Safety instrumented systems (SIS) operated in ‘low demand mode’ require a regular functional check. The safety characteristics of the components (sensor, logic and final control element) determine the interval.
To verify its function in such an application, the sensor usually has to be dismounted or brought in contact with the measured medium – a procedure that in most cases cannot be accomplished while production is in progress. Moreover, this kind of test is usually very cost-intensive, and filling a vessel up to the overfill protection trigger point to verify sensor functionality also carries risks.
A function test on Vega’s vibrating level switches Vegaswing, Vegavib and Vegawave can be done via keystroke from the Vegator 636 signal conditioning instrument, or shortly interrupting the signal line. The scope of this test, which can be carried out in the middle of a running process, fulfils the requirements of both WHG (Water Resource Act) and SIL (Safety Integrity Level) as a periodic check.
What about continuously measuring sensors?
Previously, continuous level transmitters could not be checked extensively without influencing the ongoing process and without a reference measurement. There is now a testing method available for Vega’s guided microwave and radar sensors to allow a ‘recurring function test’ to be performed without influencing the ongoing process. The operator of a plant can thus reduce the work and costs that are otherwise involved in such checks – eg, dismounting the sensor or bringing the sensor and the measured product into contact – and so avoid the production stoppages resulting from these checks.
The function check reduces test overheads considerably and consists of:
* A restart of the sensor.
* The simulation of 4 and 20 mA current.
* A check of the parameter settings and echo data.
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