What is the difference between availability and reliability?

Availability measures the ability of a piece of equipment to be operated if needed, while reliability measures the ability of a piece of equipment to perform its intended function for a specific interval without failure.

While both metrics are essentially measuring the length of time that an asset is operational, they differ in how the interval is being measured. The difference between these measures allows for different perspectives on a plant’s ability to perform. The distinct importance of one from the other is shown by their individual definitions.

Availability

Availability, also known as operational availability, is expressed as the percentage of time that an asset is operating compared to its total scheduled operation time. Alternatively, availability can be defined as the duration of time that a plant or a particular equipment is able to perform its intended task.

Calculating availability

Availability is calculated by dividing the actual operation time by the total scheduled operation time. In equation form it can be written as:

Availability (%) = (Actual operation time/Scheduled operation time) x 100%    

Actual operation time is defined as the total length of time that the asset is performing its intended function. The scheduled operation time is the total period when the asset is expected to perform work. The scheduled operation time excludes idle time (i.e. time when an asset is not scheduled to operate). Ideally, assets should have as close to 100% availability as possible.

Reliability

Reliability quantifies the likelihood of equipment to operate as intended without disruptions. In other words, reliability can be seen as the probability of success and the dependability of an asset to continuously be operational, without failures, for a period of time.

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How to measure reliability

Because reliability is expressed as a duration of operation without failure, reliability can be measured using the mean time between failure (MTBF) metric. Alternatively, the inverse of MTBF, also known as failure rate can be used. MTBF quantifies the average duration that an asset operates as intended without failures.

In equation form:

MTBF = Operating time (hours)/Number of failures

and

Failure rate = Number of failures/Unit of time (i.e. hours, week, month, etc.)

The operating time is the total time interval during which the asset is intended to be functional, and number of failures is the number of occurrences of failures or breakdowns.

Relationship of availability and reliability

Generally, availability and reliability go hand in hand, and an increase in reliability usually translates to an increase in availability. However, it is important to remember that both metrics can produce different results. Sometimes, you might have a highly available machine that is not reliable, or vice versa.

Take for example a general-purpose motor that is operating close to its maximum capacity. The motor can run for several hours a day, implying a high availability. However, it needs to stop every half an hour to resolve operational problems. Despite its high availability, this motor is not a highly reliable piece of equipment.