arrange deliveries such that agents do not backtrack,and that every customer is served within 30 minutes of order;
•To ensure that agents greet customers, smile, deliver the pizzas, and collect payments courteously.
These clear definitions of requirements enable the analyst to determine the success or failure of the system quite easily. The IDEF methodology promotes such clarity, and Figure 2.2 shows the Level 0 FBD of the pizza delivery system. Note that we have not thus far talked about equipment used, only what they have to do to satisfy their functional requirements.
For example, the agents could be using bicycles, scooters, motorcycles,or cars to do their rounds. Similarly, they may use an insulated box to carry the pizzas, or they may use some other equipment. The only requirement is that the pizzas are delivered while they are still hot. We can break this down to show the sub-functions, as shown in Figure 2.3. Note that the inputs, outputs, controls, and facilities/equipment retain their original alignment, though they may now be connected to some of the sub-function boxes.
Figure 2.2 Level 0 FBD of pizza delivery system
Figure 2.3 Level 1 FBD—Relationship of intermediate functions pizza delivery system.
We are now ready to address more complex industrial systems, and use a gas compressor in a process plant as an example (see Figure 2.4). We have broken down the main function A0 into sub-functions A1, A2, A3, and A4 in Figure 2.5. Thereafter, we have expanded one of these sub-functions A2 further, as illustrated in Figure 2.6.
Figure 2.4 Level 0 FBD of a gas compression system.
The method is applicable to any business process. We can use an FBD to describe an industrial organization, a supermarket chain,the police force, or a pizza franchise. The diagram itself may appear complex at first sight, but after some familiarization it becomes easier. The clarity and definition it brings makes it a good communication tool.
Figure 2.5 Level 1 identification of sub-system.
Figure 2.6 Level 2 FBD of a gas compression sub-system.
2.3 FAILURE MODES AND EFFECTS ANALYSIS (FMEA)
The performance standards embedded in the definition of the function allows identification of the success or failure of each of the systems or sub-systems. If there is a failure to achieve the objective, it is possible to identify how exactly this happens. In doing so, we identify the mode of failure. Each failure may have several failure modes.
As an example, consider engine-driven emergency generators. An important function is that they must start if the main power supply fails. They have other functions, but let us focus on this one for the moment. What are the causes of its failure to start and how can it happen? We have to establish fuel supply and combustion air, and crank the engine up in order to start it. Several things may prevent the success of the cranking operation. These include weak batteries or problems with the starter motor or the starting-clutch mechanism. If any of these failures occurs, the engine will not be able to start. These are called failure modes.
We can take this type of analysis down to a lower level. For example, the clutch itself may have failed due to a broken spring. At what level should we stop the analysis? This depends on our maintenance policy. We have the following options:
•Replace the clutch assembly, or
•Open the clutch assembly at site and replace the main element damaged, for example, the broken spring.
We can carry out the FMEA at a sub-system functional level, for example, fails to start or stopped while running, as discussed above. It is also feasible to do an FMEA at a level of the smallest replaceable element, such as that of the clutch spring. When designing process plants, a functional approach is generally used. When designing individual equipment, the manufacturers usually carry out FMEAs at the level of the non-repairable component parts. This enables the manufacturer to identify potential component reliability problems and eliminate them at the design stage. Davidson4 gives examples of both types of FMEA applications.
In a functional analysis, we identify maintenance significant items, failures of which can cause loss of system or sub-system function. In this case, we stop the analysis at assembly level because we will replace it as a unit, and not by replacing, for example, its broken spring. Unlike the manufacturers, we cannot usually justify analysis at the lower level, because the cost of analysis will exceed the benefit. The volume of work in a component level FMEA is much higher than in a functional FMEA.
For each failure mode, there will be some identifiable local effect. For example, an alarm light may come on, or the vibration or noise level may rise. In addition there can be some effect at the overall system level. If the batteries are weak, the cranking speed will be slow, and there will be a whining noise; this is the local effect. The engine will not start, and emergency power will not be available. This may impair safety in the installation, leading to asset damage, injury or loss of life; this is the system effect.
We can identify how significant each failure mode is by examining the system effects. In this case, failure to start can eventually cause loss of life. However, if we have another power source, say a bank of batteries, the failure to start of the engine will not really matter. There may be some inconvenience, but there is no safety implication. The failure is the same; that is, the engine does not start, but the consequences are different.
The purpose of maintenance is to ensure that the system continues to function. How we maintain each sub-system will depend on the consequences, as described by the system effects. For example, if the failure of an item does not cause immediate loss of function, we can limit the maintenance to repairing it after failure. In each situation, the outcome is dependent on the configuration of the facility. The operating context may differ in seemingly identical facilities. The FBD and FMEA will help identify these differences and take the guesswork out of decision making.
The elegance of the functional approach will now be clear. For every business, we can define its objectives at the top level, or its overall functions. We can break these down to identify the related systems and sub-systems. Next, we identify the functions of each system and sub-system, and carry out an FMEA. The analysis is applicable to an operating plant or to one that is still on the drawing board. As a result of this top-down approach, we can concentrate the planning effort on what really matters to the organization.
Individuals and organizations can fall into the trap of rewarding activity instead of the results achieved. Movement and activity are often associated with hard work. Sometimes this
