A client operates a reciprocating compressor at their terminal facilities, which is integral to their gas processing operations. During routine operations, excessive vibrations were observed within the system, raising concerns regarding equipment reliability, safety, and compliance with relevant industry standards. The compressor system in question features a 6-cylinder design with three stages, powered by a variable-speed gas engine. Currently, one cylinder operates in a fully unloaded state, while all other cylinders function in single-acting mode. The gas processed by this system has a high methane content, ranging from 84 to 92 mole%.
The client engaged the team at Dynaflow to perform a thorough review of the existing pulsation analysis and to conduct a complete API 618 (latest edition) Design Approach 3 analysis of the as-built system. The primary objectives included identifying the root causes of the observed vibrations and recommending effective mitigation strategies to optimize system performance while ensuring compliance with industry standards.
Methodological Approach to Pulsation Analysis
The main technical challenges identified during the study were excessive vibrations and loading across various components of the compressor system. Specific issues included:
- Pipe Stress: Two areas of the piping system exhibited stress ranges that exceeded allowable limits.
- Flange Loading: Nineteen flanges showed excessive loading under the Kellogg equivalent pressure method, with two remaining non-compliant even after applying the Koves factor.
- Nozzle Loading: Excessive loads were observed on vessel nozzles, air coolers, and pulsation bottles, particularly under cyclic operating conditions.
- Support Limitations: The widespread use of clamp-type supports in the system resulted in undefined support flexibility, which contributed to high thermal expansion stresses and vibrations.
The analysis had to account for operational constraints, including the system’s configuration with one compressor cylinder unloaded and the remaining cylinders in single-acting mode. The gas composition, with 84–92 mole% methane, and the varying operating temperatures presented additional complexities.
Methodological Approach to Analyze System Issues
To address the identified challenges, the engineers at Dynaflow adopted a structured methodology involving advanced modeling and simulation tools:
- Static Flexibility Analysis
The piping system was modeled using CAESAR II to evaluate static stresses, flange loading, and nozzle loads. Finite Element Analysis was performed using NozzlePRO to assess critical nozzle loadings and stress intensification factors. - Pulsation and Vibration Analysis
A complete API 618 Design Approach 3 study was conducted to evaluate acoustic pulsations and mechanical response. This included a review of pressure pulsations, shaking forces, and vibration-induced stresses. - Site Visit
A site visit was conducted to verify the as-built configuration, assess the functionality of supports, and incorporate real-world data, such as material thickness and corrosion allowances, into the models. - Mitigation Strategy Development
Based on the findings, design modifications and operational adjustments were proposed to address the identified issues and optimize system performance.
Findings from the Static and Pulsation Analysis
The static flexibility analysis revealed two critical stress issues. For Line L24, the stress range between the minimum and maximum design temperatures was 543% of the allowable value, while Line L26 exceeded the allowable range by 114% due to thermal expansion. These overstressed areas indicated the need for redesign or additional flexibility in the piping system.
The flange load assessment identified 19 overloaded flanges under the Kellogg equivalent pressure method. Incorporating the Koves factor reduced this number to two, but a more detailed analysis using ASME VIII Appendix 2 was required to confirm their acceptability.
Nozzle load evaluations showed excessive forces on vessel nozzles and air cooler nozzles, particularly under cyclic conditions. The use of clamp-type supports further exacerbated the problem by contributing to undefined load distribution and limited flexibility.
The pulsation and vibration analysis revealed that pressure pulsations and shaking forces were contributing significantly to the observed vibrations. The interface between pulsation bottles and compressor cylinders was identified as a critical area requiring design modifications.
Proposed Design Modifications and Solutions
To address the identified issues, Dynaflow proposed a series of targeted solutions to optimize the system:
- Pipe Stress Mitigation
The overstressed piping sections (L24 and L26) were addressed by introducing additional flexibility and optimizing support configurations. This included re-routing pipes in critical areas and reducing the thermal expansion loads by incorporating flexible supports. - Flange Load Optimization
The two remaining overloaded flanges were deemed acceptable after a detailed assessment using ASME VIII Appendix 2. This analysis demonstrated that the existing design could sustain the applied loads with no risk of failure. - Nozzle Load Reductions
Vessel and air cooler nozzle loads were reduced by implementing detailed FEA models to account for the flexibility of nozzle-header interfaces. Updated stress intensification factors for gusseted weldolets were applied to ensure accurate assessments. - Support Improvements
Clamp-type supports were modified to improve their functionality, including the removal of guide functionality at two locations and the relocation of supports to redistribute loads. Rest supports underneath the discharge pulsation bottles were lowered or removed to reduce theoretical loads during operation. - Pulsation and Vibration Mitigation
Structural reinforcements, such as diagonal bracing and stiffener plates, were added to pulsation bottles to minimize vibrations. Additionally, operational adjustments were made to maintain the system’s balance and reduce pulsation-induced stresses.
The implementation of these solutions effectively addressed the key challenges highlighted during the analysis. All static pipe stresses were brought within allowable limits as per ASME B31.3 and EN13480 standards. Flange loads were validated as acceptable using advanced analysis techniques, and nozzle loads were reduced to safe levels.
The modifications to support configurations and structural reinforcements significantly mitigated vibrations and improved the system’s overall reliability. Furthermore, the system achieved full compliance with API 618 Design Approach 3, ensuring safe and efficient operations moving forward.
Conclusion
The pulsation and vibration analysis conducted by Dynaflow successfully identified and resolved critical issues in the client’s reciprocating compressor system. Through detailed modeling, advanced simulations, and targeted design modifications, the system was optimized to meet industry standards. Key outcomes included improved operational reliability, compliance with API 618, and reduced risk of equipment fatigue or failure.
