Structural steel which is exposed to a fire can lose its tensile strength and eventually fail, depending on the quantity of heat generated and the duration of the fire. If steel failure occurs, equipment and piping could rupture or fall, potentially releasing additional fuel and other hazardous materials. It is desirable therefore to protect structural steel where the potential exists for the release of large quantities of flammable or combustible materials. Water spray (deluge sprinkler) protection can be used to provide this protection, but fireproofing is often preferred because of its passive nature and improved resistance to explosion overpressures.
Like fire barriers, fireproofing is designed to protect steel for a specified period of time as designated by a “fire resistance rating.” Construction details for various fireproofing system designs with different fire resistance ratings can be found in the UL Fire Resistance Directory (1992). Resistance ratings for fireproofing typically range from 1 to 4 hours and are determined based on one of two test methods, ASTM E-119 (UL 263) or UL1709. ASTM E-119, as discussed previously, simulates the heat developed from fires involving ordinary combustibles and is not reflective of the rapid and high heat release from burning hydrocarbons. UL 1709, also known as the Rapid Rise Fire Test, on the other hand, simulates the heat release from burning hydrocarbons and thus is better suited for determining the fire resistance rating of fireproofing used in chemical process plants (A comparison of ASTM and UL test methods is illustrated in Figure 16-3).
Figure 16-3 Comparison of methods to test fireproofing. Time-temperature curves show the reaction of concrete encased columns to fire severities represented by ASTM E-119 and UL 1709. Preparation of samples and test conditions are described in IM.2.5.1. (IRI 1990).
There are three basic types of fireproofing systems:
• Spray-on/Coated Systems—These systems consist of fireproofing materials which are sprayed or coated directly onto the structural steel, often with some means of reinforcement. The materials used are one of two types: heat reactive or inert insulating (e.g., concrete, vermiculite, gunite, and cementitious mixtures). Heat reactive materials absorb heat via their reactive mechanisms and are sacrificial in nature.
• Wrap Systems—These systems consist of flexible sheeting or mats which are wrapped around structural steel members. The sheeting or mats are then secured in place.
• Box Systems—These systems consist of a “box” which is constructed around the structural steel member using noncombustible insulating materials such as mineral, fiber or gypsum boards; mineral wool bats; gypsum; or cementitious mixtures. Laths are needed for some of these materials.
The extent of fireproofing, including the fire resistance rating, depends on a number of factors, including: volume of flammable/combustible liquid which could be released, release scenarios, hazards of materials in the process (toxicity, flammability, reactivity, etc.), criticality of operations, liquid drainage systems, elevation and proximity of steel to potential fire, building code and insurance company requirements, company standards, and other protective features. Initially, however, consideration should be given to fireproofing load-bearing steel 9-12 m (30-40 feet) above grade (or above other solid surfaces where flammable/combustible liquids could pool) and within 4-8 m (15-25 feet) of a potential fire (including the drainage path of burning liquids). It should be emphasized that these are general guidelines and that more or less fireproofing may be warranted depending on the specific situation and hazards. For this reason, persons experienced in fire protection should be consulted when specifying the installation of fireproofing. Additional information can also be found in API 2218, “Fireproofing Practices in Petroleum and Petrochemical Processing Plants”; NFPA 30, “Flammable and Combustible Liquids Code”; and various insurance company publications.
Load-bearing steel for which fireproofing should be considered includes structure columns and beams. (Wind and cross bracing will generally not warrant fireproofing if failure of such members will not adversely affect the structural integrity of the structure).
It is common practice not to apply spray-on/coated type fireproofing on the top flanges of beams which will be used to support open-type floor grating. This is due to the difficulty in achieving adequate fireproofing application without affecting the quality (and safety) of the walking surface and without creating corrosion problems. According to some sources (Castle and Castle 1987) this is acceptable since the principal fire exposure is often from pool fires underneath the beam. It is important however that the interface between the fireproofing and the steel at the flange edge be properly sealed with caulk or other materials to prevent water and chemicals from penetrating beneath the fireproofing.
• Equipment supports, such as vessel legs, skirts, and saddles (skirts may warrant fireproof ing on the inside if there are internal leakage sources or if there are sufficiently large openings in the skirt which would allow heat from an external fire to enter; also, fireproof ing of saddles is not necessary if the height of the saddle at its lowest point is 12 inches or less).
• Pipe supports (racks).
Fireproofing is also sometimes applied to process equipment and electrical/instrument cables to protect them from fire.
While installation in general is important to the performance of fireproofing, two aspects are especially critical with respect to spray-on/coated fireproofing: surface preparation and final finish (top) coating. If the steel surface is not properly prepared, the fireproofing material may not adhere to adequately to the steel and could even delaminate during a fire. It is therefore important to follow the manufacturer’s recommendations for surface preparation and to test the adhesion of the applied fireproofing. Top coating is not always necessary, but it maybe desirable where highly corrosive materials are present. Top coating also allows for easier cleaning of the fireproofing.
Fireproof ing should routinely be inspected for physical damage, delamination or other deterioration. Any deteriorated fireproofing should be completely removed and promptly repaired.