Case Study: Reducing Excessive Vibration to obtain API 619 Vibration Compliance at a gas compressor unit

Overview: Vibration Control for API 619 Vibration Compliance

Roots Systems approached us with a critical issue at one of their gas pumping units: it was failing to meet the vibration limits set by API 619. The vibration levels at the station exceeded the allowable limits, raising concerns about the system's reliability and longevity. This case study details our investigation, analysis, and the effective solution we implemented to bring the station into compliance with API 619 vibration standards.

Initial Site Observations: Vibration Levels exceeding API Standard 619 Limits

Upon arriving at the pumping station in Wales, we observed that wooden blocks had been placed under the pump and motor assembly frame. These were part of a trial-and-error approach by the on-site engineers to reduce vibrations. However, the issue persisted. The pump's differential pressure was set at 0.2 bar—significantly lower than the intended 2 bar—and the vibration worsened as the differential pressure increased incrementally. Our objective was to pinpoint the structural element responsible for the excessive vibration and address it in a way that would bring the station into compliance with API 619 limits.

a gas compressor assembly mounted on blocks to mitigate the vibration as a vibration level trial and error approach

Identifying the Vibration Source: Root Cause of Non-Compliance

We conducted precise measurements at the station’s operational speed and discovered that the blower's pulsation frequency was 47.2Hz. While the wooden blocks helped reduce some vibration, they weren't sufficient to meet the API 619 compliance thresholds.

API 619, similar to other compressor standards such as API 617, 618, 672, and 681, specifies allowable vibration levels at the main bearings of compressors. After thorough analysis, we ruled out the blower and motor's critical speed as the cause of the issue. The problem stemmed from the natural frequency of the frame, which was too close to the blower's pulsation frequency. This resonance resulted in excessive vibrations that violated the screw compressor vibration limits at the bearings

the mode shape of a gas compressor, frame and motor assembly

FEA Vibration Analysis: Verifying the Source with Simulation

To further investigate, we used Ansys software to perform a Finite Element Analysis (FEA) vibration simulation on the pump, motor, and frame assembly. The analysis confirmed that the frame’s natural frequency was dangerously close to the blower’s pulsation frequency of 47.2Hz, which was causing the excessive vibrations and preventing the system from meeting API 619’s vibration requirements.

Solution: Stiffening the Frame to Achieve API 619 Compliance

Given the urgency of the situation and the project’s significant delays, we recommended a swift solution: stiffen the frame to move its natural frequency away from the blower’s pulsation frequency. This modification would eliminate the resonance effect and reduce vibration levels to meet API 619 standards

Validation of Results: Confirming API 619 Compliance

After reinforcing the frame, we conducted Experimental Modal Testing to validate the changes. The results showed that the natural frequency of the frame had shifted to a higher range, moving away from the blower's pulsation frequency. The vibration levels were now within the limits set by API 619.

Further Frequency Response Function (FRF) testing revealed no resonances near 47.2Hz, confirming the vibration had been effectively reduced.

overlaid frequency response functions (FRFs) showing resonances taken on a screw compressor pumpimg station frame using an impact hammer, accelerometer and data acquisition system to meet API 619 vibration compliance

Model Updating: Final Confirmation of Vibration Improvements

Following the frame stiffening, we performed Model Updating to refine our results and ensure the modified frame performed as intended. This iterative process enabled us to align the physical changes with the modal analysis model, providing final confirmation that the system's vibration levels adhered to API 619 standards.

Conclusion: Achieving API 619 Compliance

By addressing the frame’s natural frequency, we successfully reduced the vibration levels at the pumping station, bringing the system into full API 619 compliance. Through FEA vibration analysis, experimental modal testing, and model updating, we accurately identified the cause of the excessive vibration and implemented an effective solution. Following the modifications, the customer conducted an API 619 vibration audit, confirming that the vibration levels had been significantly reduced, allowing the gas pumping station to operate efficiently and within the required vibration standards.

Key Technical Details:

Blower pulsation frequency: 47.2 Hz
Differential pressure causing extreme vibration: 0.2 bar (operational pressure: 2 bar)
FEA Software used: Ansys
Frame mode shape: Close to 47.2 Hz, causing resonance
API 619 Compliance Target: Achieving acceptable vibration levels at the main bearings

Experimental Modal Analysis Results:

Post-modification FRFs confirmed that the natural frequency of the frame shifted to a non-critical range, ensuring vibration levels met API 619 specifications.

Why Vibration Control is Critical for API 619 Vibration Compliance

API 619 sets specific vibration thresholds to protect gas compressors from excessive wear. Unchecked vibration at the main bearing can accelerate component degradation, reducing the system's operational life. By adhering to API 619 vibration limits, we help ensure that gas pumping stations operate efficiently, safely, and with a longer lifespan..

This case study underscores the importance of thorough vibration analysis and effective engineering solutions in meeting industry standards. With the help of simulation, testing, and model updating, we successfully achieved API 619 vibration compliance for Roots Systems, ensuring optimal performance at their gas pumping station.