Flare System Design: A Case for Dynamic Simulation

ABSTRACT Flare and relief systems are inherently dynamic process. Relief devices (PSV, rupture disc and flare tips) depend on system pipage for specified performance. These dependencies cause both transitory and lasting impacts. These impacts can only be understood by dynamic analysis. The past 10years of application confirm the need for and credibility of dynamic models. This paper details the theoretical development, validation and utilization experience of flare system dynamic models. INTRODUCTION Steady state flowsheet simulators are fairly routine software in today's processplant design environments. Pipeline transients and control system design are recognized dynamic problems and are best analyzed by simulation. However, dynamic simulation is still infrequently practiced. with cost to benefit ratio rapidly in favor of computerization, applications of dynamic simulations in process industries will undoubtedly multiply into the future. This paper describes a class of design problems in which we saw advantage in applying dynamic simulation oil field flare systems are inherent dynamic processes. Relief devices depend on system pipage for specified performance - relief valve capacity dependent on discharge back pressure, header peak pressure dependent on rupture disc burst conditions, radiation imposed limits dependent on peak relief rate, flare equipment size dependent on relief load characteristics and particulars of the staging philosophy. These dependencies cause both transitory and lasting impacts, e. g. temporal overpressure, flow cycling, and flame out events. And these impacts can only be understood by dynamic analysis. The past 10years of application confirm the need for dynamic models and the credibility of this technology improved in close association with better understanding of the complex flare system interactions. FLARE SYSTEM DESIGN CONSIDERATIONS Flare systems are final safeguards against overpressure, during plant transient conditions. The flare system must be designed to relieve excessive pressure to insure safety, specifically not to exceed levels allowed by "Code". API Design Practice 520 & 521 are widely observed as voluntary guidelines on the design and operation of flare systems in the American Petroleum Industry. Two key concepts are particularly applicable to this work. "Pressure relieving devices are installed to ensure that a process system or any of its components are not subject to pressures that exceed the Maximum Allowable Accumulated Pressure." "Fail safe device and automatic equipment should not replace pressure relieving devices as protection for individual process equipment." The first paragraph specifies the requirement of relief devices in the protection of equipment against overpressure. The second paragraph further stipulates that pressure relieving devices cannot be replaced nor substituted. This second paragraph therefore excludes all but pressure relieving devices in flare systems. API (ASME) pressure relieving devices for flare systems are clearly defined as PSV (Pressure Safety Valve), rupture disc and flare tip. ASME section VIII, Div. 1 Code stipulates the pressure relief requirements on unfired pressure vessels.