3Initial Screening and

Consequence Screening

This chapter provides guidance on the initial evaluation of buildings inprocess plants. Screening tools will be discussed as a means of identifyingbuildings at sufficient risk to require more detailed analysis. At eachsuccessive stage of the screening methodology, additional informationabout the materials, processes, buildings, and other site-specific factors isincorporated into the analysis. These stages have the effect of applyingsuccessively "finer" screens. Buildings that are eliminated by screening areconsidered appropriate for their intended purpose without modification orrelocation.

Use of a screening methodology such as that presented here will allowthe user to quickly eliminate from consideration many plant buildings onthe basis of no hazard present, low consequence, or conformance withestablished design and spacing criteria for adequate protection from possi-ble consequences. As a result, resources can be focused on other processplant buildings that present greater risk to personnel and business objec-tives. Screening can be a particularly effective tool to prioritize efforts forhigher-risk situations, especially when multiple buildings and processesneed to be evaluated. This allows resources to be applied more effectively.

3.1. Process and Plant Documentation

The first step in the screening procedure is to obtain sufficient informationabout the facility to support the analysis. Table 3.1 shows the typicaldocumentation requirements as a function of the level of analysis beingperformed.

The information requirements for each successive step in the screeningprocess build upon those for the preceding steps. The user has the choiceof collecting this information all at one time or in phases. Because theinformation necessary for the more detailed analyses, such as quantitative

TABLE 3.1Process and Plant Documentation for Evaluating Explosion and FireRisks to Process Plant Buildings

Type of Analysis

Initial screening (Section 3.2)—Through identification of materials

and conditions present at thespecific site

—Through applying occupancy orfunctional criteria of concern

Consequence screening (Section 3.3)—By comparison to design and

spacing criteria

—By modeling site-specific conditions

Risk screening (Section 4.3)

Risk assessment (Chapter 5)—Through qualitative risk assessment—Through quantitative risk

assessment

Typical Information

Material Safety Data SheetsProcess conditionsTotal inventory of materials being

handledInformation on site conditions as

needed to evaluate explosion or firepotential

Building occupancyBuilding function criteria

Information used in initial screeningApplicable industry or company

standardsBuilding construction typeDistances between process units and

buildings

Inventories of material havingexplosion or fire potential that can bereleased

Plant volumes and degree ofconfinement or obstruction (ifMultienergy or similar method isused) if vapor cloud explosion is aconcern

Information used in consequencemodeling

Generic frequency data on events ofconcern from similar plants

Risk tolerance criteria or methodologies

Depending on the type of analysis,information required may include, inaddition to all the information requiredabove:

—Detailed process safety informationon plant design and construction

—Operating information includingoperating procedures

—Information on passive and activemitigation systems

—Maintenance and inspectionstandards

—Site incident history—Information on building design and

construction—Additional information required for

quantitative risk assessment,including but not limited to failurerate data, and equipment repairand testing data (Ref. 4)

risk assessment (QRA), can be extensive, the phased approach to datacollection will often be the most effective from a resource standpoint. Thephased approach involves collecting only the information required for theparticular level of analysis being conducted. If a screening removes abuilding from further analysis, no effort to obtain additional informationis necessary.

3.2. Initial Screening

The next step in the screening is to determine if a process plant buildingis, in fact, potentially subjected to an event of concern and if the buildingexceeds criteria of concern, such as occupancy. If no event that couldsignificantly impact the building can occur, or if the building does notpresent a concern for other reasons, such as low occupancy, no furtherevaluation is necessary. When the potential for an event of concern doesexist, the building under study may still be excluded from further evalu-ation if the estimated degree of damage to the building would not likelyresult in an intolerable level of personnel injury, impact the safe shutdownof the process, or result in substantial business interruption.

Initial screening requires each facility to identify or develop genericscreening criteria against which the user can compare the specific informa-tion applicable to the building under study. Screening criteria might bebased upon types and quantities of materials, propess types and conditions,occupancy levels, and building functions.

3.2.L Initial Screening for Events of Concern throughIdentification of Materials and Conditions Present atthe Specific Site

As discussed in Chapter 1, the incident outcomes of concern for processplant buildings include:

• Release of flammable or combustible materials resulting in:—Vapor cloud explosions (VCEs)—Pool fires—Jet fires—Flash fires—FireballsCondensed-phase explosionsUncontrolled chemical reactions (runaway reactions)Boiling liquid expanding vapor explosions (BLEVEs)Pressure-volume (PV) rupturesPhysical explosionsConfined explosions

As a first step in the screening process, buildings can be removed fromfurther consideration if the materials being handled, or the site conditionspresent, cannot produce these incident outcomes under any reasonablecircumstances. Further, in some situations, the materials handled may notbe present in sufficient quantities to result in an event of concern. Becausesome materials have the potential to produce one or more of the above incidentoutcomes, each possible incident outcome may need to be considered.

3.2.1.1. ExplosionsAn evaluation of materials and site conditions that can lead to an explosionrequires an understanding of the chemical and physical properties of thematerials being handled, a determination of the quantities handled, andan assessment of the site conditions that can contribute to an event. Referto Appendix A for additional information on explosions.

Vapor Cloud Explosions. Lenoir and Davenport (Ref. 16) have summa-rized some major VCEs worldwide from 1921 to 1991. The materialsinvolved in these incidents suggest that certain hydrocarbons—such asethane, ethylene, propane, and butane—demonstrate greater potential forVCEs. Several factors may contribute to these statistics. These materialsare prevalent in industry and are often handled in large quantities, increasingthe potential for an incident. Certain inherent properties of the materials alsocontribute to their potential for explosion. These include flammability, reac-tivity, vapor pressure, and vapor density (with respect to air).

These light hydrocarbons are not the only materials exhibiting thepotential for a VCE. Under certain conditions, other materials, includingheavier hydrocarbons such as cyclohexane, benzene, or gasoline, can causeVCEs with blast effects similar to those of LPG and other low-molecular-weight materials. For example, if large quantities of heavier hydrocarbonsare released at elevated temperatures, a vapor cloud may form. Theoverpressure from such a VCE can be significant, such as that involvingcyclohexane at Flixborough (see Table 1.1 in Chapter 1 of this book andRef. 28).

The key point here is to determine if flammable or combustiblematerials are being processed under conditions of temperature and pressuresuch that, if a release occurs, a significant quantity of the material may bereleased into the air as either a gas, vapor, mist, or aerosol. If suchconditions are present, the user should assume that the potential for a vaporcloud explosion exists. Otherwise, VCE hazards can be ignored.

Determining the release quantity of material that is required for a VCEimpacting process plant buildings is extremely site-specific. Importantfactors are the release conditions, the physical and chemical properties ofthe released material, the degree of confinement, obstacle density, and thegeometry of the release area. Two references (Refs. 29 and 30) suggest thatrelease quantities of flammable material in gaseous, vapor, or aerosol formgreater than 10,000 Ib. (4,500 kg) have sufficient explosion potential to be

of concern. However, another reference (Ref. 5) suggests that releasequantities as small as 240 Ib. (110 kg) can result in significant levels ofblast overpressure given certain site-specific conditions such as confine-ment or a high degree of obstacle density (referred to as congestion}.

Confined Explosions. In situations where the vapors are confined withina building, vessel, or other such enclosure, flammable materials with flashpoints below the temperature within the enclosure may have the potentialfor an explosion. Similarly, in confined situations, combustible materials,regardless of temperature, can pose a potential for explosion if dispersed asan aerosol, mist, or dust.

Condensed-phase Explosions/Other Uncontrolled Chemical Reactions.Processes that handle materials with high heats of decomposition orundergo other exothermic chemical reactions are candidates for explosiveevents. The user should also consider a chemical reaction that is exother-mic may have an increased reaction rate under certain conditions (reactionrunaway), as might result from process upsets or other system failures.Except in the case of detonating materials, such as TNT, decomposing orreactive chemicals generally need some degree of confinement for signifi-cant explosion effects to occur. Reference 31, CCPS's Chemical ReactivityEvaluation Guidelines, and Reference 32, Emergency Relief Systems UsingDIERS Technology, by the Design Institute for Emergency Relief Systems(DIERS), provide guidance on chemical reactivity and on relief systemdesigns for emergency venting of systems where the potential for explosionexists.

If the above type of materials and/or conditions exist in the area ofprocess plant buildings, the buildings should remain in the analysis poolfor further study.

BLEVEs/Pressure-volume Ruptures/Physical Explosions. Rapid loss ofcontainment of materials confined under pressure at temperatures abovetheir normal boiling point may result in a BLEVE, with blast, radiant heat(if flammable material is involved), as well as fragment effects. These effectscan be experienced for considerable distances, depending upon the typesand volumes of material stored.

Catastrophic rupture of a pressure vessel as a result of a PV rupture orphysical explosion may also result in blast and fragment effects.

3.2.1.2. FiresWhen handling flammable or combustible material, the resulting conse-quences could involve fire. Also, it is not uncommon for explosionsinvolving flammable or combustible materials to be followed by fire,increasing the potential effects to building occupants. A detailed discussionon fire has not been included in this book because substantial literature isavailable on the effects of fire. Table 1.2 provides a number of references,

including industry and insurance standards for guidance on spacing ofequipment. Most of these references address potential fire impacts. Lees(Reference 33, Chapter 16, Fire, page 48) provides an extensive listing offire references, including information on design considerations for build-ings, and guidance on plant layout.

In addition to spacing criteria, many standards provide requirementsfor building design and construction to provide fire resistance and protectoccupants. Table 3.2 summarizes typical key references for fire protectionand evaluation.

TABLE 3.2Sources of Fire Protection information

Ref.NO.

22

23

24

25

26

34

35

36

37

38

39

42

Title

National Fire Protection Association. Flammable and Combustible LiquidsCode. ANSI/NFPA 30: An American National Standard. Prepared inconjunction with the American National Standards Institute. Quincy, MA.

National Fire Protection Association. Standard for the Production, Storageand Handling of Liquefied Natural Cases. NFPA 59A. Quincy, MA.

American Petroleum Institute. Standard 2510, Design and Construction ofLiquefied Petroleum Cas (LPC) Installations. Prepared in conjunction withthe American National Standards Institute. Washington, D.C.

Industrial Risk Insurers !!̂ Information Manual 2.5.2. Hartford, CT.

Factory Mutual Engineering and Research Loss Prevention Data Sheets.Norwood, MA.

American Institute of Chemical Engineers. Dow's Fire & Explosion Index:Hazard Classification Guide. New York, NY.

American Petroleum Institute. Standard 251OA, Fire ProtectionConsiderations for the Design and Operation of Liquefied Petroleum Cas(LPC) Storage Facilities. Washington, D.C.

American Petroleum Institute. Standard 2508, Design and ConstructionEthane and Ethylene Installations at Marine and Pipeline Terminals,Natural Cas Processing Plants, Refineries, Petrochemical Plants, and TankFarms. Washington, D.C.

National Fire Protection Association. Standard for the Storage andHandling of Liquid Petroleum Cases. NFPA 58. Quincy, MA.

National Fire Protection Association. Recommended Practice for theProtection of Buildings from Exterior Fire Exposures. NFPA 8OA. Quincy, MA.

Imperial Chemical Industries PLC, Explosion Hazards Section, TechnicalDepartment. The Mond Index: How to identify, assess and minimizepotential hazards on chemical plant units for new and existing processes.Second Edition. Winnington, Northwich Cheshire.

National Fire Protection Association. Code for Safety to Life from Fire inBuildings and Structures©. NFPA 101. Quincy, MA.

In general, fire falls into the following categories:

Pool Fires. Flammable and combustible liquids processed at temperaturessuch that they remain in a liquid state with limited evaporation uponrelease will form a pool. These materials, which have the potential for poolfire upon ignition, include NFPA Class I flammable liquids, such asgasoline, and NFPA Class II and Class III combustible liquids.

Jet Fires. Any flammable material and many combustible materials proc-essed at elevated pressures may have the potential for a jet fire, dependingupon the release conditions. If the processing pressures are low, and thebuilding is sufficiently far away, little, if any, potential may exist for thebuilding to be impacted by the jet flame.

Flash Fires. The same materials that can create a VCE can result in a flashfire if the conditions necessary for a VCE are not present, as discussed in3.2.1.1.

Fireballs. A fireball results from releases that have limited mixing withair prior to ignition. Materials that can produce VCEs may also have thepotential for fireballs, depending upon the release quantity and dispersioncharacteristics. A BLEVE involving flammable or combustible materialsalso produces a fireball.

The damage caused by fire may be due to direct flame contact orexposure to radiant heat. Fire damage is time dependent: The amount ofdamage will increase with the level of heating and the duration of theexposure. The amount of damage will also depend on the materials ofconstruction and the orientation of the exposure. Potential fire damage canbe mitigated by increasing separation distances between potential sourcesof hydrocarbon, applying fire proofing to the surface of exposures or byapplying water sprays to cool exposed surfaces. Radiant heat does not havean immediate effect on most process plant structures because they aredesigned for fire resistance. Typical construction materials offering fireresistance include reinforced concrete, and reinforced or unreinforcedmasonry (with limited window space).

Time-temperature curves for fire resistance for different types ofmaterials are available from American Society for Testing and Materials(ASTM) Standard E 119 (Ref. 41).

Radiant heat can be calculated using the SFPE Handbook of FireProtection Engineering (Ref. 40) or CCPS's Guidelines for Evaluating theCharacteristics of Vapor Cloud Explosions, Flash Fires, and BLEVEs (Ref.5). If the expected radiant heat load exceeds the capacity of the buildingmaterials to resist it, further evaluation should be performed. References104 and 105 provide additional guidance on fire.

Factors influencing the selection of materials include spacing (distancefrom fire exposure), the presence of fire barriers, adequate drainage, and

potential exposure times. For example, main structural steel members canbe covered with fireproofing materials, often designed for 1 or 2 hours offire resistance. Alternatively, structural steel can be protected through theuse of water spray systems.

If process plant buildings are constructed of fire-resistant material,there is often time for occupants to evacuate, if escape routes are availableand if means are present to reduce smoke ingress to the building. Oneimportant consideration in fire evaluation is the fact that fire has thepotential to impact building occupants through products of combustionsuch as smoke and carbon monoxide. Properly designed ventilation sys-tems may prevent smoke or products of combustion from entering thebuilding. For further guidance, the reader is referred to Reference 42, theNFPA 101 Code for Safety to Life from Fire in Buildings and Structures®.

EXAMPLE 1Initial Screening through Identification of Materials andConditions Present at trie Specific Site

Background

An indoor packaging facility handles lubricating oil (an NFPA Class HIBliquid) in drums at atmospheric pressure and temperature. Prior to shipment,the drums are stored in pallets in a warehouse section of the facility. Aconcern was raised about the explosion and fire potential that may bepresent from handling combustible liquid inside a building. A lunchroom isadjacent to the packaging facility.

Approach

A Class NIB is defined as one having a flash point above 930C (20O0F). Sincethe lube oil is not reactive and is handled at atmospheric temperatures, nopotential exists for explosion. However, the potential for fire exists.

The facility is designed, constructed, and operated as outlined by NFPA 30,Flammable and Combustible Liquids Code (Ref. 22). For example: All walls,floors, and roofs are designed with the appropriate fire ratings; suitablefire-rated doors are provided; barriers are installed to prevent the flow ofliquids between adjacent building sections; and container storage is inaccordance with NFPA 30 guidance for location, as well as maximum heightand quantity. Additionally, automatic sprinkler protection is installed in thebuilding. Multiple means of egress are available, and an emergencyevacuation plan is in place, with drills conducted routinely.

Based on the above evaluation, it was concluded that the fire concern wasadequately addressed, and no further action was needed.

3.2.2. Conduct Initial Screening through Applying Occupancyor Functional Criteria of Concern

Additional initial screening may be performed by identifying buildings thatshould be considered for evaluation because of either their occupancy ortheir function (e.g., importance to an orderly and safe shutdown in theevent of a major incident, or because their loss would result in significantbusiness interruption). Organizations should establish appropriate criteriafor classifying buildings. Some considerations include:

• Occupancy. How many people occupy the building and for whatduration? For example:a. Control rooms are usually occupied on a 24-hour basis by one

or more persons.b. Engineering or administration buildings house activities that

often take place during the day and may accommodate manypeople depending upon the work being performed.

c. Instrument shelters may only be visited twice a day by a singleoperator.

• Function. Does the building house a safety- or business-criticalfunction? For example:a. Some buildings may house equipment that could be critical to a safe

and orderly shutdown or emergency response. A determinationshould be made of the building's function in an emergency, andfurther evaluation should be considered if it is critical.

b. Certain buildings may house equipment critical to continuedoperation of other process units or even of the entire facility, andtheir damage could result in significant business interruption.These might include some central control buildings, centralelectrical switchgear buildings, or critical utilities.

Some factors to consider in developing occupancy criteria include:

• The time the building will be in the location. Buildings may be eitherpermanent or temporary. In some cases, temporary buildings mayhave a low contribution to long-term risk because of the reducedtime of exposure to potential explosions or fire. These might includebuildings that are scheduled to be demolished, or removed, in thenear future. On the other hand, some temporary buildings may beplaced on location when the highest risk of an incident is present,such as during plant startup or shutdown. The benefits of locatingthese temporary buildings near process units should be weighedagainst the potential increased risk.

• Ability to evacuate. Fire often develops over a time period sufficientto provide warning to occupants, and some buildings may provideadequate fire resistance or means of egress to permit evacuation. Forother events that can develop over a period of time, such as a BLEVEor, in some circumstances, a VCE, and for which means of detection,

alarm, and evacuation are available, consideration may also be givenfor the ability to evacuate.

• Peak period occupancy. Large groups of people may be present in abuilding for short periods of time.

• Increased occupancy during an emergency. Some buildings mayserve as emergency shelters or emergency response centers, in whichcase they may have a higher occupancy when an incident is mostlikely to occur.

No single factor clearly defines occupancy criteria for every situation.The user should determine which of the above considerations pertain tospecific site conditions. These factors should then be applied consistentlyto all process plant buildings within a site.

Table 3.3 presents some examples of buildings that are generallyconsidered to be occupied and unoccupied. This list is not exhaustive andis meant to provide representative examples only.

EXAMPLE 2Initial Screening through Applying Occupancy or FunctionalCriteria of Concern

Background

A company has established building occupancy criteria for internal purposes.These are based on a determination of a building's "occupancy load/'defined as the number of person-hours in the building in one week. Thecompany's criteria for occupancy load are 400 hours per week. If a buildinghas an occupancy load greater than 400, and it could be impacted by an

TABLE 3.3Examples of Occupied and Unoccupied Buildings

Increasinglylikely to beclassified asunoccupied

Increasinglylikely to beclassified asoccupied

Office buildingsControl roomsProcess buildingsMaintenance shopsGuard shacksEmergency response facilitiesTemporary buildings/trailersPurchase and stores buildingsWarehousesFire housesField laboratoriesSatellite control roomsChange roomsUninterruptible power supply (UPS) sheltersMotor control centersScale housesElectrical substationsAnalyzer shelters

explosion or fire, then it must be evaluated to ensure appropriate protectionof building occupants.

Approach

In applying the occupancy criteria at one of the company's facilities, fourbuildings were reviewed.

Building 1—A control room within a process unit

A control room is situated in a process unit handling material that has thepotential for an explosion and/or fire. The control room houses three boardoperators plus a shift supervisor, 24 hours per day. Two 24-hour-per-day"outside" operators are also in the control room, about half-time. OnMonday through Friday, the unit superintendent and one administrativesupport person are in the building for 8 hours each day. Further, two unitsupport engineers also have offices in the building, normally working 8 hoursa day, five days a week.

The unit occupancy load is determined as follows:

3 operators x 168 hours/week = 5041 shift supervisor x 168 hours/week = 1682 outside operators x 84 hours/week = 1684 others x 40 hours/week = 160

1,000

Since this exceeds the company occupancy load criteria of 400, furtherevaluation is needed.

Building 2—An analyzer building in the process unit

An analyzer building in the process unit is visited by a technician for 2 hoursa day, seven days a week. The occupancy load is:

1 technician x 14 hours/week = 14

Since this does not exceed the occupancy criteria, no further study isneeded.

Building 3—A motor control center building (MCC) adjacentto the process unit

A motor control center building (MCC), located adjacent to the process unit,is also visited by a technician for 2 hours a day, seven days a week. As perBuilding 2, the occupancy load for the MCC is 14, which does not exceedthe company occupancy criteria.

However, in evaluating the MCC it was noted that the building housedelectrical equipment critical to the operation of another, remotely locatedprocess unit. Thus, loss of the MCC would result in a shutdown of a processunit that would otherwise not be affected by an explosion or fire in theprocess unit being evaluated. Based on this functional consideration of the

MCC, it was decided that further evaluation would be warranted as a meansfor improving overall plant reliability.

Building 4—Proposed project trailers

As part of an expansion project, it is proposed that three trailers be installedapproximately 200 ft (60 m) from the boundary limits of the unit. The trailerswould be located close to each other. Each trailer would be occupied byfour engineers, 40 hours per week. The occupancy load is:

12 engineers x 40 hours/week = 480

Since this exceeds the 400 personnel hours per week, and since it was notnecessary to have the engineers located close to the unit, the decision wasmade to install the trailers at a remote distance.

In the evaluation of the proposed project trailers, it is noted that the companycould circumvent their own occupancy criteria by treating each trailer as aseparate building, rather than evaluating the three trailers together as a singleoccupied structure. By treating the trailers individually, the overall risk to thetrailer occupants may not be accurately identified. All three trailers, because oftheir proximity to one another, are likely to be impacted by a common event.Thus the location of the trailers, together, presents a potential for multipleinjuries or fatalities. As such, they should be evaluated as a single structure. In asimilar fashion, clusters of permanent buildings that are detached from oneanother may be more appropriately evaluated as a single structure so that therisk of multiple injuries or fatalities can be appropriately identified.

EXAMPLE 3Initial Screening through Applying Occupancyor Functional Criteria of Concern

A company has defined an occupied building as:

• Any building in which any individual spends more than 50% of his orher time, or

• Any building with an occupancy load greater than 500 person hours perweek

The company felt that these criteria, together, help identify risks to occupants ofsmall buildings that are nearly always occupied, yet exclude from considerationlarger buildings that only occasionally have many people in them.

3.3. Consequence Screening

Whereas an initial screening focuses primarily on the potential risksassociated with the types of materials handled or the function of the

building, consequence screening assumes that an event has occurred andevaluates site-specific conditions, such as building location and design, thatcan mitigate the consequences. This can be done either by comparingsite-specific factors to building design and spacing criteria or as a stand-alone study, either qualitative or quantitative, evaluating site-specificconsequences for an event of concern.

TABLE 3.4Example of Design and Spacing Standards (Ref . 43)

Plant Categorization

Low Explosion PotentialHandles endothermicreactions andnonflammable materials.

Moderate ExplosionPotentialHandles exothermicreactions, and/or releaseof flammable vaporlikely to be less than20,000 Ib. (9,000 kg) in 5minutes.

High Explosion PotentialHandles highlyexothermic reactionsof highly reactivematerials, and/or it ispossible to release morethan 20,000 Ib. (9,000kg) of flammable vaporin 5 minutes.

Spacing Criteriafor Control Rooms

Adjacent to plant.

Greater than 50 ft(15 m) or meetingelectrical areaclassificationrequirements.

Greater than 100ft (30 m).

Examples of Building DesignConsiderations

Conventional design meetinglocal/plant requirements andbuilding codes.

Single-story construction ispreferred, construction materiallimitations, window glazing typeand size limitations, fragmentmissile penetrationconsiderations, no heavyequipment on roof. Use structuralconfiguration and details thatenhance blast protection, but"blast-resistant" design is notrequired.

Blast-protective performance—-for the design blast loadingconditions— moderate structuraldamage, without collapse,consistent with personnel safety.Housed facilities remain operable.

Design blast loading— incident(side-on) overpressure:a. 10 psi (0.69 bar)— For 20 ms

triangular load with shockfront [corresponds toTNT-equivalence of 2,000 Ib.at 100 ft (900 kg at 3Om)I;

b. 2.9 psi (0.20 bar)— For 100 mstriangular load (blast effectsfrom 200 ft (60 m) dia., 13-ft-(4-m-) high, ethane-airpancake-shaped VCE);

c. 14.5 psi (1.0 bar)- 30mstriangular shock loadrepresenting extreme "upperbound" VCE scenario.

3.3.1. Consequence Screening by Comparisonto Design and Spacing Criteria

Explosion risks to occupied buildings in process plants can be categorizedin terms of (1) the types and quantities of materials handled or severity ofprocess conditions, (2) the distance from the explosion to the buildings(separation distance), and (3) the degree or type of blast resistance for whichthe buildings were designed. The results of the evaluation are then com-pared with pre-established criteria, which are usually based on design andspacing standards such as those summarized in Chapter 1, Table 1.2. Ifthe criteria are not met, design or siting modifications may need to be made,or other risk-reduction measures may need to be performed.

Table 3.4 shows a composite example of one type of standard that canbe used to perform an explosion consequence screening by comparison topre-established design and spacing criteria.

Table 3.4 can be used to illustrate both the advantages and the potentialpitfalls of using pre-established design criteria for initial screening. Theadvantage is that once the explosion potential of the plant is defined, aquick judgment as to the adequacy of a given building's design and locationcan be made. The potential pitfall in this type of comparative approach isthat design and siting standards, as a rule, do not take into accountsite-specific conditions. As a result, the particular standard may not provideadequate protection or, alternatively, may not be cost-effective because itresults in overdesign for the specific facility under review.

For example, referring to Table 3.4, significant blast resulting innear-total destruction of the plant buildings has occurred with releases offewer than 20,000 Ib. (9,000 kg) of flammable material (Ref. 15). On theother hand, depending upon the materials handled, the process conditions,and the plant layout (with no confinement or congestion), releases ofgreater than 20,000 Ib. (9,000 kg) of such material may have little potentialfor blast effects.

EXAMPLE 4Consequence Screening by Comparison to Designand Spacing Criteria

Background

A control building is located 125 ft (38 m) from a facility that handles highlyreactive materials having the potential for explosion. The building canwithstand a 12 psi (0.83 bar) side-on overpressure, 20 ms blast load.

Approach

The company standards classify this facility as "high explosion potential/'requiring control buildings that are located at this distance [125 ft (38 m)]from a potential explosion be designed to a minimum load of 10 psi (0.69bar) side-on overpressure and 20 ms blast load. It was determined that this

standard was appropriate for the facility. Since the building design exceedsthe requirements of the standard, no further evaluation is needed.

3.3.2. Consequence Screening by Modeling Site-Specific Conditions

Site-specific consequence screening for explosion can be performed eitherqualitatively or quantitatively, depending upon the explosion potential ofthe materials being handled, as well as processing conditions and othersite-specific factors. In performing a consequence screening, it is necessaryto select "Evaluation-case" events for consideration. This is defined asfollows:

Evaluation-case event—the scenario resulting in the most severe conse-quences, considering all incident/incident outcome combinations, that isconsidered plausible or reasonably believable.

Another definition of an evaluation-case event is "that incident out-come for which an engineered solution or a management system solutioncould be found that would reasonably prevent its occurrence/7 or moresimply put, an event that could be preventable. All human-caused accidentsand the consequences of some natural occurrences (earthquake, wind,waves) cap be defined as preventable. However, we are only interested inevents where engineering and management system parameters could haveprevented the accident provided they are within reasonable control of plantpersonnel.

Selection of an evaluation-case event helps ensure that buildings arenot inappropriately removed from the analysis pool because insignificantincidents were chosen for the evaluation. It also helps ensure that unreal-istic or highly improbable events are not chosen for the consequencescreening, requiring extensive and unnecessary analysis for incidents that,in fact, are very unlikely to occur.

3.3.2.1. Qualitative Consequence Screening Basedon Site-Specific Conditions

In a qualitative evaluation, the inherent properties of the materials areassessed for explosion potential, in combination with a review of siteconditions. An assessment is then made as to the potential for explosion,based on the experience level and judgment of the assessor.

EXAMPLE 5Consequence Screening by Qualitatively ModelingSite-Specific Conditions

Background

A facility stores NFPA (National Fire Protection Association) Class 1B liquid ina diked atmospheric storage tank equipped with an external floating roof.

The material is stored at ambient temperatures. Spacing meets internalcompany standards based on radiant heat fluxes.

A fire-resistant building is located in the facility, 75 ft (23 m) from the nearesttank, and 25 ft (7.5 m) from the nearest edge of the dike wall. The buildingcontains tank field monitoring and control equipment, provides shelter tothe operators, and also houses administrative support activities. The buildingis equipped with multiple means of egress.

A concern was raised about potential impacts to the building occupants froman explosion or tank fire resulting from a Class 1B flammable liquid spillwithin the tank dike. There was also a concern about a potential tank explosion.

Approach

Class 1B flammable liquid, if released at atmospheric pressure and ambienttemperatures, forms a pool of flammable liquid that, if ignited, develops intoa pool fire. If delayed ignition occurs, experience has shown a flash fire mayresult. For the example under consideration, explosions are extremelyunlikely because:

1. The inherent properties of Class 1B liquids, under the storage and releaseconditions specified (lack of confinement, congestion, and release ofmaterial at low pressure), preclude formation of a well-mixed, turbulentvapor cloud that can support rapid flame propagation. Thus, thepotential for VCE is low.

2. The open-top floating roof tank design eliminates the potential forBLEVE. Further, the material being handled has no potential for chemi-cal reactions or for condensed-phase explosions. Thus, these types ofexplosions can also be eliminated from consideration.

3. When the tank is initially filled, or emptied, for cleaning, or if a significantoperational upset occurs causing the tank to be emptied, a potentialflammable vapor mixture can form below the floating roof. At this time,the potential for an explosion cannot be entirely eliminated. To addressthis, alarms and operating procedures are in place to reduce the chanceof a flammable mixture forming. Further, procedures are in place tominimize potential ignition sources and limit access into the vicinity ofthe tanks.

It was concluded no scenario could be identified for an explosionimpacting the building. The building is of fire-resistant design and hasmultiple means of egress. Emergency response procedures were in placefor building evacuation in the event of a fire. Consequently, the impactto building occupants from a tank fire was low, and no further evaluationwas required.

It should be emphasized that qualitative consequence screening shouldonly remove from further consideration those buildings where there issignificant industry experience to support the assessment. In Example 5cited above, a long and successful operating history exists to support the

low explosion potential of NFPA Class IB flammable liquids, when handledunder the conditions indicated.

3.3.2.2. Quantitative Consequence Screening Basedon Site-Specific Conditions

A quantitative consequence screening can be performed by evaluating theinherent properties of the materials being handled, in conjunction with anestimate of the quantities available, and consideration of the actual con-figuration and layout of the process equipment. A calculation is thenperformed to determine potential blast effects, taking into account site-spe-cific factors contributing to or mitigating the potential consequences (e.g.,for VCEs, degree of confinement and plant layout and spacing). The resultsare then used to determine which buildings are appropriately designed andsited and which require further evaluation.

Vapor Cloud Explosion ModelingFor VCEs, consequence screening is performed in a number of differentways, depending upon the level of detail desired and the tools available tothe analyst. VCE consequence screening is based on an estimate of theblast parameters that could reasonably occur, given conservative assump-tions about the quantity and nature of possible releases.

Calculating the Mass in the Flammable Range in the Vapor Cloud. Intheir most rigorous form, VCE calculations use detailed dispersion model-ing, with estimates of cloud size, cloud mass, and distances the releaseswill travel. This information is then used to determine the potential blastparameters that can be achieved based on an evaluation of site-specificconditions, using either the TNT equivalency method, the Multienergymodel, the Baker-Strehlow model (Ref. 5), or other available models. Insome cases, after a detailed dispersion modeling and a review of the sitelayout, it may be revealed that the release has little, if any, explosionpotential because of the lack of congestion or confinement of the cloud.

EXAMPLE 6Consequence Screening by ModelingSite-Specific Conditions

Background

A flammable hydrocarbon gas that is lighter than air is processed at a smallfacility. An office building is located 75 ft (15 m) from the processingequipment. Because of the size of the facility, no pipe racks or othersignificantly sized equipment are in the area that can create confinement orcongestion if a release occurs. Further, the flammable gas is processed at lowpressures and ambient temperatures.

Approach

It was concluded, based on dispersion modeling [the vapor cloud waspredicted to travel only 30 ft (9 m) to its lower flammable limit] and the lackof confinement within the cloud, that the risk of a VCE was low. Since thebuilding was designed to be fire resistant, no further study was required.

In some cases, detailed dispersion modeling tools may not be availableor their use is not warranted. To calculate the size of the flammable portionin the vapor cloud, other less precise, though sufficiently conservative,methods are available. Reference 5 cites a number of company, insurance,and governmental practices for estimating quantities of materials thatcould become involved in an explosion or fire. Some conservative ap-proaches for determining the quantities of materials released include:

For a gas or vapor release, the potential amount of material in the cloudcan be determined by one of the following techniques:

a. [Rate of release] x [time required to stop the leak], where the timerequired to stop the leak is a function of isolating points, emer-gency valves, depressuring systems, and other mitigation sys-tems as appropriateor

b. [Release rate] x [time for cloud to achieve delayed ignition]or

c. The total inventory of material (as defined by a single contain-ment system or by an interconnected system) of material

For a nonflashing liquid release, the material in the cloud equals:

[Liquid evaporation rate] x [time for cloud to achieve delayed ignition]

It is noted that the evaporation rate depends upon the quantity of materialspilled, the surface area, and meteorological conditions. The quantityspilled approximated by:

[Rate of release] x [time required to stop the leak]

The time required to stop the leak is a function of isolating points,emergency valves, depressuring systems, and other mitigation systems asappropriate.

For a two-phase or flashing release, the material in the cloud is the lesser of:

a. 2 x [fraction vaporized] x [rate of release] x [time required to stopthe leak]or

b. 2 x [fraction vaporized] x [rate of release] x [time for cloud toachieve delayed ignition]or

c. 2 x [fraction vaporized] x [total inventory of material]or

d. Total inventory

Note: The factor of 2 in items a, b, and c above is to account for the aerosolformation and entrainment in the flashing release. Also, when determiningthe material in the cloud resulting from a two-phase or flashing release,consideration should be given to the contribution to the cloud from liquidpool evaporation.

There is an upper limit to the mass of material that can contribute toan explosion, regardless of release duration. This upper limit results fromdispersion effects, which dilute the flammable material at the cloudboundaries to concentrations below the lower flammable limit. Thesedispersion effects can apply to all release modes.

Other guidelines cited in Reference 5 suggest methods for estimatingadditional release factors such as release duration or inventories. It shouldbe noted that the above methods are again only approximations. Site-spe-cific designs and process conditions should be evaluated. For example,some facilities may have few, if any, emergency block valves available toisolate a release. Other facilities may be designed for rapid isolation.

Other techniques that take into account some site-specific conditions,such as the Dow Fire and Explosion Index (Ref. 34) and the Mond Index(Ref. 39), have been used to prioritize buildings for evaluation. The resultsof these indices should be used in conjunction with consideration of otherfactors, rather than as stand-alone criteria. These other factors mightinclude an evaluation of the effects of confinement and/or congestion-in-duced turbulence on the potential for blast.

Care must be taken to ensure that the criteria or methods used closelymatch the site-specific conditions under study. Failure to do so may resultin early elimination of at-risk buildings from the analysis pool. Further,when standard criteria are being applied, care must be taken to ensure thatthe objective of the standard matches the intent of the study. A standarddeveloped for equipment protection or loss prevention may not be appro-priate for personnel protection.

Calculating Blast. Once the mass of the flammable portion in thevapor cloud has been estimated, several methods are available for charac-terizing the blast effects from a VCE. These include the TNT equivalencymethod, the Baker-Strehlow model, and the Multienergy model. Eachmethod requires information on the properties of the material beingreleased. Both the Baker-Strehlow and Multienergy models require anevaluation of plant layout and spacing to determine the quantity offlammable material that can participate in an explosion. Characterizationof the confinement and congestion within the plant's equipment conges-tion can be made to determine its contribution to the blast. These and othermethods for calculating blast parameters are reviewed in Reference 5.

Other ExplosionsAs discussed in Section 3.2. 1, other explosion events can occur that impactprocess plant buildings, including condensed-phase explosions, uncon-trolled chemical reactions, PV ruptures, and BLEVEs. Appendix A andReference 5 describe the information needed and the methods available forcalculating blast parameters from these events.

Determining Explosion Impacts to BuildingsOnce the potential blast effects are calculated, an approximate evaluationof the impact to the building can be made using established data on buildingstructural response to blasts. Table 3.5, Table 4.8, and Figure 4.5 inChapter 4 provide additional information on building construction and thepotential consequences from the blast. (Note that the overpressures givenin these tables and figures are peak side-on overpressure and not reflectedoverpressure.) As with the screening methods discussed in Section 3.3.1,once the consequences to the building are determined, a decision can thenbe made on whether a potential risk to building occupants exists andwhether additional study is warranted.

TABLE 3.5Overpressure vs. Consequences Correlation for RepresentativeBuildings (Refs. 5 and 33)

Peak Side-onOverpressure,

psi (bar)

-0.2 (-0.015)

>0.5 (>0.03)

>1 (>0.07)

>2 (>0.14)

>3 (>0.21)

>10 (0.70)

Consequences to Buildings

Threshold of glass breakage

Significant repairable cosmeticdamage is possible

Possible minor structural damage tobuildings and severe damage totrailer-type buildings andunreinforced masonry load-bearingwall buildings

Local failure of isolated parts ofbuildings and collapse of trailer-typebuildings and unreinforced masonryload-bearing wail buildings

Collapse of buildings

Probable total destruction ofnonblast-resistant buildings

Possible BuildingOccupant InjuryConsequences

No injury to personnel

Possible personnelinjury from glassbreakage, falling lightfixtures, etc.

Personnel injury fromdebris is likely

Possible serious injury orfatality of someoccupants

Probable serious injuryor fatality of manyoccupants

Probable 100% fatalities

The peak side-on overpressures and corresponding consequences givenin Table 3.5 have been developed from References 5 and 33. The data inTable 3.5 are based on conservative assumptions, such as long durationblast waves, and low-strength building construction. This helps ensuresufficient conservatism for coarse screening efforts for a range of repre-sentative buildings.

It should be noted that consequence screening is performed withoutregard to the likelihood of an event's occurring. As a result, consequencescreening does not determine risk. Furthermore, the consequence evalu-ation performed may not represent a detailed evaluation of consequencesto the process plant. Instead, it is an approximation of expected conse-quences, given an estimate of potential blast overpressure and anticipatedresponse of representative building types. The user should not mistake thisevaluation for a detailed consequence assessment.

Again, since the purpose of both initial and consequence screenings isto establish which buildings in process plants require further evaluationand which buildings do not require further consideration, the assumptionsmade in the studies should be conservatively chosen. This conservatismwill help ensure that all buildings potentially exposed to significant blastare appropriately evaluated.

Finally, when screening for consequences based on expected blastparameters, or when performing any risk assessment of process plantbuildings, consideration should be given to other equipment in the areathat could impact the building, even though the building may withstandthe blast effect. An example might be a tall structure near a building thatmight not survive a blast. The subsequent collapse of the structure couldimpact the building.

EXAMPLE 7

Background

The control room discussed in Example 2 is located 150 ft (45 m) from thenearest process equipment in the unit. Since the building occupancyexceeds the company criteria, further evaluation is required. The buildingwas designed to be blast resistant to 5 psi (0.34 bar).

Approach

The total inventory of flammable material that could be released wasdetermined, and the TNT equivalence method (from Reference 5) wasapplied. Using this information, an incident side-on overpressure of 3 psi at150 ft (0.21 bar at 45 m) was calculated. On this basis, it was determinedthat the building could sustain the maximum anticipated blast overpressure,and no further evaluation was needed.

EXAMPLE 8

Background

A steel-frame maintenance building with sheet-metal siding is located 300 ft(90 m) from the edge of process unit handling ethylene. The building has anoccupancy load of 800 person-hours, with 20 personnel present 40 hours aweek. This exceeds the company's occupancy criteria.

Approach

In the course of evaluating the risk to a nearby control room, blastparameters were calculated using the Multienergy method. At 300 ft (90 m),the peak side-on overpressure was determined to be 1.5 psi (0.10 bar). Bythe estimates shown in Table 3.5 and Table 4.8, at 1.5 psi (0.10 bar) sheetmetal can be ripped off and internal walls can be damaged. It was felt that,at this level, the building could sustain sufficient damage to cause seriousinjury to the occupants, and further study evaluation should be performed.

EXAMPLE 9

Background

A small engineering building is located 350 ft (107 m) from the process unitdiscussed in Example 8. It has an occupancy load of 500 person-hours,which exceeds the company's occupancy criteria. The building isconstructed of unreinforced concrete and contains several windows. Earliercalculations estimated the incident side-on overpressure to be 0.5 psi at 350ft (0.069 bar at 105 m).

Approach

By the estimates shown in Table 4.8, the calculated incident side-onoverpressure of 0.5 psi (0.069 bar) should not cause significant damage tothe building. However, glass breakage could occur. It was determined thatthe windows should be eliminated or strengthened.