The EVO-42 compressor package, featuring a screw compressor, oil separator, motor, dampener, and associated piping, was subject to a comprehensive evaluation by Dynaflow Research Group. This study aimed to assess its mechanical and acoustic performance under operating conditions, following the guidelines set out in API 619 and ASME VIII Div. 2. The evaluation was initiated to address concerns regarding the system’s mechanical integrity and resonance due to pressure pulsations as well as to ensure compliance with industry standards.
The scope of the project encompassed three key areas: a mechanical study to assess the impact of dynamic loads on the oil separator, a pulsation study to evaluate pressure fluctuations in the system, and a mechanical response analysis to determine the effect of these pulsations on structural integrity. This case study outlines the methodology, findings, and recommendations derived from the analyses.
Identification of Mechanical and Pulsation Challenges
The EVO-42 screw compressor inherently generates pressure pulsations due to its operational characteristics, with frequencies tied to rotor speed and the number of lobes. These pulsations can lead to mechanical vibrations and stresses, potentially compromising the integrity of the system. Specific concerns revolved around:
- Excessive bending stresses in the oil separator due to compressor and motor loads.
- Resonance phenomena in the piping and components caused by pressure pulsations.
- Mechanical modes of vibration coupling with pulsation frequencies, particularly at low RPM, leading to cyclic stress exceedance and potential system damage.
The analyses aimed to evaluate these issues and propose solutions to mitigate their impact.
Methodology for Analyzing Mechanical and Pulsation Effects
The evaluation was divided into three key activities, each utilizing advanced modeling tools and codes to achieve reliable results:
- Mechanical Study:
The oil separator’s structural response to dynamic loads was analyzed using Finite Element Analysis. The loads from the compressor and motor were modeled, with cyclic forces applied to assess fatigue stresses. The analysis adhered to ASME VIII Div. 2 standards, focusing on stress concentrations at welded connections that are particularly sensitive to fatigue. - Pulsation Study:
Pressure pulsations in the compressor skid, including the downstream oil separator and small-bore connections, were analyzed using BOSpulse software. The study involved modeling the screw compressor and silencer to predict pulsation amplitudes and assess their compliance with API 619 limits. The analysis considered the first two harmonics of the Pocket Passing Frequency (PPF), which typically contain the highest pulsation energy, and examined the effects of varying gas properties and compressor speeds. - Mechanical Response Analysis:
The mechanical response to pulsations was evaluated by constructing a detailed three-dimensional FEA model. This analysis identified natural vibration modes in the system and assessed their interaction with pulsation-induced forces. Both beam modes (swaying motions) and shell modes (vibrations of the vessel wall) were studied, with a focus on low RPM conditions where resonances were most likely to occur.
Findings from Mechanical and Pulsation Analyses
Mechanical Study Results:
Initial FEA results indicated excessive bending stresses in the oil separator wall, exceeding ASME VIII Div. 2 limits. These stresses were attributed to the torque transmitted by the compressor and motor. However, incorporating a conical coupling between the motor and compressor significantly reduced these stresses by enhancing the system’s torsional rigidity. With this coupling, the stresses were brought within acceptable limits, negating the need for additional reinforcements to the compressor support.
Pulsation Study Results:
The pulsation analysis revealed standing waves in the inlet piping and main chamber of the oil separator. These acoustic modes were found to coincide with the first two harmonics of the PPF, particularly at low RPM. While the oil separator effectively reduced pulsations downstream, resonance at specific frequencies led to increased shaking forces on the system. The analysis confirmed that pulsations were within API 619 limits for most operating conditions, but low RPM scenarios posed higher risks of resonance-induced stress.
Structural Response Analysis:
The structural response analysis identified several natural vibration modes that coupled with pulsation frequencies, leading to high displacements and stresses at low RPM. These stresses exceeded cyclic stress limits specified for API 618, particularly in small piping sections with insufficient restraints. At higher RPMs, the system operated well within allowable limits, with no significant vibrations or stresses observed.
Recommendations for Mitigating Identified Issues
To address the identified issues, the following solutions were proposed:
- Compressor Support:
The addition of the conical coupling between the motor and compressor eliminated the need for further reinforcement of the compressor support, as it effectively redistributed loads and reduced bending stresses. - Piping Restraints:
An additional restraint was recommended for a small oil return line to limit bending stresses and bring cyclic stresses within allowable limits. This measure would mitigate the risk of fatigue failure in the piping. - Operational Adjustments:
To minimize resonance risks, it was recommended to prioritize operating the system at medium and high RPMs, where pulsation amplitudes and mechanical vibrations were significantly lower. Adjusting system dimensions to address resonance for one operating condition was deemed impractical, as this could exacerbate issues at other conditions. - Future Design Considerations:
For future designs, incorporating advanced damping mechanisms and optimizing structural supports may further reduce susceptibility to resonance and cyclic stress.
Conclusions and Broader Implications
The comprehensive evaluation of the EVO-42 compressor package identified critical issues related to mechanical stresses and pressure pulsations, particularly at low RPM operating conditions. By incorporating a conical coupling and additional piping restraints, these issues were effectively mitigated, ensuring compliance with ASME VIII Div. 2 and API 619 standards.
The study highlights the importance of detailed mechanical and acoustic analyses in the design and operation of compressor systems. It underscores the need for robust support structures, precise pulsation modeling, and consideration of operating conditions to prevent resonance and fatigue-related failures. By implementing the recommended measures and prioritizing mid-to-high RPM operations, the EVO-42 compressor package can achieve reliable and efficient performance across its operational range.
