Flare Systems
The height of a flare stack is not decided by the flare. It is decided by the radiation contour that must not be exceeded at grade, at the platforms, at the fence line and on the escape routes — and, if the flame fails to ignite, by the distance the released gas needs to fall below its flammable limit.
This is the calculation that turns a flare into a constraint on the plot plan, and the one that most often sends a layout back for revision. Techso produces it early, so that the constraint is known while the layout can still absorb it.
Radiation at a receiver depends on the heat released, the fraction of it emitted as radiation, the position of the flame centre, the distance, and how much the atmosphere absorbs on the way. Solar radiation is added, because personnel are exposed to both.
API 521 sets permissible radiation levels by how long a person can be exposed and what protection they have: continuous exposure of personnel, emergency action lasting minutes, escape lasting seconds, and equipment with no personnel present. Each level applies to a different part of the plant, which is why one contour is never enough.
The consequence is a set of contours, not a number: one for the fence line, one for roads and walkways, one for manned buildings, one for the platforms of adjacent structures.
A flare that fails to ignite releases a cloud instead of a flame. The stack must be high enough for that cloud to fall below its lower flammable limit before it reaches any point where people can be, or where an ignition source exists.