Why not? What's MWT? What formula do you use to calculate Availability and Reliability?

Hi,

Mwt is the Mean wait time, is the time between the breakdown and the start of the intervention on the machine.
To calculate the Availability and Reliability i use statistics formulas.

Silva

Silva,

Looks to me like you need more data to be able to work out statistics for some of the equipment. Cannot work out MTBF if you haven't had any failures. You could alternatively use your judgement to estimate realistic failure rates for the unknown elements.

To work out system stats you would also need to know more about the system. Including is there redundancy in the system - for example does the system require both D & E to be working or just one of them.
If there is redundancy is it spared redundancy? - Are both D & E running all the time or does E run only when D fails and vice versa. Your No. work hours would indicate that all equipment runs all the time.
You also indicate that D = E & F = G & H = I. However, there has been 8 failures of I and only 1 of H. Similarly there have been 5 failures of G and none of F. This seems to indicate that maybe they are not identical or there is preferential running of one equipment over the other.
Once you have either enough real data to calculate failure rates of each equipment, or have made a judgment on an assumed failure rate, then the system can be evaluated. One way would be to reduce the configuration by combining each of the parallel branches and then analysising the resulting series configuration.
There are many texts which can assist.

Hi David, thank you for your reply.

The equipments D&E are equals, we are using both because for doing our production quantity’s we need both working at same time during all year (same applies to F&G and H&I). Is not because the redundancy. The number of fails is different because although the equipments are equals the age is different.

For example if during one year one equipment fails only one time my MTBF is the time that he works, and if the number of fails is zero the MTBF is zero, it’s correct?

I make the calculation on my line, for example if calculate my MTBF per equipments, the MTBF of my system (production line) is the average of all MTBF. It’s correct?

The preventive time, will enter in the calculations of MTBF and so on?

Please check the attachment if my calculations are corrected.

Thanks

Joaquim

9171031443_production_line.xls (26 Kb, 39 downloads)

quote:

if the number of fails is zero the MTBF is zero, it’s correct?

No, if the number of failures is zero then the MTBF is infinity (#Div/0! error in your worksheet).

quote:

the MTBF of my system (production line) is the average of all MTBF. It’s correct?

Again, no. You cannot average when you have elements that are equal to infinity.

For your system as the failure of any one element is also a system failure, then the MTBF is equal to the the No. of work hours of system divided by the total number of failures for all equipments = 1832/18 = 100h.

I have assumed that
a) all the failures are independent
b) when fixing one equipment failure you did not take the opportunity to fix another equipment and counted this as a failure too
c) All equipment failures were also system failures

Hi David,
Thanks for your help; i understand the “philosophy” for the system calculation.

For the individual equipments, the calculations are well done?

Thank you,

Silva

The best way to calculate MTBF is by the use of Weibull distribution. For the analysis, is important to have historical information of the failures or up times and down times of the equipment. With this method you get the BETA factor and the MTBF. The BETA factor is important to know the kind of maintenance that is required for the equipment. Attach a file to calculate MTBF with some distributions.

VALRAMOR4.xls (544 Kb, 61 downloads)

Hi,

Thanks for your help, but the file give error when you try to make the calculations.

Silva

Hi,

Do you take into consideration the trip of a machine due to process upsets also in your calculation for MTBF

Karthik

Hi,

yes i take.

Silva

Hi all,
can anyone help me in finding the formula for calculating MTBF.

Zaidi,

If you can read around 20% of this thread then you will find the answer to your inquiry.

My Warm Regards,

I would like to ask if anyone can share the most common/appropriate method of calculating MTBF and Availability in the Oil and Gas Industry - with relation to Rotating Equipment.

How do the big Oil Companies calculate and present these metrics?