At refineries, chemical plants and other process facilities, the people most at risk from an accidental explosion, fire or toxic release are often not operators at the unit but workers in nearby offices, control rooms and temporary trailers. Facility siting is the discipline of deciding where those occupied buildings can safely go and how they must be built. This guide explains the API recommended practices that govern siting, how blast-resistant modules (BRMs) fit in, and what to ask before buying or relocating one. It stays at the conceptual level; site-specific evaluation requires qualified process safety and structural professionals.

Why siting became a central process safety issue

On March 23, 2005, explosions and fires at the BP Texas City refinery killed 15 workers and injured 180 others. The US Chemical Safety and Hazard Investigation Board reported that many of the victims were in or around work trailers located near an atmospheric vent stack. The CSB recommended that API revise RP 752 or issue a new practice so that occupied trailers and similar temporary structures are placed safely away from hazardous process areas, protecting occupants from heat, blast overpressure and projectiles, with trailer siting assessed separately from permanent buildings.

The industry responded. API published RP 753 for portable buildings in 2007 and a third edition of RP 752 for permanent buildings in 2009, and later added RP 756 for tents. Under OSHA's Process Safety Management standard, a covered facility's process hazard analysis must address facility siting, which is why these practices are so widely used.

The three API recommended practices

All three consider three main hazards: explosion, fire and toxic material release. Each recognizes that the same building can be acceptable for one hazard and unacceptable for another. A building that withstands a design blast load may still need protection from smoke, heat or a toxic gas cloud.

How a siting evaluation works, conceptually

1. Inventory buildings and occupancy. List every permanent and portable building and tent, who uses it, how many people and for how long. Occupancy criteria determine which buildings need evaluation.
2. Identify process hazards. Using the facility's process hazard analysis, determine credible explosion, fire and toxic release scenarios that could affect each building.
3. Choose an evaluation approach. The practices allow consequence-based approaches, risk-based approaches and, for some portable building cases, spacing-based approaches. The facility selects the approach and the criteria it will apply.
4. Evaluate building response and occupant vulnerability. Engineers estimate how each building would respond and how occupants would be affected, considering structure, windows, doors, furnishings and ventilation.
5. Manage the risk. Options include relocating the building, reducing occupancy, upgrading or replacing the building, or reducing the process hazard itself.
6. Maintain the program. Re-evaluate when processes change, buildings are moved or added, or occupancy patterns shift, through management of change.

What a blast-resistant module is

A blast-resistant module is a portable or modular building, usually built from heavy steel framing and plate, engineered by its manufacturer to resist a specified design load and remain protective for its occupants. BRMs are common as control rooms, operator shelters, offices and break rooms at plants and during turnarounds, because they can be moved as needs change. They range from small single-unit shelters to multi-module complexes.

"Blast resistant" is a description of an engineered design for specific criteria, not a universal rating. Two BRMs that look alike may be designed for very different loads and building response levels. Comparing them requires the design basis documentation.

Buyer's checklist for blast-resistant modules

Turnarounds, tents and temporary occupancy

Plant turnarounds and major projects bring large numbers of contractors on site for weeks at a time, along with trailers, tool cribs, lunch tents and laydown areas. That is exactly the situation the CSB highlighted at Texas City. API RP 756 recognizes that tents offer occupants little protection from explosion, fire or toxic hazards, so siting and occupancy control carry most of the burden. Good practice includes:

• Laying out temporary facilities as part of turnaround planning, with review by the site's process safety team before anything is placed.
• Keeping break, meal and meeting tents out of areas identified as hazardous in the siting study.
• Limiting how many people gather near operating units and for how long.
• Making sure muster points, alarms and evacuation routes are known to every temporary worker on day one.
• Removing temporary buildings promptly when the work ends, so they do not quietly become permanent occupied spaces.

Permanent buildings: retrofit, relocate or rebuild

For existing permanent buildings that do not meet the facility's criteria, owners typically compare three paths. Relocating occupants to a safer building is often the fastest and cheapest. Retrofitting may involve strengthening walls and roofs, upgrading windows or eliminating them, and improving doors and anchorage of interior items; the options are similar to those in our retrofit comparison, adapted to industrial loads. Rebuilding is reserved for critical buildings like central control rooms where long-term occupancy near the process is unavoidable. Whatever the choice, the analysis should follow recognized references, discussed in our standards guide, and be performed by engineers with specific petrochemical experience.

Siting is one layer of process safety. The others start with preventing loss of containment in the first place: mechanical integrity, safe operating limits, and containment of liquid spills, a topic covered in depth by our sister site SecondaryContainment.org. The Center for Chemical Process Safety at AIChE also publishes guidance on evaluating process plant buildings.