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Mixing Flow and Items 423
Controlling flow with items and items with flow
controls the opening and shutting of valve blocks; valves impact the level of flow; and the level of flow controls the item's location.
Step The Flow Process model
Flow controlling the item
The top section of the model is where flow controls the item. The I (Indicator) connectors on both the Reception and Processing tanks (Rate library) control the three Gate blocks (Item library) used in the item stream at the bottom part of the model. Gate 1 is open only while the Reception tank is in the full state. Conversely, Gate 2 is open only while the Reception tank is in the empty state. Finally, Gate 3 is open only while the Processing tank is empty.
Item controlling the flow
The bottom section of the model is where the item controls the flow. In this case, the three Valve blocks (Rate library) in the top part of the model are controlled by the lengths of the three Queue blocks (Item library) in the item stream in the bottom portion. The item stream has only one item which cycles through the item-based blocks in a loop. Consequently, the queue lengths for each Queue in this loop will alternate between 1 and 0. For instance, Valve 1 is open only when the length in the Stage 1 queue equals 1. Similar logic applies to Valve 2 and Valve 3.
The activity in the model is divided into four stages: open Valve 1 and release flow into Recep- tion; open Valve 2 and release flow into Processing; process the flow; open Valve 3 and release flow into Sink
Stage 1: Open Valve 1 and release flow to Reception
The I (Indicator) connector on the Reception tank is controlling Gate 1. At the start of the sim- ulation, Gate 1 is closed because the Reception tank is not full (it's actually empty at this point), and the cycling item is blocked from leaving the Stage 1 queue. Notice the highlighted
Discrete Rate

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