Pressure Dependent Demands

Pressure Dependent Demands (PDD) allows you to perform hydraulic simulation by treating the nodal demand as a variable of nodal pressure. Using PDD you can perform hydraulic simulation for:

Pressure dependent demand at a node or a set of nodes
Combination of PDD and volume based demand
Calculate the actual supplied demand at a PDD node and demand shortfall
Present the calculated PDD and the associated results in a table and graph.

In order to access PDD choose Components > Pressure Dependent Demand Functions or click Pressure Dependent Demand Functions to open the Pressure Dependent Demand Functions dialog box.

New	Creates a new pressure dependent demand function.
Duplicate	Copies the currently selected demand.
Delete	Deletes an existing demand. You can hold down the Ctrl key while clicking on items in the list to select multiple entries at once.
Rename	Renames an existing pressure dependent demand function.
Report	Generates a pressure dependent demand report based on the selected demand.
Synchronization Options	Browses the Engineering Library, synchronizes to or from the library, imports from the library or exports to the library.

Properties tab

Function Type - Either Power Function or Piecewise Linear. Power Function is used to define the exponential relationship between the nodal pressure and demand. The ratio of actual supplied demand to reference demand is defined as a power function of the ratio of actual pressure to reference pressure.

Power Function Exponent - The coefficient that defines the power function relationship between the demand ratio and pressure ratio.

Has Threshold Pressure? - Turn on to specify if a threshold pressure is to be input.

Pressure Threshold is the maximum pressure above which the demand is kept constant.

If the function type chosen is Piecewise Linear then the following opens.

Piecewise Linear is a table of reference pressure percentage vs. reference demand percentage. The last entry value of reference pressure is the greatest that defines the threshold pressure. If the last pressure percentage is less than 100%, the threshold pressure is equal to the reference pressure. If the last pressure percentage is greater than 100%, the threshold pressure is the multiplication of the reference pressure with the greatest pressure percentage.

Percent of Reference Pressure % - defines the percentage of a nodal pressure to reference pressure.

Percent of Reference Demand - defines the percentage of a nodal demand to reference demand.

The Reference Pressure is the pressure at which the demands are fully met at a node. In the graph below, the demand assigned to the node is 18 gpm and the reference pressure is 40 psi. As the pressure deviates from 40 psi, the actual demand at the node changes in response to the pressure dependent demand curve (blue line).

In some cases, there is an upper limit to the amount of water that will be used as pressure increases (users will throttle back their faucets). In this case the pressure at which demand is no longer a function of pressure is called the Pressure Threshold. In the graph below the pressure threshold is 50 psi.

The pressure threshold must be equal to or greater than the reference pressure. A reference pressure must be specified to use pressure dependent demand. The threshold pressure is optional. The user can optionally set the reference pressure to the threshold pressure. These values can be set globally or the global value can be overridden on a node by node basis.