(44r) High-Fidelity Combined Dynamic Modeling of Depressurizing Vessels and Flare Networks: Accurately Assess the Effects of Process Changes On Mechanical Integrity and System Capacity

The use of high-fidelity dynamic modeling for combined depressurization and flare network analysis can result in identification of additional capacity in flare networks without compromising process safety, presenting potentially significant opportunities for reducing or saving capital expenditure (CAPEX). The same analysis can simultaneously provide accurate assessment of temperatures in vessels and pipework, to identify areas where close approach to or violation of minimum metal temperatures may occur, thus compromising vessel and pipework mechanical integrity.

The flare networks for major plants represent a non-negligible part of the overall capital investment. Current industrial practice for their design is primarily based on steady-state analysis, as described in standards such as API 521 and supported by widely-used software tools. However, it is a widely recognized fact that the application of steady-steady considerations to what is fundamentally a dynamic system inevitably requires the use of conservative assumptions which often result in significant oversizing of flare headers and other components of the network. Another major contributor to capital cost is the use of special materials for the parts of the system that may be exposed to low-temperature fluids – typically the tail pipes attached to high-pressure process vessels – and which are therefore at risk of embrittlement. The key to being able to limit the additional capital expenditure is the accurate estimation of the length of piping that is subject to “abnormal” temperatures.

In addition to dynamic flare network analysis, the detailed dynamic modeling and simulation of the rapid depressurization (“blowdown”) of high-pressure vessels is a key factor in determining both the load imposed on the pressure relief system (e.g. flare network) and also the temperatures of the vessel and downstream pipework walls. Current methods typically use ‘lumped’ equilibrium approaches that do not adequately predict these quantities; indeed, in contrast with the current over-conservative practice in flare network design, they may provide non-conservative solutions. By contrast, the high-fidelity approach incorporates a multi-phase non-equilibrium representation of the vessel fluid contents, as well as a 3-dimensional model of the metal walls that takes into account the transfer of heat between regions of the wall in contact with different phases. This allows a much more accurate estimation of the wall temperatures, and the direct computation of thermal stresses.

In the past it has been difficult to assess these important design criteria effectively because of the challenges in modeling such complex systems. This paper describes how the combination of high-fidelity depressurization capabilities and an advanced model-based system for flare system network design, coupled with evolving best practice in using these tools, represents a transformation for process safety management. The new techniques allow accurate assessment of the effects of process changes on mechanical integrity, thus enabling informed safety and CAPEX decision support.