What is the difference between availability and reliability?

While both availability and reliability metrics measure uptime or the length of time that an asset is operational, they differ in how the interval is being measured. 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.

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 particular equipment is able to perform its intended tasks.

How to calculate 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. the time when an asset is not scheduled to operate). Ideally, assets should have as close to 100% availability as possible.

Steps to improve availability

Central to increasing your equipment’s availability is streamlining your maintenance and operational practices. These steps will help you begin the process of improving equipment availability in your facility.

1. Measure your current availability

The first step to any kind of improvement is knowing where you currently stand. Determine how many hours out of your scheduled time your equipment is in operation, and render that as a percentage.

Once you know where you stand in terms of each asset’s actual availability, you’ll be in a better position to determine how much work you’ll need to do in order to improve it.

2. Determine your achievable availability

Achievable availability isn’t the same as operational availability since it’s based on an ideal situation. It’s how much availability your equipment would have if limiting factors, such as manpower, spare parts, and maintenance practices, were handled absolutely perfectly.

It’s also limited by the current design of your facility’s systems. As such, any flaws in your processes’ design will limit your achievable availability.

To determine your achievable availability, you’ll need to benchmark yourself against similar facilities within your industry. Once you know how well others in your industry are doing, you can adjust that for current design constraints—such as distance to and from shops and stores, limitations on equipment accessibility, etc.—and arrive at an estimate of how high you could push your availability.

It’s important to note here that using your own plant data can skew your results since it doesn’t control for organizational effectiveness (i.e. how efficiently your team carries out tasks).

3. Update operational practices

The majority of limitations on equipment availability come from operational procedures, not maintenance practices. As such, your focus should be on making sure your operational procedures don’t put unnecessary limits on your equipment’s availability and performance.

Take a look at your current practices for operating your equipment and see how potential failures, product defects, and costs relate back to those practices. Then make adjustments as needed. While this may take a bit of digging, it doesn’t require a significant investment in new tools or equipment, so it’s a highly cost-effective way to start improving availability.

4. Implement effective PM practices

While operations do impact availability, maintenance still plays a vital role. Often, reactive maintenance practices leave equipment exposed to potential failures that cause massively expensive—and totally avoidable—downtime. Even a preventive approach may result in excessive downtime if too many PMs are performed.

In either case, too many hours are put into maintenance when the equipment could be up and running. To reduce those hours, the following tips can help:

• Implement an effective PM plan to avoid equipment breakdowns.
• Focus PMs on preventing the most impactful downtime events.
• Perfect the timing on recurring PMs.
• Streamline PM scheduling.
• Improve MRO inventory management to make sure parts are available.
• Continuously strive to optimize PMs in your facility.

As you work to establish and improve your preventive maintenance program, you’ll see fewer avoidable or unnecessary hours spent on maintenance.

5. Improve scheduling practices

Airtight scheduling practices are key to eliminating logistical delays when it comes to operating and maintaining your equipment. When it comes to the maintenance schedule, some potential improvements might include:

• Considering locations of equipment, machines, storerooms, etc.
• Arranging for equipment and parts to be available on time.
• Communicating with operations crews to have assets offline when needed.
• Accounting for skill limitations for each task.
• Scheduling tasks based on priority.

A well-constructed maintenance schedule will make sure PMs are handled in an efficient manner while avoiding costly breakdowns.

6. Implement predictive maintenance

In an effort to further streamline preventive maintenance, it helps to have predictive analytics and sensors in place. Predictive maintenance uses sensors to monitor assets and predict when important PMs are needed based on your equipment’s needs. That way, PMs are performed exactly as often as necessary—no more, no less.

To use PdM effectively, your team will need to have a proactive outlook on your maintenance processes, so it’s worth implementing a preventive maintenance plan first before investing in sensors and analytics software. That said, a predictive maintenance strategy is well worth reaching for in the long-term.

Reliability

Reliability quantifies the likelihood of equipment to operate as intended without disruptions or downtime. 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.

How to measure reliability

Because reliability is expressed as the 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, weeks, months, etc.)

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

Steps to improve reliability

To achieve world-class reliability in your facility, it’s not enough to just keep equipment up and running as much as possible. You need to minimize disruptive breakdowns. These steps can help you accomplish that.

1. Collect data on equipment health and failure modes

To begin preventing downtime events, you’ll need to collect data on your equipment’s health and common failure modes. If you already have a database in place, draw upon that information. If not, you’ll need to start collecting data.

A CMMS can help you by logging work order data and monitoring asset health through meter readings, maintenance reports, and even sensors. Over time, you’ll accumulate the data you need in order to move on to the next step.

2. Perform FMEA on critical assets

Once you have some decent data on your equipment, it’s time to perform failure mode and effects analysis (FMEA). FMEA involves analyzing potential failure modes for each piece of equipment and determining which ones are most impactful. You’ll take measurements in four categories:

• Equipment costs
• Production
• Safety
• The environment

As you analyze each failure mode, you’ll be able to determine which ones are most important to prevent.

3. Prioritize preventive maintenance tasks

Once you know the failure modes you need to prevent most, it’s time to prioritize your preventive maintenance tasks. This step is fairly straightforward, but it does require knowing what tasks are needed to prevent the most severe failure modes.

You may need to perform a bit of root cause analysis here. In order to avoid wasted PMs, you’ll want to make sure the tasks you plan actually treat the equipment failures you want to prevent.

4. Optimize MRO inventory management

Your MRO inventory should be stocked with appropriate quantities of the right items. While it is important to keep inventory costs down—meaning you shouldn’t keep too many items in stock—you do need to make sure you have enough of each item in stock.

That means analyzing your work order history on each asset and determining what spare parts and tools are used when, how many parts are needed, and how long it takes to replenish your stock of those parts. Involve your maintenance and operations teams in this process since they’ll help you steer away from making decisions solely on cost alone.

5. Train your team in best practices

Many equipment failures result from human error, so it’s important to make sure your operators and maintenance technicians are well versed in best practices. Alongside having operating procedures in place that maximize equipment availability, train your personnel on following those procedures with precision.

In addition, consider adding checklists to work orders and other documents used by your personnel. Doing so—and enforcing their use—will help keep your team on track and avoid costly errors.

With the use of consistent training and checklists, you’ll minimize the impact that human error has on your production processes.

6. Focus on continuous improvement

As you work on improving reliability in your facility, don’t stop after each step. It’s a continuous process, and you’ll need to keep working to improve upon each new procedure, practice, and task you implement.

Be constantly on the lookout for ways to streamline your maintenance and production processes, improve quality, and eliminate defects. Perform regular audits on your equipment and processes. As you develop a more effective preventive maintenance plan, start implementing new technologies into your processes to take your maintenance plan to the next level.

Through it, all, keep careful records. Doing so will give you the baseline knowledge you need to keep moving forward with continuous improvement.

Relationship between 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.

Conclusion on availability and reliability

As you focus on improving both availability and reliability in your facility, you’ll help improve the overall quality and effectiveness of your processes. You’ll see fewer defects, more productivity, and greater profitability in your facility.