Product(s):	WaterCAD, WaterGEMS
Version(s):	08.11.XX.XX and higher
Area:	Modeling

Problem

How do WaterGEMS and WaterCAD treat the Discharge to Atmosphere element during a steady state or EPS?

Solution

This element is primarily for use with HAMMER (see this article), but can also be used for a steady state or EPS in WaterGEMS and WaterCAD. The reason why this element (and other transient related elements) are present in WaterCAD and WaterGEMS is because these programs share a file format with HAMMER. Meaning, you can lay out your transient analysis protective equipment when developing your steady state/EPS hydraulic model, so that you can later directly open it in HAMMER and have it ready for transient modeling.

In order to establish the pressure-dependent relationship between pressure and flow out of the opening, the user must enter a pair of "Flow (typical)" and "Pressure drop (Typical)". From these values a discharge coefficient (orifice coefficient) is calculated, which the program uses to calculate other flows through the element during the simulation as the pressure changes. Based on the pressure from the steady state or EPS calculations, the discharge coefficient is used to calculate the updated flow (some iteration may be required by the solver), and you can observe the calculated pressure and calculated discharge.

Note that the "drop" in this case ("pressure drop (typical)" field) refers to the difference between pipeline pressure (inside the D2A) and atmospheric pressure (outside the D2A). So, it is not referring to any headloss across the node element but rather the pressure reported at the D2A element (since the pressure results in WaterCAD and WaterGEMS refer to the gauge pressure inside of the pipeline with atmospheric pressure as the datum)

How do I determine both the typical flow and pressure?

To determine the "typical flow" and "typical pressure drop" for your D2A, you can either assume a flow and solve for head, or assume a head and solve for flow.

Typically it is best to assume a pressure, based on the normal or expected hydraulic grade in the surrounding area. For example if you would normally expect to have an HGL of around 500 ft in your system (for example based on nearby tanks or reservoir, or from a pressure zone), subtract the D2A physical elevation to determine the "typical" pressure. Then, use one of the methods below (minor loss equation or orifice equation, as examples) to solve for flow based on that pressure. Enter the resulting flow as the "Flow (Typical)". An outside application could also be used for this such as Bentley FlowMaster's orifice worksheet.

For example assume you have a six inch opening to the atmosphere at an elevation of 100 feet with no tailwater effects and the typical HGL in the area is 110 ft. Using the orifice equation (see below FlowMaster worksheet), the pressure drop is 10 feet and the corresponding typical flow is roughly 1500 gpm. By setting these values in the D2A properties, it means that if the pressure at the node is 10 feet, the outflow will be 1500 gpm. If the pressure at the node drops below 10 feet, the outflow will drop accordingly based on the orifice equation.

Method 1: If there is no restriction/contraction at the pipe outlet, then consider using the minor loss equation

H = K * (V^2) / 2g

K: Minor loss coefficient. 1.0 is a typical value to assume for a pipe exit (to atmosphere, not submerged).

V : Velocity at the pipe exit (ft/s, m/s)

g : Gravitational acceleration (32.2 ft/(s^2), 9.81 m/(s^2))

V = Q/A

V : Velocity (ft/s, m/s)

Q : Flow (cfs, cms)

A : Cross-sectional Area (ft^2, m^2)

Method 2: If the outlet orifice is smaller than the pipe diameter, then consider using the orifice equation.

Q = C A (2 g H)^0.5

Known

C : Orifice coefficient. This value is assumed by the engineer. A typical value for a typical orifice is 0.6. For in-depth information on orifice coefficients for different situations, see Brater and King's Handbook of Hydraulics (1996).

A : Orifice Area (ft, m)

g : Gravitational acceleration (32.2 ft/(s^2), 9.81 m/(s^2))

Unknown

Q : Discharge (cfs, cms)

H : Head (ft, m)

The program will not produce valid results if the "Flow (Typical)" and a corresponding "Pressure Drop (Typical)" are set to zero. If you are using this element, you will need to enter valid values for these two fields.

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

Problem

Solution

See Also