Documented evidence with regard to the adherence to the required safety integrity level (SIL) within the scope of the safety life cycle has to be delivered in order to proof that the imple-mentation of safety systems (Safety Instrumented Systems SIS) in the process industry has been executed according to professional standards. When carrying out the hazard analysis and the risk assessment, safety functions (Safety Instrumented Function SIF) will be estab-lished and evaluated against a required SIL. The achievable SIL both for systematic defaults and for random failures can be established for each safety function being carried out by means of a safety system. The established SIL has to be in conformity with or better than the required SIL. The engineers of the weyer group will establish the respective SIL-level of the plant, taking the data delivered by the manufacturers as the calculation base.

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2. 2 Verification of random
failures
Accidentally occurring hardware failures
will be evaluated by calculating the medium
Prob-ability of Failure on Demand, Average
(PFDavg) and the Average Probability of a
dangerous Failure per Hour (PFHavg)
respectively.Indoingso,themanufacturer’s
specifications con-cerning the default rate
of the appliances, as documented in the
declaration of conformity, will be deployed.
In case that these data might not be
available, the experts of the weyer group
will make use of statistical results compiled
Documented evidence with regard to the adherence to the required safety integrity level (SIL) within the scope of the
safety life cycle has to be delivered in order to proof that the imple-mentation of safety systems (Safety Instrumented
Systems SIS) in the process industry has been executed according to professional standards. When carrying out the hazard
analysis and the risk assessment, safety functions (Safety Instrumented Function SIF) will be estab-lished and evaluated
against a required SIL. The achievable SIL both for systematic defaults and for random failures can be established for each
safety function being carried out by means of a safety system. The established SIL has to be in conformity with or better
than the required SIL. The engineers of the weyer group will establish the respective SIL-level of the plant, taking the data
delivered by the manufacturers as the calculation base.
Our services:
À Verification of systema-
tic defaults
À Verification of random
failures
À Evaluation according to
standards
À SIL – project
implementation
1 Verification of systema-
tic defaults
The weyer group will check the achievable
SIL, referring to the Hardware Fault
Tolerance (HFT) and the percentage
of Safe Failure Functions (SFF) of
component systems. Hence, this check
will result in a qualitative evaluation of the
protective circuit of the plant. Depend-
ing on necessity, improvements might be
obtained by means of redundant design
or selec-tion of appropriate appliances and
diversity respectively.
SIL - Safety instrumented systems
in the process industry
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weyer special | sil

3. that the currently valid safety standards will
be met.
4 SIL – project
implementation
Having previously carried out a Hazard and
Operability Study (HAZOP), the safety
functions (SIF) and the required SIL have
been established and documented. The
component systems Sensor(s), Logic
Systems and Actuator(s) will be assigned
to the safety system (SIS) as Loop for each
safety function, equipped with the
necessary instrumentation. The architecture
and the fault tolerance of the safety system
will be established by the engineers of the
weyer group. Hereupon, the appropriate
appliances (sensors and actuators) as well
as their opera-tional deployment and the
control unit (logic system) will have to be
by organizations like Exida or Namur. When
performing these calculations, the
engineers of the weyer group will deploy
software tools which are certified by the
Technical Control Board (TÜV).
That software can be used for the
purpose of establishing the interval of the
repeated inspec-tions and it also shows the
percentage of the component systems with
reference to the fail-ure probability of the
entire system. Redundant structures and/
or the selection of appropriate appliances
might allow for an optimization of the
modules of the component systems in order
to achieve the required SIL. The engineers
of the weyer group will gear their efforts,
when deciding the appropriate deployment
of appliances being on a lower SIL-level in
redundant structures, particularly aiming
at cost savings in your plant. The herewith
established data might as well be used for
documentation purposes.
3 Evaluation according to
standards
In the process of evaluating your plant, the
engineers of the weyer group will refer to
the standards set by DIN EN 61508, DIN EN
61511 and VDI/VDE 2180, in which the
require-ments referring to safety functions
in the process industry, being implemented
by means of safety systems, are specified.
Hence, the plant operator can feel certain
weyer special | sil | page 3
weyer group
allround. smart.
specified (e.g. explosion protection). In case
that the manufacturer’s data (HFT, SFF and
failure rate) are not available to perform the
calculations of the failure probability for all
modules of the component sys-tems, the
necessary data for the execution of such a
calculation will be obtained by the engi-
neers of the weyer group. As soon as all
data are available, the intervals for the
repeated inspections can be established. In
the course of this process, all calculations
will be docu-mented by the engineers of
the weyer group. When applicable,
alternative appliances or structures will
have been chosen in order to extend the
inspection intervals at your plant. In the
process, non-identical inspection intervals
of safety systems might be synchronised
and deployed for a standardized
maintenance schedule.
Your benefits:
À We will evaluate your plant/systems
with regard to the reliability of safety
functions
À Increase of the operational availability
of your plant in terms of safety
relevance
À Our experts will assist the plant operator
within the scope of delivering the proof
of risk reduction required by law. Thus,
you will gain legal certainty.

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allround. smart.
Your contact
www.weyer-groupe.com
www.weyer-group.com
References:
À DHC Solvent Chemie GmbH,
Mülheim a.d. Ruhr
À EnBW Kraftwerke AG, Heilbronn
À LOESCHE GmbH, u.a. am Standort
Dillingen
À LORD Germany GmbH, Hückelhoven
À Sonderabfallverbrennungs GmbH,
SAVA Brunsbüttel
À RWE, Köln
À Hitachi Power Europe, Duisburg
À LOESCHE GmbH, Düsseldorf
Image source: LT Gasetechnik 04/2013
The weyer group is a group-independent corporate association of engineering and con-
sulting companies in Germany, Austria, Switzerland, Poland and the Netherlands.
Since its establishment over 30 years ago, the weyer group has developed a wide range of
expertise which has always been based on the expectations and wishes of our clients.