Turbo Tips: Why is Root-Cause Analysis Important for Turbomachines?

Root cause analysis (RCA) plays a major role in the reliability, maintenance, operation, and performance of a turbomachine. It is a problem-solving method used to identify the root causes of faults and identify corrective actions to prevent the problem/failures/faults from reoccurring—simply repairing or replacing a part is not enough.

For example, if a bolt is broken, there may be an incorrect bolt torque at installation, wrong bolt selection, inappropriate bolt material, etc. Any failed or damaged part should not necessarily be replaced with the same bolt. Based on RCA, a different part with superior material or a stronger structure might be used. Fixing the root cause usually prevents the whole sequence of events that led to a failure (or trip). In many cases, the failure is because of two or multiple deviations. All should be addressed.

CONSIDERATIONS FOR MAJOR OVERHAUL & REPAIRS

An important milestone for turbomachinery is a major overhaul (major shutdown), which is ideally planned every 5 or 6 years. Major overhauls are planned due to fouling, excessive vibration, misalignment issues, chronic problems, or malfunctions. During repair and maintenance, it is necessary to keep track of the tasks hour-by-hour. Before shutting down, take note of the final set of operational key parameters, such as hot alignment data. For failures or damaged parts, proper RCA should be completed, and root causes should be properly dealt with.

Special care is needed for the disassembly, inspection, and repair of key parts, such as lubrication systems and seal systems. Delicate or tiny parts including small valves, instruments, sensors, small orifices, etc. in turbomachinery systems or associated subsystems can be easily missed or damaged. Dirt or foreign materials can be introduced in lubrication oil or seal systems at major maintenance works that later cause serious problems at start-up.

It is useful to get a sample of fouling and deposits on internal parts to determine how they formed and necessary to conduct a proper analysis to eliminate them. Rubs can be identified at major overhauls when turbomachinery is disassembled and could indicate a problem or malfunction. For example, operation at critical speeds at high vibration amplitudes can result in some rubs. Too often some details or data, such as clearances, matches, etc., at disassembly are lost, and it is sometimes difficult to re-construct events and properly re-assemble the turbomachinery package. Hour-to-hour notes, proper match marks, and neat/orderly writing can help to avoid such difficulties. Component clearances, such as bearing clearances, are important to check and report during re-assembly. Bow check of the rotor is also important.

KEY CHALLENGES IN RCA

Several conditions can make RCA more time-consuming. Important information is often missing because it is generally not possible, in practice, to monitor everything and store all data for a long time. Secondly, gathering data and evidence and classifying them along a timeline of events can be difficult and time-consuming. Thirdly, there may be more than one root cause for a given problem/failure, and this multiplicity can make the conclusion difficult to establish. Fourth, the root causes often have several levels. Looking at many examples of turbomachinery RCAs, a deeper investigation could reveal that specification at the design stage, the operational procedure, or the maintenance procedure might be the root cause.

Amin Almasi is a Chartered Professional Engineer in Australia and the U.K. (M.Sc. and B.Sc. in mechanical engineering). He is a senior consultant specializing in rotating equipment, condition monitoring and reliability.