API 618 defines three levels of pulsation analysis, each with different scope and rigor. Choosing the wrong level — either too light or unnecessarily heavy — costs time and money. Here is how to determine which one your project actually requires.
Design Approach 1 — Simplified Acoustic Filter Design
DA1 is the simplest level. It applies simplified acoustic filter calculations to size pulsation suppression bottles based on cylinder bore, stroke, and operating speed. No digital acoustic simulation is performed.
DA1 is appropriate only for low-horsepower, low-speed applications where pulsation risk is inherently low. For most modern high-speed separable compressor packages, DA1 alone is not sufficient to demonstrate compliance.
Design Approach 2 — Acoustic Simulation Without Mechanical Response
DA2 adds a full digital acoustic model of the compressor system, simulating pulsation levels across the operating speed range and comparing them to API 618 limits. Mechanical natural frequency avoidance is checked — the analysis confirms that piping and structural resonance frequencies are separated from compressor excitation frequencies.
DA2 is the most common level for standard reciprocating compressor packages. It covers the majority of gas gathering, pipeline, and processing applications.
Design Approach 3 — Full Acoustic and Mechanical Response
DA3 adds forced response analysis on top of the DA2 acoustic simulation. The unbalanced forces from the acoustic model are applied to a finite element model of the piping system, and stress levels are calculated directly.
DA3 is required when frequency avoidance cannot be achieved — when acoustic and mechanical resonances cannot be separated by the required margin. It is also typically specified for high-horsepower installations, offshore applications, and complex multi-unit configurations.
How to Choose
If your package is above 75 kW per cylinder, involves complex piping geometry, or is installed offshore — DA3 should be your default assumption. For standard onshore packages in the 500–3,000 BHP range, DA2 is typically appropriate. When in doubt, contact your analysis firm early. The cost of choosing the right level upfront is far lower than the cost of reanalysis after the fact.
