Process Plants have always had the challenge of diagnosing what happened after a plant shut down. Finding the root cause of a particular problem is difficult with perhaps 100’s or 1000 plant shutdown interlock contacts involved.
Finding the fault and rectifying the fault as quickly as possible is of the essence as plant downtime is lost revenue. Finding these faults means tracking what is happening in the plant so that the conditions leading up to the trip can be reviewed and analysed.
So every change of state needs to be captured i.e. Interlock contacts and trip or alarm statuses are continuously monitored.
Sequence of Event Recorders deal with Events - An important concept to note is that although we are monitoring a relay contact which may be open or closed in it’s normal state when it changes state it becomes an event in the system implying that the time it changed state is captured as part of the history to be useful for post shut down diagnostics.
The Sequence of Events recorder chronologically Time Stamps every change of state pinpointing its occurrence in time.
- Change of State (COS) = Open to Closed or vice versa
- Event = Open to Closed or vice versa and Time and Date Stamp
The Benefit of a Sequence of Event Recorder
The role of the Sequence of Event recorder is to compliment the supervisory and control system within the plant. Sequence of Event Recorder’s primary purpose is to assist in the diagnosis of root cause analysis of plant faults. Sequence of Event Recorder does this by chronologically logging all monitored changes of state of the inputs on a continuous basis. Eg 1milli-second vs 1second or more on supervisory systems.
Record Pre and Post Event at 1msec resolution
The SOE recorder captures changes of state on a continuous basis so the plant status can be reviewed before and after the trip event. By logging the events to a PC based Supervisory system database at the plant events can be stored over long periods of time.
Resolve input changes of state to 1msec
Current benchmark standard for Sequence of Event Recording is milli-second enabling a chain of events to be chronologically captured with enough resolution to discriminate between Changes of state down to a single milli-second apart.
Analyse Downtime using Time Stamped at Source data. An essential component to accurate analysis of root cause leading to plant downtime.
Event recorders need to time stamp a change of state at the source and as they happen so that a list of events can be reviewed and sorted into chronological order even on the other end of a slow communications network. This Time and Date Stamp cannot be added later without introducing errors due to communications latency.
Referencing all events to a single time base
An essential criteria of root cause is to analyse all events relative to the same time base and number of key features are required to ease the job of engineers analysing the system
First Understand your Plant
It is not enough to simply install a Sequence of Events recorder on a plant and expect the results to yield the route cause. An understanding of the plants operation and the resultant latency that process interlocks, alarms and trips have is essential.
The SOE system will faithfully capture events down to millisecond resolution but cannot uncover primary latency in the system such as relay pick up or drop out time. This is a crucial phase in the implementation and configuration of the SOE system and essential to producing accurate results.
Key Features of SOE recorders
Input Sense detection
Reading the status of inputs as open or closed is important in understanding the interlock arrangement and reviewing the status of the system.
Adjustable Delay Timers
One of the many issues in monitoring plant contacts is the disparity between pull in or drop out times on different types of relay. Individual delay Timers are essential in aiding the processing of input conditions and correcting known plant timing mismatches.
Negative Time delays facilitate the easy adjustment of known individual slower events to the system time base reference obviating the need to shift all other inputs forward to compensate for a slow event.
Adjustable Bounce and Chatter Filters
One problem with such high resolution is that contact bounce and chattering inputs can also be detected and logged as events. While one needs the best possible resolution to pinpoint the change of state in time the bounce transitions are a nuisance and the first occurrence is the one that matters.
An SOE system needs to provide a means to filter both the bounce and chatter out so that event lists are not clogged with irrelevant events.
Buffers for Avalanche conditions
Most shut downs or trips can mean a lot of events over a short period of time. The SOE system must be able to buffer or queue the events until they can be processed into the history database normally a PC based supervisory and reporting system.
The ability to inhibit certain inputs for periods of time is useful in eliminating a known nuisance input while monitoring or undergoing maintenance.
Integration of Analogue Inputs
Most plants are combinations of Analogue and Digital signals that are controlled or monitored. Traditionally SOE recorders monitor digital inputs resulting in a signal conditioning requirement with alarm setpoint contacts for any analogue process.
Integration of Analogue Signals with Soft Setpoints directly into the SOE system provides a simple one stop approach with all monitored points on a common system.
Sequence of Event Recorder is available from Omniflex.