Introduction to FGS

1. Introduction to Fire &
Gas Systems (FGS)
Design & Mapping
(Rev. 2.0)

2. Intro
Basic
Concepts
Design Bases
Concluding
Remarks
Detailed Design,
Construction &
M&O
FGS Model
2
FGS Overview & Basic Architecture
 FGS are employed in the O&G industry for the purpose of
detecting loss of containment of hazardous materials from the
process and initiating response to mitigate the release impact.
 Loss of containment can be a small leak or a catastrophic
release. It may be detected directly by measuring the presence of
the released materials (e.g., gas concentration) or inferred from
the effects of the release (e.g., thermal radiation from a fire).
 Detection may include flammable gas, toxic gas, smoke, flame,
acoustic emission, or rapid heat rise in areas adjacent to the
process itself. Detector coverage and associated detection
capability varies substantially depending on the hazard scenario.
 Actions taken by the FGS may be manually or automatically
initiated and may affect a wide variety of systems, such as
audible and visual alarm indications, water deluge, fire
suppressant initiation, HVAC system equipment, process
isolation, or process de-pressurization. Similar to detection
capability, mitigation effectiveness is highly scenario-dependent.

3. Intro
Basic
Concepts
Design Bases
Concluding
Remarks
Detailed Design,
Construction &
M&O
FGS Model
3
Position of FGS in the PSM System
FGS systems are one risk reduction measure to protect facilities from
major accidents. There are many other technical and organizational
barriers of defense being applied to prevent damage to life and/or facilities.
The figure provides a broad view of common “barriers of defense” to clarify
the position of FGS within this context.
FGS systems come into action when several other layers of protection
have already failed. A FGS cannot prevent the occurrence of a gas release.
It is designed to warn the personnel of a site as early as possible about a
gas release taking place and it can start automatic actions if designed so.
Consequently, according to the author´s view a FGS should be considered
as part of the plants´ emergency response. However, whether FGS are
viewed as a prevention or mitigation measure depends on the point of view
of an individual O&G company. Some companies would deem that FGS
are an element of Plant Emergency Response, while some may view FGS
as a layer of Critical Alarm and Operator Intervention. The latter is not easy
to justify.
Gas Detectors and KPIs: Some O&G Companies include the activation of
safety devices as leading or lagging indicators for Process Safety. Such
activations are reported through their IT system. It seems that considering
the triggering of gas detectors as a Process Safety Indicator is a smart way
to raise awareness of the FGS network among personnel.
FGS

4. Intro
Basic
Concepts
Design Bases
Concluding
Remarks
Detailed Design,
Construction &
M&O
FGS Model
4
Can I Take Credit for a FGS Loop in a LOPA ?
 In a LOPA the intent is to prevent a Loss Of
Containment (LOC) from occurring
 FGS loops are generally NOT preventive layers
and normally do NOT qualify as an Independent
Protection Layer (IPL) in a LOPA, i.e. assigning a
SIL to a FGS is not in compliance with the
definition of an Independent Protection Layer
(within LOPA). Specifically ‘A layer of protection
which will interrupt a hazardous scenario
irrespective of the performance of another layer of
protection or the initiating failure.’
 Additionally, gas leaks are generally not caused by
the process but by equipment/pipe failure and no
additional layers of protection can prevent these
failures.
 Note –It depends on what one is trying to prevent.
If the hazardous event is considered as “serious
personnel injury” instead of LOC, then FGS may
qualify as a prevention layer (almost impossible to
demonstrate)
Prevention
(reduces frequency)
BPCS
ESD (SIS)
PSV
Operator
procedure
Hazardous Event
(LOC)
Mitigation
(reduces severity)
Fire and Gas
Fire suppression
systems
Emergency
Procedures

5. Intro
Basic
Concepts
Design Bases
Concluding
Remarks
Detailed Design,
Construction &
M&O
FGS Model
5
Prescriptive and Performance Based Standards
 Use of risk-based design is rapidly
becoming the norm for FGS within the
O&G industry. Traditionally, prescriptive
practices were use to design FGS.
These prescriptive practices do not
require evaluation of the risk reduction
capability of the FGS.
 Prescriptive practices often suffered
from two main flaws:
 FGS were often unable to detect
hazards due to an insufficient
number of or poorly located
detectors (not coverage
evaluation).
 A relatively high frequency of
spurious activation. This has been
in part due to poor instrument
selection and installation
Prescriptive standards specify the requirement to
meet the code while performance based
standards only give a guideline to the designer /
end user.
 NFPA 72 and EN 54 codes are prescriptive
standards, primarily intended for fire protection
in enclosed areas.
 IEC 61511 and ISA 84.00.01 are performance
based standards, which have been written to
help analyze, design, realize, install,
commission and maintain SIL loops for the
process industry.
 The ISA 84.00.01 committee has released a
FGS technical report for the process industry
in 2010, and a new edition was issued in 2018
(ISA TR 84.00.07)

6. Intro
Basic
Concepts
Design Bases
Concluding
Remarks
Detailed Design,
Construction &
M&O
FGS Model
6
What’s in the New Version of IEC 61511?
There are many changes in the new version of
IEC 61511. However, we shall emphasize in only
one substantive:
1. Demand mode functions are no longer
separated into mitigation and preventive
functions.
2. Main reason for this change being that SIF, as
they are designed in accordance with IEC61511
and its associated approaches, are necessarily
preventive.
3. Everything in the standard and associated
annexes and technical reports assumes that if
the SIF operates properly, then no consequence
will occur.
4. Even something as fundamental as the Risk
Reduction Factor is invalidated by a SIF that is
not preventive.
5. Mitigation systems require a much more
rigorous analysis if they are to be quantitatively
designed. Simple techniques such as LOPA are
useless in assessing the requirements of these
types of systems.

7. Intro
Basic
Concepts
Design Bases
Concluding
Remarks
Detailed Design,
Construction &
M&O
FGS Model
7
Risk-Based Approach
 A risk-based approach is difficult to apply to FGS due to three factors:
 FGS are generally implemented to reduce the risk of LOC. These hazards may be difficult to define and analyze
without using advanced risk analysis techniques, such as gas dispersion modeling or fire modeling associated
with a given scenario.
 FGS do not prevent a hazardous situation, but rather minimize the effects of an event that has already occurred
(i.e. residual risk to be tolerable)
 Even properly designed and managed FGS can provide poor risk reduction in the operating environment due to
inadequate detector coverage and mitigation effectiveness.
 It is therefore difficult to develop a sound technical justification for allocating risk reduction to FGS functions in LOPA.
Consequence modelling, residual risk considerations, evaluation of detector coverage and mitigation effectiveness are
all required, (this means a risk-based approach),which are all beyond simplified risk assessment tools (e.g. LOPA).
 The main fundamental aspects of this risk-based approach are:
 Design and implementation of a FGS can be performed in a manner that is consistent with the underlying
principles of both ANSI/ISA-84.00.01-2004 and IEC 61511.
 The fundamental approach is to examine the hazard and risk in order to establish required FGS performance, and
then to specify a design that achieves that performance.
 This performance-based FGS design process integrates into the relevant portions of a Safety Lifecycle for safety
functions.

8. Intro
Basic
Concepts
Design Bases
Concluding
Remarks
Detailed Design,
Construction &
M&O
FGS Model
8
Summary
The risk-Based approach can be summarized as follows:

9. Intro
Basic
Concepts
Design Bases
Concluding
Remarks
Detailed Design,
Construction &
M&O
FGS Model
9
FGS Concepts
 FGS Effectiveness - The ability of the FGS function to
detect and mitigate a design-basis hazard under a
demand condition. FGS effectiveness is dependent on
a number of factors associated with design, installation,
site-specific operating conditions, and maintenance.
FGS effectiveness is a function of the selected FGS
performance metrics, including detector coverage, FGS
safety availability, and mitigation action effectiveness,
accounting for common cause, common mode, and
systematic failures.
 FGS Safety Availability - The availability of the fire and
gas function designed to automatically mitigate the
consequences of hazards. FGS availability is equal to
one minus the probability of failure on demand
(PFDavg) for the FGS safety function (sensor, logic
solver, and/or final element).
 Mitigation Effectiveness – The probability that the
results of activating the final element(s) will mitigate the
consequence of a defined hazard as expected (e.g.,
prevents a small fire or gas accumulation from
escalating to a large fire or accumulation). The FGS
must be activated in a sufficiently timely fashion to
reduce the event severity.
Leak or
Fire
Detection
Coverage
Mitigation
Effectiveness
YES
NO
YES
NO
YES
NO
0.15
0.85
0.98
0.02
0.85
0.15
F&G System
Availability
X/yr.
Consequence