In some cases, uncontrolled chemical reaction can present a potential causes of overpressure. Intended reactions can become uncontrolled and unintended reactions can occur without control due to any of several root causes:
a. Loss of temperature control, due to an external fire, loss of cooling, or loss of mixing.
b. Loss of reaction mixture control due to loss of mixing, loss of a feed stream, incorrect feed of the correct compounds, or feed of incorrect compounds.
Estimation of the relief flow rate required in a chemical reaction contingency is a complex process, usually requiring bench-scale experiments simulating the expected reaction upset conditions. This is necessarily highly dependent on the specific substances involved, the specific process being evaluated, and the specific conditions expected to produce the reaction upset.
Publications from the Design Institute for Emergency Relief Systems (DIERS) and the Center for Chemical Process Safety (CCPS) of the American Institute of Chemical Engineers (AIChE) provide further detail and additional sources of information on the subject of the chemical reaction causes of overpressure.