(138a) Dynamic Analysis of Flare System for Series of Multiple Unrelated Failures

Proper design and verification of depressurization systems are critical in ensuring overall safety of the LNG facility. During an emergency, equipment failure or a planned maintenance event, pressure vessels or process systems are normally depressurized. The primary function of depressuring is to provide equipment with a controlled means of pressure reduction to prevent the possibility of over pressuring and rupturing the equipment or to reduce release rates if a loss of containment occurs.

Depressuring system consists of multiple components such as

• Source of overpressure - Equipment to be protected (vessels and pipe)
• Relieving device - Depressuring valve
• Disposal or flare system - Primarily flare header and flare stack.

Depressuring systems are generally designed for a single major relieving event and to depressurize certain section of the plant with one or minimum number of depressuring valves. The cause of the relieving event can be fire, major leak, power failure or a instrument air failure. Each of these individual failures would result in a catastrophic outcomes or series of multiple unrelated equipment failures.

General practice is not to design the flare system for a series of multiple unrelated failures. For a recent LNG project, Bechtel has undertaken an analysis of the flare systems to determine the effect of opening all depressuring valves simultaneously during a plant wide power outrage. The assumptions for a series of multiple unrelated failures during power outage are the inability to start the backup generator followed by complete discharge of UPS serving the depressuring valves

Steady state simulation to analyse the flare system for this scenario is not appropriate as the relieving rate not continuous and the flare system volume and accumulation are not accounted for. In recent times, dynamic simulation has found significant importance in carrying out such analysis of flare system due to several reasons.

• It takes into account initial accumulation and packing due to volume in flare network
• Peak flow out is typically delayed and at reduced rate relative to peak flow in
• More realistic and less conservative results are obtained

This article discusses the dynamic simulation analysis of flare system for a general power outrage scenario. The software used and the method employed in modeling the flare system are presented. The key results and the recommendations to enhance the safety are also identified and discussed.