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Rates, Constraints, and Movement
Meeting the critical constraint requirement
When a Merge or Diverge block is set to certain modes, flow can be blocked from moving through one or more of its branches. For example, if one branch of a Diverge block in Distribu- tional mode has been assigned a blank value or a value <=0, flow through that branch is halted. Similarly, flow through all but the selected branch is blocked when the Merge block is in Select mode. In both of these cases, the maximum rate would be 0 for the affected branches.
The “Mode table” on page 387 lists each mode for the Merge and Diverge blocks. The column labeled “Parameter values that always block the flow” indicates which conditions would always cause a branch to have an implied constraint of 0.
Meeting the critical constraint requirement
As discussed earlier, 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. Otherwise, the rate of flow would approach infinity.
☞ By definition, residence blocks always delineate the boundary between two rate sections. A general rule is that there must be at least one critical constraint between every two residence blocks. (The critical constraints can be provided by the Convey Flow, Diverge, Interchange, Merge, Tank, and Valve blocks. The residence blocks are the Convey Flow, Interchange, or Tank.) The exceptions to the general rule include certain situations where a Merge or Diverge block is between two residence blocks.
The following examples illustrate some ways the required critical constraint mechanism can be met in discrete rate models.
Valve or Convey Flow
The No Merge or Diverge model illustrates two typical ways to provide a critical constraint to the rate of flow between two residence blocks (in this case, Tanks) that don’t have a Merge or Diverge block between them.
The example to the right uses a Valve to constrain the flow between two Tanks. This is the most straight forward and most common situation. In order for the Valve’s maximum rate to provide the critical constraint it must be:
• Greaterthanorequalto0(zero)
• Less than 1e10 (the defined infinite rate) • Not a blank
The example to the left uses a Convey Flow block to meet the requirement for a critical con- straint. A Convey Flow block derives the criti- cal constraint for its inflow from its dialog settings and the critical constraint for its outflow from its dialog settings and model conditions.
Because it has critical constraints at both its inflow and outflow connectors, the Convey Flow block limits the rate of flow from the first Tank to the second to a number that is less than infinite.
Tank or Interchange
Instead of using a Valve to provide the critical constraint between two residence blocks, you can specify maximum inflow and maximum outflow rates for an intervening Tank or Inter-
Discrete Rate

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