Does This Formula Need To Be Revised?

SMRP has made a great KPIs book called SMRP Best Practices and I always refer to this book as a main reference for maintenance and reliability KPIs.

There is one formula in this book I couldn’t digest, and I think that it needs either to be clarified or to be revised by SMRP.

The above-mentioned formula is the “Availability” formula.

If you generally search the net for the Availability formula, you’ll get the formula that we all know and use:

Where:

Uptime = Total Available Time – Downtime

Downtime = Scheduled Downtime + Unscheduled Downtime

But for some reason the SMRP definition for Availability is a bit different as shown below:

In SMRP’s formula, there’s a new element which is the “Idle time” and it is defined as shown above, and calculated as shown below:

So, what’s wrong with the SMRP formula?

Let’s consider an asset which has been kept as standby for one month (the availability calculation period).

When an asset is kept as standby (without any failure or problem), it is considered available because we can use it at any time and hence the uptime can be calculated as follows:

Uptime = Total Available Time – Downtime
Please note that Standby time (Sometimes also called Available Not Require (ANR)) is NOT a downtime.

So, Uptime = 720 – 0 = 720 hours

So, according to the normally used formula, the availability calculation for this standby asset will be:

Availability % = Uptime ÷ (Uptime + Downtime) x 100

Availability = 720/720 = 100% available

Which is logical as it means that our asset is 100% available because at any time we can run it.

While if we use the SMRP formula the calculation will be as follows:

Uptime = Total Available Time – (Idle Time + Downtime)

Uptime = 720 – (720 + 0) = 0 Hours

Idle Time = ND + AIT

Idle Time = 720 + 0 = 720 Hours
because the above-mentioned Idle Time definition says “or waiting to run” which is the standby time!

Accordingly, the Availability of this asset will be:

Availability % = {Uptime (hrs.) / [Total Available Time (hrs.) – Idle Time (hrs.)]} x 100

Availability % = 0 / [720-720] = 0/0 = 0%

The logical question here now is, if you keep your asset (which has no failures and is ready to work at any time) as standby for one month will you consider it totally unavailable, i.e. its availability is 0%?

I think the answer is obviously NO because it is illogical and, hence, I think that SMRP’s Availability formula needs to be revised.