4

51-650 Echotel SIL Safety Manual

1.2.1 Transducer Design

Magnetrol’s advanced transducer design performs in difficult

process conditions. Model 961 has a tip-sensitive transducer

with an arched gap increasing its performance in aerated or

foamy liquids. Model 962 has a tip-sensitive lower gap and

flow-through upper gap permitting separations of 125 inches

(318 cm).

1.2.2 Transducer Materials

A broad selection of transducer materials is available for the

Model 961/962. Metallic transducers include 316 SS,

NACE construction option for sour gas service, and can

also be built per ASME B31.1 and B31.3 piping codes.

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)

Tables 1 and 2 define the criteria for the achievable SIL

against the target mode of operation in Demand Mode

Operation.

Table 1 shows the relationship between the SIL and the

Probability of Failure on Demand Average (PFDavg).

Table 2 can be used to determine the achievable SIL as a

function of the Hardware Fault Tolerance (HFT) and the

Safe Failure Fraction (SFF) for the complete safety system

(Type B– complex components as per IEC 61508 Part 2) of

which the level transmitter is one component.

Table 2

Minimum Hardware Fault Tolerance

SFF

Hardware Fault

Tolerance (HFT)

0

1

2

None: <60%

Not

Allowed

SIL 1

SIL 2

Low:

60% to <90%

SIL 1

SIL 2

SIL 3

Medium: 90% to <99%

SIL 2

SIL 3

High:

≥99%

SIL 3

Table 1

SIL vs. PFDavg

Safety

Integrity Level

(SIL)

Target Average

Probability of Failure

on Demand (PFDavg)

4

3

2

1