Those that perform a primary protection or
measurement function:
In this instance, the gas detector needs a
supporting performance test for the measure-
ment function, as covered in additional BS EN
standards. Where these additional BS EN
standards include the measurement function of
the control equipment, the control equipment
must also be tested and approved.
If in doubt as to whether the complete gas
detection system requires full ATEX type and
performance approval, there is one question that
should be asked: "Is the gas detector removing
the risk of explosion by, for example, switching off
power, closing down valves or shutting off ventila-
tion?" If the answer is yes, then both ATEX type
and performance approval is required.
SIL compliance
Otherwise known as BS EN 61508, SIL is the
international standard for electrical, electronic and
programmable electronic safety related systems. It
sets out the requirements for ensuring that
systems are designed, implemented, operated and
maintained to provide the required SIL level.
Many plants have to conform to a certain SIL
rating. Whilst this rating comprises a number of
factors such as the risk, severity and the likelihood
of a significant accident, the products that are
being used in such installations have to be within
strict PFD value limits. PFD stands for probability of
a failure on demand. There is a correlation between
the PFD values of products and the SIL rating of an
installation in that the smaller the PFD value, the
higher the SIL rating of the installation will be.
The aim of a good system designer will be to
use systems which, if subjected to testing just
once a year, will fall as far below the maximum
permissible PFD as possible. Not only will this save
on system downtime, it will save on maintenance
and replacement costs.
By offering a complete system approach, includ-
ing installation as well as the controllers, cables,
input and output devices and service and main-
tenance procedures, fixed gas detection users
should be able to achieve a guaranteed Life Cycle
Safety reliability in accordance with SIL require-
ments, for as long as the integrity of the system
is maintained. Choosing the right system and
supplier will mean that SIL compliance can be
achieved at, almost, the touch of a button.
For this to be achieved, system integrity must be
essential. For instance, should a user change the
system parameters by, perhaps, performing service
and maintenance at seven months instead of the
recommended six, the probability of failure will
have been changed and SIL will no longer be valid.
For this reason, users should ask their suppliers to
support each of their SIL installations with a set of
comprehensive user notes. These should combine
recommended guidelines with comments and
examples to ensure that the ramifications of any
changes are clearly understood.
No-one can or should put a price on safety. The
good news, however, is that, with recent advance-
ments in technology, suppliers should be able to
provide systems that meet both safety and legisla-
tive requirements and that, in addition, reduce
system downtime and minimise maintenance
costs. These systems really do offer much more
than money can buy.
*Note: 94/9/EC Directive is currently under review
IFP
56 INTERNATIONAL FIRE PROTECTION
Draeger Sensor
Calibration
SAFETY IN GAS DETECTION � MORE THAN MONEY CAN BUY
GAS DETECTION
Further information is
available from:
Marion MacKenzie
Draeger Safety UK Limited
Ullswater Close
Blyth Riverside Business Park
Blyth
Northumberland NE24 4RG
Tel: 01670 561413
Fax: 01670 544475
"Is the gas detector removing the risk of explosion by, for
example, switching off power, closing down valves or
shutting off ventilation?"

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