What is availability?
Availability refers to the duration of time that a plant or a particular equipment is able to perform its intended task.
The amount of operational time of an equipment can directly impact the performance of a plant. While availability as a metric can be expressed in various ways, it generally quantifies the probability that an equipment is in working condition.
Availability is often measured by looking into an equipment’s uptime – that is the amount of time that the equipment is performing work. To characterize the availability of an asset, it is therefore important to identify the instances of downtime or any duration when operations are stopped. Downtime can be classified as planned or unplanned. While downtime can be a strategic step to perform proactive maintenance activities, it would provide useful insights to compare how much downtime is contributing to the overall performance of the equipment or even the whole plant.
With the use of maintenance software such as a computerized maintenance management system (CMMS), the process of gathering such metrics can done more easily and accurately.
Differentiating availability from reliability
Another metric that relates to the duration that an equipment operates is reliability. Reliability differs from availability by quantifying the probability that an equipment will work as intended without any failures, as opposed to just measuring the time it is in operation.
As a hypothetical example, imagine a faulty mechanical mixer. Say the mixer shuts off every hour, while a properly functioning mixer can continuously work for 24 hours without issues. While the availability of the faulty mixer may still be quite high if operated over a couple of hours, it can’t be considered a reliable piece of equipment as it breaks down every time.
Intuitively, availability and reliability are related metrics. If an equipment is expected to operate without failures, you can expect the same equipment to increase availability.
How to calculate availability
Availability, by definition, is expressed as the percentage of actual operation time that the equipment is used out of the total time being observed. In the form of an equation, availability is expressed as:
Availability (%) = [Actual operation time (hours)/Total time (hours)] * 100
Where the actual operation time can be obtained by deducting any planned or unplanned downtime from the total time:
Actual operation time = Total time (hours) – Total downtime (hours)
For example, say a working mechanical mixer is observed for 10 hours. A mechanical breakdown causes it to stop abruptly. Investigations and repairs take 2 hours before the mixer was up and running again. The actual operation time can be calculated by the following equation:
Actual operation time (hours) = 10 hours – 2 hours = 8 hours
Now using the obtained actual operation time with the availability formula, we can calculate the availability to be 80% as shown below:
Availability (%) = [8 hours/10 hours] * 100 = 80%
How to calculate availability from other metrics
Given that other metrics that also concern the time of operation are being measured as data points, availability can be calculated by these data sources. The mean time between failure (MTBF) and mean time to repair (MTR), are common metrics that can also be used to calculate an equipment’s availability.
MTBF and MTR describe the effects of breakdowns on the operational time that an equipment can be used. Assuming by definition that the MTBF is a period when the equipment is performing under good working conditions, and MTR is the downtime it takes to repair the equipment, availability can be calculated using the following formula:
Availability = MTBF/(MTBF + MTR)
How to improve availability
From the given equations that calculate availability, it becomes obvious that reducing downtime is a key step to improving availability. Note that downtime is composed of planned and unplanned events. Downtime due to unplanned breakdowns usually cause more adverse impacts to the system’s availability. Planned downtime, on the other hand, should be used as invested time to perform proactive measures that keep equipment in good working condition.
Improving the availability of the whole plant should be done in a systematic way that involves the organization’s top management as well as staff. Promoting a culture towards reducing defects and reducing downtime can impact the plant’s availability, and even the plant’s overall effectiveness.
Relating availability to the overall equipment effectiveness (OEE)
Availability is one of three factors that define the overall equipment effectiveness (OEE) of a plant. The two other factors being efficiency and quality. The product of the three factors, all expressed in percentages, is how the OEE score is obtained.
OEE is a metric that has been the standard of measuring plant performance. Being one of the factors contributing to OEE only reinforces the importance of availability to the performance of a plant.