Page 401 - ExtendSim User Guide
P. 401

Rates, Constraints, and Movement 375
Flow rules
• Howitsvalueconnectionshavebeenconnected.Forinstance,theValveblock’sR(maxi- mum rate) input connector can be used to dynamically modify the block’s maximum flow rate.
• Howitsflowconnectionshavebeenconnected.ATankisasourceifonlyitsoutflowcon- nector is connected; it is a sink if only its inflow connector is connected.
These flow rules completely describe the events or conditions under which a particular block may constrain the movement of flow through it. However, changes in a block’s constraints during a simulation cause its effective rates to be reevaluated and can cause a connected block’s effective rates to be reevaluated, propagating calculations throughout an LP area. When recalculation is required, the Executive block (Item library) uses the aggregated set of flow rules from all the blocks in the LP area to calculate a new set of effective rates for the area. Thus a particular block’s flow rules can be superseded by the global calculations of the Executive.
Critical and relational constraints
There are two primary types of flow rules: critical constraints and relational constraints.
Critical constraints
While all flow rules cooperate to constrain the rate of flow,
some blocks provide special rules called critical con-
straints. If a rate section contains one or more critical con-
straints, they place an upper bound to the rate of flow for the blocks within that section. A critical constraint is unconditional – no matter what happens in the simulation, the effective rate of flow cannot be higher than the lowest critical constraint of any block in that rate section. For example, the Maximum rate field is a Valve's critical constraint; the entry in that field defines an upper limit on the rate of flow through the block. If that entry is the lowest critical constraint in the rate section, the effective rate for every block in that section cannot be higher.
The blocks with the potential to set a critical constraint for flow are the Convey Flow, Diverge, Interchange, Merge, Tank, and Valve; of these, the Valve is most commonly used. As will be shown in “Meeting the critical constraint requirement” on page 380, these blocks must be placed at critical locations in order for the model to run properly.
ExtendSim's discrete rate system attempts to move flow through the model as fast as possible. Without any mechanism to impede its progress, the effective rate would theoretically approach infinity and the flow would move from one part of a model to another instantaneously. In order to avoid this error condition, each LP area of the model must contain one or more constraints (typically a Valve) to restrict the flow to a number that is below infinity. If the required mini- mum set of critical constraints is not present in a model, ExtendSim stops the simulation and displays an error message.
Relational constraints
Relational constraints define the way the effective rates of different sections are related to each other, creating dependencies between rate sections. For instance, the relational constraint between one rate section (effective rate x) and another rate section (effective rate y), could be defined as xy, x=y, 2x-3=y, or any other expression. Relational constraints get updated when the block reacts to new parameters or to changes in its state, but they don’t affect a block’s crit- ical constraints.
Critical constraint in a Valve
Discrete Rate

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