The Tesoro Salt Lake City Refinery initiated a project to reduce the sulfur content in its products, necessitating modifications to the piping system on the discharge side of compressors K-701A/B. These changes prompted concerns about excessive pulsations and vibration-induced stresses, particularly as no previous pulsation study had been conducted. The client sought to ensure compliance with the American Petroleum Institute (API) 618 5th Edition Design Approach 3 (DA3) standards, which govern pulsation and vibration analysis for reciprocating compressors.
This case study outlines a comprehensive pulsation and vibration assessment performed to verify the system’s compliance with API 618 DA3 requirements. The study encompassed both acoustical and mechanical analyses, along with a site inspection to address discrepancies between as-built piping and available drawings. The objective was to ensure that the re-rated system operates safely and efficiently without introducing vibration-induced failures.
Definition of The Technical Challenge
The primary technical challenge involved assessing the discharge piping of compressors K-701A/B for compliance with API 618 DA3 standards under re-rated operating conditions. Key concerns included excessive pulsation amplitudes, shaking forces, and vibration-induced stresses that could compromise the mechanical integrity of the piping system.
Constraints included:
- A lack of prior pulsation studies for the system.
- Discrepancies between as-built conditions and existing isometric drawings, necessitating a site visit for verification.
- The need to ensure conformance without major alterations to the existing piping layout.
Methodological Approach to Pulsation Analysis
The study followed API 618 DA3 protocols, using a combination of advanced simulation software and field inspections. The methodology was divided into the following key steps:
- Acoustical Analysis:
An acoustic model of the discharge piping was developed using BOSpulse, a pulsation analysis program. The model extended from the compressor piston valve and pulsation bottle to suitable termination points. Input parameters included gas composition, mass flow rates for four load cases, and the presence of a bypass on the coalescer. The pulsation amplitudes were calculated and assessed against API 618 pulsation criteria. - Mechanical Response Analysis:
Using the dynamic module of CAESAR II, the shaking forces from the acoustical analysis were applied to a detailed mechanical model of the discharge piping. This analysis evaluated cyclic stresses and displacements, ensuring they remained within API 618 DA3 limits. The supporting structures’ stiffness and functionality were incorporated based on on-site observations and API 618/688 guidelines. - Site Inspection:
A field visit was conducted to verify the as-built conditions of the piping system, particularly the pipe supports and structural elements relevant to the mechanical study. Any deviations from the design drawings were documented and incorporated into the analysis.
Acoustic Assessment of Discharge Piping
The acoustical and mechanical analyses yielded the following key findings:
- Pulsation Analysis Results:
For all load cases, the pulsation amplitudes were well below the allowable limits specified in API 618. The maximum pulsation level observed was 41% of the allowable value, confirming compliance with Step M3 of DA3. - Mechanical Response Results:
The forced mechanical response analysis demonstrated that cyclic stresses in the piping remained below the allowable limits per API 618 Step M8. The maximum observed stress was 34% of the allowable value. While some vibration amplitudes exceeded the API 618 Step M7 displacement criteria, these did not result in stresses high enough to cause fatigue failure, and thus compliance was achieved based on stress limits. - Site Inspection Observations:
The routing of the discharge piping matched the isometric drawings, but deviations in pipe support functionality were noted. These discrepancies were addressed in the mechanical model, ensuring the analysis reflected the as-built conditions.
Recommendations for System Improvements
Although the system was found to conform to API 618 DA3 requirements, certain recommendations were made to mitigate potential risks associated with noticeable vibration amplitudes:
- Support Structure Stiffness:
Verification of the stiffness of new support structures was recommended, with a minimum stiffness of 5000 lbf/in assumed in the analysis. Ensuring this stiffness would help maintain compliance with displacement criteria. - Mitigation of Vibration Amplitudes:
Specific locations with noticeable vibration amplitudes were identified. Recommended measures included:- Filling gaps in pipe supports near high-displacement areas.
- Strengthening bracing supports, such as extending L-shaped braces to U-shaped configurations.
- Replacing guided supports with guided clamp supports in critical areas.
These measures were deemed optional and could be implemented proactively or monitored during commissioning to assess their necessity.
Conclusion
The pulsation and vibration study for the discharge piping of compressors K-701A/B at the Salt Lake City Refinery confirmed compliance with API 618 DA3 standards. The analysis showed that pulsation amplitudes and cyclic stresses remained well within allowable limits, ensuring the system’s mechanical integrity under re-rated operating conditions.
While the system met all conformance criteria, recommendations were provided to address potential vibration issues proactively. These included verifying support stiffness and implementing minor structural modifications if necessary.
