Problem

How can I model or design a water system that flows based on assumed demands? (demand driven)

How can I model or design a water system whose outflow depends on the pressure? (pressure driven)

How can I model or design a water system that flows based on gravity? (gravity driven)

What is the difference between these types of analysis?

Solution

A demand-driven analysis refers to the use of assumed demands (outflow) at node elements, where the model solves results like flow and pressure based on those assumed outflows. 

A pressure-driven analysis refers to a network whose demands (outflow) are sensitive to pressure. (higher pressure = higher outflow)

A gravity-driven analysis refers to a network without pumps where flow travels in pipes by gravity, based on one or more assumed hydraulic grades at reservoirs and tanks.

Demand Driven Analysis

In a "demand-driven" analysis, the modeler simply enters demands at junctions, and the other results are "driven" based on those "knowns" in the overall equation. For example in a simple system with one source reservoir at a lower elevation with a pump feeding downstream customers, you would determine the appropriate consumption/outflow to assume, and enter those as fixed demands on the downstream junctions or customer meters elements (which may have a pattern of change over time for an Extended Period Simulation). The numerical solver then determines the pump operating point, flow and pressure in network elements based on assuming that those demands must be satisfied.

The fundamentals training workshops and sample files include examples of demand driven systems.

Pressure Driven Analysis

In a situation where outflow is sufficiently sensitive to pressure, pressure-dependent demands (PDD) can be used to vary outflow with pressure (higher pressure = higher demand). This can be referred to as "pressure driven". See the following article for more details: Using Pressure Dependent Demands

Gravity Driven Analysis

A "gravity driven" system refers to a network that does not use pumps to supply customers/demands. Instead, water flows by gravity based on a source (assumed hydraulic grade at a source reservoir or tank at a higher elevation than the consumption points). You would simply need to have a source at a higher hydraulic grade than the customers and water would flow by gravity with little or no need for pumping.

Note that WaterGEMS and WaterCAD assume pipes are flowing full. So, they can only model a gravity driven system where the pipes are still pressurized. In a case where water flows by gravity and is part full (the pipes are not pressurized and open channel or closed channel flow occurs), you would need to use different software that has a numerical solver that can handle a mix of full and partly full pipes, such as OpenFlows SewerGEMS or OpenFlows CivilStorm. Despite the word "sewer" in the name, SewerGEMS can handle almost any system whether the water is clean or dirty. If you have a partly full supply line and a pressurized water distribution system, you may be better off creating two models.

A network can be both "demand driven" and "gravity driven" if assumed demands are supplied by a source at a higher elevation, with no pump involved.

See Also

Disconnected Demand Nodes user notification when computing model

Modeling Air Valves At High Points in WaterCAD or WaterGEMS

Modeling intermittent water supply

(Book) Advanced Water Distribution Modeling and Management