Orifice Link

An Orifice models a fixed, uncontrolled circular opening punched through a tank or reservoir wall -- a drawdown port, low-level outlet, or orifice plate that starts passing water the moment the tank's surface rises above the opening's centerline, with no gate or moving parts involved.

An Orifice always starts at a Tank -- the wall or structure the opening is set into -- since it represents a fixed hole punched through that structure's shell. It discharges using the standard orifice equation rather than pipe or Manning's-equation channel flow, and like an Open Channel Conduit it is not part of the pressurized (EPANET) network.

How It's Programmed

An Orifice's flow depends only on how far the tank's water surface has risen above the opening's centerline -- it has no gate setting, schedule, or downstream dependency the way a Gate does:

Q = Cd · A · √(2g · H)

where A is the opening's cross-sectional area (computed from its diameter), H is the head (depth of water above the centerline elevation, clamped to zero when the tank is below centerline), and Cd is the discharge coefficient, which absorbs the opening's actual edge condition -- sharp-edged, rounded, short tube, etc. A tank below the centerline elevation contributes no flow at all; once it rises above the centerline, discharge grows with the square root of that head, the standard orifice relationship.

Because an Orifice is only ever driven by the upstream tank's own head, it is inherently one-directional and free-discharging -- it cannot back up, throttle, or reverse the way a Gate or Open Channel Conduit can. If you need a controlled structure instead (one that can be opened, closed, or scheduled), use an Open Channel Gate rather than an Orifice.

UI Workflow and Configuration

To add an Orifice to your model:

1. Use the link-drawing tool starting at the Tank whose wall you're modeling, dragging to the downstream node (another Tank, an open-channel Junction, or an Outflow Node).
2. In the link dialog that appears, set Link Type to Orifice.
3. Set the Centerline Elev. -- the elevation of the opening's center, which the tank's water surface must rise above before any flow occurs. Defaults to the tank's own elevation.
4. Set the Diameter -- the diameter of the circular opening.
5. Set the Discharge Coeff. -- defaults to 0.61 (a typical sharp-edged orifice value); use a higher value (around 0.8) for a smooth, rounded, or short-tube opening.
6. Confirm the connecting nodes and save.

[!NOTE] An Orifice must originate at a Tank -- the model will reject any other starting node, since a fixed opening is only meaningful punched through a reservoir's wall.

Configuration Fields