This case study examines a reciprocating compressor system operated by a leading energy company, used for compressing natural gas. The system consists of three identical compressor units, each with two stages (suction and discharge pulsation suppression devices and connected piping), supplying gas to a common header. This system had been in operation for several years but started showing large vibrations after modifications had been made, which included rerouting. Â The primary objective of this study is to identify the root causes of severe vibrations experienced in the system, particularly in the 1st and 2nd stage suction lines, assess the system’s compliance with API 618 (6th ed. 2024) Design Approach 3 standards, and propose modifications to mitigate these issues.
The compressor system’s non-compliance with API 618 standards has led to severe vibrations and performance issues, potentially compromising the system’s reliability and safety. This case study aims to provide a comprehensive analysis of the problem and propose effective solutions to ensure compliance and improve overall system performance.
Addressing Vibrations in a Natural Gas Compressor System
The technical issue at hand is the failure of a compressor system to meet multiple API 618 checks, including:
- Pressure pulsation amplitude in the piping
- Pressure pulsation at the compressor flange
- Vibration magnitude
- Cyclic stress
The primary constraints and requirements identified in the system include:
- The pulsation suppression device has a Helmholtz frequency of 25 Hz, which is higher than the operating frequency of the compressor (13–16 Hz), rendering it ineffective in dampening pulsations.
- The existing supports are too flexible, contributing to excessive stresses in the system.
These issues collectively result in the system’s non-compliance with API 618 Design Approach 3 standards, leading to severe vibrations and potential performance degradation.
System Analysis for API 618 Compliance
To analyze the problem and develop effective solutions, the following methodology was employed:
- System Modeling: The compressor system was modeled using industry-standard pulsation analysis software to conduct a pulsation and vibration analysis conforming to API 618 Design Approach 3. The model encompassed the entire system from the 1st stage inlet to the 2nd stage discharge pulsation bottle outlet.
- Helmholtz Frequency Analysis: The Helmholtz frequency of the pulsation suppression device was calculated and analyzed to determine its effectiveness in dampening pulsations at the compressor’s operating frequency.
- Scenario Evaluation: Multiple scenarios were set up to evaluate the impact of potential modifications, including moving orifices and reinforcing supports, on the system’s performance and compliance with API 618 standards.
- API 618 Compliance Checks: The model was used to assess the system’s compliance with various API 618 checks, including pressure pulsation amplitude, vibration magnitude, and cyclic stress.
Critical Findings from System Performance Analysis
The analysis of the compressor system revealed several critical findings:
- API 618 Compliance: The current system fails all API 618 checks, including pressure pulsation amplitude, vibration magnitude, and cyclic stress. This non-compliance is a significant concern for the system’s reliability and performance.
- Pulsation Suppression Device: The Helmholtz frequency of the pulsation suppression device (25 Hz) does not effectively dampen the compressor’s operating frequency (13–16 Hz). This mismatch results in ineffective pulsation control, contributing to the observed vibration issues.
- Support Flexibility: The existing supports for the piping system are too flexible, exacerbating the stresses in the system. This lack of adequate support, particularly in the 2nd stage suction line, leads to excessive vibrations.
- Orifice Location: The current placement of orifices at the compressor flange contributes to the system’s inability to dampen pulsations effectively. This suboptimal location impacts the overall pulsation control strategy.
How To Address Pulsations, Vibrations, and Supports
Based on the analysis, the following solutions are proposed to address the identified issues:
1. Orifice Relocation:
- Move the orifices from the compressor flange to between the pulsation bottles (original as-built location).
- This change is expected to improve the Helmholtz frequency and dampen pulsations more effectively.
2. Support Reinforcement:
- Replace or reinforce the structural steel supports for all lines (1st and 2nd stage suction and discharge) to achieve stiffness equivalent to an industry-standard steel profile.
- Orient the C-beam structure in the direction of the pipe for additional stiffness.
3. Additional Support:
- Install an additional support on the 2nd stage suction A line to reduce vibrations in the long unsupported section.
Assessing the Impact of Proposed Modifications
The effectiveness of the proposed solutions was evaluated using BOSpulse. The results indicate:
- Significant Reduction in Pulsations: Implementing the recommendations, particularly the orifice relocation and support reinforcement, leads to a substantial reduction in pressure pulsations throughout the system.
- Improved Cyclic Stress Performance: The modified system complies with API 618 Design Approach 3 by meeting the cyclic stress criteria, which is a critical requirement for overall compliance.
- Enhanced Vibration Control: The addition of supports and reinforcement of existing structures significantly reduces vibration magnitudes, particularly in the problematic 1st and 2nd stage suction lines.
While some API 618 checks may still exceed acceptable limits, the overall improvement in system performance and the achievement of cyclic stress compliance represent API 618 conformity. Following Design Approach 3, some checks (such as the pipeline pulsation levels) may be exceeded, as long as the cyclic stress assessment is passed.
By addressing these vibration issues and improving compliance with industry standards, the compressor system achieved enhanced reliability, reduced maintenance requirements, and improved overall performance.
