Question

Demonstrate a working knowledge of selected hazard analysis techniques.

Answer 1

Hazard Analysis Techniques.

Hazard analysis is used to identify and quantify the risks associated with a hazard. Hazard analysis techniques helps to estimate how often an hazardous event will occur; estimating the consequences to persons, environment, and plant; and deciding the required amount of risk reduction (if any). The widely used hazard analysis techniques include Failure mode and effects analysis (FMEA) and Fault Tree Analysis (FTA). FMEA is used to identify equipment or system failures arising from component faults, evaluate the effect of failures, and prioritize the effects of failures according to severity of defects. FMEA techniques work by listing all possible failure modes of devices or components in the plant which is then followed by an evaluation of the effect of each failure mode. On the other hand, FTA looks at a hazard event and resolves the causes into basic events. If the incident rate can be asserted, the hazard rate can be predicted. Choosing the most appropriate method for the job depends on a basic knowledge of the methods and seeing how they fit the application.

Failure mode and effects analysis (FMEA)

The purpose of FMEA is to identify potential hazards associated with a process by investigating the failure modes for each process item. But FMEA has difficulty in identifying hazards that require the failure of more than one process item, due to the complex interaction of the failures. The steps performed during the analysis are,

1. A detailed description of the system.

2. Establishing basic principles and purpose of the study.

3. Carrying out the study.

4. Preparing the result report.

Advantages of FMEA	Disadvantages of FMEA
Performs a systematic review of the process	Requires a large amount of data and Difficulty finding all the failure modes.
Puts forward a detailed methodology to assess the plant item by item.	Can be time consuming and expensive.

When performing a Process or while designing FMEA, the Risk Priority Number (RPN) is a calculation to sort the risks from highest to lowest. The RPN is calculated by multiplying the three scoring columns: Severity, Occurrence and Detection.

RPN = Severity x Occurrence x Detection

Types of FMEA’s

Ø Concept FMEA

Ø Design FMEA

Ø Process FMEA

There is another variant of FMEA known as FMECA (Failure Mode, Effects and Criticality Analysis) in which a criticality analysis to evaluate the FMEA parameters is also included.

Concept FMEA

Formulated in the very early stages in the lifecycle of a product or a process when they are still at a conceptual stage. It is basically done at a system or sub-system level before the key requirements have been decided. Concept FMEA targets the potential failure modes of the factors under consideration or proposed stage. This technique helps in identifying the factors that can be included in a product or process and It often acts as a precursor to the Design and Process FMEA.

Design FMEA

Design FMEA is used to analyze factors in the design stage of a product or process and helps to identify the potential failure modes caused by faulty design. It is usually performed at the lowest possible level so that all the potential defects are identified in the design stage itself. Design FMEA is one of the most critical elements in the lifecycle of a product as it helps in identifying the defects before they are actually developed. Whenever the product or process goes through a change or revamp, then it is crucial to update the Design FMEA also.

Process FMEA

Process FMEA is done for the process through which a product is manufactured or assembled or a service is delivered to the customer and it aims at identifying the potential failure modes of the process caused by the frail sections in the production process. Process FMEA equips business organizations to make their processes robust and dynamic to deal with any negative eventuality. All the possible failures are identified and prioritized so that suitable actions could be initiated and control measures can be put forth to eliminate or reduce these failures or defects. Like Design FMEA, Process FMEA is also active throughout the lifecycle of a process. It also gets updated whenever the process is changed or renovated.

Fault Tree Analysis (FTA)

FTA is a deductive analysis technique depicting a visual path of failure. The deductive analysis starts with a general conclusion and further attempts to determine the specific causes of the conclusion through the construction of a logic diagram called a fault tree and is also known as taking a top-down approach. The main objective of the fault tree analysis is to help identify potential causes of system failures before the failures actually occur. This approach can also be used to evaluate the probability of the top event using analytical or statistical methods and these calculations involve system quantitative reliability and maintainability information, such as failure probability, failure rate and repair rate. As product and process technology becomes more complex, the visual FTA approach has proven to be a stand-alone risk technique or a supplement to Failure Mode and Effects Analysis (FMEA).

Following are the steps for Fault tree construction.

Ø Defining the fault condition, and write down the top level failure.

Ø Using technical information and professional judgments to find the possible reasons for the failure to occur.

Ø Proceed to break down each element with additional gates to lower levels.

Ø Finalize and review the ultimate diagram.

Ø Analyse the probability of occurrence for each of the lowest level elements and follow up by calculating the statistical probabilities from the bottom up.

Advantages of FTA	Disadvantages of FTA
Deals well with parallel or alternative fault paths.	Requires a separate fault tree for each top vent and makes it difficult to analyse complex systems.
Looks into possible causes of an end effect which may not have been foreseen.	Fault tree developed by different individuals can differ in structure, producing different cut set elements and results.
Very useful technique for focused analysis where analysis is required for one or two major outcomes.	More difficult than other techniques to document.

Applications

· Finds tremendous applications in field of safety engineering and reliability engineering to determine the probability of an hazardous event or a particular system failure.

· Aerospace Engineering