Mean time between failure (MTBF)1, is one of the most widely used metrics in physical asset management. Generally, companies use it as a guide to evaluate the performance of their physical assets, helping them identify assets or processes that are causing lost revenue and other cost-related issues.
Although widely applied, MTBF is still the subject of some confusion. MTBF is useful for a range of different purposes, giving organizations greater ability to increase the net present value of their physical asset base.
When companies first look at implementing MTBF, they tend to ask three fundamental questions:
- What can MTBF tell us about our assets
- What levels can it be applied at
- How can MTBF be used to add value to our reliability initiatives
What MTBF can tell us
Figure 1. Example system
The standard use of MTBF in industry is to tell us the performance of the primary function of an asset or system.
For example, a pumping system consists of a duty/standby pump arrangement, a pressure relief valve, piping, a tank and associated level switches. The primary function of this system is to pump water to tank B at a rate of between 900 liters/minute and 1,000 liters/minute. In this case, a failure occurs when the pump system is unable to pump water at the required rate for whatever reason.
Here, we can calculate the MTBF as follows:
So, if the total time required to deliver this function was five years and there were four failures in that time, the average time between failures would be 5/4 = 1.25.
If this were the mean time between failures, then the failure rate for one year would be 1/MTBF or 1/1.25, which is 0.8, or 80% likelihood of experiencing a failure of the primary function in one year.
If we then wanted to convert this into months, we would first convert the MTBF figure to months, 1.25 years = 15 months, then again determine the likelihood of this occurring in one month: 1/15, or 0.066.
This means there is a 6% likelihood of experiencing a failure resulting in the loss of the primary function in any given month. We could do the same for a week, a day or any other given period.
The above example shows us that initial uses of MTBF can provide us with the average time between failures2 for a given time period, and that this can then be manipulated to give us a failure rate3 for any specified period of time.
Thus, for one measurement of MTBF, we are able to calculate the following information:
- Likelihood of a failure in one year = 1/1.25years (80% or 8 x 10-1)
- Likelihood of a failure in one month = 1/15 months (6% or 6 x 10-2)
- Likelihood of a failure in one day = 1/456.25 days (0.22% or 2.2 x 10-3)
- Likelihood of failure in one hour = 1/10,950 hours (0.009% or 9 x 10-5)
The formula takes into account the total time of the function, not of the asset itself. This means that regardless of the number or type of assets in the system, the calculation always uses the total time required of the function, or five years in this example.
At what level can we apply MTBF?
Like many other metrics in physical asset management, MTBF is applicable at any level throughout the asset base.
For performance measurement, there are two rules for its application:
- It is always used to measure the function of the asset where it is being applied.
- It always uses the total time required of the function of the level where it is applied.
In the example given above, we determined that the MTBF for the pumping system was 1.25 years, and we were then able to derive failure rates for various other periods.
In addition, we can also apply this to the assets in the system as demonstrated in table 1.
|Asset||Function||Total Time Required||Number of Failures||MTBF||Annual Failure Rate|
|Duty Pump||To pump water to tank B at a rate of between 900 l/minute and 1,000 l/minute||5 years||7||0.714 years||140%|
|Stand By Pump||To maintain 800 l/minute to 1,000 l/minute if the duty pump fails||5 years||2||2.5 years||40%|
|Piping||To provide clear access for 800 l/minute to 1,000 l/minute from the pump sets to the tank||5 years||1||5 years||20%|
|High-High Level Switch||To trip the pumping system when water reaches the high-high level||5 years||1||5 years||20%|
|High Level Switch||To shut off the pump when the tank level reaches the high level||5 years||1||5 years||20%|
|Low Level Switch||To turn on the pump when the tank has been drained to the low level||5 years||1||5 years||20%|
|Low-Low level switch||To alarm when the tank level has been drained to the low-low level||5 years||0||5 years||20%|
|Tank||To contain up to 250,000 liters of water||5 years||0||5 years||20%|
|Pumping System||To pump water to tank B at a rate of between 900 l/minute and 1000 l/minute||5 years||4||1.25 years||80%|