The placement of gas detectors has traditionally been an imprecise field of engineering. With no detailed prescriptive rules on when and where to place gas detection equipment, designs have been left to experts who use their judgment along with rules of thumb to set designs. These ad hoc methods have left industry in a position where different process units within the same refinery have vastly different gas detection designs for equipment in similar operating profiles. Furthermore, often no documented basis for the selection exists making it difficult to justify the differences in designs between units to stakeholders and regulators. In 2011, ISA released a technical report describing performance based methods for fire and gas system (FGS) design. This technical report laid out a safety lifecycle and introduced the new metric of “coverage” to define FGS designs. The approach presented in ISA TR84.00.07 was applied to the problem of H2S gas detection on a refinery Sulfur Recovery unit. All of the process equipment was assessed using calibrated semi-quantitative techniques, resulting in graded areas with associated coverage targets. Fire and gas mapping software was then utilized to confirm that the assigned coverage targets were achieved. This paper describes how that project was executed, presents an overview of the results, and compares the resulting design against other process units and expectations.

1.
ADIPEC 2013 Technical Conference Manuscript
Name: Edward Marszal
Company: Kenexis Consulting Corporation
Job title: President
Address: 3366 Riverside Dr, Columbus, Ohio, 43221, USA
Phone number: 614‐451‐7031
Email: Edward.Marszal@kenexis.com
Category: Case Studies
Abstract ID: 555
Title: Case Study: Implementing Performance Based Gas Detector Placement per ISA TR 84.00.07 on a Sulfur Recovery
Unit
Author(s): Edward Marszal, Elizabeth Smith
This manuscript was prepared for presentation at the ADIPEC 2013 Technical Conference, Abu Dhabi, UAE, 10‐13 November 2013.
This manuscript was selected for presentation by the ADIPEC 2013 Technical Committee Review and Voting Panel upon online submission of an
abstract by the named author(s).
Abstract:
The placement of gas detectors has traditionally been an imprecise field of engineering. With no detailed
prescriptive rules on when and where to place gas detection equipment, designs have been left to experts
who use their judgment along with rules of thumb to set designs. These ad hoc methods have left industry
in a position where different process units within the same refinery have vastly different gas detection
designs for equipment in similar operating profiles. Furthermore, often no documented basis for the
selection exists making it difficult to justify the differences in designs between units to stakeholders and
regulators.
In 2011, ISA released a technical report describing performance based methods for fire and gas system
(FGS) design. This technical report laid out a safety lifecycle and introduced the new metric of “coverage”
to define FGS designs. The approach presented in ISA TR84.00.07 was applied to the problem of H2S gas
detection on a refinery Sulfur Recovery unit. All of the process equipment was assessed using calibrated
semi‐quantitative techniques, resulting in graded areas with associated coverage targets. Fire and gas
mapping software was then utilized to confirm that the assigned coverage targets were achieved. This
paper describes how that project was executed, presents an overview of the results, and compares the
resulting design against other process units and expectations.
1

2. Introduction
A Fire and Gas System (FGS) performance‐based design basis was developed for the Sulfur Recovery Units (SRUs)
operated by a US Gulf Coast Refinery. The study was performed to determine the hazard posed to personnel from
process material releases in the vicinity of the SRUs. The Sulfur Recovery Units are responsible for treating gases
that have elevated levels of Hydrogen Sulfide (H2S) present, e.g. acid gas and sour water stripper gas produced as a
byproduct of other refining processes. As a result, the SRU has a significant inherent risk if a release were to occur.
The study included analysis of the H2S hazards, dispersion modeling, an assessment of detector coverage, and
recommendations for detector placement.
The case study was performed based on guidelines from ANSI/ISA‐TR84.00.07‐2010 Technical Report Guidance on
the Evaluation of Fire, Combustible Gas and Toxic Gas System Effectiveness. The Performance‐Based FGS Lifecycle
Process, as defined by this technical report, is shown in Figure 1.
Figure 1 FGS Safety Lifecycle (from ANSI/ISA‐TR84.00.07‐2010)
Step 1 can be determined by either the facility if the project is limited in scope, e. g. SRU included in the Case Study,
or can be defined during execution of a project if an entire facility is being analyzed, e.g. offshore platforms.
Steps 2 through 6 are discussed in the section of this paper that addresses the Hydrogen Sulfide Hazard Analysis,
whereas Steps 7 through 11 are discussed in the Coverage Assessment section, although each step is not explicitly
identified. Note that Step 7, in this particular case study was provided as an existing H2S Detector Array that will
remain in place, and Step 9 was not within the scope of this case study.
Hydrogen Sulfide Hazard Analysis
Hydrogen sulfide‐containing process streams are treated in the Sulfur Recovery Unit to convert gaseous hydrogen
sulfide to elemental sulfur. The Claus reaction was used in this particular application.
H2S + O2  S2 + 2 H2O
As the H2S is being converted and recycled, there are various concentrations of H2S present in the SRU, ranging
from less than 1% (v/v) to greater than 75% (v/v), or 1,000 ppm to 75,000 ppm. These concentrations are hazardous
to personnel. Hydrogen sulfide is a broad spectrum toxin, meaning that it damages several different body systems
simultaneously. The most pronounced effects are as a pulmanotoxin resulting in pulmonary edema in
concentrations in the low 300 ppm range and a neurotoxin resulting in rapid and then sudden loss of breathing at
concentrations as low as the 500 ppm range.
2

3.
Design Basis Scenarios and Dispersion Modeling
The design was based on analysis of location and magnitude of hazard scenarios. In this study, gas dispersion
modeling was conducted for the identified credible hazard scenarios to determine the potential size of gas clouds
and to determine which equipment had the potential to release hazardous concentrations of H2S. Equipment of
interest was identified based on this analysis, and existing detectors were modeled to determine coverage of
identified hazard scenarios. Since every possible hazard scenario cannot feasibly be analyzed, a subset of those
scenarios that have been chosen as the basis of the design, those scenarios are herein designated as “Design‐Basis
Scenarios”.
Dispersion modeling was conducted to provide incident outcomes of design basis hazards. In the case of H2S gas,
the design basis hazard has been defined as an initial incipient stage gas cloud which can be detected at a point
which allows for personnel egress from the affected area as well as preventing personnel entry into the area if
unoccupied at the time of release.
Dispersion modeling was performed using commercially available consequence analysis software and general
parameters reflecting the condition at the facility.
In selecting appropriate design‐basis scenarios, consideration was given to the predominant H2S‐containing streams
present in the SRU. In sulfur recovery units, release scenarios associated with 5 mm equivalent hole diameter were
selected as credible design‐basis scenarios. The released streams that were considered included: Acid Gas, Sour
Water Stripper Gas, Tail Gas, and Rich Amine used for both Tail Gas Treatment and Acid Gas Treatment. Tail Gas
was modeled as a 25mm hole size due to the less concentrated process conditions and H2S concentrations.
Credible hazard scenarios were identified that have the potential to result in personnel injury due to H2S exposure.
A small leak (5 mm equivalent hole diameter for excepting Tail Gas) was identified as having the capability to create
a hazardous situation while simultaneously being more difficult to detect than larger leaks.
For H2S exposure, the level‐of‐concern for injuries is 100 ppm (parts per million v/v) airborne concentration (IDLH as
per the United States National Institute of Occupational Safety Hygienists [NIOSH]) and life‐threatening effects at
700 ppm airborne concentration or higher (immediate exposure). In addition, lower concentrations associated with
occupational exposure of 10 ppm were modeled, this concentration is the limit of detection / alarm capability for
many common H2S detectors. The Short‐Term Exposure Limit (STEL) as established by the American Conference of
Government Industrial Hygienists (ACGIH) is 5 ppm as of 2010 publication (the STEL was previously 15 ppm). The
STEL is the exposure concentration not to be exceeded for up to 15 minutes, not more than 4 times per 8 hr day.
The justification for lowering the STEL was to protect against minor irritation of the respiratory tract and a “brief
change” in rate of oxygen uptake. For each representative release scenario, H2S dispersion was modeled to four
endpoints, as shown in Table 1.
Table 1 – H2S Dispersion Endpoints
Endpoint Concentration
(ppm)
700
100
10
Notes
Potentially Fatal – Consistent w/ Course H2S Dispersion Study
IDLH – Consistent w/ Course H2S Dispersion Study
H2S Detection High Alarm Concentration
3

4. Commercially available computerized consequences analysis software was used to characterize the extent of toxic
dispersion and the potential impact on personnel. Table 2 shows the release rate of H2S, the duration of the release
modeled, and the anticipated distance the H2S will travel downwind of the release point. The reported distances
are the IDLH concentrations and detectable concentrations, respectively.
Table 2
Design Basis Consequence Modeling
Model
Description
Release Rate (lb/hr)
Duration
(min)
Distance to Endpoint (ft)
Distance to
100 ppm
Distance to 10
ppm
S‐01
Acid Gas ‐ 5mm vapor leak
13
60
100
540
S‐02
SWS Gas ‐ 5mm vapor leak
9
60
60
240
S‐03
Tail Gas ‐ 25 mm vapor leak
170
60
50
80
S‐04
Tail Gas Clean‐Up Rich Amine
27 (1)
60
35
160
S‐05
(1)
Acid Gas Clean‐Up Rich Amine
144 (1)
60
90
350
Release Rate of H2S only
Figure 2 shows the results of the Acid Gas design basis scenario dispersion modeling when taking a vertical cross‐
section of the dispersion profile. This view represents the height of the cloud as well as the downwind distance
reached by various concentrations of H2S.
Figure 2 Acid Gas ‐ 5mm leak ‐ Vapor Dispersion ‐ Sideview
Figure 3 shows the results of the Acid Gas design basis scenario dispersion modeling when taking a horizontal cross‐
section, or “footprint” of the dispersion profile. This view represents the width of the cloud as well as the downwind
distance reached by various concentrations of H2S at or near ground level. This model type is used during the
detector coverage assessment.
4

5.
Figure 3
Acid Gas ‐ 5mm leak ‐ Vapor Dispersion ‐ Footprint
The gas dispersion analysis is also sensitive to meteorological conditions. A wind rose was obtained to determine
the relative probability of the release being blown in any specific direction.
Also, the following atmospheric stability and wind speed condition was modeled after considering the range of
atmospheric conditions that are likely to occur at the facility.
 Adverse Nighttime: F class atmospheric stability with 1.5 m/s wind speed
Toxic gas cloud sizes resulting from releases are very sensitive to the selected wind speed. In addition, the wind
direction distribution (wind rose) for the case study area was used to determine the probability of gas dispersing in
any given direction around the point of release. This is shown in Figure 4.
5

6. Figure 4
Case Study Wind Rose
Design Basis Consequence Assessment
In order to assess the coverage provided by toxic gas detectors, the location, size and frequency of the design basis
releases must be determined. This is performed on an equipment‐by‐equipment basis, by first determining which of
the representative release scenarios most accurately reflects the process material within each piece of equipment.
The equipment must then be assigned a location in relation to other equipment and also gas detectors in the
vicinity, which is typically done using plot plans. The frequency of release is based on the type of equipment being
analyzed. Generally rotating equipment, such as pumps and compressors, is assigned a release frequency that is
greater than that assigned to pressure vessels or shell and tube heat exchangers. These frequencies can be obtained
from publicly available databases of leak rates as well as proprietary data from specific process plants and operating
companies.
Table 3 shows some of the major pieces of SRU equipment considered for this study, the representative scenario
applicable to that equipment, and the chosen release frequency (assuming a 5mm leak. Please note, Table 3 does
not include all leak sources included for this study.
Table 3
Sample of SRU Equipment and Associated Release Scenarios / Frequencies
Representative
Release Frequency
Equipment Description
Scenario
(/yr)
Amine Stripper Tower
Stripper Reflux Pumps
Acid Gas Scrubber
SRU Scrubber Pumps
Reactor
SWS Gas Scrubber
S01
S01
S01
S01
S01
S02
1E‐03
1E‐02
3E‐03
1E‐02
3E‐03
3E‐03
6

7.
Representative
Scenario
Release Frequency
(/yr)
SWS Gas Scrubber Pumps
SWS Gas Knock‐Out
Sour Water Pumps
Tail Gas Treatment Reactor Feed Cooler
Tail Gas Treatment Reactor
Tail Gas Treatment Stripper
Tail Gas Rich Solvent Pump
Tail Gas Rich Solvent Flash Drum
Acid Gas Rich Solvent Pumps
S02
S02
S02
S03
S03
S03
S04
S05
S05
1E‐02
1E‐03
1E‐02
1E‐03
3E‐03
3E‐03
1E‐02
1E‐03
1E‐02
Acid Gas Lean / Rich Solvent Exchanger
S05
1E‐03
Equipment Description
The magnitude and frequency of releases has been determined based on the design scenarios and the types of
equipment that process the hazardous material. This is as inputs to the hazard assessment and the gas coverage
assessment discussed in the following section.
Gas Coverage Assessment
The Gas Coverage Assessment consisted of the following tasks for the case study discussed in this paper:
 Define Performance Targets

Mapping of Gas Releases

Assess Detector Coverage
o
o

Unmitigated Risk Assessment (without benefit of detectors)
Mitigated Risk (first‐pass or existing detector design / layout)
Modify FGS Design
A Gas Detection System performance assessment typically also includes an assessment of the FGS Safety Availability.
The Case Study on which this paper is based did not include this in the scope of the H2S hazard assessment, only an
assessment of coverage was carried out.
Defining Performance Targets
The performance grade is a specification that defines the ability of an FGS function to detect, alarm, and if
necessary, mitigate the consequence of a fire or gas release upon a demand condition. In concept, a higher hazard
installation should require higher levels of performance; while a lower hazard installation should allow for lower
levels of performance, so that FGS resources can be more effectively allocated.
Specification of target performance grade for fire and gas systems is an exercise in risk assessment. For the Case
Study, a semi‐quantitative risk assessment was performed to specify risk reduction requirements for each area using
a scoring procedure. The goal of this risk assessment was to assign a performance grade for each leak source within
the Sulfur Recovery Unit zone.
Each process area containing toxic gas hazards was characterized by one of four performance grades. These grades
are listed in Table 4.
7

8.
Table 4
Gas Performance Grades
Grade
Exposure Definition
Required
Detector Coverage
A
Hydrocarbon processing, with high exposure.
90%
B
Hydrocarbon processing, with moderate exposure.
Hydrocarbon processing, with low or very‐low exposure.
80%
60%
C
No Grading
~
Risk is tolerable w/o benefit of FGS
Each of the grades serves to define a relative level of gas risk, with grade A being the highest risk areas and grade C
being the lowest risk areas necessitating detection. The grades correspond to required levels of FGS performance
(i.e. detector coverage) as shown in Table 4.
The Required Detector Coverage metric was chosen for this project. Alternatively, the release frequency, detector
coverage, FGS availability, and other factors (e.g. occupancy) can be assessed and compared verified against
allowable risk targets (e.g. Target Mitigated Event Likelihood) as defined by the organization or facility.
Gas performance grades were specified for gas functions which protect H2S processing areas. Performance targets
were selected toxic gas detection for each major equipment item. A summary of the results are presented in Table
5, which provides the selected performance target for gas functions in the SRU for the equipment containing the
defined design basis scenarios. The grade selections are based on equipment type, occupancy, process conditions,
and toxic concentration.
Table 5
SRU Grade Selection Summary
Plant: 1. Sulfur Recovery Unit
Zone ID
SRU
Zone Description
Zone Type
Sulfur Recovery Unit X - Toxic
Area Grading
Equip Tag
Acid Gas
Equipment Description
Acid Gas
79% H2S
<50 psig
Area Grading
Hazard Type
Grade
H2S Gas
A
Sour Water Stripper Sour Water Stripper Gas
Gas
40% H2S
<50 psig
H2S Gas
A
Tail Gas
H2S Gas
B
H2S Gas
C
Amine Regeneration Unit Rich H2S Gas
Amine
B
Tail Gas
0.8% H2S
<50 psig
TGU Rich Amine
Tail Gas Unit Rich Amine
1% H2S
50 - 100 psig
ARU Rich Amine
2% H2S
100 - 150 psig
Note: the grades reported here are only indicative of design basis scenarios within the zone. FGS detection for all
major equipment within each zone is defined independently based on the process stream conditions and equipment
type. As the detector coverage is relevant to an entire zone and ell equipment contained within the zone, this Case
8

9. Study chose the highest grade, A, as the criteria that must be met, i.e. the detectable scenarios must be greater than
or equal to 90% of the total scenarios.
Mapping Gas Releases
The gas releases, as shown in Figures 2 and 3 and Table 3 are represented graphically using a commercially available
FGS assessment tool. The mapping analysis involves obtaining an equipment layout or plot plan of the zone being
analyzed. Figure 5 presents the Unmitigated Gas Risk.
Figure 5 Unmitigated Gas Risk
This is a composite of all of the equipment that were considered leak sources, a sample of this equipment is listed in
Table 2. The major equipment is shown here (with all identifying information removed) – the plot plan and overlay
can be as detailed as the plot plans provided, resulting in varying degrees of detail in the geographic risk image.
The FGS mapping software aggregates the release scenarios, consisting of concentration, frequency, and dispersion
size, and plots the “footprint” overlaid on a plot plan of the unit. The location of the release is defined by the user to
coincide with the equipment that is the source of the leak. The software output also shows the wind distribution
and release frequency.
As shown in Figure 5, the highest hazard frequency typically occurs in areas that contain the most equipment of
concern. Alternatively, it can occur in the vicinity of higher risk (i.e., higher leak rate) equipment, such as pumps or
compressors. Although not utilized in this case study, the release scenarios can also be represented in either the
upwind or downwind direction from equipment by an appropriate distance – this distance is a property of the
release orientation, release height, process conditions, and the material being released.
9

10. Detector Coverage Assessment
FGS detector coverage was assessed in order to determine the risk posed by an H2S release in the SRU to personnel
located in the SRU and ensure that the performance coverage targets are achieved.
The existing layout of detectors was assessed to ensure the coverage footprint is sufficient to provide the required
hazard alarms and control actions. The effective range of selected detectors considers the expected environment
based on test data.
The existing design was analyzed to determine the gas scenario coverage. This involves an assessment to determine
how effective the proposed array of detectors will be in detecting an incipient hazard at a level that will alert
personnel to a toxic gas release. Using the FGS assessment software tool, Kenexis has reviewed the existing design
and modified as necessary to deliver acceptable gas coverage. As the detection of H2S within the process area of
this Case Study does not initiate any automatic actions, and is instead used only for alarm, the coverage numbers
were based on any single detector being in alarm state, or 1ooN. It is also possible to report one or more detector’s
ability to detect a hazard, i.e. 2ooN coverage, in instances where appropriate. This is often used for fire detection as
well as automated actions (e.g. shutdown, isolation, blowdown, and fire mitigation measures) based on gas
detection as 2ooN vote‐to‐trip will reduce spurious activation.
The initial design, which accounted for the location of existing detectors, included 11 gas detectors. This resulted in
an achieved coverage of 58.1%, which is graphically represented in Figure 6 and reported in Table 6.
Table 6
Detector Coverage Summary
Layout
Number of
Achieved Coverage
Zone
Hazard
Detectors
(%, 1ooN)
Existing
8
58.1%
SRU
Toxic – H2S
Recommended
15
90.1%
10

11. Figure 6
Mitigated Gas Risk – Existing Detector Layout
Figure 7 represents detector placement after inclusion of the recommended detectors. These detectors are in
addition to those detectors already existing. The Case Study focused on the inclusion of existing detectors with the
addition of detectors to meet the performance targets.
11

12. Figure 7
Mitigated Gas Risk – Recommended Detector Layout
It is likely that the performance targets could have been achieved using fewer detectors had the Case Study included
provisions for an “optimized design”, i.e. achieving desired coverage with the fewest installed detectors. This is not
meant to imply that the additional detectors do not offer hazard reduction to the facility, as they may be used for earlier
detection, may be placed at locations that are heavily trafficked and therefore represent increased hazard to personnel,
or represent areas in which increased awareness is desired.
12

13.
References
ISA‐TR84.00.07 The Application of ANSI/ISA 84.00.01‐2004 Parts 1‐3 (IEC 61511 Parts 1‐3
Instrumented Functions (SIFs) in Fire & Gas Systems
Modified) for Safety
Kenexis Case Study on H2S Hazard Analysis and Assessment performed for a US Refinery’s Sulfur Recovery Unit.
13