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This shows that BS EN 61511, and by implication it’s concepts, is for end users and not for
manufacturers. The issues raised by the Specialist Inspector relate to the site / operating
environment specific to the terminal operator.
The HSE provides guidance on ‘Demonstrating prior use of elements of a safety instrumented
function in support of BS EN 61511’ (CDOIF, 2012). This reinforces that in the case of field equipment
the demonstration (of Prior Use) is achieved by confirming extensive operating experience in an
equivalent (or near equivalent) process related system, which may or may not be safety related.
It is difficult to understand how the radar level transmitter manufacturer can take into account the
individual site / operating environment required to develop the Prior Use claim. The most important
factor in the ‘Prior Use’ route is the similar operating profiles and physical environments. If the end
user finds that the sensor is not working due to process induced factors such as fouling, sticking or
drifting out of calibration then realistically they will just clean, lubricate, reset or calibrate the
sensor, they will not report the issue back to the manufacturer. Further discussion of the issues
regarding certification and prior use claims can be found in a number of sources, for example
Dearden, 2016.
It is understandable and correct of the Specialist Inspector to reject the BS EN 61511 based claim
made by the manufacturer and certification body.
It is also disappointing that certification bodies issue SIL certificates for manufacturer’s products
quoting the BS EN 61511 based ‘Prior Use’ requirements. The end user and their design contractor
believed that they were buying a SIL 2 ‘certified’ transmitter based upon the certificate and report.
Would BS EN 61511 Ed 2 change the assessment?
In February 2016, Edition 2 of IEC 61511 Part 1 (IEC, 2016a) was released, although BS EN 61511 Ed 2
is not yet available. The supporting Part 2 of IEC 61511 Ed 2 (IEC, 2016b) has recently been released
in July 2016.
There are many changes in Edition 2, but we will concentrate on those that relate to the Specialist
Inspector’s decision to reject the SIL 2 certificate.
The two tables relating to HFT in BS EN 61511 Ed1 have been replaced by a single table in IEC 61511
Ed 2, reproduced below.
SIL
Minimum required HFT
1 (any mode)
0
2 (low demand mode)
0
2 (continuous mode)
1
3 (high demand or continuous mode)
1
4 (any mode)
2
Figure 7 – IEC 61511-1 Ed 2 Table 3 – Minimum HFT requirements according to SIL
At first inspection, a number of changes are apparent: -
·
SFF (the old Table 5) is no longer considered
·
Both low demand and continuous demand modes are explicitly detailed
o
Possibly a typo for SIL 3 which does not consider low demand – this was labelled as
‘any mode’ in the previous FDIS version
·
HFT requirements have been reduced; SIL 2 in low demand mode now requires HFT 0