| Product(s): | WaterGEMS, WaterCAD |
| Version(s): | 08.11.XX.XX and higher |
| Area: | Modeling |
Water networks for buildings and campuses can be a special case for hydraulic modeling. When designing these systems, pipes are typically sized based on reasonable peak demands. The usual way of doing this is the “Fixture unit method” where you total the fixtures downstream of any point and use “Hunter’s Curve” or a similar approach to determine peak flow. In some cases the term "hydrosanitary design" is used.
The fixture unit method was developed by Roy B Hunte in the 1930s. Over the years plumbing practice has changed and as have peak demands. Some have tried to update it. The latest version of AWWA Manual 22 Sizing Service Lines and Meters, has a modified method for estimating peak demands. You need to understand how the method works to make sure it is applicable to your situation. You can find this information in the Manual or in an article in the Journal AWWA: Walski and Buchberger “New M22 Edition Features improved Method for Determining Peak Demands for Residential Buildings,” Journal AWWA, Vol. 117, No. 1 , p. 61.”
This is also covered in the book Advanced Water Distribution Systems Modeling and Management (pages 399-401).
To design this type of system using OpenFlows Water (WaterCAD or WaterGEMS), you need to first lay out the pipe network. Please be sure to account for the head loss through any backflow preventers and flow meters. Carefully choose the boundary condition (reservoir), establishing a hydraulic grade which you can reasonably expect from the supply system (consider a conservative value).
Then, the important part is the selection of demands. OpenFlows Water does not use the fixture unit method, it calculates the actual flow and pressure based on the network and boundary conditions.
In order to make a decision on peak demand, you need to understand the nature of the water usage. Is it typical domestic/commercial use where the fixture unit method may be applicable? Or, maybe it is some batch process that uses a huge amount of water for a short time and drops down to near zero. Or is it a constant industrial or cooling process that uses the same flow rate almost all the time?
Talk to the engineers/architects working on the building to get an understanding of water use patterns which will help you decide on a way to determine peak demand. Consider what the consequences are for over or under designing the distribution pipes. If there is great uncertainty, think “When in doubt, build it stout”.
https://www.iapmo.org/water-demand-calculator/