Dp15k

SAFETY IN PLANT DESIGN DESIGN PRACTICESFLAMMABLE GAS, TOXIC GAS,

AND FIRE DETECTION SYSTEMSSection

XV-KPage

1 of 17EXXONENGINEERING PROPRIETARY INFORMATION - For Authorized Company Use Only

DateDecember, 1999

EXXON RESEARCH AND ENGINEERING COMPANY - FLORHAM PARK, N.J.

CONTENTSSection Page

SCOPE ............................................................................................................................................................ 3

REFERENCES ................................................................................................................................................ 3DESIGN PRACTICE ............................................................................................................................... 3INTERNATIONAL PRACTICE ................................................................................................................ 3OTHER LITERATURE ............................................................................................................................ 3

GENERAL ....................................................................................................................................................... 3

RISK FROM FLAMMABLE AND TOXIC MATERIAL RELEASES ................................................................ 3

TOXIC GAS DETECTORS.............................................................................................................................. 4APPLICATION IN H2S DETECTION....................................................................................................... 5APPLICATION IN HF DETECTION ........................................................................................................ 7PERIMETER MONITORING................................................................................................................... 7

FLAMMABLE GAS DETECTORS .................................................................................................................. 8TYPES OF FLAMMABLE GAS DETECTORS........................................................................................ 8LOCATING HYDROCARBON DETECTORS ......................................................................................... 8

ALTERNATIVES TO TOXIC AND FLAMMABLE GAS DETECTORS ......................................................... 10

FIRE DETECTORS ....................................................................................................................................... 10SMOKE DETECTORS.......................................................................................................................... 11HEAT DETECTORS ............................................................................................................................. 11FLAME DETECTORS........................................................................................................................... 11FLAME DETECTOR PERFORMANCE................................................................................................. 12DUAL SENSOR PERFORMANCE ....................................................................................................... 13DETECTOR RESPONSE ..................................................................................................................... 13WHEN TO INSTALL FIRE DETECTORS? ........................................................................................... 14DESIGN GUIDELINES FOR DETECTORS.......................................................................................... 14

TABLESTable 1 Substances Exhibiting Significant UV Absorption ............................................................. 12Table 2 Potential UV and IR Flame Detector False Alarms ........................................................... 13

FIGURESFigure 1 Radiation Spectra from a Hydrocarbon Fuel Fire.............................................................. 15Figure 2 Flame Detector vs Sunlight Transmission......................................................................... 15Figure 3 Typical Flame Detector Response to Gasoline Pan Fire .................................................. 16Figure 4 Flame Detection Distance vs. Fire Size ............................................................................ 16Figure 5 Flame Detection Distance vs. Viewing Angle.................................................................... 17

Changes shown by ➧

DESIGN PRACTICES SAFETY IN PLANT DESIGN

SectionXV-K

Page2 of 17

FLAMMABLE GAS, TOXIC GAS,AND FIRE DETECTION SYSTEMS

DateDecember, 1999 PROPRIETARY INFORMATION - For Authorized Company Use Only

EXXONENGINEERING


Revision Memo

12/99Page 3 Updated references.Page 5 Added clarification to second bullet on availability of toxic gas detector

technologiesPages 5 and 6 Added "Potential" to the definitions of the H2S Hazard Areas. Changed

the upper limit of the Low Potential H2S Hazard Area and the lower limitof the Medium Potential H2S Hazard Area from 1000 vppm to 250 vppmalong the lines of the EBSI proposal in report MR.35DQ.97. Clarifieddefinitions of all H2S Hazard Areas. Added flowchart to aid in thedetermination of the potential H2S hazard areas.

Page 7 Added section on Application in HF Detection. Updated the perimetermonitoring technology.

Page 8 Updated the catalytic point detector technology. Updated the IRAbsorption point detector technology. Updated the Open-Path IRdetector technology. Removed the following incomplete andunnecessary sentence: "For a fixed size hole, the amount of gasreleased at 200 psig (14 barg) is roughly equal to the amount of vaporreleased from liquid butane at 100°F (38°C)." Added "higher risk" todefinition of materials involved.

Page 9 Added fifth bullet: "Control valves operating above 200 psig (14 barg)" tothe list of leak sources. Clarified the air monitoring recommendation for"Cooling tower exhaust' (5th bullet) and removed the followingstatement: "Hydrocarbon analyzers in the return water may besubstituted for hydrocarbon vapor detectors.

Page 10 Added "toxic gas" to detectors that are not required.Page 11 Added statements related to plastic tubes that may be used as

alternatives to traditional fire-sensing devices.Page 16 Added "Flame Detector" to title of Figure 3 for clarification and removed

the "6 in. (150 mm) Diameter". Also corrected Figure 4 distance scale'slowest value to 10 from 1.

Page 17 Corrected the degree signs on the viewing angles shown.



XV-KPage


DateDecember, 1999


SCOPEThis section describes the use of detection systems for flammable or toxic gas releases and for fires. Early detection is a key tocontrolling a release or fire and mitigating its effects.

➧ REFERENCESDESIGN PRACTICEX-G Pumps - Shaft Sealing

INTERNATIONAL PRACTICEIP 4-3-1, Plant Buildings for Operation and StorageIP 4-3-3, Shelters for Process AnalyzersIP 10-8-1, Combustion Gas TurbinesIP 15-8-3, Combustible and Toxic Vapor Detection SystemsIP 16-6-1, Substation Layout

OTHER LITERATUREOutdoor Installation of Flammable Gas Detectors, ER&E Report No. EE.77E.78.Guidelines on Safety Requirements for H2S Monitoring, ER&E Report No. EE.15E.85.Guidelines for the Use of Fire Detectors, ER&E Report No. EE.43E.87.Marketing Terminal Fire Detection Systems - Engineering Standard, ER&E Report No. EE.2M.90.National Fire Alarm Code, National Fire Protection Association - NFPA 72HF Alkylation Consequence Reduction Technology; Best Proven Reasonable Design for Detection, Automation and Initiation,ER&E Document No. 99SR37.Hydrogen Sulfide Handling Practices at refineries and Chemical Plants: A Benchmarking Study, EBSI Report No. MR.35DQ.97.Pump Sealing Technology Manual, ER&E Report No. TMEE-023.Proof of Principle, Development and Field Test of a New Commercially Available Hydrogen Sulfide Open Path Monitor UsingTunable Diode Lasers, ER&E Report No. EE.125E.98.

GENERALWhile the goal in design and operation of refineries and chemical plants is a plant free of gas releases and fires, the risk of arelease or fire is never zero. Some areas of the plant have a greater risk than others. The risk is influenced by the materialcontained, the conditions under which it is contained, and the equipment and procedures used to contain it.The risk from flammable or toxic gas releases or fires can be reduced by early detection and subsequent mitigating actions.Detection and response to a release or fire, is often effectively provided by the process operators, in conjunction with their dailyactivities. However, sometimes the nature of the event is such that a more rapid detection and response is required than can beexpected from the operator. Additionally, the increased use of remote monitoring and automation can reduce the time operatorsspend on the unit, possibly increasing the time to detect a leak. When more rapid detection is necessary than can be provided bythe process operator, an instrumented monitoring system can be used.

RISK FROM FLAMMABLE AND TOXIC MATERIAL RELEASESThe risk from a flammable or toxic material release is the combination of the probability of a leak occurring and the potentialconsequences should the leak occur. In determining the probability of a leak, the type of operation, the operating conditions, andthe equipment involved are all considered. In determining the consequences, the material contained, the size of the leak, theproximity of the leak to other equipment or the fence line, and speed with which the leak might be detected without instrumentsare all considerations.All releases occur as a result of some failure. These failures can generally be classified as follows:• Human errors (e.g., failure to close a drain valve, incorrect valve opened).• Active equipment failures (e.g., pump seal or bearing failure).• Passive equipment failures (e.g., pipe gasket failure or pipe rupture).


SectionXV-K

Page4 of 17



EXXONENGINEERING


RISK FROM FLAMMABLE AND TOXIC MATERIAL RELEASES (Cont)In addition to this general classification, data on typical refinery and chemical plant manufacturing indicate certain areas whichhave historically caused release problems. Areas of higher probability include:• Open sampling or drain points.• Equipment opened for maintenance.• Seals on rotating equipment, i.e., pumps and compressors.• Atmospheric seals on sulfur units.• Flange gaskets.• Tanks, due to overfilling.• Small lines in vibrating service.• Lines in corrosive/erosive service.• Plate and frame heat exchangers.When a leak does occur the consequences may range from inconsequential effect to personnel health effects, fire, or vapor cloudexplosion. The severity of the consequences depends on the following factors:• The material leaked, specifically its toxicity or flammability.• The mass rate leaked which is a function of the material vapor pressure, the operating temperature and pressure, and the

cross-sectional area of the opening.• The proximity of the leak to in-plant personnel, ignition sources, and the property line.• The ability to detect the leak and stop or reduce the rate.• Atmospheric conditions, especially wind velocity and direction.Toxic gas, flammable gas, and fire detectors can reduce the risk of a release by reducing the severity. Severity is reducedbecause faster response is possible. The response may be carried out by the operator or through an automated response.Automated responses include stopping and/or isolating equipment and automated foam, water spray, or deluge systems.While vapor and fire detectors reduce risk by decreasing the severity of a release, other design changes and/or instrumentationmay provide equivalent risk reduction by reducing the probability of a release. Designs which reduce the probability of a releasemay be considered as an alternative to detectors, e.g., pump seal leak detection in place of toxic or flammable vapor detectors.Also see ALTERNATIVES TO TOXIC AND FLAMMABLE GAS DETECTORS later in this section.

TOXIC GAS DETECTORSToxic gas often is present in refinery or chemical plant streams. Hydrogen sulfide (H2S) is often present in refinery streams, buthydrogen fluoride (HF), chlorine (CI2), carbon monoxide (CO), anhydrous hydrogen chloride (HCI), bromine (Br2), and ammonia(NH3) may also be present at a site. Small releases of toxic gas are a threat to personnel working directly where the leak occurs.Medium releases may affect other personnel working on the unit but not directly at the release source. Large releases mayproduce toxic concentrations outside the unit or even outside the plant fence line.Toxic releases may be generally classified by the cause of the release as follows:• Releases resulting from venting or draining equipment, typically for sampling or as part of maintenance. Toxics are not

intentionally vented but equipment may not be properly gas freed or the toxic concentration may not be properly identified.• Releases from "active" equipment. From previous experience, active equipment which is a likely release point includes:

pump and compressor seals, sulfur plant seals, and H2S incinerators which have lost flame. Since the amount of activeequipment is low, the number of likely release points is relatively few and known in advance.

• Releases from "passive," higher integrity equipment, such as pipe flanges and pipe/vessel wall ruptures. These are lowerprobability release points than for active equipment. Unlike active equipment, the amount of passive equipment is very largeand the most likely release points are not easily identified.

Different detection methods are employed for each type of release.• Since releases which occur from opening equipment immediately expose personnel, the absence of toxics must be

confirmed beforehand. Effective control of releases is by training, effective work permits including contractor monitoring,personal or portable gas detectors, and the use of personnel protection like breathing apparatus when appropriate.Sampling systems should be designed for closed transfers whenever possible.



XV-KPage


DateDecember, 1999


TOXIC GAS DETECTORS (Cont)

➧ • Releases from higher potential, active equipment should be monitored with toxic gas detectors. Toxic gas detectorsavailable for use in refineries and chemical plants are based on a wide range of technologies including electrochemicalsensors, solid state sensors, ion mobility sensors, and FTIR and laser diode open path systems. Detectors should belocated close to the potential leak source. This will signal the problem to those in the general area or those who might enterthe area.

• Releases from higher integrity, passive equipment usually do not require monitoring. Because of the lower likelihood of arelease and the very large area covered by passive equipment (e.g., pipe flanges) complete monitoring is not possible.Manifolds with a large number of valves and flanges, representing a concentration of leak sources, may be locally monitoredif the toxic concentrations are high.

➧ S APPLICATION IN H2S DETECTIONH2S is the toxic gas most commonly handled in refineries. Three hazard levels are defined below for H2S operation: high,medium, and low. A flowchart that aids in the determination of the potential hazard areas is also provided.High potential hazard areas contain BOTH a potential release source AND high H2S concentrations. Examples of potentialrelease sources include floating roof tank seals, pump and compressor seals, sulfur plant seal legs, H2S incinerators, vents,drains and sampling connections. High H2S concentrations are defined as those where the H2S concentration in the vaporphase of the contained stream is above 2 volume%. If H2S is dissolved in a liquid, and an isenthalpic flash of the liquid atatmospheric pressure produces a vapor with more than 2 volume% H2S, the stream would also be considered highconcentration. Typical sources with higher release potential and high H2S concentrations include, but are not limited to:• Amine regenerator overhead pumps.• Sour water stripper overhead pumps.• Compressors for H2S rich gas.• Claus plant atmospheric seals.• Air intakes in Claus plants.• Burners of H2S combustors and incinerators.• Water seals on flare drums which often have a high H2S concentration.Medium potential hazard areas require BOTH a potential release source (see examples under High Potential Hazard Areas)AND contained H2S concentrations between a minimum of:a) 250 volume ppm, if potential exposure involves possible direct exposure to the vessel vapor space, such as sampling or

opening vessels, orb) 500 volume ppm, for dispersed process exposure not involving possible direct exposure to the vessel vapor spaceand a maximum of 2.0 volume%.

Typical examples for such areas include:• Sour water drawoff facilities.• Sour crude tanks and slop tanks.• Asphalt and heavy fuel oil tanks.Low potential hazard areas contain EITHER a potential release source (see examples under High Potential Hazard Areas) withH2S concentrations of between a minimum of 10 vppm and a maximum of:a) 250 volume ppm, if potential exposure involves possible direct exposure to the vessel vapor space, such as sampling or

opening vessels, orb) 500 volume ppm, for dispersed process exposure not involving possible direct exposure to the vessel vapor space,ORhigh H2S concentrations within passive, high integrity equipment. High H2S concentrations are defined as those where the H2Sconcentration in the vapor phase of the contained stream is above 2 volume%. If H2S is dissolved in a liquid, and an isenthalpicflash of the liquid at atmospheric pressure produces a vapor with more than 2 volume% H2S, the stream would also beconsidered high concentration. Examples of passive, high integrity equipment includes process vessels, piping, flanges, andother static equipment (no moving parts).


SectionXV-K

Page6 of 17



EXXONENGINEERING


TOXIC GAS DETECTORS (Cont)Typical examples of Low Potential Hazard areas include:• Light ends pumps handling fluids with low H2S concentrations (e.g., untreated propane).• Sour fuel gas at fired boilers as long as the gas contains less than 500 vppm H2S.• Heavy fuel loading.• Acid gas transfer lines (typically high H2S concentration but no potential release source).

Start

Is there a potentialrelease source?

(Note 1)concentration >20,000 vppm?

(Note 2)

High PotentialHazard Area

IsH2S vapor

concentration >20,000 vppm?

(Note 2)

concentration >500 vppm?

(Note 2)

Medium PotentialHazard Area

concentration >250 vppm?

(Note 2)

Is direct exposurepossible?

Low Potential HazardArea

Negligible PotentialHazard Area concentration >

10 vppm?(Note 2)

No Yes Yes

Yes

Yes

Yes

No

YesNo

NoYes

FLOWCHART TO DETERMINE H2S POTENTIAL HAZARD AREAS

Note 1: Potential Release Sources are defined above, under "Application of H2S Detection"Note 2: H2S Concentrations are defined above, under "Application of H2S Detection" DP15Kf0

No

No

No

IsH2S vapor

IsH2S vapor

IsH2S vapor

IsH2S vapor

Additional information on H2S hazard levels may be obtained from the EBSI Report No. MR.35DQ.97, titled "Hydrogen SulfideHandling Practices at refineries and Chemical Plants: A Benchmarking Study."Fixed H2S detectors are recommended for use within high potential hazard areas. The detection points should be locatedat each high potential release point. Ideally, the detectors should be located within 5 ft (1.5 m) of the leak point, and between thecenterline and 2 ft (60 cm) below. Access may require some adjustment to the location. The detector should alarm between 10and 20 ppm. When multiple sources are located together, e.g., pumps, a single monitor can service two sources, provided thesources are located within 10 ft (3 m) of each other.



XV-KPage


DateDecember, 1999


TOXIC GAS DETECTORS (Cont)Fixed detectors are not recommended for medium and low potential hazard areas. While fixed detectors are notrecommended for medium hazard areas, other protective measures may be specified by the local SOC. The area may bemarked, personal H2S detectors may be required, and/or authorization before entering the area might be required. In the lowhazard area, it is unlikely that a person will be exposed to dangerous concentrations as a result of a leak. Dispersion calculationsfor a 0.5 ft3/sec (14 liter/sec) leak of 1000 ppm H2S show that the concentration is reduced to 100 ppm in 3 ft (1 m) and 14 ppmin 10 ft (3 m).The alarm system for high potential hazard areas should allow for rapid identification of the leak source. Three options, whichwill allow rapid identification, are:1. Locate all detector outputs in the control room on a panel or distributed control computer display. The panel/display should

indicate all detectors in alarm, and the first detector which went into alarm. A local common alarm should sound in theprocess area.

2. Locate a panel with all alarms in the field and a common alarm to the control house. The panel should indicate all detectorsin alarm and the first detector which went into alarm. A local common alarm should sound in the process area.

3. Provide each detection point with a flashing light which would indicate an alarm. All detectors in the area should be tied to acommon audible area alarm and a common control room alarm.

An H2S detector should also be placed in the air intake duct for normally occupied buildings in close proximity to high potentialhazard H2S areas. Normally occupied buildings are those to which persons are regularly assigned and in which they perform themajority of their duties such as unit operating control rooms, office buildings, laboratories, and central maintenance workshops.The local Safe Operations Committee (SOC) will determine whether areas occupied for part of the day such as local operatorshelters or change houses are classified as “normally occupied." Close proximity will be determined by the size and scope ofpotential releases. Where release scenarios and dispersion modeling have already been carried out, the results will indicatewhich buildings can be subject to high concentrations. When dispersion results are not available, nominally occupied buildingswithin 200 ft (60 m) should include a detector.Normally H2S detectors in air inlets should alarm at 10 ppm H2S. When the building is a multi-unit control house or designatedsafe haven, the air system should also shut down upon alarm. The shutdown concentration may be higher than 10 ppm,provided there is an alarm at 10 ppm. Higher shutdown levels should be based on discussion with the local hygienist.In addition to locating fixed detectors at potential leak sources, other toxic gas detectors may be used for additional protection ofpersonnel from H2S leaks in high potential hazard areas. Personal monitors may be provided to those working in the high H2Sarea. Although H2S has a characteristic rotten egg odor, continued exposure at low levels or exposure in conjunction withhydrocarbons may deaden the sense of smell. The personal detector provides additional protection that H2S is detected.Personal monitors may be coupled with the use of escape devices. The escape device may be a small air cylinder or a respiratorto ensure that the person is not overcome while exiting. (The effectiveness of respirators decreases as H2S concentrationincreases.) A decision to use personal monitors and escape packs is made by the local site based on their assessment of therisk reduction it would provide at the site.

➧ S APPLICATION IN HF DETECTIONFor detailed information on the latest technology on hydrogen fluoride (HF) detection refer to the ER&E Document No. 99SR37titled "HF Alkylation Consequence Reduction Technology; Best Proven Reasonable Design for Detection, Automation andInitiation". The document includes information on detector types, range, selectivity, response time, stability, reliability, anddeployment.

S PERIMETER MONITORINGPerimeter monitoring of the unit with a number of fixed monitors is usually not recommended. By placing detectors at potentialhigh hazard leak sources, most leaks should be detected before reaching the perimeter. The remaining leaks will occur fromhigher integrity equipment and may still not be detected by fixed perimeter monitoring. Certain environmental conditions mayallow a narrow plume to pass between adjacent detectors, or an elevated leak may not be detected by monitors at grade. Underthese conditions, perimeter monitoring might even provide a false sense of security.

➧ Perimeter monitoring might be considered in special circumstances such as close proximity to a fence line or occupied buildings.If a site determines that perimeter monitoring is required, an open path detection system should be considered. These systemscan provide more effective coverage than a series of point detectors and depending on the number of point detectors that wouldbe needed, may also be more cost effective. In open path detection systems, as described in EE.125E.98, a light beamtraverses a relatively long path using a transmitter/receiver system and an average concentration along the path is based on theamount of light absorbed by the component being monitored. Open path detection systems utilizing tunable diode lasers haverecently been demonstrated for H2S and HF. Paths up to 330 ft (100 m) can be monitored.


SectionXV-K

Page8 of 17



EXXONENGINEERING


FLAMMABLE GAS DETECTORSThe detection of flammable vapors is similar to that for toxic vapors. The most likely release points are similar, as is the behaviorof the vapor as it disperses. The concentration of flammables required for ignition is generally much higher than theconcentration of toxic gas required for an acute health threat. However, the use of liquefied hydrocarbons above their normalboiling points can lead to much larger leaks and higher concentrations. Although detection is similar, the consequences offlammable hydrocarbon and toxic releases can be quite different. Since detectors are installed to assist in mitigation of releases,the different consequences of flammable gas and toxic gas releases can result in different deployment strategies.

TYPES OF FLAMMABLE GAS DETECTORSThere are three distinct types of flammable gas detectors for use in refineries and chemical plants: catalytic point detector, shortpath infrared point detector, and infrared open path detector. There are advantages and disadvantages for each.

➧ The most common gas detector is a catalytic point detector. These detectors operate by sensing the resistance change of aheated catalytic element caused by the catalytic oxidation of flammable gases on the surface of the element. They have provento be mechanically robust, highly specific, and are low cost. As with all point detectors, they have the disadvantage of monitoringonly the surrounding air. As the area to be monitored increases, additional detectors are required. A second disadvantage of thecatalytic point detector is that the catalytic element can be attacked or coated resulting in a loss of sensitivity. Halogens,hydrogen sulfide, lead compounds, and silicones can all reduce the sensitivity. To ensure that sensitivity is maintained, monthlyfunctional checks and full calibration checks on a quarterly to semi-annually basis are typically performed. Finally, if theflammable level exceeds the upper explosive level (UEL), the reduction in oxygen reduces the catalytic conversion. When theUEL is exceeded, there may be an apparent reduction in the concentration, even registering below the lower explosive limit(LEL), due to the lack of complete catalytic conversion.

➧ A second type of point detector uses infrared (IR) absorption to detect hydrocarbons. These detectors operate by passing aninfrared beam through a short path sample cell and measuring the transmitted light at the hydrocarbon absorption wavelengthand at a non-absorbing reference wavelength. The ratio of the reference signal to the hydrocarbon signal is proportional to thehydrocarbon content in the beam path. The reference wavelength can also be used to continuously monitor detectorperformance, signaling when maintenance is needed. While the cost of the IR detector is 1.5 to 2 times greater than the catalyticdetector, internal self-checking and negating the effect of contaminate gases reduces the scheduled maintenance while providingthe same reliability. Unlike catalytic detectors, there is no loss in sensitivity at concentrations higher than the UEL. However, thepoint detector still samples only the surrounding environment, and multiple detectors are required for an area.

➧ Larger areas can be covered by the use of an open-path IR detector. These detectors use the same detection concept as theIR point detector. However, instead of using a short path, the beam is projected over 100 to 300 ft (30 to 90 m) and can bereflected using mirrors. The open path detector provides information about the average flammable concentration along the path.Therefore, the detector will produce the same response for a 5 ft (1.5 m) wide plume of 50% LEL as it will for a 25 ft (8 m) wideplume of 10% LEL. Its cost is considerably more than point detectors and therefore will prove cost effective when it replaces anumber of point detectors, including their associated maintenance cost.All types of flammable gas detectors are calibrated for one composition and their response to other compositions may vary.For example, a catalytic detector calibrated to the LEL of methane will read 65% of the LEL when propane is present at its LEL.Many gases do not exhibit as pronounced a difference in sensitivity. IR detectors will have similar sensitivity issues. Wheremultiple gases are present, the calibration standard should be selected to allow sufficient detection for as many other potentialgases as possible. Detailed requirements for calibration of flammable gas detectors are given in IP 15-8-3.

S LOCATING HYDROCARBON DETECTORSSeveral factors must be considered when determining the need for flammable gas detectors. They include: the potential forvapor cloud formation upon release, the potential for equipment to develop a leak, and the proximity of the potential leak point toignition sources.Contained materials with higher potential for vapor cloud formation upon release include:• Liquid with an atmospheric boiling point below ambient temperature, (C4 and lighter).• Liquid which is contained at a temperature above its atmospheric boiling point such that 25% or more vaporizes upon

release to the atmosphere.• Gas at high pressure, above 200 psig (14 barg).

➧ Materials which are contained above their autoignition temperature are not likely to form a flammable vapor cloud, since theywould most likely ignite immediately upon release. Therefore monitoring is not appropriate.

➧ When equipment contains higher risk materials, i.e. materials with a higher potential for vapor cloud formation, and the equipmenthas a higher potential for leaks, fixed detectors are recommended. The following equipment should be considered:• Pump seals.• Compressor seals.



XV-KPage


DateDecember, 1999


FLAMMABLE GAS DETECTORS (Cont)For pumps, a detector may be located at each seal. One detector may be used between two seals provided the detector iswithin 5 ft (1.5 m) of each seal. The detector should ideally be located at or below the centerline of the pump and at least 18 in.above the ground. Some changes to location may be necessary for accessibility.For compressors, a detector may be located as close as possible to each seal, at or below the centerline for heavier than airgases and above the centerline for lighter than air gases.

➧ There are other lower probability sources of hydrocarbon leaks. The leak point from these sources is not as localized as withpump and compressor seals. Generally hydrocarbon detectors are not deployed. However, when they are in close proximity toignition sources and contain the higher risk materials described above, detectors may be considered. Lower probability sourcesinclude:• Small pipes and connections subject to failure due to vibration.• Manifolds with a large number of flanges.• Pipes subject to external corrosion (i.e., cooling tower overspray).• Drain and sample points.• Control valves operating above 200 psig (14 barg).Detectors may be considered if these potential leak points are within 100 ft (30 m) of ignition sources such as:• Furnaces.• Hot, uninsulated metal surfaces above 600°F (315°C).• Vehicle traffic.• Other internal combustion engines.• Electrical substations.• Railroads.• Areas with high levels of construction or maintenance.• Designated smoking areas.

➧ Hydrocarbon detectors should also be used to monitor the air of enclosures to prevent the buildup of an explosive atmosphere.These detectors may also be coupled with an automated shutdown system to prevent a possible confined space explosion. Airmonitoring of the following is recommended:• Air intake to control rooms and "safe haven" buildings, per IP 4-3-1.• Gas turbine acoustic enclosures, per IP 10-8-1.• Gasoline octane analyzer buildings, per IP 4-3-3.• Hydrocarbon process analyzer buildings, per IP 4-3-3.• Cooling tower exhaust, monitoring for presence of hydrocarbons from a cooling water exchanger tube leak.• Compressor shelters, which do not provide free ventilation as per IP 4-3-1.Detectors should also be considered for monitoring of offsite pressurized storage. The offsite location results in reducedsurveillance by process operators and leaks are less likely to be detected. The very large volume of pressurized liquids couldresult in severe consequences. The need for flammable gas monitoring would be further increased when the pressurized storageis relatively close to the fence line and developed areas. If the storage area is diked, one detector may be placed within the dikealong each wall. This will take advantage of the wall which will tend to contain the hydrocarbon vapors (excluding LNG). Forsites with prevailing wind patterns, fewer detectors may be required, as determined by the local SOC. Open-path detectors mayalso be used to monitor the perimeter, especially when there is no dike.Hydrocarbon detectors may be set between 20 and 60% of the LEL. A setting of 40% is nominal. In some cases H2S detectorscan also provide adequate warning of a hydrocarbon leak. Where H2S is present in hydrocarbon vapor streams at greater than0.5 mole%; a 10 ppm detection of H2S typically also provides a warning of approximately 10% of the LEL.As with toxic gas detectors, the alarm system should also help the process operator to identify the source of the leak. The use ofcontrol room display, local panel, or individual flashing lights, as described in the toxic gas section, are all options.


SectionXV-K

Page10 of 17



EXXONENGINEERING


S ALTERNATIVES TO TOXIC AND FLAMMABLE GAS DETECTORSThere are alternatives to toxic and flammable gas detectors which might be considered in specific cases. These alternativesmonitor the conditions which may lead to a release. By anticipating the release, these systems reduce the probability of arelease occurring. Since they prevent, rather than mitigate a release, these systems should be applied first, whenever possible.Level detectors on atmospheric and pressurized storage are simple instruments which are used to prevent a leak or spill.Rather than overfill a tank and detect the released hydrocarbon, level instruments detect overfilling before it occurs. When usedas a safety device, this level instrument should be an independent alarm, separate from the normal tank indicator level.Although small bore piping, especially when used in vibrating service, is considered a higher leak risk, proper reinforcing of theconnections reduces the risk of failure. With proper support, toxic or flammable gas detection is not required. If there are a largenumber of small bore connections in a concentrated area, some gas detection may still be advantageous. This would bedetermined only on a case-by-case basis.Leaks from pump and compressor seals will also be reduced with proper seal leak detection. On pumps, a leak detectionsystem utilizes primary and secondary seals and monitors the space between the seals for leakage. Leakage can be detected bya rise in pressure, a change in liquid level when a barrier liquid is used between the two seals, or a change in temperature whencryogenic fluids are pumped.

➧ For pumps in high hazard H2S service or C4 and lighter liquid service, hydrocarbon or toxic gas detectors are not required whendual seals with leak detection (control level 5 or 6 as defined in Section X-G) are installed. Section X-G also recommends,but does not require, bearing vibration or bearing temperature monitors. Experience has shown that the majority of pump sealfailures which result in significant flammable releases or fires are the result of a bearing failure. Detecting the possibility of abearing failure before it occurs could prevent seal leaks and possible resulting fires or vapor clouds. Therefore bearing monitorsare also recommended, but not required, when gas detectors are not used.If seal leak detection and not gas detection is used, it is increasingly important to ensure proper support of all small bore pipingaround the pump. If the small bore piping were to fail, there would be no gas detection, so it is important to reduce the risk offailure by proper support. The use of bearing monitoring will also reduce the risk of small bore pipe failure by warning ofexcessive vibration.On compressors, differential pressure on seal buffer gas will warn of a potential leak. Provided that the InternationalPractices on compressors are followed to reduce and monitor vibration, flammable gas detectors would not be required on theseals. However, toxic gas detection is still recommended for compressors because of the much lower level of leakage which istolerable and the additional benefit provided by detecting other small leaks around the compressor.

S FIRE DETECTORSFlammable gas detectors may not be appropriate for all types of hydrocarbon releases. In operations where gases mayoccasionally be present at 10% of LEL or more as part of normal operation, detectors could result in numerous false alarms, e.g.,well head areas of offshore platforms and loading racks. In other cases, the release may readily ignite, e.g., material above itsautoignition point, or high concentrations of hydrogen. In these cases, fire detection systems may be used. Early detection offire will provide the best chance for control.Fire detectors may also be used even if hydrocarbon or alternative leak detection is already in use, when it is desirable to furtherreduce risk. The combination of fire detection with automated actuation of a water deluge or spray will mitigate the effect of a fireand reduce risk. Pump retrofits which might require deviation to spacing standards may be able to offset the increased risk ofreduced spacing with an automatic deluge system.There are also fires which do not develop from flammable vapors which may also benefit from the use of fire detection as follows:1. Electrical equipment fires.2. Mechanical equipment fires as a result of failures and increased friction and heat.3. Fires from flammable dusts or solids, as in exhaust ducts or filter baghouses.Fire detectors can be classified as one of three broad types:1. Smoke Sensing - These detectors respond to the presence of smoke particles. These detectors are primarily used indoors

and in enclosed spaces.2. Heat Sensing - These detectors respond when the sensing device becomes heated to a predetermined level.3. Radiant Energy Sensing - These detectors respond to the radiant energy produced by burning substances. Flame

detectors sense the radiant energy from open flames with background sunlight or ambient light. Spark/ember detectorssense sparks or embers in dark environments such as ductwork.



XV-KPage


DateDecember, 1999


FIRE DETECTORS (Cont)

S SMOKE DETECTORSSmoke detectors are usually employed only in confined spaces such as control house buildings or electrical substations.There have been numerous fires in electrical substations, despite proper electrical design. All new substations should havesmoke detection, tied to a central control house alarm. Smoke detection should also be considered for retrofit of any existingsubstations. The large amount of electrical equipment in control house buildings is a potential source of fire too. Smokedetection is not employed in outdoor process areas because wind currents make detection unreliable.There are two types of smoke detectors, ionization and photoelectric. Ionization detectors are usually preferred for electrical orhigh energy, flaming fires which generate small smoke particles. These are specified for control rooms, IP 4-3-1, and electricalsubstations, IP 16-6-1. Photoelectric detectors are usually preferred for low energy, smoldering fires. Additional detail on smokedetectors is available in NFPA 72.

S HEAT DETECTORSHeat sensing detectors should be employed when the potential source of the fire is well known. Temperature sensitivedevices should be placed near each fire source. These heat sensing devices are usually not subject to false alarms. This makesthem ideal for initiating automated response to fires such as water deluge sprays.There are three types of heat detectors: temperature sensing point detector, rate-of-rise point detector, and temperature sensingline detector. Temperature sensing point detectors use a fusible link or plug which melts at a pre-determined temperature todetect a fire. Rate-of-rise detectors respond to a sudden increase in temperature, typically 12-15°F/min (6-8°C/min). The rate ofrise is typically detected by the rapid expansion of gas within one or more detecting heads. Line detectors depend on localizedheating of a temperature sensitive line. In common line detectors, heating will cause the insulation between two wires within theline to melt creating a short. Other line detectors may measure an increase in electrical resistance of wires or a decrease inoptical transmission of optical fibers to detect heating.

➧ Point detectors can be applied to specific locations where the likelihood of fire is higher. Pumps with flammable liquids orliquids operating above their autoignition points and exchangers operating above the process material auto ignition point arepossible locations. In certain cases air-pressurized, long-lasting, UV-resistant plastic tubes may be used as alternatives totraditional fire-sensing devices. The tubes are wrapped around the equipment they protect and, when they melt during a fire, theloss of their contained air pressure initiates an alarm. This type of fire-sensing devices may be used, for example, in remote orcongested areas, in pumps with liquids above their autoignition temperature, or in pumps utilizing dual sealing systems andmeeting the criteria for possible monitoring due to fire potential per the Pump Sealing Technology Manual (TMEE-023). Plastictube-type detectors have been used in the past also in LPG and in low flash service pumps, as well as in the diked areas ofspheres, on the spheres themselves, and on vapor recovery lines containing flammable air hydrocarbon mixtures. While heatsensing point detectors are typically not used outdoors, the process pressure and flammability of liquids and vapors in refineriesand chemical plants often make for intense fires which can be easily sensed. Temperature sensing detectors are more oftenemployed than rate of rise detectors.Line detectors may be useful for equipment which covers a large area and in which a fire is likely to develop anywhere in thearea with equal probability. Equipment such as long conveyor belts and switch gear are possible locations for line detectors.Because temperature sensing point detectors monitor a small local area and have been demonstrated to have low false alarmrates, they have been coupled with automatic actuation of a deluge system. Two systems which have been used are:• Pilot heads with a fusible plug are typically located 4-6 ft (1.2-1.8 m) from possible fire sources such as higher risk pumps or

exchangers. The fusible plug can be selected to melt from 135 to 580°F (57 to 304°C). The pilot heads are part of a pipingsystem, which contains pressurized air so that the melted plug releases the air, which actuates the deluge valve.(1)

• A similar system replaces the pilot heads with nylon tubing. The tubing, instead of a fusible plug, melts and actuates theemergency response. This is similar to the plastic tubing described above for point detectors.

Note:(1) Baytown refinery and chemical plant have large number of deluge water systems coupled with pilot heads which have

demonstrated response to fires and few false alarms. Maintenance costs are low.

S FLAME DETECTORSRadiant energy (flame) detectors should be used when there are multiple fire sources in an area. The flame detector covers awide area and may be more economic than point sources. Flame detectors also respond faster than heat sensing or smokedetectors. When rapid spread of the fire is expected, and automated response is required, the flame detector will provide fasterresponse than the heat sensing device.


SectionXV-K

Page12 of 17



EXXONENGINEERING


FIRE DETECTORS (Cont)Flame detectors respond to the radiant energy, which is emitted from all open flames. Flames emit energy in wavelengths fromthe ultraviolet (UV) through the infrared (IR) wavelengths, about 0.18 to 5 microns. To discriminate between fire and othersources of radiation, particularly sunlight, narrow wavelengths are monitored. The wavelengths are typically emitted from burningmaterial but absent from other sources. Based on the wavelength monitored, there are two types of flame detectors, UV and IR.UV detectors monitor radiant energy from about 0.18 to 0.27 microns. All flames emit radiation in this region. At thesewavelengths, radiation from sunlight is absorbed by the atmosphere so that there is no background solar radiation, and thedetector is not affected by the sun.IR detectors monitor radiant energy from about 4.1 to 4.6 microns. All hydrocarbon flames produce CO2 which has an emissionspike at this band, see Figure 1. The amount of CO2 determines the intensity of the radiation. Radiation from the sun is partiallyabsorbed by the atmosphere in this region, see Figure 2. The partial absorption is sufficient to allow the detector sensitivity to beadjusted to zero out the effect of the sun.

S FLAME DETECTOR PERFORMANCEBoth UV and IR detectors have reduced ability to detect fires under certain conditions. UV radiation is more easily absorbed thanIR. Dense smoke or high concentrations of dirt or oil on the lens reduce UV sensing more than IR. UV absorbing vapors, seeTable 1, which might be present will reduce UV sensing but have no effect on IR. At least two UV instruments provide internalreflectance checks to warn that the glass is dirty. IR, unlike UV, does not have the ability to detect all types of fires. If the firedoes not generate sufficient CO2, e.g., hydrogen or ammonia, an IR sensor will not detect it. IR radiation can also be absorbedby ice or water which might cover the lens.

TABLE 1SUBSTANCES EXHIBITING SIGNIFICANT UV ABSORPTION

Acetaldhyde Cumene 2-NitropropaneAcetone Cyclopentadiene 2-PentanoneAcrylonitrile O-Dichlorobenzene PhenolAlpha-Methylstryene P-Dichlorobenzene Phenyl Clycide EtherAmmonia Ethanol PyridineAniline Ethyl Acrylate StyreneBenzene Hydrogen Sulfide Tetrachloroethylene1,3 Butadiene Methyl Methacrylate Toluene2-Butanone Naphthalene TrichloroethyleneButylamine Nitroethane Vinyl TolueneChlorobenzene Nitrobenzene Xylene1-Chloro-1-Nitropropane NitromethaneChloroprene 1-Nitropropane

Both UV and IR detectors also are subject to false alarms due to the presence of non-fire UV or IR sources within the facility.Potential false alarms for each type of detector are given in Table 2.



XV-KPage


DateDecember, 1999


FIRE DETECTORS (Cont)

TABLE 2POTENTIAL UV AND IR FLAME DETECTOR FALSE ALARMS

POTENTIAL FALSE SIGNAL UV IR

Welding Positioning and AimingVoting with multiple detectorsBypass during hot work

No effect

Direct sunlight No effect Positioning and aiming to avoid direct beam,e.g., sunrise

Lightning Time Delay of 3 seconds eliminates No effectRadiation from x-rays Bypass during inspection work No effect"Friendly fires," flares, furnaces,etc.

Positioning and aiming Positioning and aiming

UV lamps for fluorescentpenetrant and magnetic particleexams

Shield UV lamps from detector sightBypass during inspection

No effect

Flashing lights: engine timers,emergency vehicles, flashcameras

Positioning and AimingBypass during use

Positioning and Aiming Bypass during use

High pressure sodium, mercuryvapor, or quartz-hydrogen lamps

Glass (not plastic) shields over lamps toabsorb UV

No effect

Hot surfaces No effect Flicker circuit tests for flicker from flames whichis absent from hot surfaces with steadyradiation levels. However, steam cloudspassing between the hot surface and thedetector may simulate flicker.

Reflected sunlight No effect Positioning and aiming. Reflected sunlight cancause false alarms.

Although IR detectors claim to be unaffected by solar radiation, false alarms within Exxon have been attributed to the sun.Because the IR detector zeros out background solar radiation, direct or reflected sunlight, e.g., glare at sunrise or reflection offwater may trigger a false alarm. False alarms are more likely when flames with low amounts of CO2 are to be detected, forexample in hydrogen rich POWERFORMING streams. For these streams, the detector sensitivity is usually increased,increasing the potential for false alarm.The selection of a detector will be site dependent. It should consider the potential non-detection of the flame due to interferenceby dirt or vapors for UV or due to the type of flame for IR. In addition the frequency of false alarm conditions at the site and thedifficulty of designing around them should be considered.

S DUAL SENSOR PERFORMANCEThe field experience with simple UV and IR devices has led to the development of dual wavelength sensor to reduce theincidence of false alarms.IR-IR detectors monitor a second narrow band typically near 3.9 microns. Hydrocarbon fires produce less radiation here than atthe CO2 peak. The two bands are compared to increase the rejection of false signals due to flickering radiation from flashinglights, reflected sunlight or heated equipment with flickering.UV-IR detectors combine standard single band UV and IR sensors. Fire must be sensed by each device to provide an alarm.Since the false alarms from UV and IR sensors are generally mutually exclusive, the number of false alarms is reduced.Generally dual sensors type detectors are preferred as field experience has shown that they do reduce the number of falsealarms.

S DETECTOR RESPONSEThe response of a flame detector will affect its placement. For a given detector, the response time will be a function of the size ofthe fire and the distance to the fire. Response time usually quoted by vendors based on detection of a 1 ft2 (0.1 m2) gasoline firein a shallow pan. Figure 3 shows a typical response time to a small gasoline pan fire (EE.43E.87). Each detector has aminimum response time below which reducing the distance to the flame has no effect.


SectionXV-K

Page14 of 17



EXXONENGINEERING


FIRE DETECTORS (Cont)As the size of the fire increases, it can be detected with no change in response time at increasing distances. Quadrupling thediameter of the fire roughly doubles the distance at which it can be detected, at the same response time, see Figure 4. Flamedetectors should be able to detect a 1 ft2 (0.1 m2) pan fire at 35 ft (10 m) in ten seconds (EE.43E.87). Most commercial units arecapable of this.Detector response is also effected by the position of the fire within the detector field of view. Response times or distances arequoted for fires in the direct line of sight of the detector. However, detectors have a field of view of at least 90°, and up to 120°.Sensitivity is reduced as the fire moves away from the central axis, as shown in Figure 5 for a typical detector. In Figure 5, thedetection distance of the "standard" fire at 45° is half that of the straight-ahead distance (EE.43E.87).

S WHEN TO INSTALL FIRE DETECTORS?The need for flame or heat sensing detectors is determined by reviewing the potential hazards, risk exposure, and appropriateresponse to fire. The following conditions would support the installation of fire detectors:• High fire risk facility; quick response is needed to avoid rapid escalation and endangerment of personnel or equipment.• Sufficient personnel not available for regular surveillance.• Early warning by other means not feasible: gas detectors, seal leak detectors, protective equipment systems, process

instrumentation, etc.• Capability exists for prompt response to alarm: shutdown, isolation, and fire suppression.• Ongoing maintenance support available in the case of UV and IR detectors.The primary use of flame detectors within Exxon has been for truck loading racks for flammable products such as gasoline.When the loading operation is attended only by the driver, the detection system actuates a fire suppression system toautomatically control and extinguish the fire.Detectors have also been applied in a limited way for special situations in the refinery. Equipment containing hydrocarbonsabove their autoignition point, which historically was subject to periodic leaks and fires, have been placed under surveillance. Inthis case, the detectors serve an alarm function only.Heat sensing devices in combination with deluge systems have been successfully used with low maintenance costs andessentially zero false alarms both onsite and offsite.

S DESIGN GUIDELINES FOR DETECTORSWhen a fire detection system is needed, the following guidelines should be followed to ensure acceptable performance:• Review possible fire scenarios: what fuels are involved, where might the fire start, how fast might it spread.• Where the rapid spread of the fire is likely, automatic actuation of protective systems should be specified.• When a flame detector is used, a dual sensor IR-IR or UV-IR flame detector, is preferred to reduce the potential for false

alarm and is required when the detector will automatically activate a suppression system.• IR flame detectors are preferred for hydrocarbons. When the fuel contains little or no carbon, a single UV detector or heat

detector is preferred. Heat sensing devices are viable alternatives in either case provided the potential flame location is wellknown and the sensing device can be located nearby.

• Flame detectors should be located no greater than 35 ft (10 m) from possible fire sources. At 35 ft (10 m), the detectorshould respond in ten seconds to a 1 ft2 (0.1 m2) pan fire of the expected material on fire.

• Flame detectors should be positioned to see the base of the fire not just the flames above it.• Enough flame detectors must be deployed to avoid blind spots and to account for loss in sensitivity away from the detector's

central axis.• To avoid false alarms from sources outside the risk area, flame detectors should not have a view of the horizon.



XV-KPage


DateDecember, 1999


FIGURE 1RADIATION SPECTRA FROM A HYDROCARBON FUEL FIRE

0 1 2 3 4

Wavelength (Microns)

Spec

tra R

adia

nt

Inte

nsity

Wat

tsM

icro

n x

Ster

adia

n

5 6 7 80

1

2

Single FrequencyInfrared SensorResponse

DP15KF01

FIGURE 2FLAME DETECTOR VS SUNLIGHT TRANSMISSION

Wavelength (Microns)UltravioletSensorResponse

0.2

Ultraviolet Visible Infrared100755025

0

0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.5 2.0 3.0 4.0 5.0

Solar RadiationReachingThe Earth

InfraredSensor

Response

DP15KF02


SectionXV-K

Page16 of 17



EXXONENGINEERING


➧ FIGURE 3TYPICAL FLAME DETECTOR RESPONSE TO GASOLINE PAN FIRE

0 2 4 60

10

20

30

Response Time, SecDP15KF03

Dis

tanc

e to

Fire

, ft.

(1 ft

= 0

.305

m)

FIGURE 4FLAME DETECTION DISTANCE VS. FIRE SIZE

1 10

Fire Diameter, ft(1 ft = 0.305 m)

Dis

tanc

e, ft

(1 ft

= 0

.305

m)

10010

100

1000

DP15KF04



XV-KPage


DateDecember, 1999


FIGURE 5FLAME DETECTION DISTANCE VS. VIEWING ANGLE

45°

30°

15°

25%

50%

75%

100%

0°

Viewing Angle

Nor

mal

ized

Det

ectio

n D

ista

nce

DP15KF05

Dp15k

Documents

Transcript of Dp15k