Annunciators overcome plant safety limitations
An
Omniflex product story
Edited by the Engineeringtalk editorial team
Feb 24, 2005
A dedicated annunciator offers far greater flexibility than PLCs, DCSs or Scada systems in handling critical plant alarms.
The last two to three years have seen a resurgence in the use of annunciators as dedicated devices in industry.
This has come largely as a result of failure of PLCs, DCSs and Scada systems to provide the kind of integrity and alarm functionality that the Omni16 annunciator can provide.
A PLC is generally designed to control, and anything that deviates from that affects its performance.
Providing alarm annunciator functionality with alarm sequences and operator intervention becomes part of its program cycle which the PLC must service to the detriment of its control functions.
With 100 or more I/Os, the PLC has to have considerable resources to process all those alarms and provide control.
With a single point of failure, if the PLC fails the alarm system also fails and the plant is vulnerable.
A separate alarm system can also provide the key parameters for shutdown conditions should the plant process become unsafe.
PLCs have to be programmed for these applications each time with the inherent risk of new code.
Omni16c alarm annunciators integrate tightly with a PLC via the Modbus RS485 or RS232 port the PLC can deliver the status of alarm points and read back the pushbutton status while leaving all the annunciation functions to the Omni16c alarm annunciator.
The PLC has full access to all parameters on the annunciator.
Omni16 annunciators can be used to reduce the number of I/Os a PLC needs to monitor by providing group alarm functionality.
Scada systems can easily suffer from information overload.
It's easy to keep adding monitor points for a plant, and many plants suffer the consequences of to much information.
For operators, this includes too many alarms and statuses coming through as events on the system.
An alarm annunciation approach provides the means to display important and indeed critical alarms in conjunction or independently of the Scada.
The operator can easily have his attention drawn to these alarms with his intervention being required for acknowledgement and manual resetting of alarms.
An operator cannot inadvertently miss an annunciator alarm.
Omni16c annunciators can be connected to a PC serial port to display critical alarms that demand attention cannot be missed.
Distributed control systems suffer from overload: generally they run using the limits of their resources for control, safety shutdown, followed by alarms and monitoring.
The burden of this means that finding out which alarm came first can often not be determined because the I/O scan cycle is so long.
Using Omni16c annunciators ensures critical alarms can be displayed off the DCS to ensure operators respond accordingly.
The annunciator can also be used as front end I/O with inputs hardwired to the annunciator and read back to the DCS via the Modbus RS485 connection.
If the scan cycle is to long the annunciator can discriminate first-up alarms to with 4ms.
The annunciator will indicate the first-up alarm within the defined annunciator groups and this can be read by the DCS.
Omni16c annunciators have easy interfacing and integration.
The Omni16c annunciator compliments all the systems above in a number of ways.
Users can connect the Omni16 to a PLC, DCS or Scada system via the optional Modbus port.
Using the annunciator can save PLC or DCS I/O by bringing alarm statuses in via Omni16 with serial connections to the supervisory systems.
ISA standard alarm sequences are provided on DIP switches.
The Omni16 conforms to international standards: it is IEC61508 rated for integration into functional safety related systems (SIL 1).
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