Page 397 - ExtendSim User Guide

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Rates, Constraints, and Movement 371
Rates, rate sections, and the LP area
Valve
Controls and monitors the flow, limiting the rate of flow passing through. This block can also be used to set a goal for the duration or quantity of flow.
Rates, rate sections, and the LP area
One of the most important aspects of a discrete rate model is the rate of flow – the speed of flow movement. The flow rate is represented as the ratio of flow units to the model’s time units. This is displayed in block dialogs as units/time, gallons/minute, transactions/second, boxes/hour, and so forth.
Discrete rate models can be thought of as being divided into rate sections and LP areas. The flow connectors within each rate section have the same effective rate, which is the speed at which flow moves through those blocks. The LP area is composed of one or more rate sections whose effective rates might change during the simulation.
The types of rates considered during the model building process, rate sections, and the LP area, are discussed below.
Types of rates
The following rates are taken into consideration by ExtendSim and by a block’s flow rules. (Flow rules will be discussed on page 374.)
Maximum rate
ExtendSim's discrete rate architecture
attempts to move flow through the model
as fast as possible. In the absence of any
constraints, the flow rate would theoreti-
cally approach infinity and flow would move from one part of a model to another instanta- neously; this would be a modeling error.
The maximum rate puts an upper limit on the movement of flow through a block. Six Rate library blocks have the ability to set a maximum rate. You can set an explicit maximum rate in the Interchange, Tank, and Valve blocks. The maximum rate for the Convey Flow block is mathematically derived. Maximum rates may also be implicitly specified under certain condi- tions in the Merge and Diverge blocks. In each case, the maximum rate is the highest rate of flow those blocks will allow, and hence the highest potential rate of flow for that part of the model.
☞ An inflow connector for a Convey Flow, Interchange, or Tank block can have one maximum rate while the block’s outflow connector can have a different maximum rate. The maximum rate for the Convey Flow block’s inflow is derived from settings in its dialog; the maximum rate for its outflow is derived from dialog settings and model conditions. The maximum inflow and maximum outflow rates for the Interchange and Tank blocks can be entered directly in their dialogs.
In order to avoid an error condition, each area of a model must have some mechanism in place to restrict the rate of flow to a number that is less than infinity. If the required minimum set of constraints is not present, ExtendSim stops the simulation and displays an error message.
Default maximum rate for Valve
Discrete Rate
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