PDD and demand driven analysis difference

Question

Hello, I have modeled an agricultural water distribution network using water gems in which 10 similar pumps are operated in parallel. when I run the model based on the conventional approach of demand-driven analysis all those 10 pumps are operated delivering a flow rate of 2900 lps. unlike demand driven method when I run the model using pressure dependent demand method (PDD), only 8 pumps(2 pumps are off) are operated delivering 2600 lps flow with a deficit of supplied demand in junctions. does anyone know the reason? thank you in advance

Jesse Dringoli · Accepted Answer

Shahram, 
 
When using PDD, demand will vary with pressure, based on the PDD function that you have set up. Lower pressure = lower demand, higher pressure = higher demand. So in the example case, since the calculated pressure is lower than the reference pressure, the demand is lower than the base demand (following the PDD function you entered). 
 
You can read more about how PDD works in the related Help topics and in this article: 
 
 communities.bentley.com/.../2671.setting-up-pressure-dependent-demand-tn

Jesse Dringoli · Answer

Hello Shahram, 
 In the scenario where PDD is applied, the base demands are reduced per your PDD function. 
 As an example, J-644 has a base demand of 69 L/s. In the PDD scenario, the calculated pressure is 38.3 m, which is below the Reference Pressure of 60 m which you have set in the PDD alternative. So, the calculated demand is less: 55.2 L/s. 
 With the reduced total demand, the total flow from all the pumps will be less, so they'll be able to operate at lower speeds to maintain the same traget HGL when compared to the fixed demand scenario. Eight pumps are able to achieve the target HGL within the maximum relative speed factor. With the higher demand in the fixed demand scenario, two more pumps need to turn on to maintain the same target HGL while staying within the maximum relative speed factor. 
 When using variable speed pumps in parallel, the program will automatically calculate how many "lag" pumps need to turn on in order to meet the target. You can read more about this in the Help topics regarding Variable Speed Pumps and APEX. 
 As an example, if you increase the reference pressure from 60 to 80 m, the total demand will be even less and only seven pumps will be needed to achieve the same target HGL. 
 Note: I see that you're using an older version of WaterCAD/WaterGEMS - in the latest release, you may need to adjust the piping around your pump station per the bottom part of this article 
 You may also want to read this article for information on how the variable speed pump battery element can be used to replace all the individual pumps.