Speed monitoring system
SENSONICS
The Sentry 8608 Speed Monitor is an API670 compliant module which accepts either an eddy current probe or magnetic probe speed sensor input measured from a key / slot or toothed wheel (TTL also accepted) up to a frequency of 10 kHz.
Measured speed is displayed on the front panel in either RPM or Hz to an accuracy of <0.01% and four independent speed level alarms are available for the overspeed protection function. Additional alarms are also available to indicate the integrity of the speed sensor and to provide a zero speed alarm.
The speed monitor can be operated in an overspeed monitor mode, selectable either from the front panel or automatically. This will cause the module to record the maximum speed and trip speed levels during an overspeed protection event or during the use of the speed monitor online testing facility.
SIL calculations of the 8608 module in single channel and 2 of 3 voted arrangement according to IEC 61508 analysis requirements have been completed and the results detailed below.
The MO8608 speed monitor has a significant installation base in the UK and analysis demonstrates an MTBF of 34 years. As can be seen the calculated theoretical MTBF is lower (worse) than that measured which adds substance to the SIL analysis and calculations.
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