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Every process plant has a dedicated disposal system to ensure that gases from equipment, instruments and pressure safety valves are flared to follow environmental norms for the gases vented to the atmosphere from the flare stack.
To estimate the combined relief load for the external fire scenario, the maximum extent of the fire should be estimated. API 5211 defines a fire impact area of 230 m2 to 460 m2 around the source of fire. However, it does not define the exact shape of the area. For a simplified approach, the shape is always considered as a circle, often referred to as a fire circle. This ensures that every piece of equipment in the fire circle radius is considered for estimating the combined relief load.
Various considerations, such as actual layout of the plant, the location of the fire source and provision of the drainage and dikes, need to be addressed while defining fire circles. Basic steps for defining fire circles and fire zones are outlined in the following sections.
Identification of fire source. Equipment that handles flammable fluid or combustible liquid above flashpoint is considered a source of fire. A list of such equipment should be prepared, and these items should be identified on an equipment layout. Note: API does not specify the height (elevation from the grade) limit to identify equipment as a source of fire. It dictates only the limit of the fire, which is 7.6 m from the grade or any solid floor where a pool fire can occur.
In the absence of an international code, the user may assume that, even ifthe equip - ment is elevated 7.6 m above fire height, it can still accumulate enough liquid on the ground to trigger...