Throughput Analysis with Phase Diagrams

If you are performing throughput analysis for a phase diagram, you will need to specify, for each operational phase, how its possible throughput is calculated. The phase throughput properties in the Phase Properties window allow you to specify the maximum throughput that the entire system can process during the particular phase. (For a detailed discussion of throughput at the block level, see the Throughput Analysis topic.)

A phase's throughput can be thought of as the initial throughput that enters the system modeled in the linked RBD. For example, imagine a textile factory that receives different quantities of raw materials during different seasons. These seasons could be treated as different phases. In this case a phase may be seen as sending a certain quantity of units to the first component of the system (the textile factory in this case). Depending on the capacity and availability of the factory, these units may be all processed or a backlog may accumulate.

Phase throughput can be one of the following:

• Unrestricted throughput specifies that the amount of output the system can process during the phase will be equal to the amount that the linked diagram can process during the specified phase duration.
• Constant throughput allows you to specify an amount of throughput per unit time that is the same throughout the entire simulation.
• Variable throughput allows you to specify an amount of throughput per unit time that varies over the course of the simulation. For example, the flow of oil from a well may drop over time as the oil reserves are depleted so the amount of throughput per unit time will decrease as the simulation time increases. If you select this option you must also specify the following:
  • Throughput model allows you to choose the model that describes the change in throughput, y, as a linear function of time, x. You can choose from three general models for this purpose:
    • Linear
    • Exponential
    • Power

The parameters, a and b, for each of these models are chosen by the user based on engineering judgment and on any previous data.

• Maximum throughput capacity allows you to specify the maximum throughput capacity of the phase. Once this maximum throughput capacity value is reached, the throughput per unit time becomes constant and equal in value to the maximum throughput capacity specified by the user. In this situation, the variable throughput model would then act as a constant throughput model. For example, suppose a machine has a constant throughput when functioning normally. When an outage happens, it takes about 12 days to restore the machine. Every time the machine fails, it is repaired and then the throughput goes through a warm up period of 5 days until it reaches normal production capacity again. During this warm up period, the machine is not operating at its maximum level, but after the warm up period and the machine returns to its normal function and achieves its maximum and constant throughput.

Even though only three models are available, they provide considerable modeling flexibility. This flexibility is achieved by using the functions as building blocks for more complex functions. As an example, you can create a step model by using multiple phases that each have a constant throughput. A ramp model would use phases with linearly increasing functions in conjunction with constant phases, and so forth.