Flow Emitters are devices associated with junctions 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 junction 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 used to model flow through sprinkler systems and irrigation networks. They can also be used to 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 junction, the demand that WaterGEMS CONNECT reports in its output results includes both the normal demand and the flow through the emitter.

The flow through an emitter is calculated as:

Where

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.