2.16).
2.8.8 Two Different System Functions
The fact that different system functions give rise to different RBDs is illustrated in Example 2.2.
Example 2.2 (Pipeline with safety valves)
Consider a pipeline with two independent safety valves
Figure 2.17 Two safety valves in a pipeline: (a) physical layout, (b) RBD for the safety barrier function, and (c) RBD for spurious closure.
The two blocks in Figure 2.17b represent the valve function “stop flow” for valve 1 and 2, respectively. This means that each valve is able to close and stop the flow in the pipeline. To achieve the system function “stop flow,” it is sufficient that at least one of the individual valves can “stop flow.” The associated RBD is therefore a parallel structure with respect to the system function “stop flow.”
The valves may close spuriously, that is, without a control signal, and stop the flow in the pipeline. The two blocks in Figure 2.17c represent the valve function “maintain flow” in the pipeline, for valves 1 and 2, respectively. Because the flow in the pipeline stops when one of the valves closes, the system function “maintain flow” is fulfilled only when both valves function with respect to the valve function “maintain flow”. The associated RBD is therefore a series structure for the system function “maintain flow.”
Example 2.2 shows that two different functions of a single system give rise to two different RBD. Observe also that the blocks in the two RBDs represent different component functions in (b) and (c).
Remark 2.3 (Terminology problem)
Many authors use the term “component” instead of block. There is nothing wrong with this terminology–and we also use it later in this book–but we have to be very careful when, for example, saying that “component
2.8.9 Practical Construction of RBDs
A specific system function, SF, usually requires a long range of subfunctions. For the essential function of a car, for example, we need the functions of the engine, the brakes, the steering, the ventilation, and many more. The RBD for the SF is then a long series structure of the required subsystem functions, as shown in Figure 2.18. Each of the required subfunctions may again need sub‐subfunctions.
Figure 2.18 Construction of the RBD in levels.
How many levels are required to depend on how complicated the system function is and the objectives of the analysis. RBDs are further discussed in Chapter 4. Chapter 6 deals with quantitative reliability analysis based on RBDs.
2.9 Problems
1 2.1 Identify and describe briefly the main subsystems of a family car and establish a system breakdown structure for the car.
2 2.2 Establish a function tree for a (domestic) refrigerator.
3 2.3 List the environmental, operating, and maintenance factors that should be considered when defining the operating context of a family car.
4 2.4 List some information functions that are available in a modern car.
5 2.5 Identify the main functions of a family car and establish a function tree for the car.
6 2.6 List some safety functions of a modern car. Are the identified functions online or offline functions?
7 2.7 Identify and describe the functions of the front door of a house.
8 2.8 Describe the functions of a vacuum flask (thermos) and suggest relevant performance criteria.
9 2.9 Describe briefly a system you consider to be complex.
10 2.10 Refer to the SADT functional block (see Figure 2.3) and list all the inputs, controls, and resources you need to bake a pizza. The output from the function is the new‐baked pizza. How would you set up the performance criteria for your pizza?
11 2.11 Based on an Internet search, explain what is meant by a CONOPS and list its main elements.
12 2.12 Based on an Internet search, list the main elements that are typically included in a system requirements document (or a system requirements specification).
13 2.13 Establish an RBD of the braking system of a family car.3
14 2.14 Consider a voted oo structure. The voting can be specified in two different ways:– As the number out of the components that need to function for the system to function.– As the number of the components that need to fail to cause system failure.In the first case, we often write oo:G (for “good”) and in the second case, we write oo:F (for failed).Determine the number such that a 2oo4:G structure corresponds to a oo4:F structure.Determine the number such that a oo:G structure corresponds to a oo:F structure.
15 2.15 Are there any examples of standby redundancy in a family car? Justify your answer.
References
1 Aslaksen, E.W. (2013). The System Concept and Its Application to Engineering. Heidelberg: Springer‐Verlag.
2 Blanchard, B.S. and Fabrycky, W.J. (2011). Systems Engineering and Analysis, 5e. Boston, MA: Pearson.
3 Einstein,
