How to Avoid over Designing of Flare System (Headers, Flare Stack and KO drums)

Relief devices are reliable means of overpressure protection commonly used in industry. OSHA requires all potential overpressure scenarios be documented for each piece of equipment. Required relief rate calculations are performed to determine how much a system will need to relieve in order to not exceed pressure above what is allowable by code. Due to lack of data or time pressure, designers often tend to make simplified conservative assumptions which can lead to overdesigning of new relief system or render existing system inadequate. Just like under design, overdesign has its own negative consequences in terms of performance and cost. Overestimation of required relief rate results in bigger PSVs and bigger headers for global scenarios. In addition to flare headers, these over-estimated relief rates have direct impact on sizing of effluent handling system components such as Knock Out (KO) Drums, KO pumps and Flare Stack. This paper presentation sheds light on some of these over conservative assumptions and in order to avoid overdesign, how a designer can refine relief calculations. Further a flare case study has been presented which delves deeper into refinement of relief rate calculations which saved significant modifications to the existing flare header system and eliminated need for a new flare.