Experience with Similar Assets: Although probability of failure is asset and site specific, some guidance regarding probability of failure can be gained by examining experience with similar assets at your facility or other facilities. For example, if there is a history of a certain type of pump failing frequently after 2 years of use, and a utility has that type of pump and it is currently 18 months old, the asset should be given a higher probability of failure than it would if there was no general experience of this type.

Knowledge of Factors Affecting Physical Mortality: There are two types of assets in water and wastewater utilities - vertical or active assets and horizontal or passive assets. Vertical assets are typically visible plant assets, and include assets such as pumps, blowers, mixers, diffusers, and chlorinators. Horizontal assets are field assets, generally buried, and include pipe, valves, manholes, meters, and service lines. It's important to know the difference between the two types because different factors affect their physical mortality.

In vertical assets, the asset usually has moving parts so the asset fails with use rather than age. Failure is related to overall run time, frequency of starting and stopping, quantity and type of routine and preventative maintenance, conditions of exposure (corrosive environment, extreme heat or cold, severe weather,) improper alignment, and lubrication.

In horizontal assets, the assets are passively providing service. These types of assets fail with age because they are in constant service. Failure is related to soil characteristics, saturation level of soil, physical loads, bedding conditions, asset material and related attributes, construction conditions, exposure to weather, and quantity of internal and external corrosion.

It is important to consider all of the relevant factors when assessing an asset or class of assets' probability of failure. The factors taken together provide an overall assessment of the asset's likelihood of failure due to mortality. If, however, a given asset is more likely to fail based on one of the other modes of failure (financial inefficiency, capacity, or level of service) before it fails due to mortality, the probability of failure should match that mode of failure instead. The assets should be assigned a probability of failure rating based on how likely the asset is to fail on a scale of 1 to 5 or 1 to 10 or some other scale of the utility's choosing. An example rating using 1 to 5 is shown below.

The ratings can be developed by a team of people who are knowledgeable about the assets, gathered together in a room to decide how the assets should be ranked. This ranking does not have to be a long, time-consuming activity. A small utility should be able to complete this process by meeting a few times for a few hours. A larger utility may take a little longer.

An important consideration is the fact that the assets should be ranked relative to each other. The rankings should not be compared to other utilities; this system is meant as an internal tool only. The goal is to determine which of your assets are more likely to fail than other assets in your utility.

Once the assets are ranked according to the chosen scale, the results can be reviewed to see if they make sense. If you believe the assets that are ranked highest in terms of probability of failure are the ones most likely to fail, then the results are fine for a starting point. If not, then adjustments can be made until the rankings make sense.

A more sophisticated approach can be used, but a simple ranking like 1 to 5 works very well and will not take a lot of time to accomplish.