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Merging, Diverging, and Routing Flow 391
Merging and diverging flow
• In the case of the Diverge block, when the upstream supply is greater than or equal to the downstream demand, the block passes as much flow through each branch as the downstream demand will allow and the priorities are ignored. However, when the cumulative down- stream demand exceeds upstream supply, the priorities that have been assigned to each branch are used to calculate the appropriate effective rates for the outflow branches.
• In contrast, the Merge block passes as much flow as possible through each inflow branch when downstream demand exceeds upstream supply, ignoring the priorities. However, when the cumulative upstream supply exceeds downstream demand, the priorities assigned to each branch are used to calculate the appropriate effective rates for the inflow branches.
Special cases apply to the use of the Priority mode in a Merge or Diverge block:
• If the priority for a particular branch has been set to blank, the effective rate for that branch will be zero and the flow will stop for that branch.
• Iftheprioritiesoftwoormorebranchesareequal,theflowwillbedividedamongthemina “distributional” manner with equal proportions (see Distributional mode, below.)
☞ The priority entries in a Diverge block’s dialog are not fixed rules but instead are situational; they are only used to resolve discrepancies when downstream demand exceeds upstream sup- ply. For a Merge block, the entries are used to resolve discrepancies when upstream supply is greater than downstream demand.
Merge/Diverge blocks in Priority mode are not always compatible with Merge/Diverge blocks in Sensing mode. Consequently, an area of the model with some blocks in Sensing mode and others in Priority mode are prone to error. See “Cautions when using potential rates” on
page 455 for more information.
Priority Mode Diverge model
In this example, the
constraining rates in
the valves have been
set such that the
upstream supply of 8
flow units per time
unit through Valve 1
exceeds the cumula-
tive downstream
demand of 6 set by
Valves 2, 3,and 4.
Because a large
enough supply of flow
exists to satisfy down-
stream demand, the
priorities in this case
are ignored and have
no impact on the set of effective rates defined for each outflow branch.
However, if a “supply scarcity” is introduced by changing the constraining rate in Valve 1 from 8 to 4, the Diverge block will calculate a set of effective rates that distributes the now limited supply of flow according to the defined priorities. Since the priorities have been assigned in descending order (top outflow branch has highest priority), the Diverge block will do its best to
Priority Mode Diverge model
Discrete Rate


































































































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