The spall and breach limits calculated by these equations are shown in Fig. 38.
5.5. FIRE PERFORMANCE OF FIRE BARRIERS AND COMPONENTS
minutes the component has been able to withstand a standardized test. The rating can also indicate the criteria that were tested. In Europe, for instance, the three criteria are denoted as ‘R’ (load bearing), ‘E’ (integrity) and ‘I’ (insulation). The requirements and details of the test procedures are defined on a national basis. In addition, the fire barrier components need to be rated for a pressure difference to account for the overpressure generated by fireballs and vapour cloud explosions.
All of the components of the fire barrier have the same rating. Load bearing capacity is, of course, only required for the load bearing structures, such as walls and horizontal slabs (ceiling/floor). Examples of fire barrier components are fire doors, fire dampers and penetration seals for cables and ducts.
Most fire barrier components are passive by nature, that is, they are not expected to change their state in case of fire. The most significant group of active barrier components are the fire dampers which close during the fire, either by thermal or electrical activation.
5.5.1.2. Conservative screening approach for the fire performance of barriers A conservative approach, developed for aircraft crash assessment [8], is to assume that the fire will propagate to an area surrounded by:
(a) A single fire barrier rated for at least 3 h and a 30 kPa pressure difference;
(b) Two fire barriers rated for at least 3 h and less than a 30 kPa pressure or difference.
This approach acts as a first level screen. It assumes that ventilation ductwork with a less than 30 kPa pressure difference rating will provide a pathway for fire product propagation due to the induced overpressure of the fire.
5.5.1.3. Simple engineering approach for the assessment of the fire performance of barriers
The performance of the structural components forming the barriers can be assessed by using fire safety engineering methods to estimate the severity and duration of the fire. Fire analysis needs to determine:
— Whether the fire will lead to flashover or whether it will remain local;
— The heat release rate of the fire;
— The temperature of the flashover fire or local fire;
— The fire duration in hours.
The methods for addressing these points can be found, for instance, in Refs [22, 40], which provide the necessary spreadsheet tools. Finally, the performance of the barrier can be assessed by comparing the fire duration to the fire rating (hours) of the structural components. If the rating is less than the estimated fire duration, the barrier will fail.
5.5.1.4. Detailed approach for fire barrier performance assessment
Computational analysis can be used as a detailed approach for fire barrier performance assessment. The performance of the barrier components is based on the temperature failure criterion or detailed mechanical analysis, as explained in Section 4.4.
Simple failure criteria can be based on the calculated temperatures. Concrete structures may experience adverse effects due to extreme heating rates. Owing to the capillary water present in concrete, explosive spalling may occur, which leads to rapid loss of concrete cover and the possibility of a direct fire attack of the reinforcing steel. The steel reinforcement bars lose their strength above 400°C and the whole structure may lose its load bearing function. Pre-stressed concrete members may also lose their load bearing function if pre-stressing cables are heated above 250°C.
Steel structures or components are also sensitive to fire exposure. Owing to their high thermal conductivity and heat absorption, steel failure may be assumed at 500°C for structural steel. The supports of large components may fail in the case of intensive fire exposure due to loss of strength of columns, hangers, ribs, supports, etc. early on in the fire scenario.
5.5.1.5. Fire damper performance
The capacity of the fire dampers to perform their barrier function depends on both their thermomechanical stability (capacity to maintain leak tightness and thermal insulation under rapid and non-uniform heating and pressure difference) and the response of the closing mechanism. As with any other barrier components, the fire dampers need to be rated for a sufficient length of time for a standard fire test and pressure difference.
A special feature of human induced external events, such as explosions and aircraft crashes, is the rapid time evolution of the exposure. Fast activation and response are needed for:
— Closing the air intake channels feeding fresh air to the various parts of the plant, e.g. main control room, electronics rooms and diesel engines;
— Closing the ventilation ducts outside the physical damage footprint to prevent the spreading of the fire.
The response time of the damper depends on the physical characteristics and closing mechanism. The reliability of the closing mechanism under a pressure wave may be difficult to verify. The margin between the damper closing and the fireball arrival times depends on the speed of the activation mechanism, the physical distance between the event and ventilation shaft entrance, and the length and shape of the shaft leading from the entrance to the damper. In aircraft crashes, for instance, the speed of the fireball expansion is about 40 m/s. Inside a ventilation shaft, the fireball will proceed with a speed that is lower than the free air value, although the air speed inside the shaft can be significantly higher.
The activation of the damper is to be based on as early detection of the external event as possible. Optical methods that react to the conditions outside the plant are needed.
5.5.2. Fire performance of safety related structures, systems and