Owning physical assets means maintaining them in an efficient and cost effective way. Key to understanding asset management is the P-F curve, which we explained in a previous blog, and the net P-F interval.

The net P-F interval is the minimum interval likely to elapse between the discovery of an asset's potential failure and the occurrence of the functional failure. The net P-F interval must be long enough to take action to avoid or minimize the consequences associated with the functional failure. This is illustrated in the following two examples. Both these have a P-F interval of six months.

Figure 1

The figure above shows that if the item is inspected monthly, the net P-F interval is five months. On the other hand, if it is inspected at four month intervals as shown in Figure 2 below, the net P-F interval is two months.

So, in the first case the minimum amount of time available to do something about the failure is three months longer than in the second, but the task of inspecting the asset must be done four times more often. The minimum time required to act may be influenced by factors such as logistics, repair time, secondary damage, operating parameters and asset condition. These should all be captured in the Operating Context.

Figure 2

The net P-F interval governs the amount of time available to take whatever action is needed to reduce or eliminate the consequences of the failure. Depending on the operating context of the asset, warning of incipient failure enables the users of an asset to reduce or avoid consequences associated with downtime (logistics and corrective action), repair costs and secondary damage or even safety.

How to determine the P-F interval

For an on-condition task to be technically feasible, the net P-F interval must be longer than the time required to take action to avoid or reduce the consequences of the failure. If the net P-F interval is too short for any sensible action to be taken, then the on-condition task is clearly not technically feasible.

This explains why so much energy is being devoted to finding potential failure conditions and associated on-condition techniques which give the longest possible P-F intervals. For P-F intervals that are very short, continuous monitoring may be the only solution.

The P-F curves illustrated in this article indicate that the P-F interval for any given failure is constant. However, this of course is not the case - some vary over a wide range of values. Clearly, in these cases a task interval should be selected which is substantially less than the shortest of the likely P-F intervals. In this way, one can always be reasonably certain of detecting the potential failure before it becomes a functional failure.

It should be noted that the criticality of the equipment should not be used to “adjust” the P-F interval since the P-F curve is a function of the equipment behavior and not its criticality since the equipment does not know it is critical.  Whether to use condition based maintenance should be based on if it is worth doing and technically feasible and then the P-F interval is determined from the P-F curve.

If the net P-F interval associated with this minimum interval is long enough for suitable action to be taken to deal with the consequences of the failure, then the on-condition task is technically feasible. On the other hand, if the P-F interval is very inconsistent, then it is not possible to establish a meaningful task interval, and the task should again be abandoned in favor of some other way of dealing with the failure.