Mechanical Analysis

For the mechanical analysis model developed using AutoPIPE Advanced software, connection stiffness cannot be accurately represented using beam-type elements. Therefore, ANSYS software is employed to create a 3D structural model at these connection locations to calculate the connection stiffness. Each connection stiffness is represented by an equivalent spring with six degrees of freedom, which is then incorporated into the AutoPIPE model for the overall mechanical analysis. The figure below depicts the AutoPIPE mechanical analysis model, including three connection stiffness values derived from the 3D FEA models.

Forced Mechanical Response Analysis (Steps 3b1 and 3b2) in API 618 DA3

API 618 (6th Ed.) specifies two criteria for performing a complete DA3 analysis:

• Meeting Separation Margins and Shaking Force Guidelines: If the system meets the requirements for excitation frequency separation margins and shaking force amplitude guidelines, the DA3 analysis can be completed without the need for a forced mechanical response analysis.
• Requirement for Forced Mechanical Response Analysis: If the system does not meet the excitation frequency separation margins or shaking force amplitude guidelines, a forced mechanical response analysis must be performed in addition to the mechanical natural frequencies analysis.

Excitation Frequency Separation Margins Guidelines require that the minimum mechanical natural frequency of the compressor package system is at least 2.4 times the maximum compressor operating speed frequency, and the predicted mechanical natural frequencies shall be separated from significant excitation frequencies by at least 20%.

In practice, meeting the separation margins requirements can be challenging. Achieving a ±20% frequency separation margin is particularly difficult for frequencies above the second harmonic, as frequency overlaps are common in higher harmonics, especially in compressor packages with variable operating speeds. Additionally, the definition of "significant" excitation amplitude is often unclear. In some cases, a shaking force that is only 10% to 20% of the guideline may still be substantial enough to induce vibration. Moreover, cylinder gas forces, which are typically significant excitations, often have dominant harmonic components that coincide with the 1X or 2X compressor speed frequencies. These forces may be overlooked if a forced mechanical response analysis is not performed.

Given the difficulties in meeting excitation frequency separation margins and shaking force amplitude guidelines as outlined in the API 618 standard, a forced mechanical response analysis is required in such cases to complete a comprehensive API 618 DA3 analysis.

Conflicting Design Requirements from Piping Stress Analysis and Mechanical Vibration Analysis

Piping stress analysis and mechanical vibration analysis often lead to conflicting design requirements. Piping stress analysis typically demands a flexible system to accommodate thermal expansion and prevent excessive thermal stress. In contrast, mechanical vibration analysis requires a stiffer system to avoid mechanical resonance at low frequencies. These conflicting needs present a significant challenge in system design.

Given that most piping and nozzle failures are due to piping vibration, it is generally preferable to design on-skid piping systems to be stiffer, rather than making them "soft" to accommodate thermal expansion. For off-skid piping, where pulsation and other shaking forces are typically attenuated, thermal stress can be managed effectively with thermal loops, flexible bracing, and similar measures.

To address these conflicting design requirements, it is beneficial for both stress and vibration analyses to be performed by the same analyst. This approach ensures that both aspects are considered together and facilitates the development of a balanced solution.

Mechanical Analysis of Equipment such as Screw Compressors, Rotary Pumps and Diaphragm Compressors

The methodology for the mechanical analysis of equipment such as screw compressors, rotary pumps, and diaphragm compressors is generally similar to that used for reciprocating compressors, with the primary difference being adherence to specific standards. For screw compressors, the applicable standard is API 619, and the analysis typically involves 3D Finite Element Analysis (FEA) of the vessel using shell models. For rotary pumps, the relevant standard is API 674. The diaphragm compressors generally follow the API 618 Standard. Each type of equipment requires adherence to its specific standard to ensure accurate and compliant mechanical analysis.

The following figures illustrate the mechanical analysis models for a screw compressor package, a rotary pump package and a diaphragm compressor package, and the partial results from the mechanical analysis.