API 618 5th Edition introduced several significant changes from the 4th Edition that affect how pulsation analyses are scoped, performed, and documented. If your engineering team is still working from 4th Edition requirements, there are gaps in your analysis that need to be addressed.
Vessel Dynamic Analysis — New in 5th Edition
The most significant new requirement in API 618 5th Edition is the mandatory vessel dynamic analysis for pulsation suppression vessels. The 5th Edition requires that the mechanical natural frequencies of the pulsation bottles, scrubbers, and separators be evaluated to ensure they do not coincide with compressor excitation frequencies.
This is a modal finite element analysis of the vessels and their supporting structure, separate from and in addition to the acoustic pulsation simulation. It is required for all DA2 and DA3 studies under the 5th Edition and was not explicitly required under the 4th Edition.
Changes to Shaking Force Assessment
The 5th Edition revised the methodology for calculating and applying pulsation-induced shaking forces. The approach for evaluating force transmission through the piping system was updated, and the requirements for documenting force calculations were clarified.
Documentation Requirements
The 5th Edition placed increased emphasis on analysis documentation, requiring that assumptions, input data sources, and sensitivity studies be explicitly documented in the analysis report. This is a meaningful change from informal practice under the 4th Edition.
Implications for New Projects
For new compressor packages specified under API 618 5th Edition, the vessel dynamic analysis requirement adds scope and cost to the analysis program. Projects that budget based on 4th Edition scope will be underfunded.
For existing installations that were analyzed under the 4th Edition, the vessel dynamic analysis has not been performed and the 5th Edition requirement is not retroactive. However, owners considering modifications to existing systems should consider whether the 5th Edition requirements should be applied.
