Facilities that handle highly reactive chemical have a potential for a runaway reaction due to mixing in the flare header. Usually these reactions are not reviewed prior to an incident or PHA Action Item.
Usually the facility hires an engineering firm that tries to model the reaction based on information from the process at normal conditions or through readily available kinetic data in literature. The problem with reactive or incompatible fluids in a single flare header is being able to understand the variability with the inherent uncertainty of the scenario. Most don't even explore these variability's.
Our reactive hazard experts are able to determine the potential for a system to mix incompatible chemicals. We take the analysis one step further and use past reaction and literature data to try and estimate the effect of mixing your incompatible chemicals on the facilities safety. Oftentimes, due to the wide variety of circumstances that can lead to unsafe mixing, safety instrumented systems, physical barricades are employed to greatly minimize the risk.