(29a) Dilemmas Faced during Review of an LNG Flare System and the Use of Unisim Dynamic Simulations for Verification.

1) Double jeopardy scenarios for sizing PSVs. Oversizing PSVs may increase confidence in a design from the perspective of equipment protection; however, drawbacks such as chatter, instability, and shorter lifetime of the PSV, among others, need to be taken into account before deciding on an oversized PSV.

2) Overuse of the Heat Absorption Equation for Vessels Containing Liquids when designing the relief flow rate in fire cases. When a vessel containing a liquid is exposed to a fire, designers usually apply the Heat of Absorption Equation for Vessels Containing Liquids given in the API 521 standard. This may be an accepted approach when determining the maximum relief flow rate. However, questions arise regarding the following scenarios: a) Does the vessel wall thickness impact the relief flow? b) I got a relief flow rate; in which circumstances do we need more than just a PSV to protect the system? What about the temperature of the wall of the vessel interacting with the vapor phase? c) LNG involves several cryogenic processes; what API 521 correlations are better suited to deal with cryogenic fluids?

3) Designers may decide to create fewer large PSVs to reduce costs. This may generate high sound levels that cause Acoustically Induced Vibration (AIV). DNV will present several mitigating measures that could be applied to reduce the risk of failure.

4) When sizing depressurization valves (DPVs), designers may group vessels using different approaches to reduce their number. DNV reviews the considerations and strategies that increase confidence in the design.

DNV will also show how the features of the most recent UniSim versions (like R492) can help design relief and depressurization systems by using dynamic simulations.