(133c) Relief Systems Design: Simplifying Assumptions Gone Wrong

The process of evaluating an entire facility’s relief and flare systems design can be extremely time consuming and costly.  In most cases, simplifying assumptions or generalities are utilized in order to make this process less painful.  Unfortunately, doing this can result in even more costly or painful results if the team executing the work does not know when to forego these assumptions and generalities.  This paper will look at some of the common areas where the generally reasonable engineering assumptions do not apply.  The paper will delve into these two case studies to give a more in depth account of what might be missed and how to properly account for the cases. First, a Fluid Catalytic Cracker Unit (FCCU) will be examined to show how some of the simplifying assumptions could actually lead to costly mistakes; however, taking extra time during the initial analysis would have prevented the mistakes.  Secondly, when looking at high pressure reactor train, the recycle gas compressor system needs to be analyzed in extensive detail.  The usual simplifying assumption is to account for hydrogen going through each possible pressure safety valve (PSV) during a relief case instead of actually looking at how the hydrogen in the system may be relieved. These two cases are the first in a series of situations where it is important to put the time and energy into performing sound engineering analysis rather than applying simplifying assumptions.