Why Facility Siting Is One of the Most Overlooked Parts of a PHA
Process Hazard Analyses are usually very good at following the process. Teams examine vessels, piping, valves, instruments, alarms, interlocks, relief devices, operating procedures, and potential human errors. They work systematically through each node, identify deviations, and recommend safeguards.
What often receives far less attention is everything surrounding the process.
Where is the control room? Who works in the maintenance shop next door? Could a vapor cloud reach the laboratory? Is a temporary trailer being placed inside a potential blast zone? Could a fire block the only evacuation route? Could one equipment failure damage an adjacent process and create a larger event?
These are facility-siting questions, and they are an explicit part of the PHA requirement.
Facility Siting Is Not an Optional PHA Topic
The OSHA Process Safety Management standard requires every PHA to address facility siting under 29 CFR 1910.119(e)(3)(v).
For an existing facility, OSHA explains that facility siting primarily concerns the location of components within the establishment. It is not limited to the original decision about where the plant was built. The evaluation should consider the relationships among process equipment, occupied structures, supporting operations, emergency systems, and other onsite activities.
EPA has a similar requirement for Program 3 processes under the Risk Management Program. EPA refers to it as stationary source siting. In addition to onsite hazards, the EPA interpretation of this requirement says the evaluation should consider whether the location of covered vessels creates risks to offsite public and environmental receptors.
The regulatory language is short, but the expected evaluation can be extensive.
Why Facility Siting Gets Missed
Facility Siting, like Human Factors, appears explicitly under 29 CFR 1910.119(e)(3), but it rarely receives the attention its regulatory status warrants. Also like Human Factors, OSHA does not define the term or provide much context within the standard itself. Employers are given a requirement without a clear explanation of its intended scope. As a result, facility siting is often reduced to a single generic question at the end of a PHA checklist: “Are there any facility-siting concerns?”
The team says no, checks the box, and moves on.
That approach misses the purpose of the requirement. Facility siting is not a single hazard scenario. It is a separate perspective that must be applied across the facility.
Most PHA methodologies are organized around equipment nodes. The team follows a process line from one vessel to the next and evaluates deviations such as high pressure, low flow, reverse flow, contamination, loss of cooling, or operator error.
Facility siting is spatial rather than sequential. It asks what happens beyond the equipment boundary when containment is lost.
A process deviation may begin in one node, but its consequences may extend across several units, enter occupied buildings, damage utilities, block access roads, or disable the equipment needed to respond. Those relationships are easy to overlook when the PHA team concentrates exclusively on the process flow diagram.
Facility siting may also be neglected because the facility has not physically moved. The team assumes that because the process and buildings have existed in the same locations for years, their placement must be acceptable.
Age is not proof of acceptable risk. The process inventory may have increased. A building may now contain more people. A storage room may have been converted into an office. A portable building may have been added during a turnaround and never removed. An air intake may have been relocated. New equipment may have increased congestion or created a more severe release scenario.
A siting decision that was once acceptable may no longer be acceptable under current operating conditions.
The Consequences Facility Siting Should Address
A meaningful facility-siting review begins by identifying the credible consequences that could extend beyond the immediate point of release.
Explosion Overpressure
A flammable vapor cloud explosion can damage buildings, collapse walls, break windows, dislodge equipment, and generate projectiles.
The evaluation should consider:
Occupied buildings within potential overpressure areas.
The construction and condition of those buildings.
Portable buildings and trailers that provide limited blast resistance.
Windows facing process equipment.
Control rooms that must remain operational during an emergency.
Structures whose failure could injure occupants or damage adjacent equipment.
A building does not automatically provide protection simply because employees are inside it. In some circumstances, structural failure, shattered glass, or internal debris can make building occupants more vulnerable.
In my experience, typical blast pressure ratings are 2, 5 or 10 PSI. Do you know the blast pressure rating for your structures? Do you know where to find them? Do you know the potential blast pressure your structures may be subjected to? A proper facility siting study will assist you in identifying answers to these questions.
Fire and Thermal Radiation
Pool fires, jet fires, vessel fires, and fireballs can expose personnel and equipment to hazardous heat.
The PHA should consider whether thermal radiation could:
Affect normally occupied work areas.
Prevent employees from evacuating.
Damage emergency shutdown equipment.
Compromise structural supports.
Cause the failure of adjacent vessels or piping.
Prevent emergency responders from reaching isolation points.
Escalate into a larger multi-equipment event.
Equipment spacing, drainage, fireproofing, water supplies, remote isolation, and depressurization systems may all influence the result.
Toxic Releases
A toxic release can travel well beyond the equipment that failed. The potential effect depends on the chemical, release rate, physical state, weather conditions, release elevation, and surrounding structures.
The review should evaluate:
Building air intakes.
Control-room ventilation and pressurization.
Doors, windows, and other potential infiltration points.
Locations where employees routinely gather.
Evacuation routes and muster areas.
Gas detection and alarm coverage.
Shelter-in-place capabilities.
Nearby public or environmental receptors when EPA RMP applies.
A designated muster point is not safe simply because it is far from the process. Wind direction, terrain, release behavior, and access to alternate locations must also be considered.
Projectiles and Equipment Collapse
Explosions and equipment failures can produce fragments that travel beyond the immediate process area. Structural collapse, falling piping, vessel fragments, and damaged supports can expose employees or initiate additional releases.
These hazards may be difficult to predict precisely, but they should not be ignored merely because the evaluation contains uncertainty.
Escalation and Domino Effects
Closely spaced equipment can allow one release to initiate another.
A fire may heat adjacent vessels. An explosion may rupture nearby piping. A vehicle collision may damage multiple systems. A toxic or flammable release may disable operators before they can isolate the process.
A good facility-siting review evaluates the potential for an initial event to spread across unit boundaries and challenge multiple safeguards at the same time.
Occupied Buildings Must Be Evaluated Individually
One of the most common weaknesses is relying on a global statement that all occupied buildings are acceptably located.
Different buildings have different construction, occupancy, and functions. An administrative office, laboratory, maintenance shop, control room, warehouse, operator shelter, and temporary trailer should not automatically receive the same conclusion.
For each occupied structure, the team should understand:
Its distance and orientation relative to hazardous equipment.
The number of people present and the duration of occupancy.
Whether occupancy increases during turnarounds or major maintenance.
The building’s construction and ability to resist expected hazards.
The location of windows, doors, and outdoor work areas.
Ventilation systems and fresh-air intakes.
Whether personnel can detect a release and evacuate.
Whether the building performs a critical emergency function.
In refinery enforcement guidance, OSHA has specifically stated that facility-siting considerations include both permanent and temporary occupied structures. A short checklist may be used, but only if it actually identifies, evaluates, and controls the relevant siting hazards.
Temporary Buildings Can Create Permanent Risk
Portable trailers, tents, contractor offices, break areas, and maintenance shelters deserve particular attention.
These structures are often placed for convenience. They may be located close to the work, utilities, or available parking without a formal review of process hazards. What begins as a temporary arrangement can remain for months or years.
The consequences can be severe. During the 2005 BP Texas City refinery explosion, 15 workers were killed and 180 were injured. Many of the victims were in or near temporary work trailers located close to the process unit. According to the Chemical Safety Board investigation, all fatalities occurred in or near trailers, even though the work performed by many of the occupants was unrelated to the unit startup that initiated the event.
The lesson is not limited to refineries. Temporary structures should be included in facility-siting procedures at any facility handling materials capable of causing a significant fire, explosion, or toxic release.
Current industry guidance includes API Recommended Practice 752 for permanent buildings, API RP 753 for portable buildings, and API RP 756 for tents. API published updated editions of all three documents in 2024 to expand their treatment of explosion, fire, and toxic-release hazards.
Facility Siting Is More Than Occupied Buildings
Occupied structures are important, but they are not the entire facility-siting review.
The PHA should also consider the location of:
Process vessels and storage tanks.
Loading and unloading areas.
Relief-device discharge points.
Flares and atmospheric vents.
Fired equipment and other ignition sources.
Electrical substations and emergency generators.
Firewater pumps and water supplies.
Emergency shutdown stations.
Chemical detectors and alarms.
Control-system equipment.
Roads, rail lines, and vehicle routes.
Drainage systems and containment areas.
Emergency access routes.
Muster points and evacuation routes.
Adjacent process units.
Critical safeguards should not be located where the same event they are intended to control can disable them. An emergency generator, control room, firewater pump, shutdown station, or response-equipment cache provides little protection if it is inaccessible or damaged during the initiating event.
Qualitative Review Versus Consequence Modeling
Neither OSHA nor EPA requires every facility-siting review to become a full quantitative risk assessment. EPA specifically recognizes that stationary source siting may be evaluated qualitatively.
However, qualitative does not mean unsupported. The team should have enough technical information to understand the credible consequences and make a defensible decision. This may include chemical properties, inventories, process conditions, release history, building information, plot plans, occupancy data, drainage patterns, and existing consequence analyses.
Additional modeling may be appropriate when:
High-hazard occupied buildings are close to process equipment.
The facility handles significant quantities of toxic or flammable materials.
The team cannot determine whether a building is outside the hazard area.
Existing controls depend on assumptions that have not been verified.
A proposed project will increase inventory or operating severity.
Temporary buildings will be occupied near an operating process.
The potential consequences include significant offsite effects.
Blast, fire, or toxic-dispersion modeling should answer a specific decision question. The objective is not to create an impressive technical report. It is to determine whether people, buildings, equipment, and emergency systems are appropriately located and protected.
Facility Siting Must Be Kept Current
Facility siting should not be treated as a one-time design review.
The PHA must be updated and revalidated at least every five years, but siting concerns can develop between revalidations. Management of change and project-review processes should identify changes that could affect the siting analysis.
Examples include:
Adding a trailer, tent, office, or break area.
Changing the normal occupancy of an existing building.
Increasing chemical inventory.
Installing new process or storage equipment.
Changing process pressure, temperature, or chemistry.
Relocating a vent, flare, air intake, or emergency system.
Modifying site access or evacuation routes.
Constructing a new building near an existing process.
Bringing large contractor workforces onsite during a turnaround.
Changing neighboring operations or land use.
Temporary changes require the same discipline as permanent changes when they place people inside a credible hazard area.
What a Defensible Facility-Siting Review Looks Like
A strong facility-siting review should leave a clear record of what the team evaluated and why it reached its conclusions. At a minimum, the documentation should identify:
The hazardous process areas and credible release consequences.
Permanent and temporary occupied structures.
Building occupancy and critical functions.
Potential fire, explosion, toxic, projectile, and escalation hazards.
Emergency access, evacuation, and shelter considerations.
The technical basis for concluding that existing locations are acceptable.
Additional studies required when the available information is insufficient.
Recommendations for relocation, occupancy control, structural protection, detection, isolation, mitigation, or emergency planning.
The process for reviewing future changes.
OSHA requires employers to promptly address PHA findings, document how recommendations are resolved, establish completion schedules, and communicate the resulting actions to affected employees. Facility-siting recommendations should receive the same formal resolution as any other PHA recommendation.
The Question Every PHA Team Should Ask
The purpose of facility siting is not to prove that the existing layout is acceptable. It is to challenge the assumptions built into that layout. The PHA team should ask:
“If this release occurs, who and what will be in its path?”
That question connects the process hazards to the people, buildings, utilities, safeguards, and emergency systems that surround the process. It changes facility siting from a checklist item into what OSHA intended it to be: an evaluation of whether the physical arrangement of the facility contributes to risk or helps control it.
Facility siting is easy to overlook because it exists outside the process node. The consequences of overlooking it, however, can extend far beyond the node, the unit, and sometimes the facility boundary.
Appalachian EHS can help facilities evaluate facility-siting practices, strengthen PHA scopes, review occupied-building and temporary-structure risks, and develop a practical plan for resolving identified siting concerns.