While analysing an equipment for degradation it becomes necessary to understand as to whether an observed degradation of an equipment’s condition is irreversible or reversible. This post deals with this issue and is illustrated by a case.
An irreversible degradation process would call for either a complete overhaul of a machine or modular replacement of a sub-assembly or replacement of a spare.
On the contrary, a reversible degradation would need minor adjustment of machine or process parameters to correct the situation like greasing, topping up of oil, bolt tightening or minor cleaning, change of fluid level etc.
So the first decision to make is whether degradation of a system is reversible or irreversible so that appropriate follow up decisions on the right maintenance actions may be taken.
The easiest way to do that is by observing the graphical trend of a parameter that monitors the on-going condition of an asset, for example, vibration readings in velocity (in rms or peak).
So, when a graphical trend of a selected parameter is monotonically rising (moving in one direction without fluctuating) it indicates an irreversible degradation. Similarly, when a graphical trend of a selected parameter is not monotonically rising (that means when it fluctuates or changes direction) it indicates that the degradation is reversible. This to my mind is the easiest way to understand the nature of degradation to determine the right maintenance actions. And this insight is based on thermodynamic principle (1st and 2nd Law of Thermodynamics).
Case to illustrate the insight:
I happened to visit a power plant in northern India to examine a vertical ACW pump. It was vibrating at 54 mm/sec, which was abnormally high. When I looked at the trend I saw the unmistakeable monotonic rise of the vibration from a level of 3 mm/sec to its present value in six months time. So I reasoned that the white metal bearing was slowly wearing out through rubbing and as the clearance increased so did the vibration. Clearly, the degradation was irreversible which called for a replacement of the spare, i.e. the bearing.
As soon as the bearing was changed and the clearance was set through “run in”, vibration came down dramatically from 54 mm/sec to 1.5 mm/sec. It was nothing short of a magic to the engineers and managers of the plant.
But all that was really done was to model the degradation by invoking fundamental physical laws or principles.