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HydroCAD Stormwater Modeling - Since 1986

Vortex Valves

Vortex Valves

HydroCAD provides direct support for vortex control valves from Hydro International, Mosbaek and Hydrovex.


Vortex valves are sometimes used as a pond outlet device in order to maximize the storage utilization. Unlike standard outlet devices, where the discharge increases steadily with head, vortex valves are designed to reduce the discharge at intermediate and high heads. This allows higher discharge rates early in the storm, while detaining a greater volume as the flow increases. With proper pond design, this can reduce the peak discharge and allow the use of smaller storage volumes.

Principle of operation

Under low head conditions, a vortex valve will operate as a traditional weir or orifice. But as the head increases, the device creates a circular flow pattern which induces a vortex in the control opening. This creates an air core at the center of the opening, which reduces the discharge in comparison to the flow that would be predicted by the standard orifice equation. In effect, the device acts like a smaller orifice under high-head conditions, without requiring any active control mechanisms. Compared to standard orifice controls, Vortex valves can produce a significant reduction in peak flow. Due to the larger size of the clear opening, they are also less prone to blockage than an orifice with an equivalent discharge.

Modeling in HydroCAD

Each vortex valve has a unique stage-discharge relationship provided by the manufacturer. This information may be used to configure a special outlet device to simulate the discharge. Although these values may be entered manually, HydroCAD also provides a library of rating tables for some commonly-used devices, including the vortex Valves produced by Hydro International and Mosbaek.

To use one of these tables, create a special outlet device and click "Load From File".  After selecting the desired CSV file the rating rating table will read into the table, with the values automatically converted to match the current project settings. This means that you can use a metric (SI) or English (US) rating table for any HydroCAD project.

Note: For models or sizes not found in the redefined tables, please contact the manufacturer for a custom rating curve.  This data can be manually entered into the special outlet screen, or loaded from a properly formatted CSV file.

Tailwater Issues

Standard rating tables are based on the assumption of free discharge. If HydroCAD determines that the device is being subject to a tailwater condition, a suitable warning message will generally be issued. In this event, you may be able to select the "Head Loss" option (on the Special Outlet screen) in order to provide tailwater sensitivity. This option will use the headwater/tailwater difference, rather than the headwater alone, in determining the device discharge. However, you must ensure that the tailwater conditions will not adversely effect the device characteristics. This and other device requirements should be obtained from the device maker and followed carefully.

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