Page 398 - ExtendSim User Guide

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372 Rates, Constraints, and Movement
Rates, rate sections, and the LP area
Effective rate
One of the most important reasons for cre-
ating a discrete rate model is to determine
the actual rate of flow movement. The
effective rate is the calculated actual rate
of flow between Rate library blocks during
the simulation run. It is the result of an internal calculation taking into account the maximum rates and all the constraints of the process. In some situations the effective rate is the same as the maximum rate; in others it is lower. One effective rate is associated with each rate section in a model, as discussed “Rate sections” on page 373.
In a rate section, the effective rate of flow cannot be higher than the lowest maximum rate for all the blocks in that section. In fact, it can be lower than the lowest maximum rate, and could even be zero (0), depending on model conditions.
While each rate section can have only one effective rate, a section can have more than one block that has a maximum rate. In fact, it is common to have several Valve blocks, each with their own maximum rate, in a rate section.
Infinite rate
An infinite rate is a theoretical rate that would cause the flow to instantaneously move from one location in the model to another.
The Executive block’s Discrete Rate tab specifies that a rate equal to or greater than some number is consid- ered infinite; the default is that a rate 1e10 is considered infinite, as shown above.
From the Executive’s Discrete Rate tab
Effective rate (Valve’s Results tab)
You can change the infinite number to be anything that you want. However, because of the 12 digit precision limitation of the effective rates, the number should be set as close as possible to the highest possible effective rate which would ever reach this limit. (Setting a correct infinite rate is more critical in the case of potential upstream supply and potential downstream demand calculations, an advanced topic discussed on page 455.)
Infinite effective rate
Since instantaneous movement is not possible in the real world, the Rate library does not sup- port an infinite effective rate. The infinity number specified in the Executive establishes an upper limit on the model’s allowable set of effective rates. If the simulation calculates an effec- tive rate that equals or exceeds that number, ExtendSim will stop the simulation and generate an error message. This could happen, for instance, if a source tank is directly connected to a sink tank, without any intervening constraint.
Infinite maximum rate
The Executive’s infinity number (by default, 1e10) can be used in a block's rule set when con- ditions are such that it cannot in any way constrain the movement of flow. For example this would be accomplished by checking the (infinite)checkbox for a Valve’s maximum rate. With an infinite maximum rate, the Valve will not limit the speed of flow passing through it.
If you set the Valve’s maximum rate field to blank or to a number 1e10, the block would also not limit the speed of flow passing through it.
Discrete Rate
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