Equipment Reliability

What is equipment reliability?

The term reliability is defined as the probability of equipment failure in a given time and under set conditions. Put simply, it is the likelihood that equipment will continue to deliver its intended function for a specific period of time (without failure). 

A decade ago, the equipment was deemed “reliable” as long as it continued to operate and produce output. With the growing maturity of reliability and maintenance engineering strategies in the industry, the goalpost has changed. Today, reliability is evaluated in the context of how well is the equipment being utilized to achieve success at both strategic and operational levels.

Table of Contents

How to measure equipment reliability?
Interconnection between reliability and maintainability
Developing an equipment reliability program
Equipment reliability is a team effort

How to measure equipment reliability?

Reliability is generally measured by the failure-free duration of the operation.

For equipment that has built-in redundancy, all possible scenarios or modes of those redundancies have to be accounted for when calculating the probability of failure. 

For example, if a piece of equipment is designed to operate for 5,000 hours continuously and it indeed continues to operate without failure until this time, the equipment could be characterized as 100% reliable. If that equipment partially or completely fails within 5,000 hours of operation, its reliability would obviously be less than 100%.

It is important to note that most industrial equipment consists of several subsystems and components, each having its own designation of reliability. The overall reliability of the equipment, in this case, would be the combination of the reliability of its subsystems and components. 

A good practice for calculating the reliability of the equipment is to understand the functional relationship of its subsystems and understand the impact of their failures on the overall equipment’s reliability. Since reliability is best characterized by equipment uptime and the duration of operations, the industry developed certain metrics that incorporate both of these parameters. 

Mean Time Between Failures (MTBF)

MTBF is a measure that is used to measure the reliability of repairable equipment.

MTBF represents an average time between two failures occurring in a given period. To have enough data points to calculate MTBF, the equipment has to undergo at least two failures. As with all statistical calculations, the more data points you have, the more accurate your “averages” are going to be.

Mean Time to Failure (MTTF)

MTTF, on the other hand, is a measure of reliability that is applicable for non-repairable equipment.

Put simply,  the MTTF tells the average lifespan of a device. It is calculated by averaging the time of failure of one type of equipment in a given population of production or operation.

Interconnection between reliability and maintainability

Reliability and maintainability are closely related statistical terms that are often studied together. Maintainability is defined as the ease of performing maintenance. The easier it is to perform repairs and maintenance on an asset, the higher its maintainability. 

People with a lacking background in reliability theory tend to confuse them, saying that if the equipment has high reliability it will also have high maintainability (and vice versa). While it may be true for some equipment under specific conditions, that is not always true.

For example, mean time to repair (MTTR) – a measure of maintainability – can be improved by reducing the number of actions the technician has to perform during the repair. While this will cut down the repair time, skipping important steps will eventually lead to more failures, reducing reliability in the process. 

An optimized maintenance program understands the difference and balances trade-offs between reliability and maintainability of the equipment. 

Developing an equipment reliability program

Developing an equipment reliability program is the foundational activity to systematically achieve the reliability of any asset. The equipment reliability program will be different for equipment manufacturers and plant managers as they have different objectives. 

The manufacturer would look to improve reliability at the design stage while the plant manager would look to improve the reliability during the operations (in-service stage). 

The following steps, in general, should be taken to develop a well-articulated reliability program.

1) Planning 

The planning activity would involve understanding the context of the equipment and the operation within which the equipment is or will be installed. During the planning stage,  the performance requirements will be studied and the corresponding level of reliability desired will be established. 

For example, to ensure that the train does not exceed more than five (5) min delays in every subsequent train station, the locomotive equipment and subsystems have to attain a specific level of failure-free operation. In short, the plan of action has to consider the end-user requirements. 

2) Analysis

A detailed equipment level reliability analysis can be performed to understand the possible failure modes – and failure history – that can arise throughout the operations phase of the equipment. Some of the common analyses include FMECA, Fault Tree Analysis (FTA), and Reliability Centered Maintenance (RCM). 

At this stage, the organization will develop maintenance strategies and schedules that correspond to the criticality of the equipment – all in an effort to achieve the desired level of reliability, availability, and maintainability. 

3) Implementation

Once maintenance is strategized, the next step is to put those plans into action.

Organizations that are serious about reliability will often seek help in the form of CMMS software. They will use it to set up and organize timely execution of maintenance work, as well as to gather and store valuable data along the way – with optimization being the ultimate goal.    

At the end of the day, we can all agree that plans which can’t be followed aren’t worth very much.

4) Continuous improvement

The purpose of continuous improvement is to track reliability and maintainability metrics. As equipment gets older and becomes more worn out, maintenance schedules and reliability improvement initiatives have to be adjusted to prolong its useful life. 

Since the operating context and the equipment performance characteristics often change over time, continuous improvement is here to ensure the equipment continues to be reliable over its entire life cycle.