Page 483 - ExtendSim User Guide

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Advanced Topics 457
Upstream supply and downstream demand
• Demand B2 is 400 gallons/minute because the effective rate on branch B1 is 600 and Demand C is 1000: Demand B2=Demand C-effective rate B1. (Verification: If a Tank with flow is present at branch B2, it could provide 400 gallons/minute because the total Prioritize 2 can accept is 1000 and the block already takes 600 from the top priority branch.)
• SensorAreportsademandof1400gallons/minutebecauseDemandB1is1000and Demand B2 is 400. However, this is incorrect. (Verification: If a Tank with flow is placed right before Prioritize 1, the outflow effective rate would be 1000 gallons/minute and not 1400 gallons/minutes. The erroneous result comes from the fact that the association of “cor- rect” local rules doesn't guarantee a “correct” global result.
☞ By understanding flow rules and how they can affect the global result, you can avoid this prob- lem. The most important step is verifying this model from left to right for the supply and from right to left for the demand. Two solutions are 1) having the Prioritize 2 block be in Neutral mode, and 2) placing a Valve with a maximum rate of 1000 between Sensor A and Prioritize 1.
Mixing Merge/Diverge block modes
Situations where Merge or Diverge blocks in Sensing mode are mixed with Diverge or Merge blocks in Distributional, Neutral, or Priority modes within an LP area should be avoided as they are prone to give inaccurate results.
In these types of mixed-mode situations, the LP area is going to be recalculated multiple times to provide the effective rates and the upstream supply and downstream demand rates. All the blocks in the LP area must provide one set of constraints to the Executive so it can solve the effective rates and a second set of constraints so the Executive can solve the supply and demand rates. This causes repeated intermediate LP calculations, the results of which are affected by constraints which are applied for the supply and demand rates. This may yield inaccurate results.
Unexpected effects of bias order
As discussed on page 434, the Distributional, Priority, and Sensing modes require bias order to be defined. This affects their effective rates. However, the only mode whose proportions are influenced by the bias ranking is the Sensing mode.
When effective rates within an LP area have to be recalculated, the Executive makes a list of blocks ranked from the top bias order to the bottom. Depending on a Merge or Diverge block’s ranking in the Executive’s list, the effective rates might be different. If the block is in Sensing mode, the proportions will also be influenced. The two models that follow illustrate these issues.
Combine Priority Sensing model
In this model, a Diverge block (Prioritize 1) is set to Priority mode and a Merge block (Sensing 2) is set to Supply Sensing mode. The blocks’ Model Settings tabs are set to Each block
Discrete Rate
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