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  • Writer's pictureJD Solomon

How Do You Define Reliability? (and a case example from Tampa)

Reliability is an overworked word. How do you evaluate reliability?
Reliability is an overworked word. How do you evaluate reliability?

Reliability is one of the most overworked and misunderstood words in the world of facilities and infrastructure. Reliability is also one of the most powerful and important words. Properly evaluating reliability resolves risk, safety, quality, compliance, and human performance. Reliability also frames a powerful and effective systems approach instead of leaving gaps by cobbling together individual assets and components. These are a few fundamentals of reliability and a case application of why it matters.


Reliability in the Dictionary

The Merriam-Webster dictionary cites reliability as a noun with two variations of the definition:

  • the quality or state of being reliable (suitable or fit to be relied on; dependable)

  • the extent to which an experiment, test, or measuring procedure yields the same results on repeated trials


Colloquially, we jumble reliability with dependability, sureness, solidness, responsible, and trustworthiness. As we will see in this article, reliability engineers assign a specific meaning to reliability.


Reliability for Educators, Behavioral Psychologists, and Human Systems

Professionals in human sciences follow the second meaning of reliability cited in the dictionary. In that world, qualitative assessments are common to determine human feelings and related aspects that are difficult to measure to decimal places. For example, validity and reliability are key concepts that apply to surveys that capture human feelings and opinions.


A survey is considered valid if it can be shown to measure the variable intended to measure and not others.


Survey reliability refers to the extent the same results are obtained with the same question when repeated to the same group of respondents.


Reliability for Physical Systems

Reliability engineers follow more closely to the first definition in the dictionary. Reliability engineers also define other synonyms of reliability, such as availability and dependability. Reliability has a very specific meaning to reliability engineers.


Reliability is the probability that an item will perform its intended function for a specified interval under stated conditions.


Key aspects of the definition include

  • Probability

  • Function

  • Time

  • Conditions


What Reliability Professionals Do

Reliability engineers make sure that systems, products, and processes work consistently and dependably over time. Their main goal is to prevent and reduce the impact of failures or malfunctions, which improves overall performance and minimizes risks. Reliability engineers make sure that products and systems are safe, work well, and last long.

Reliability is established in the design phase.
Reliability is established in the design phase.


System reliability is a product of design. Maintenance can re-establish designed functions but cannot improve the inherent design. As a result, approximately two-thirds of formally trained reliability engineers practice in the design phase and approximately one-third in the maintenance phase.


Reliability in Design

There are many tools and approaches for reliability in the design phase. Reliability Centered Design and Design for Reliability are similar concepts that apply reliability concepts throughout the design phase to ensure that systems function as intended throughout the system’s life.


Some of the activities that reliability engineers do in the design phase include:

  • Environmental and Use Factors

  • Physics of Failure, including Stress-Strength

  • Failure Modes and Effects

  • Common Mode Failures

  • Tolerance and Worst Cases

  • Design of Experiments

  • Fault Tolerance

  • Human Factors Analysis

  • Reliability Apportionment

  • Systems Optimization


Reliability in Maintenance

Maintenance and reliability are related practices that are often confused during the operations phase of the system’s life. Some of the activities that reliability engineers do in the operations phase include:

  • System Reliability Function and Policy

  • Preventative Maintenance Analysis and Optimization

  • Corrective Maintenance Analysis

  • Condition Assessment and Strategies

  • Predictive Maintenance

  • Testability (including Built-In)

  • Spare Parts Analysis and Logistics

  • Planning and Scheduling Optimization

  • Redundancy and Common Mode Failure

  • Resource Evaluations and Management

  • O&M Strategies


The Relationship of Reliability and Risk

Risk is the effect of uncertainty on objectives. An effect is a deviation from the expected – either positive or negative.


Reliability is concerned with establishing expectations, and risk is concerned with managing deviations from those expectations. Uncertainty is present in both, and that overlap ties the two together.


For financial types, if there is 100 percent reliability (let’s assume this is possible), then there is no risk. This also reminds me of the old saying, “You can have 2 of 3 things – quality, cost, and speed – but you can’t have all three.”


Case Example: Tampa Bay Water

Tampa Bay Water regional water supply authority created in 1998. Its complex regional water delivery system is comprised of groundwater sources, surface water sources, an off-stream storage reservoir, a seawater desalination facility, and pumping and piping to distribute quality water across three counties.


Tampa Bay Water's system is both complex and complicated.
Tampa Bay Water's system is both complex and complicated.

Some of the major challenges with working with the system include quantity and quality variations in the water supply (groundwater, surface water, and brackish water), variations in water demands, and variations in system design. The system is complicated due to flows in certain facilities being reversed to level and optimize system costs and performance.

For example, one of the critical aspects of regional system reliability is determining appropriate operating capacities for the facilities constructed over several decades. Straightforward engineering aspects like assigning individual pumping capacities are challenging. For scenario analysis, the pumping capacities are grouped into three areas: rated (which includes design or permitted), minimum, and sustainable operating capacities. Notwithstanding basic challenges with determining the basis of design, the pump facility operating capacities are different in one flow direction versus the other based on how separate designs were retrofitted together.


I worked intensely at Tampa Bay Water for over a decade helping the agency to launch its asset management program and integrate reliability, risk, and operational resilience. Their system defines what it means to be both complex (consisting of many parts) and complicated (difficult). The system underscores the dynamic nature of reliability (it goes on forever).


How Do You Evaluate Reliability?

Systems thinking is the foundation of effective facility and infrastructure performance. Properly evaluating reliability resolves risk, safety, quality, compliance, and human performance. This brief article provides a few fundamentals of reliability and a case application of why it matters. Moving forward, "How do you evaluate reliability?"


JD Solomon Inc. provides solutions for program development, asset management, and facilitation at the nexus of facilities, infrastructure, and the environment. Subscribe for monthly updates related to our firm.


JD Solomon is a licensed professional engineer (PE), a certified reliability engineer (CRE). and a certified maintenance and reliability professional (CMRP). He is the author of Communicating Reliability, Risk & Resiliency to Decision Makers: How to Get Your Boss’s Boss to Understand and Facilitating with FINESSE: A Guide to Successful Business Solutions.


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