Problem

When exporting a Bentley water model to an EPANET model, how are the units in the EPANET model determined?

When exporting a Bentley water model to an EPANET model, why are the demands changing?

Note:

• A "Bentley water model" refers to a model created in WaterGEMS, WaterCAD or HAMMER (.wtg).
• An "EPANET model" refers to a model that can be opened in EPANET (.inp)

Background

EPANET supports two unit systems.

Bentley water models support the notion of different unit systems, allowing new models to be created in a consistent set of either SI or US Customary units. However, a particular set of units is not required, nor is it required for models to continue to use any particular unit for any particular field. A mix of SI and US customary units can be used if you see fit.

The EPANet Export [UNITS] is inferred according to what Display Format Units the WaterCAD/WaterGems model has set for Coordinates.

• If WaterCAD/GEMS Coordinates are set to display in a Metric/SI unit like "Meters" = Export to EPANET [UNITS] = LPS
• If WaterCAD/GEMS Coordinates are set to display in a US Customary unit like "Feet" = Export to EPANET [UNITS] = GPM

This is because changing the Display Format Units does not change the units that WaterCAD/GEMS stores the actual modelling data.  All WaterCAD/GEMS models store flow values in Gallons per minute, Length values in Feet, and diameter values in Inches. For example, changing the Display Format Unit of Flow to Cubic Meters/Hour doesn't change the value stored in the model database; rather it is applying a conversion multiplier to the stored value in order to display it in the software as Cubic Metres/Hour.

Now, EPANET does not support flexible units ("FlexUnits") as WaterCAD/GEMS does.   

• If in EPANET [UNITS] is set to Eg. LPS, then the Diameter of Pipes in the INP file must be Millimetres and not another unit like Metres. 
• If in EPANET [UNITS] is set to Eg. GPM then the Diameter of Pipes in the INP file must be in Inches, and not another unit like Feet.

So, WaterCAD/GEMS infers whether the user wants a "Metric" INP file based on the units used in the Coordinates, because this is one Display Format a modeler will virtually never change in their modelling environment. An SI/Metric user will likely always leave it in "Meters", and a US/Imperial Unit user will likely always leave it in "Feet". Whereas some modelers may change their Flow Display Units back and forth between a Metric Unit and an Imperial Unit, and have say demands of pump duty flows displaying in GPM,  but everything else in the model is in metric Eg. their pipe diameters in Metric / Millimeters etc.

So WaterCAD/GEMS has to "guess" what type of INP file the user wants:  An imperial unit INP file, or a metric unit INP file.  Using the Display Unit of the WaterCAD/GEMS Coordinates is generally a reliable way to tell if the user is a Metric or a US/Imperial Unit-based user.

The downside is it means that the Display Unit of "Flow" doesn't control the [UNITS] in the INP file.  You will get either LPS or GPM in the exported INP file based on the Coordinates Display Unit. However you cannot currently choose which Metric [UNITS] value sub-option or which US/Imperial [UNITS] value sub-option to use. An improvement to this will be considered for future releases.

Solution

When exporting a Bentley water model to an EPANET model, how are the units in the EPANET model determined?

If the Bentley water model's coordinate field (ie. the unit of distance that is used to represent the model map) is set to SI units, then the exported EPANET model will have SI units (the flow will be LPS).

Likewise, if the Bentley water model's coordinate field is set to US Customary units, then the exported EPANET model will have US Customary units (the flow will be GPM).

Why are the demands changing after export to EPANET?

In CONNECT Edition versions (10.00.00.XX):

If the Bentley water model has an active demand adjustment set in the calculation options then the individual demands will be adjusted by this factor when exported.

For Example:

• Demand at a junction in Bentley water model: 0.08 L/s

• Demand adjustment in Bentley water model: 1.8

• Demand at the junction in the EPANET model: 0.144 L/s (0.08 L/s * 1.8)

Note: This adjustment can be prevented by removing the demand adjustment before exporting.

In SELECTseries 6 versions (08.11.06.XX) and earlier:

EPANET supports one and only one single global demand multiplier. That is, if there was one demand multiplier in the Bentley water model that was a multiply or divide operation that applied to every demand node (scope: entire network), then that multiplier is used in the exported EPANET model.

The demand adjustment in the Bentley water model is applied by modifying the individual demands in the EPANET model when one of the following is true:

• There is more than one demand adjustment in the Bentley water model.
• The demand adjustment in the Bentley water model is not multiply or divide.

The Bentley water model's demand adjustment scope is not system wide.

See Also

When I import my EPANET file (.INP) into WaterGEMs/WaterCAD, the units are not coming in with the optimal units. How do I fix this?

What are the limitations of importing/exporting EPANET files?

Should the EPANET solver and the WaterGEMS/CAD solver give the same results for a model?

What are the difference between WaterGEMS/CAD and EPANET?