Options for modeling an outflow that varies with pressure

 Product(s):WaterGEMS, WaterCAD
 Version(s):CONNECT Edition, V8i


What options do I have for modeling an outflow that varies with pressure, during a steady state or EPS in WaterCAD of WaterGEMS? Some examples:

- Free discharge point (to the atmosphere)
- Sprinkler
- Nozzle / orifice
- hydrant outflow
- Any other outflow (demand) that is sensitive to pressure

NOTE: Looking for information on modeling free discharge in HAMMER? See Modeling Reference - Discharge To Atmosphere



1) Emitter Coefficient - single coefficient that replaces the demand
2) Pressure Dependent Demands - more control, applies to existing demands, set up function
3) Discharge To Atmosphere - specify outflow pressure vs. flow relationship based on a pair of flow and pressure.

Emitter Coefficients

In the properties of a junction or hydrant node, you will find a field called "Emitter Coefficient" which allows you to specify a flow emitter coefficient "k".

Flow Emitters are devices associated with nodes that model the flow through a nozzle or orifice. In these situations, the demand (i.e., the flow rate through the emitter) varies in proportion to the pressure at the node raised to some power. The constant of proportionality is termed the discharge coefficient. For nozzles and sprinkler heads, the exponent on pressure is 0.5 and the manufacturer usually states the value of the discharge coefficient as the flow rate in gpm through the device at a 1 psi pressure drop.

Emitters are also used to model flow through sprinkler systems, irrigation networks and can simulate leakage in a pipe connected to the junction (if a discharge coefficient and pressure exponent for the leaking crack or joint can be estimated) and compute a fire flow at the junction (the flow available at some minimum residual pressure). In the latter case, one would use a very high value of the discharge coefficient (e.g., 100 times the maximum flow expected) and modify the junction's elevation to include the equivalent head of the pressure target.

When both an emitter and a normal demand are specified for a node, the calculated demand result includes both the normal demand and the flow through the emitter.

The flow through an emitter is calculated as:

Q = kP^n


Q is flow.
k is the emitter coefficient and is a property of the node.
P is pressure.
n is the emitter exponent and is set globally in the calculation options for the run; it is dimensionless but affects the units of k. The default value for n is 0.5 which is a typical value for an orifice.

Pressure Dependent Demands

For more control over the pressure vs. flow relationship, you can use the Pressure Dependent Demand (PDD) functionality. This applies to existing demands to make them vary with pressure. For more on setting up PDD, see the below article:

Setting Up Pressure Dependent Demand

Discharge To Atmosphere

The Discharge To Atmosphere (D2A) node element is primarly used with the HAMMER product for transient analysis, but can also be used with a steady state or EPS in WaterCAD and WaterGEMS. It is essentially a different way to specify the pressure vs. flow relationship of an outflow. You enter a "typical flow" and "Typical pressure drop" and the the program uses that to calculate a discharge (orifice) coefficient that it then uses to establish the pressure vs. outflow relationship. So, if you know the outflow (demand) of a particular node and the resulting pressure, you can enter those values and an appropriate change in demand will be calculated as the pressure varies.

As a quick estimate, you could take the diameter of the pipe (or diameter of the opening the discharges to the atmosphere) along with the initial pressure head seen with a steady state calculation using an assumed demand, then solve the orifice equation (for example by hand or with Bentley FlowMaster) for the value you'd use for the "Typical Flow".

For more on this, see:

How do WaterGEMS/WaterCAD treat the discharge to atmosphere element?

Modeling Reference - Discharge To Atmosphere [TN]

See Also

Designing and Sprinkler System