Chemical Engineering July 2016 - 46

and less than 10,000 will be achieved.
The risk reduction is partly covered by
the BPCS (up to 10, as per IeC 61511)
and by the SIS (1,000 in a SIL-3-compliant
solution).
In the integrated solution, there will be
common components for the BPCS and
SIS. Depending on the individual setup,
this will be either the central processing
unit (CPu), input-output (I/O) buses or
(parts) of the solution software (for example,
the operating system), and symbol
libraries.
The argument could be made that different
components (of the same make)
may be used for the SIS and BPCS.
However, if common elements (such as
operating systems and buses) are used,
the systematic capabilities of such components
may need to comply with the
requirements mentioned above.
It should also be kept in mind that
using components such as CPus with
freely configurable software on board -
engaging the various parties to make
sure that potential deficiencies in each
task are identified and corrected. While
integrated tools can support the effectiveness
of engineering processes, addressing
aspects like common-cause
failures requires first narrowing integration
to a sustainable level. This helps
maintain both efficient engineering processes
and functional safety at the required
level.
The previous comments about BPCS
and SIS independence and diversity
also apply to engineering tools. A potential
hidden failure of the engineering
tool may impair the desired reduction in
overall risk.
There are two types of integrated solutions
that have either a common configuration
database for SIS and BPCS,
or have independent databases for SIS
and BPCS, but use the same data access
mechanisms. Both solutions have
the disadvantage of having a common
The quality of engineering processes, tools and associated services may be even more
important to overall safety results than BCPS and SIS hardware.
and using these same components for
different tasks - may not be considered
sufficient leveraging of the integrity level
of the solution.
These commonly used components,
in order to comply with the initial risk
reduction requirements, will need to
maintain a risk reduction of greater than
1,000 by less than 10,000. Practically,
this means SIL 4, which is currently an
unachievable level.
Engineering's key role in security
The quality of engineering processes,
tools and associated services may be
even more important to overall safety
results than BCPS and SIS hardware.
Proper engineering includes the following
aspects:
* Reducing complexity by splitting tasks
into independent modules
* Properly defining and verifying
interfaces
* Testing each module intensively
* Maintain the " four-eyes " principle
when reviewing engineering documents
and results of implementation
tasks, according to IeC 60158-1,
paragraph 8.2.18
Application of this strategy requires
46
cause for potential failures, which would
infect both the BPCS and SIS.
The engineering tool for safety systems
should overcome these issues by
remaining independent (to the greatest
extent reasonably possible) from the
hardware and software environment.
This is accomplished by having the complete
functionality of the safety engineering
tool, running in a Windows software
environment, implemented in a way that
allows it to be independent from Windows
functions. This concept allows
maximum protection from errors and
creates a trusted set of engineering data
that can be used to program the SIS.
nevertheless, the engineering tool
should allow integrated engineering by
maintaining interfaces that permit automated
transfer of configuration data
(tag-oriented information as well as logic-oriented
data) from third-party systems
into the trusted set of engineering
data used for programming the SIS.
Furthermore, having the same engineers
in charge of programming the DCS
and safety system ignores the proven
benefits of the checks and balances of
independent thinking. For this reason,
IeC 61508 is setting recommendations
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