(30b) Semi-Dynamic Flare System Analysis

Flare systems are analyzed to ensure pressure drops are within safe limits for correct operation of pressure relief valves and blowdown valves and to ensure that temperatures are within safe limits for the materials of construction for the headers and laterals. A major challenge in completing this analysis is balancing rigor with computational efficiency. Blowdown processes are inherently dynamic with large variations in flow (and thus pressure drop and temperature) over the course of an event, but a full dynamic simulation of the entire flare header can be computationally prohibitive.

An alternative approach is presented where selected snapshots are taken from the dynamic simulation of sources, and these are used as inputs to a steady state flare network calculation. The resulting semi-dynamic analysis gives insight into the time evolution of the performance of the network, avoiding potentially costly over-design. Additionally, it can account for cases where design limits occur at situations other than maximum flow.

The approach is validated by a customer case study, where a low temperature limit was exceeded nine minutes after the point of maximum flow. A traditional analysis based only on running the maximum flow case would have missed the low temperature concern.