(44t) Making Relief Load Estimates Match Reality

Assumptions are a chemical engineer’s best friend; however, when an assumption is used incorrectly, it can lead to problems that do not exist. When looking at relief systems analysis, some assumptions need to be made so the system can be analyzed. Some of these assumptions, though, can lead to costly modifications to the system. The ability to know when to apply certain assumptions is paramount with the ability to accurately size the relief valve. If assumptions are always on the conservative side, the calculated relief loads may be much higher than what is to be expected in the plant compared to the relief loads that were calculated using the assumptions. Moreover, when looking at a column system, it is important to know what a conservative assumption is and what may exceed the operational limits at relief pressure. For example, when looking at the top-tower reflux failure, it may be common practice to size the relief valve based solely on losing the reflux; however, it is important to look at how the reboiler may operate during the relief scenario. Since cooling is lost to the column, the bottoms of the column would be hotter than during normal operation. If the reboiler would be modeled based on the bottom tray liquid and not assumed to have the same duty as normal operation, the actual heat input would be reduced, leading to a reduction in the relief load. This could be the difference between the relief valve being sized adequately or being undersized. A temperature pinch is also important to look at because that is how the system is expected to operate under the plant’s relief condition. By not assuming that the reboiler would operate as efficiently during relief as it does during normal operation, the relief load may be lowered, thus leading to fewer undersized valves and money saved. The application of assumptions is common practice in relief systems analysis; however, an assumption that does not match the true operational limits may lead to problems with the relief system that may not actually exist. The invalid assumptions could cost millions to fix the problem.

This paper will further discuss the problems associated with unrealistic assumptions, and will also teach ways to determine whether an assumption does not exceed the operational limits of the system.