Distributed Alarm Annunciation & Sequence of Events Recording – The Why and How.

Distributed Alarm Annunciation & Sequence of Events Recording – The Why and How. Root cause analysis is the main driving force behind Sequence of Events (SOE) Recording and the determination of what caused the plant to trip. The Cost of downtime motivates the expenditure on equipment to diagnose quickly what happened so corrective action can be taken. Costs can run into millions per hour on big plants with even a seemingly insignificant but crucial area of the plant causing a knock-on effect to the rest of the enterprise. The issue here is to have available the status of key parameters normally Relay Contacts due to the nature of safety shutdown systems/ plant trip systems. Analogue signals are often also of interest in analyzing what happened too, in many cases these are derived from Analogue set-points as relay contacts, but more modern SOE systems now have the ability to integrate the Analogue signals too directly. Will my trend screen on SCADA or DCS will show me what happened? No it will not unfortunately. Sorry, this is invariably not the case as the scan time is not good enough to give engineers the resolution of which device tripped first especially in a shutdown where many statuses change together. Scan time on SCADA is based on how many I/O on the system and how busy the Computer is with tasks. Many good systems aim for a few seconds resolution, this is simply not good enough for Sequence of events determination. My PLC has a 1micro second instruction cycle can I use this? While this specification seems impressive it also depends on how many I/O it supports and what control philosophy is running. A 1000 line program already slows this PLC to 1millisecond and it has not handled any communications delays yet. Best Practice The event I.e. change of state is date and time stamped at the front end device – the SER. This message is delivered to the network where it is transferred to a database on the SCADA or DCS system normally via an OPC server, which makes this efficient and fast. From there the sequence of events can be analyzed because they are all chronologically sorted. Network delays do not affect the data integrity. Most Important! The SOE or SER device (Sequence of Event Recorder) as they are known, process the inputs with the ability to discriminate between input changes of state to 200 micro seconds, even if there are 32 or 4800 inputs. They are designed to scan inputs in the sub millisecond range, independent of the number of inputs on the system thus the Input count has no effect on the resolution of the system. Obviously this is not how PLC architecture or SCADA and DCS architecture works. The event recorders purpose is to deliver the events with their date and time stamp to the supervisory system as fast as possible. In the event of a plant shut down the history can be reviewed as a chronological history of all monitored points with a 1milli second resolution from a time before the trip to a time after the trip. The sequence of events allows engineers to find out the first event preceding the trip and to be able to home in on that area of the plant to rectify the problem in the minimum possible time. Law and procedure often prevents them restarting the plant without first having diagnosed the cause of the trip. Time stamp at source to better than one milli-second is the mark of Quality on an SOE system and is the industry benchmark standard. Using an SOE system compliments the SCADA and DCS functionality by providing accurate data and enables plants to save by minimizing downtime. Omni-4000 & Maxiflex SOE The latest Omni-4000 package released by Omniflex is a PC/windows based Critical Real-Time Alarm Events Monitor, which performs two main functions, Distributed Alarm Annunciation and Real-Time Sequence of Event Recording. “Vital information to facilitate downtime savings and predictive maintenance” Omni-4000 integrates both the Alarm Annunci

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