Pumps & Systems - September 2007

where N is the number of machines in the populations and R are the number of repairs in the measurement period.

MTBPM (Mean Time Between Planned Maintenance): The mean number of life units between planned maintenance activities, during a particular measurement interval under stated

conditions. Planned maintenance activities only count as failures or repairs if work is required to restore the component. Here are a few examples of planned maintenance activities that are not considered repairs:

1. Oil replacement

2. Packing adjustment

3. Work planned and then cancelled

4. Periodic internal pump inspections due to known corrosion or erosion concerns

5. Periodic alignment checks due to foundation settling

6. Journal bearing inspection due to contamination concerns

SEPCO^® CAN SAVE YOU MORE THAN OUR PRODUCTS COST.

See how to reduce your water an d energy
consumption, increase equipment
reliability and improve your
product yield with SEPCO^®’s
high performance sealing
solutions. For your
business and your
own peace of
mind, check out
SEPCO^®.

Of these three metrics, MTBR is probably the most widely used for evaluating pump reliability, despite the following limitations and caveats:

• It includes the mean pump life along with the mean time for the organization to identify, plan, and repair the pump, which tends to inflate the value of MTBR.

• The MTBR metric is an amalgamation of repair data for all pumps, running and idle, that are included in the population. This also tends to greatly increase its value compared to the true mean time to failure. (Some will argue that even idle pumps are subject to failure.)

• As stated before, the repair data supplied for the calculation is subject to interpretation, therefore it is prone to errors and inconsistency.

• Wide fluctuations in the MTBR values can occur in small pump populations or when short reporting periods are used, due to the significant effect of small variations in repair numbers used in the calculations. This situation may require a 6-month or 12- month rolling average to smooth out these fluctuations. (I welcome statisticians to offer their recommendations on the minimum number of repairs or timeframe required for a given pump population to obtain a reliable MTBR value.)