Reliability HotWire: eMagazine for the Reliability Professional
Reliability HotWire

Issue 73, March 2007

Reliability Basics

Basic Steps of Applying Reliability Centered Maintenance (RCM) Part II

Although there is a great deal of variation in the application of Reliability Centered Maintenance (RCM), most procedures include some or all of the seven steps shown below:

  1. Prepare for the Analysis
  2. Select the Equipment to Be Analyzed
  3. Identify Functions
  4. Identify Functional Failures
  5. Identify and Evaluate (Categorize) the Effects of Failure
  6. Identify the Causes of Failure
  7. Select Maintenance Tasks

If we were to group the seven steps into three major blocks, these blocks would be:

  • DEFINE (Steps 1, 2 and 3)

  • ANALYZE (Steps 4, 5 and 6)

  • ACT (Step 7)

The previous issue of Reliability HotWire discussed the DEFINE stage. In this article, we examine the ANALYZE stage.


Identify Functional Failures
A functional failure is defined as the inability of an asset to fulfill one or more intended function(s) to a standard of performance that is acceptable to the user of the asset. Functional failures may include:

  • Complete failure to perform a function

  • Poor performance of a function

  • Over-performance of a function

  • Performing an unintended function

  • Etc.

We need to keep in mind that the focus of RCM is on preventing loss of function, not equipment; we should therefore avoid talking about equipment in these failure statements.


Also, remember that functional failures do not have to be simple definitions or a single statement. A function can have multiple failures. These should be distinguished, because in later steps, these different failures will relate to different failure modes, which may be due to different causes and have different effects that need to be dealt with differently. All functional failures are, so to speak, not created equal.


Furthermore, keep in mind that failure definitions of an asset are closely related to its operating context. Therefore, we should generalize neither about functions of identical assets nor about their functional failures, in case the assets operate under different contexts or are used under different requirements and expectations.


Here are a few examples of functional failures (listed with the function requirement):

 

Function Functional Failure

  1. To maintain discharge flow of 500 gpm +/- 10%.

  A. Unable to discharge at all.
  B. Fluid flow restricted.
  C. Fails to contain the fluid.
  D. Discharge flow exceeds 505 gpm.
  E. Discharge flow drops below 495 gmp.


Note that the performance limits that have been identified for the function may provide a guide to the functional failure description.


Identify and Evaluate (Categorize) the Effects of Failure

Failure effects analysis is concerned with what happens when a failure mode occurs. Revealing the effects of failure involves asking questions such as:

  • What will be observed when the failure occurs?

  • What is the impact on operations/production?

  • What is the impact on the environment/safety?

  • What physical change will occur to the equipment or adjacent equipment?

  • What alarms or indications will be observed?

Effects can be defined at three different levels:

  • Local Effect - What is observed at the individual component?

  • Next Level Effect What is observed at the sub-system level?

  • End Effect What will be observed at the system level?

For example, if you run out of gas in your car, then:

  • Local Effect: Fuel injectors fail to supply gas to the engine.

  • Next Level Effect: Engine stops working.

  • End Effect: Car stops, you are late to work.

Many RCM references contain logic diagrams that can be used to evaluate and categorize the effects of failure. For example, the following logic diagram is provided as an example in the SAE JA1012 "Guide to the Reliability-Centered Maintenance (RCM) Standard." (Other published logic diagrams may consist of 3 or 5 questions and 4 or 5 failure effect categories.)

 


[Click to Enlarge]

 

Going down the above questions tree yields the appropriate classification of a failure effect.
 

Identify the Causes of Failure (Failure Modes)
The cause of failure (failure mode) represents the specific cause of the functional failure at the actionable level (i.e. the level at which it will be possible to apply a maintenance strategy to address the potential failure). Identifying causes (failure modes) is of paramount importance. It is a time- and effort-intensive step, but it is well worth it! The day-to-day issues of maintenance are mostly managed at the failure mode level (e.g. work orders issued to deal with specific failure modes, maintenance plans designed to deal with failure modes, product recalls due to a certain unexpected failure mode or frequent failure mode, emergency design or maintenance meetings triggered because of an occurrence of a failure mode, etc.) Extensive discussions about failure mode identification in this step of the RCM process will have a great beneficial impact on the success of the RCM project. It is what could make a difference between a reactive and a proactive maintenance management plan.

 

The following is an example of failure mode identification:

 

Function Functional Failure Failure Mode

1. To deliver oil from tank T1 to tank T2 at not less than 1000L/min.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 A. Oil Leak.

 

 


 B. No oil delivered.

 

 

C. Deliver less than 1000L/min.

 

 

A.1. Crack.

A.2. Screws in the joints not tightened correctly.

A.3. Screws broke.

 

 

 

 

B.1. Impeller comes adrift.

B.2. Impeller jammed by foreign objects.

B.3. Motor burned out.

B.4. Inlet valve jammed.

B.5. Bearing seized.

 

 

C.1. Impeller worn.

C.2. Partially blocked suction line.

C.3. Motor slowing down.

C.4. Inlet valve partially clogged.

 

 

The causes of failure (failure modes) can be described to almost any level of detail. Different levels are appropriate in different situations. In some situations, it might be sufficient to state the direct reason of failure (for example: tire puncture) while in other situations, drilling down to the molecular level is needed. The SAE JA1012 guideline presents a useful demonstration of the many levels of detail that can be used to describe failure modes. For example:

Pump set fails.
 
Because: Pump fails.
   
Because: Impeller fails.
     
Because: Impeller came adrift.
       
Because: Mounting nut undone.
         
Because: Nut not tightened correctly.
           
Because: Assembly error.


The guideline recommends that "failure modes should be described in enough detail for it to be possible to select an appropriate failure management policy, but not in so much detail that excessive amounts of time are wasted on the analysis process itself." It is a tough balance to strike. It is for you to decide the required level of the root cause investigation. A common rule is to stop at the level beyond which it becomes impossible for the organization to define a failure management policy.

 

If you plan to perform statistical data analyses, another crucial piece of information that needs to be collected in this step is the time dimension. The exact times at which failure modes have occurred (or times between occurrences) need to be recorded (along with other information such as conditions in which the asset was used). Such data will be central to life data analysis and assessment of probability of occurrence. It is what will be used to quantify the "R" (reliability) in RCM. Failure modes that have not occurred but are considered to be real possibilities present a challenging aspect of the RCM process. More judgment calls and/or other reliability analysis techniques (such as accelerated testing) would be needed in this case.

NOTE: Many RCM practitioners choose to identify the Failure Mode prior to describing and categorizing the Effects of failure. The analysis logic can be applied the same way, regardless of which sequence is used.

Conclusion:
The article summarized the "analysis" or "investigative" steps in the RCM process, in which functional failures, the failure modes and their effect and causes are explored. A future article will tackle the failures head-on and discuss different alternatives for addressing failures.

 

References

ATA MSG-3 "Operator/Manufacturer Scheduled Maintenance Development," updated in March 2003.

Moubray, John, Reliability-Centered Maintenance, Industrial Press, Inc., New York City, NY, 1997.

Nowlan, F. Stanley and Howard F. Heap, Reliability-Centered Maintenance. Issued in December, 1978.

SAE JA1012 "A Guide to the Reliability-Centered Maintenance (RCM) Standard," issued in January 2002.

Smith, Anthony, Hinchcliffe, Glenn R., RCM - Gateway to World Class Maintenance, Elsevier Inc, Burlington, MA, 2004.

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