Papers and Case Studies

Piping Vibration Assessments

A Red Wolf Reliability customer had excessive piping vibration causing alternating stress, which lead to fatigue failures at the tee connection for the pressure safety valve (PSV). This failure posed significant environmental and safety hazards due to the release of high pressure, potentially hazardous gases to atmosphere..

Figure 1: Picture of the Tee that Failed Due to Excessive Vibration

A single channel data acquisition method was used to identify the piping locations with excessive vibration amplitudes. Data was collected at multiple operating conditions to identify speeds and pressures that contributed to the excessive vibration. The data indicated the top of the PSV was experiencing high vibration amplitudes primarily in-line with the discharge pipe. Vibration amplitudes were recorded as high as 3.9 in/secPK at the top of the PSV. Pressure data was also collected to measure pulsation amplitudes and identify any acoustic forces that align with vibration frequencies experienced at the PSV. This data verified pulsation was not the force leading to the piping vibration.

Modal data was collected on the compressor discharge pipe to identify natural frequencies of the pipe and PSV that may be excited by expected forcing frequencies such as running speed and its harmonics. The modal data indicated several natural frequencies within the operating range of the compressor.

Figure 2: Modal Data Showing the Bending Mode at the PSV.

This FEA model was used to make recommendations for additional pipe supports to reduce the stress at the tee connection for the PSV. The collected field modal data was used to validate a Finite Element Analysis (FEA) model. To mitigate the response of the natural frequencies and lower the vibration amplitudes experienced at the PSV, brackets were added connecting the PSV to the top of the check valve that was in line with the PSV. Additional concrete supports were added under the discharge pipe to limit the bending mode experienced at the PSV.

Figure 3: Modification Recommendations Based on FEA Model

Following the addition of the recommended supports, single channel vibration data was recollected to verify that the vibration had improved. Following the addition of recommended modifications including the concrete support, and PSV support bracket, vibration amplitudes at all measured locations along the discharge pipe and the PSV of compressor discharge pipe were found to be acceptable, with a significant decrease in the vibration amplitudes measured at the top of the PSV from 3.9 in/secPK before modifications were made to 0.9 in/secPK after.