Cooling Tower Resonance Mitigation Through Vibration Analysis.

Abstract

Condition monitoring, or CM is the process of monitoring machinery parameters such as vibration and temperature trends at routine intervals, in order to identify any significant change which may indicate developing faults. Vibration Analysis may be considered as one of the most predominant CM techniques used industry wide to carry out predictive maintenance and analysis on rotating equipment. Fault conditions such as unbalance, misalignment, resonance, hydraulic and aerodynamic forces can be detected using a variety of vibration tests and analysis methods. Saudi Aramco Shaybah NGL Recovery Plant Department experienced a failure on a cooling tower right angled gearbox during commissioning. The root cause identified was insufficient lubrication. Although this gearbox was fitted with online fixed instrumentation, set points were too lenient. However, offline vibration troubleshooting techniques highlighted a resonance issue and provided the necessary data to rectify and resolve the high vibration on the fan. Mitigation steps taken included a thorough evaluation of the existing gearbox protection design and Velometer selection. Further vibration data was collected with a portable device for two main reasons: firstly for comparison with the fixed Velometer readings and secondly to measure in the horizontal axis, which is perpendicular to the fixed position. Higher amplitudes were recorded in the horizontal axis as the vertical position where the Velometer is fixed, due to the orientation of the gearbox supporting beam. Resonance or bump tests were conducted on the gearbox to investigate the elevated amplitudes recorded in the horizontal plane. Bump tests proved that the natural frequency of the gearbox coincided with the cooling fans blade pass frequency (BPF). Resonance was proven, therefore had to be addressed in order to avoid component deterioration and imminent failure. Cost and repair or down time was paramount in finding the solution and implementation on all four cooling fans. Fundamentally, the most effective solution was to separate the fan blade pass and the natural frequencies of the right angled gearbox. Removing a resonance condition would reduce the overall vibration amplitude and extend the component life span of the cooling tower rotating parts. This paper will detail the steps taken to investigate and rectify the mechanical failure. In addition it will discuss the relevance of an effective predictive maintenance strategy and the importance of verifying the design of fixed or permanent instrumentation.