These systems and sub-systems below them are aligned to meet the overall objectives. An FBD provides an effective way to demonstrate how this works. It illustrates the relationship between the main function and those of the supporting systems or sub-systems.
We describe the functions in each of the rectangular blocks. On the left side are the inputs—raw materials, energy and utilities,or services. On the top we have the systems, mechanisms, or regulations that control the process. The outputs, such as intermediate (or finished) products or signals, are on the right of the block. Below each block, we can see the means used to achieve the function; for example, the hardware or facilities used to do the work. As a result of this approach, we move away from the traditional focus on equipment and how they work, to their role or what they have to achieve.
In the example of the pencil that we discussed earlier, let us examine failure of the third function, that is,
•It is too thin or fat to hold, or
•It has a cross-section that is irregular or difficult to grip,
•It is too short.
We then break down the main function into sub-functions. In the case of the pizza business, the sub-functions would be as follows:
•A purchasing system that will ensure that raw materials are fresh (for example, by arranging that meat and produce are purchased daily);
•A food preparation system suitable for making consistently high quality pizzas within 10 minutes of order;
•A communication system that will ensure voice contact with key staff, customers, and suppliers during working hours;
•A delivery system that will enable customers within a range of 10 km to receive their hot pizzas from pleasant agents within 30 minutes of placing the orders.
Each of the sub-functions can now be broken down, and we take the delivery system as an example:
•To deliver up to 60 hot (50–55°C) pizzas per hour during non-peak hours, and up to 120 hot pizzas per hour from 5:30 p.m. to 8:00 p.m.;
•To 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,