Reliability Tips

December 2003

Philosophy: Inspection Interval

When we work in plants and get involved in discussions of what the right inspection (condition monitoring) interval is for different equipment, we run into some interesting philosophies on how to select inspection interval.

Two of the most common arguments for selecting inspection interval for equipment are:

1. That the criticality of equipment is the key
2. That the life of equipment is a factor.

Before we will explain why both of the above philosophies are irrelevant to the inspection interval, we need to understand that equipment doesn’t fail - components of equipment do.

If the criticality of components decided the inspection frequency we would, for example, inspect a critical foundation of a turbine on the same frequency as we inspect the bearings of that turbine. To inspect the foundation each day doesn’t make any sense, because a failure of a foundation will most likely not develop into a break-down in an 24-hour period, but a failure may develop in a few days for a high speed, high load bearing.

The life of equipment is also irrelevant to the inspection frequency. If average motor life in your plant is 12 years, how often would you inspect motors? The 12 year life doesn’t provide you with any helpful information as to guide us to the correct inspection interval.

Inspection interval has to be based on the Failure Developing Period (FDP) of the components in question. For example, a particular 125 HP, 1800 rpm motor running at 60% load with little speed variation in a specific environment may be estimated to develop a bearing problem from the time of problem source until break down in 6 weeks. A guideline is to have an inspection frequency of (FDP/2), in this case 3 weeks.

Inspection interval should be selected by estimating FDP. You don’t know what the FDP is for a particular component? Neither do we, but by trending inspection and failure history your plant will learn the correct frequencies over time. The important thing is that you start with the correct reasoning.

Good luck with setting your inspection frequencie