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Rates, Constraints, and Movement
Defining a critical constraint
An example of a relational constraint is the
Change Units block, where the use of a con-
version factor causes the outflow effective
rate to be different than the inflow effective
rate. The Change Units block defines the
boundaries between one rate section and
another; the conversion factor specifies the relationship of the two effective rates.
For another example of a relational constraint, see “Comprehensive example” on page 383. For an advanced discussion of relational constraints, see “The relational constraint calculation” on page 453.
☞ You don’t enter relational constraints, they are determined by the behavior of the blocks.
Comparison of constraints
Some blocks can set a critical constraint, some can set a relational constraint, and some can do both. Even for blocks that can set constraints, the block may in some situations place no con- straint on the flow.
• The blocks that can set a critical constraint are the Convey Flow, Diverge, Interchange, Merge, Tank, and Valve.
• Relational constraints can be set by the Change Units, Convey Flow, Diverge, Interchange, Merge, and Tank blocks.
For example, a Tank where both the Maximum inflow rate and Maximum outflow rate are checked will set critical constraints for its inflow and outflow.
If neither Maximum inflow rate nor Maximum outflow rate is checked, the Tank will not have any critical constraints but could have relational constraints. If the Tank has a finite capacity but is neither full nor empty, it places no constraints on the flow. However, once the Tank reaches the full state, its inflow rate is required to be less than or equal to its outflow rate; this is a relational constraint.
The effective rate for a rate section cannot be any higher than the lowest critical constraint set for by any of the blocks in that section. Furthermore, because the aggregated set of flow rules also typically contains relational constraints, the effective rate for the section can vary any- where between zero and the smallest critical constraint.
☞ For a table that lists the blocks and which constraints they can provide, see “Types of informa- tion provided to the Executive” on page 451.
Defining a critical constraint
As mentioned earlier, a critical constraint defines the upper limit to the rate of flow through a rate section. While a particular rate section may or may not have a critical constraint, at least one of the rate sections within the LP area must have a critical constraint mechanism to limit the flow rate to a number that is less than infinity.
• YoucanexplicitlysetacriticalconstraintintheValve,Tank,andInterchangeblocks.Youdo this by entering a maximum rate in the block's dialog, obtaining a value for the maximum rate from the block’s input connector, or linking the maximum rate field to the value of a cell in a global array or ExtendSim database.
• For the Convey Flow block, the critical constraint is derived from settings in its dialog and sometimes other model values, rather than being entered directly.
Different inflow and outflow rates
Discrete Rate

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