detector placement and configuration

Protection for life…

© General Monitors 2012

Gas Detector Placement and ConfigurationEdward Naranjo Ph.D.Global Technical Product ManagerGeneral Monitors

Protection for life…© General Monitors 2012

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Content

• Introduction• Source Monitoring• Volumetric Monitoring• Enclosure Monitoring• Perimeter Monitoring• Conclusion• Appendix


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Introduction

• Fire and gas (F&G) detection systems are at the cornerstone of modern process plant safety– Alert staff to a developing hazard– Enable executive action to ensure that the plant is

placed into a safe state• F&G systems do not prevent a hazardous condition

from occurring, but rather mitigate the effects of a hazard


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F&G System Effectiveness

• F&G Systems’ ability to perform its intended safety actions in a demand condition depends on several factors– Detector geographic coverage– Detector scenario coverage– F&G system availability


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Detector Geographic Coverage

• Geographic coverage refers to the fraction of the geometric area of a monitored process area which, if a release were to occur, it would be detected by the detection equipment– Number of sensors– Location


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Detector Placement

• Methods for laying out flame and gas detectors are intended to maximize the potential for detection success in the event of a release– Source monitoring– Volumetric monitoring– Enclosure monitoring– Perimeter monitoring


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Source Monitoring

• Source monitoring involves the placement of detectors around potential release sources– Applicable to detection of moderate and large releases

of flammable materials– Useful for the detection of large or catastrophic toxic

releases that may migrate from the area


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Source Monitoring (Continued)

• Source monitoring is facilitated by a three-step process:

Possible Leak Sources Assessment Design

Layout

Determine if individual release sources should be considered individually or as one large leak source• Separated sources should

be surrounded by their own detection scheme

• Multiple sources within 10 – 15 ft (3.3 – 4.5 m) may be treated as single point source

Use computer modeling to evaluate target release• Determine set back

distance between potential source and sensors

• Determine allowable distance between sensors

Lay out sensors on plot plan using set back distances and sensor spacings• Heavier-than-air gases will

tend to move to low lying areas • Combination of point and open

path infrared (IR) detectors may provide most economical approach for achieving reasonable probability of detection success


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Example

6 m

5 m

10% LEL

6 m

5 m

Figure 1

Figure 2

If open path detector alarms at 10% LEL-m, cloud must overlap one of the beams by at least 1 m


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Volumetric Monitoring

• Volumetric monitoring is based on concept that greatest risk from flammable gas escapes is direct damage from explosions– Does not consider small releases as deemed impractical to detect

reliably

Flame velocity

Congestion

Space to achieve

maximum speed


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Detonation

• Ignited 6-m methane or propane cloud does not achieve flame speeds greater than 100 m/s in methane and 125 m/s in propane– Large unconfined volumes– Confined volumes with blockage rations less than 0.3 or 0.4

5 – 6 m (16 – 19 ft)

Pressure differential is less than required to produce structural damage (approx. 2 psi or 150 mbar)

100 m/s (330 ft/s)


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Example

≈ 5 m


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Blockages

• Structures within a space can prevent the dispersion of gas and produce localized pockets of high gas accumulations

• Structures can also generate enough resistance to a propagating flame front to generate a significant pressure front


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Blockage Ratio

• Blockage ratio is the ratio of obstructed area over total area– Considers each monitored volume a cube

Example: 11 x 55 m2 deck elevated by 15 m• Solid area is 30% of square• Three heat exchangers occupy base area of 30

m2

Deck area = 30% x (11 x 55) m2 Heat exchangers = 30 m2

Ground area = 11 x 55 m2

Total = 817 m2

Surface area of cube = 2 (11 x 55) + 2 (15 x 55) + 2 (11 x 15) = 3190 m2

Blockage ratio = 817 / 3190 = 0.26


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Volumetric Monitoring – Enclosed Buildings

• Gas clouds of certain diameter cannot exist without contacting a sensor

Type Volume, m3 (ft3) Gas Cloud Diameter, m (ft)

Small buildings < 1,000 (35,000) 4 (13)

Large buildings 1,000 (35,000) 5 (16)*4 (13)

* When an inerting system is present in compliance with NFPA 69 (Standard on Explosion Prevention Systems)


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Volumetric Monitoring – Semi-enclosed Volumes

• Considerations:– If volume does not contain a potential release source, apply

source monitoring method or monitor the perimeter of the volume– If gas is heavier than air and arises from a distant source, install

detectors near ground or grade level (18 in or 0.5 m) along perimeter

– If gas is heavier than air and comes from a nearby source or of a neutrally buoyant gas, provide multiple levels of detection at 5 m (16 ft) vertical intervals along perimeter

Volume, m3 (ft3) Blockage Ratio Gas Cloud Diameter, m (ft)

1,000 (35,000) > 0.3 5 (16)


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Volumetric Monitoring – Open Volumes

• Open volumes (blockage ratio < 0.3) usually do not require combustible gas detectors– Install detectors if open space contains multiple congested areas

• Use open path detection along perimeter if volume has no congested pockets with blockage ratios greater than 0.3 and hazard is gas migration from volume to other areas

Localized Blockage Ratio Condition Gas Cloud Diameter,

m (ft)> 0.3 NA 5 (16)*

0.3 Several congested areas 10 (33)

0.3 Isolated congested areas 5 (16)**

* Protection should be provided according to the detector deployment strategy for partially-enclosed volumes** Detection should be capable of sensing the gas cloud of 5 m (16 ft) in diameter anywhere within the pockets


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Liquid Phase and Heavier-than-air Releases

• Liquid gas releases behave differently from vapor phase releases– Less mixing with air– Gas clouds are typically dense and cold

• For areas containing inventories of process fluids, which on loss of containment remain as liquids for a significant period, a three-dimensional grid should be used– No more than 0.5 m (1.5 ft) above local deck or grade


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Liquid Phase and Heavier-than-air Releases (Continued)

• For isolated sources (ex. transfer pumps, loading installations in open areas), detectors should be placed on a 5 m (16 ft) triangular grid around potential heavy gas release points


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Enclosure Monitoring

• Enclosures are commonly found in the process industries– Analyzer buildings– Remote instrument enclosures (RIE’s)

• Detector deployment approach:– Install at least one combustible gas detector– Placement must be based on gas density

• Within 12 in of floor for heavier-than-air gases• Within 12 in of ceiling for gases lighter than air

In certain applications, gas density relative to air has little effect on where gas will accumulate compared to stronger influences like release pressure and ventilation


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Enclosure Monitoring - Heavier-than-air Gases

• If structure is naturally ventilated, install detectors in a square grip with maximum spacing of 4 m (13 ft) for a single potential source– For multiple sources, arrange

detectors in square or rectangular pattern so maximum sensor separation is 4 m (13 ft)

• If structure is mechanically ventilated ( 6 ACH), lay detectors according to expected path of gas travel

4 m (13 ft)

Potential release sources

4 m (13 ft)


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Enclosure Monitoring – Heavier-than-air Gases (Continued)

• Other considerations:– Place gas detectors in dead air spaces that encompass

potential leak sources– Place gas detectors where streams converge and

approach inlets of exhaust ventilation systems– Place detectors in pits, trenches, and other low lying

areas


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Enclosure Monitoring – Lighter-than-air Gases

• Gas detectors should be located 0.3 m (1 ft) within the ceiling if flat and 0.5 m (1.5 ft) within the apex– For naturally ventilated or

mechanically ventilated spaces, detectors should be arranged according to the guidance for heavier-than-air gases

0.5 m (1.5 ft)

0.3 m (1 ft)


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Enclosure Monitoring – Naturally Buoyant Gases

• Substances with specific gravities close to that of air should be monitored according to the volumetric monitoring technique

Gas Specific Gravity

Air 1.000

Acetylene 0.90

Ethylene 0.9683

Ethane 1.0378

Silane 1.11

≈ 5 m


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Perimeter Monitoring

• Perimeter monitoring is sometimes used where heavier-than-air or naturally buoyant gases are present

• Most effective under the following conditions:– Process unit or storage is close to plant perimeter and

there is high likelihood releases may cross the fence line– Potential release sources are outside the coverage of

local detectors


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Perimeter Monitoring (Continued)

• Considerations– Alarm set points should be as low as practical without

producing false alarms– Height of open path detector must be based on highest

point of grade along beam to prevent false alarms and beam block faults from snow, vegetation, or wildlife

– Open path detectors should be installed in locations where their paths will not be interrupted by routine operations

3 m


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Perimeter Monitoring (Continued)

• For heavier-than-air gases, the higher the detector is located above grade, the lower its alarm set point must be to remain effective

3 m

9 m

0.1 LEL-m

0.25 LEL-m


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Conclusion

• F&G system effectiveness depends on the appropriate allocation and placement of detectors

• Several detector deployment strategies maximize the chances of detection in the event of a gas release

• Source monitoring involves the placement of detectors around potential leak sources– Useful for moderate to large releases of combustible

materials and large or catastrophic toxic gas releases


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Conclusion (Continued)

• The volumetric method focuses on monitoring obstructed or congested spaces where flammable gases may accumulate– Uses a three-dimensional array of detectors to assure a

gas cloud of a certain size cannot exist in a monitored space without contacting a sensor

• Enclosure monitoring addresses gas detection within buildings or enclosures– Includes situations where the structure handles toxic

and flammable materials or where enclosures are subject to gas escapes from nearby plant areas


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• Perimeter monitoring is used to protect an area from migration of a gas plume into an area outside the owner’s control– Sometimes mandated by regulatory agencies interested in

providing increased protection from the public– Most effective in detecting process gases that are heavier

than air (ex. butane, carbon dioxide)• Regardless of detector placement method used, a credible

release may not necessarily be detectable with any degree of certainty– Obstructions at a plant may change over time, rendering the

allocation of sensors inadequate to address risks in certain areas

– Possible leak sources may not be identified during a preliminary hazard analysis


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• Fixed flame and gas detectors should be part of several safeguards that prevent or mitigate the consequences of gas releases


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References

• Continuous Monitoring for Hazardous Material Releases. 2009. Hoboken, New Jersey: John Wiley & Sons.

• ISA-TR84.00.07-2010, Guidance on the Evaluation of Fire and Gas System Effectiveness. 2010. Research Triangle Park, NC: ISA.

• Offshore Technology Report - OTO 93 002: Offshore Gas Detector Siting Criterion, Investigation of Detector Spacing. 1993. Croydon, UK: Health and Safety Executive.

• Zabetakis, M.G. 1965. Flammability Characteristics of Combustible Gases and Vapors. Bulletin 627. Washington, DC: U.S. Department of Interior, Bureau of Mines.


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Standards and Recommended Practices on Detector Placement

• API RP 14C (R2007), Analysis, Design, Installation, and Testing of Basic Surface Safety Systems for Offshore Production Platforms, seventh edition. 2001. Washington, DC: American Petroleum Institute.

• API RP 14F, Design, Installation, and Maintenance of Electrical Systems for Fixed and Floating Offshore Petroleum Facilities for Unclassified and Class 1, Division 1 and Division 2 Locations, fifth edition. 2008. Washington, DC: American Petroleum Institute.

• API RP 14FZ (R2007), Design and Installation of Electrical Systems for Fixed and Floating Offshore Petroleum Facilities for Unclassified and Class I, Zone 0, Zone 1 and Zone 0 Locations, first edition. 2001. Washington, DC: American Petroleum Institute.


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Standards and Recommended Practices on Detector Placement (Continued)

• BS EN 50073: 1999, Guide for the selection, installation, use and maintenance of apparatus for the detection and measurement of combustible gases or oxygen. 1999. London, UK: British Standards Institution.

• CXHO GP 30-85, Guidance on Practice for Fire and Gas Detection Philosophy. 2002. London, UK: BP Group.

• GP 30-85, Fire and Gas Detection. 2009. London, UK: BP Group.• ISA-RP12.13.02-2003 (IEC 61779-6 Mod), Recommended Practice

for the Installation, Operation, and Maintenance of Combustible Gas Detection Instruments. 2003. Research Triangle Park, NC: ISA.

• ISA-RP92.0.02, Part II-1998, Installation, Operation, and Maintenance of Toxic Gas-Detection Instruments: Hydrogen Sulfide. 1998. Research Triangle Park, NC: ISA.

• ISO 10418: 2003, Petroleum and natural gas industries – Offshore production installations – Basic Surface process safety systems, second edition. 2003. Geneva, Switzerland: ISO.

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