SIL Function Test Without Process Interruption

SIL function test for guided microwave and radar without dismounting or contacting sensors with medium.

Safety instrumented systems (SIS) operated in "low demand mode" require a regular function check. The safety characteristics of the components (sensor, logic and final control element) determine the test interval. To verify its function in such an application, the sensor usually has to be dismounted or contacted with medium – a procedure that in most cases cannot be accomplished during ongoing production. Moreover, this kind of test is usually very cost-intensive, and filling a vessel up to the overfill protection trigger point to verify sensor function also carries risks.

The function test via keystroke on the vibrating level switches VEGASWING, VEGAVIB and VEGAWAVE is already well known. A test cycle is initiated by simply pushing a button on 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 functional safety (SIL) as a periodic check.

But what about continuously measuring sensors? Until now, there was no continuous level transmitter on the market that could be checked extensively without influencing the ongoing process and without a reference measurement.
There is now a testing method available for VEGAFLEX guided microwave sensors and VEGAPULS radar sensors of the plics® family that allows a “recurring function test” to be performed without influencing the ongoing process. The operator of a plant can thus reduce to a minimum the work and costs that are otherwise involved in such checks, e.g. dismounting the sensor or contacting the sensor with medium, and avoid the production stoppages resulting from these checks.

The function check, which is really easy to carry out, reduces test overhead considerably. It guarantees fault detection up to 92% or 96% and consists basically of
- a restart of the sensor
- the simulation of 4 mA and 20 mA current
- a check of the parameter settings and echo data
- a minimal, undefined filling or emptying

You can find a detailed description in the supplementary information booklets, which include a test protocol form, for VEGAPULS radar sensors as well as VEGAFLEX TDR sensors.