provide much more information on the health of a field device than can be determined from a standard 4-20 mA signal. For this reason, greater SIL (by turning Dangerous Undetected failures into Dangerous Detected failures) and longer proof testing intervals can be achieved by field devices running HART diagnostics.

He also makes the strong point that HART is not a safety-rated platform and that you should never substitute HART signals for hardwired signals, when the hardwired signal is being used to detect a hazardous condition with a SIL (safety integrity level) rating. HART only should be used for diagnostic purposes.

Alan does describe ways it can be used, especially in safety instrumented systems like DeltaV SIS that can incorporate these diagnostics directly into the SIS logic. One example is upon recognition of sensor faulty status; the SIS logic can degrade the transmitter voting:

…remove the transmitter from the voting logic (i.e. a 2oo3 voted group of transmitters degrades to 1oo2 or 2oo2 with the bad transmitter viewed as faulty) or the transmitter can be simply alarmed via operator graphics.

In the case of a problem with a final control element with a HART-enabled digital valve controller:

…HART device status signals can be used to trip valves that use a HART-enabled positioner or alarm the valve via operator graphics.

Other conditions the SIS logic can monitor in sensor devices through the HART diagnostics include PV out of limits, analog-digital mismatch, PV output saturated, PV output fixed, loss of digital communications, and field device malfunction.

For safety-application rated digital valve controllers like the Fisher DVC6000 SIS, diagnostics available to the SIS logic or asset management software include: loop current, auxiliary contact status, output pressure, % travel, position, drive signal, valve setpoint, pressure, differential pressure, and DVC internal temperature. Also, this digital communications provides a path for the SIS logic or asset management software to initiate manual or scheduled partial stroke tests (PST), which:

…checks for valve movement without fully stroking the valve. Many applications will allow 10% movements to verify valve response without upsetting the critical process line. Diagnostic data is collected and an alert is given if the valve is stuck.

Alan’s whitepaper describes some of the diagnostics available in other Emerson devices such as the Rosemount 3051S pressure and 3144P temperature transmitters, and the Micro Motion Coriolis flow transmitters. He describes the purpose of these diagnostics to give the reader ideas of how they might be incorporated into their SIS logic to improve diagnostic coverage and safely increase overall availability by reducing spurious trips.

If you’re responsible for your plant’s safety instrumented system, you might consider giving this whitepaper a read.

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