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Pressure Dependent Demands (PDD) allows you to perform a hydraulic simulation in which the nodal demands can vary based on changes in nodal pressure. This TechNote describes how to set up a PDD simulation in WaterCAD, WaterGEMS, and HAMMER, and also provides suggestions for PDD input data.
Some types of water demands are volume-based, in that the demand is independent of available pressure. Examples of volume-based demand sources are washing machines, dishwashers, and toilets (the same volume is used regardless of the pressure). Other demands are pressure-dependent, meaning water usage decreases with a decrease in pressure. Pressure-based demand examples include showers, sprinklers, and leaks.
Typically, water modeling programs assume that all demands are volume-based, and maintain the user-input demand regardless of the calculated available pressure. Although this assumption works well under the normal range of pressure conditions, it loses accuracy if an episode such as a fire or pump outage causes a significant decrease in system pressure.
One option for modeling demands that vary based on pressure is to set up model nodes as simple flow emitters, using the emitter coefficient property of the node. Because the flow emitter approach places no upper limit on the amount of water demanded with increasing pressure, it is most useful for determining water consumption by a free-discharge element such as a sprinkler or broken pipe. However, other pressure-based demand types result in no additional consumption once the pressure is above a certain threshold value, such that use of flow emitters in the model could skew water consumption to be unrealistically high in higher-pressure areas. Another limitation of flow emitters is that they will result in calculation of a negative demand, or inflow, when the pressure is negative.
WaterCAD and WaterGEMS have a Pressure Dependent Demands (PDD) feature that allows for more control over demand calculation. In many instances where pressure affects water use, the PDD feature will provide a more realistic result than simply placing flow emitters on nodes.
Using PDD, you can:
WaterGEMS and WaterCAD use a formulation we call the Modified Global Gradient Algorithm. Instead of single formulation to solve PDD, WaterGEMS and WaterCAD have two methods that are both flexible in themselves:
1. Q = a P^n where n is a variable
2. Piecewise linear curve with any monotonically increasing function
The information below discussing setting up and using PDD in a water model.
Note: If you are using a piece-wise linear function make sure the changes aren't too abrupt on the curve, otherwise the solver may have a more difficult time arriving at a solution. If you have too abrupt a change you may get a "network unbalanced" user notification. To resolve this you could try to gradually decrease the demand.
This section describes how to create and configure a new Scenario and Alternative to run your PDD analysis.
Often, the Reference Pressure will be defined as the typical pressure at a node under typical demand conditions (corresponding to the full demand). However, if you are analyzing pressure-dependent demands for multiple nodes with significantly different typical pressures, you will need to override this system Reference Pressure on a node-by-node basis, as described in the next section.
Additional Resources: Pressure Management and Repair of Pipes - for Active Leakage Control (eseminar)
Pressure dependent demands and negative pressure
Modeling intermittant or closed off demands