4
51-650 Model 961/962 Echotel SIL Safety Manual
1.2.2 Transducer Materials
A broad selection of transducer materials is available for the
Model 961/962. Metallic transducers include 316 SS,
Hastelloy
®
C, and Monel
®
. The 316 SS transducer has a
NACE construction option for sour gas service, and can
also be built per ASME B31.1 and B31.3 piping codes.
Thermoplastic transducers include Kynar
®
and CPVC.
These corrosion resistant plastic transducers feature a stiff-
ening tube that runs the length of the transducer for extra
rigidity. Kynar-faced 316 SS flange options are offered with
the Kynar transducers.
1.3
Determining Safety Integrity Level (SIL)
The FMEDA report provides the failure rates and informa-
tion for the PFDavg and Architecture Constraint.
The architecture constraint was determined using the 2H
approach according to 7.4.4.3 of IEC 61508-2, 2010 edi-
tion. The Model 961/962 meets the hardware architecture
constraints for up to SIL 2 at HFT=0 (or SIL 3 at HFT=1)
when the failure rates listed in the FMEDA report are used.
2.0
Applicable Models
This manual is applicable to the following models of the
Echotel ultrasonic level switches with current shift output:
• Model 961 Single-Point Level Switches
• Model 962 Dual-Point Level Switches
3.0
Mean Time To Repair (MTTR)
SIL determinations are based on a number of factors including
the Mean Time To Repair (MTTR). The analysis for the
Echotel Model 961/962 ultrasonic level switch is based on
a MTTR that is dependent on the end-user practices.
Table 1
SIL vs. PFDavg
Safety
Integrity Level
(SIL)
Target Average
Probability of Failure
on Demand (PFDavg)
4
≥
10
-5
to <10
-4
3
≥
10
-4
to <10
-3
2
≥
10
-3
to <10
-2
1
≥
10
-2
to <10
-1
