Failure Modes & Effects Analysis (FMEA)

Failure Modes & Effects Analysis (FMEA) A hazards analysis technique specifically structured to analyze equipment items is Failure Modes & Effects Ana...
Author: Amelia Woods
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Failure Modes & Effects Analysis (FMEA) A hazards analysis technique specifically structured to analyze equipment items is Failure Modes & Effects Analysis (FMEA). The method examines the ways in which an equipment item can fail (its failure modes), and examines the effects or consequences of such failures (safety, reliability or environmental performance). If the criticality of each failure is to be considered, then the method becomes a Failure Modes, Effects and Criticality (FMECA) Analysis.

Methodology The following process for conducting an FMEA is adapted from the American Society for Quality system. 1. 2. 3. 4. 5.

Assemble a cross-functional team (how this can be done is discussed at the page HAZOP Team Selection and Management). The team should represent designers, operations, maintenance and the end user. Define the physical scope of the FMEA. For example, in the heat exchanger analysis discussed below, determine if utility systems such as the cooling water supply are to be included. Define the purpose of the equipment item being examined. Identify the failure modes for that equipment item and determine their consequences and their likelihood. Determine if the analysis is to incorporate the effect of safeguards and controls.

Team Process Like other types of hazards analysis, an FMEA should be carried out by a team. In most cases, however, only two or three team members - who are specialists in the required fields - are involved. The FMEA scribe will complete a form such as that shown below which is written for the heat exchanger shown in Figure 2 in the Examples Page. FMEA Analysis of Heat Exchanger #

Failure Mode

Cause(s)

Predicted Frequency Frequent - has happened twice in ten years.

Consequences

1

Tube failure

Corrosion from fluids (shell side).

Hydrocarbon is at higher pressure than the cooling water. Therefore flammable materials could enter the cooling tower and cause a major fire.

A

2

Tube sheet failure

See tube failure. Vibration of the See #1. tubes may cause the sheet to fail even if the tubes hold up.

Rare

See #1.

B

3

Relief valve fails open

1. Mechanical failure 2. External impact

Hydrocarbons to Rare atmosphere - fire and environmental hazard

Serious

C

4

Relief valve fails closed

1. Mechanical failure 2. Polymer build-up

None (passive failure) Uncommon

Critical

C

5

Erosion of tubes

High velocity of cooling water

See tube failure

Critical - see tube failure

B

6

Vent valve fails open Vent valve fails closed

Mechanical failure

See relief valve fails Rare open None (passive failure) Rare

Serious

C

Minor - could lead to problems for turnaround maintenance

C

8

Drain valve fails open

Mechanical failure

See "relief valve fails Rare open".

Serious

C

9

Drain valve fails closed Corrosion (tube side)

See vent valve fails closed.

7

10

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Indications/ "Announce-ment" Odours at the cooling tower. Hydrocarbon detector on the tower.

Mechanical failure

Rare

Risk

C

Incorrect process composition.

See tube failure

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Uncommon

Critical

B

Failure Modes & Effects Analysis

Failure Modes & Effects Analysis (FMEA) The Table has seven columns. The purpose of each is discussed below. 1. 2. 3.

4. 5. 6.

7.

The first column is the number of the failure mode for that item of equipment. The second column identifies the failure mode. The third column lists possible causes of the failure mode. Although identification of causes is not a requirement of the FMEA process, they do need to be identified so that appropriate corrective actions can be taken. Column four lists the signs by which operations personnel know that the event has happened. The fifth column provides an estimate for the number of times the failure mode is likely to happen. The sixth column identifies the potential consequences of the failure mode. As already noted, the consequences will vary depending on the magnitude of the failure. The consequence that is usually of most interest is injury of personnel. However, environmental impact and economic loss can also be considered. Some practitioners have two levels of consequence: immediate and 'end effect'. In the first row, the immediate effect of a tube failure is hydrocarbons in the cooling tower; the ultimate effect could be a catastrophic fire in the cooling tower. The last column provides an estimate for the level of risk associated with the failure mode.

Severity The list below provides a means of estimating the severity of the event. None. No effects observed. Minor. System operable with some loss of efficiency or quality. Low. System operation will cause some equipment damage but should not create a safety hazard. Moderate. System operation will cause equipment damage and could create a safety hazard. High. System operation will cause significant equipment damage and is likely to jeopardize safety. Very High. System operation will lead to destructive failure with a significant chance of someone being hurt and/or the creation of a major environmental problem. In all cases, the severity of the event will depend on whether it occurs with our without warning, with the second of the two obviously being the more serious. The FMEA method is one of the techniques used in Process Hazards Analyses (PHAs) to identify and risk rank hazards. The OSHA regulation lists the following allowable techniques:

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Failure Modes & Effects Analysis

Failure Modes & Effects Analysis (FMEA) Process Hazards Analysis The FMEA method is one of the techniques used in Process Hazards Analyses (PHAs) to identify and risk rank hazards. The OSHA regulation lists the following allowable techniques: a) b) c) d) e) f) g)

What-If; Checklist; What-If / Checklist; Hazard and Operability Study (HAZOP) Failure Modes and Effects Analysis (FMEA); Fault Tree Analysis; or An appropriate equivalent methodology.

Since the focus of a Process Hazards Analysis is on the identification of process or system-related issues, an FMEA would generally serve to support one of the other techniques, such as HAZOP. For example, a HAZOP may consider the system effects of a heat exchanger failure; the FMEA, as shown above, would then examine the exchanger itself in more detail.

Few Examples for FEMA Process Industry Examples Example 1: Process Flow The first example shows a simple process involving the flow of liquid from a tank into a pressure vessel. This example is used to illustrate the principles and techniques of process hazards analysis. Figure 1 Process Flow Example

Example 2: Equipment The second example is to do with an equipment item that is widely used throughout the process industries: a shell and tube heat exchanger. The example is used to illustrate equipment failure analysis techniques, particularly Failure Modes and Effects Analysis (FMEA). Figure 2 Heat Exchanger Example

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Failure Modes & Effects Analysis

Failure Modes & Effects Analysis (FMEA)

Example 3: Operations Figure 3 shows a forced draft cooling tower. Warm water from the users enters at the top of the tower then flows down the packing into the basin. Air is pulled into the base of the tower by the fans at the top of the tower. The air flows counter-currently against the warm water. Some of the air evaporates, thus cooling the water. The cooled water is pumped to the users. Make up water and treatment chemicals are added as shown. This example is used to illustrate the development of operating and maintenance procedures. Figure 3 Cooling Tower Example

Example 4: Management Workflow The fourth example illustrates the development and use of risk management systems. Figure 4 Work Flow Example

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Failure Modes & Effects Analysis

Failure Modes & Effects Analysis (FMEA) Example 5: Significant Potential Incident The fifth example is loosely based on an actual incident. The cone-roof, atmospheric storage tank shown in Figure 5 stores a non-flammable, low vapour pressure inorganic liquid. The vapour space above the chemical is air; the tank breathes in and out through a simple vent line. Also shown is a vehicle; one of the facility roads runs close to the tank. A fairly steady stream of vehicles uses the road. The fifth and final example illustrates the development and use of risk management systems. The pump stopped operating, the check valve failed to hold and light hydrocarbons flowed backward into the tank. A layer of hydrocarbons formed on top of the inorganic liquid, as shown. Figure 5 Reverse Flow Scenario

The hydrocarbons in the tank vaporized, then vented to atmosphere as shown. A hydrocarbon detector located about 100 meters from the tank detected the presence of flammable vapours. The vapours did not light off. However the potential for a serious event is high — the vapours could have ignited at a vehicle engine. The flame front could have entered the tank and caused the vapours in the tank to explode.

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Failure Modes & Effects Analysis