Will the minor losses associated with pipe bends be automatically accounted for? 

No, you must enter the appropriate minor loss coefficient in the properties of the pipe. A library of typical values for various bend angles are included, which you can choose from. 

How can I model a pipe bend without placing a junction? 

In the standalone platform, hold down the CTRL (Control) key while laying out a pipe; left-clicking will lay out a bend vertex instead of a junction. 

Can I model fluids other than Water?

Yes, you can. You can define new liquids under Components > Engineering Libraries > Liquid library, by way of the specific gravity and kinematic viscosity. You can then select your custom liquid in your calculation options (Analysis > Calculation Options). Note that only Newtonian liquids are valid, and you might need to adjust your pump definitions if the viscosity of your fluid differs from water. Furthermore, as Hazen-Williams friction method is an empirically based formula, its calculated friction losses are only applicable to Water @ 20 deg C. It should not be used for other liquid types (and you should use Darcy-Weisbach instead).

How can I add a new material such as HDPE to the material library?

1. Go to Components > Engineering Libraries and click the plus sign next to "Material Libraries"
2. Right click MaterialLibrary.xml and choose "add item" - a new material entry will appear at the bottom of the list.
3. Right click this new material entry, choose rename and enter the name (such as 'HDPE').
4. On the right side, enter the friction coefficients for your new material, then click close.

If you would like to create a new engineering library (xml file) instead of altering the default MaterialLibrary.xml, simply right click on "Material Libraries", choose "create library", choose a location to save the file, then repeat steps 2-4 above, replacing 'MaterialLibrary.xml' with the name of your library.

How can I model a steady state simulation with demands from a specific time of the day? 

If your steady state simulation represents a certain time of the day and you'd like the appropriate demands to be used, based on your demand patterns, you can use the EPS snapshot feature. Go to Analysis > Calculation options. Double click your calculation option set and choose "True" for "Is EPS Snapshot?". Then, select your date/time. When you compute the steady state simulation, it will use demands based on the multiplier on your demand patterns. Note that if your pattern timestep (increment of the time column) is less than 1hr, the computed demand will be "smoothed" based on the average of the multiplier from that timestep and the next. 

Should I use the inside diameter or outside diameter for the "diameter" field for my pipes? 

WaterCAD and WaterGEMS use the inside diameter (ID) for calculations. So, you should enter the inside diameter for each pipe's "diameter" field.

Nominal Diameter for most common material types is close enough to Internal Diameter to be used directly for modelling. However, you should be careful for some materials, as the manufacturer's expressed "nominal diameter" may refer to the outside diameter, which could be substantially different from the inside diameter. Discrepancies in model calibration are usually related to errors in roughness coefficients, not diameters.

How can I find the total volume of water through a certain element? 

Right click on the element and choose "totalizing flow meter". In this tool, you can select the timeframe and report water volume. 

How can I model a backflow preventer? 

You can do this with a General Purpose Valve (GPV) and a check valve. In the valve properties, enter a table of flow versus headloss. Make sure the orientation of the adjacent pipe is correct and then choose "True" for "has check valve?". If a certain amount of head is required to push open a closed check valve, it may be desirable to include some special entries in the GPV curve: 0,0 and 0.001,X, with ‘X' being the head value above which the valve can reopen.  

How does the Pressure Sustaining valve (PSV) work?

 A pressure sustaining valve (PSV) tries to maintain the upstream pressure or hydraulic grade at a user-defined pre-set value. In order to achieve its pressure sustaining ability, a specific headloss will be induced through the PSV, such that energy balance across the model results in the upstream pressure obeying the setting.

The valve can be in one of three states:

- Partially opened (i.e., active/throttling) to maintain its pressure setting on its upstream side when the downstream pressure is below this value.
- Fully open (inactive) if upstream pressure exceeds setting or if the downstream pressure is above the setting.
- closed if upstream pressure falls below setting or if downstream pressure exceeds upstream pressure.

Example: PSV setting is 55psi
Upstream pressure = 50 then valve closes
Upstream Pressure = 55 and downstream pressure = 60 then valve closes
Upstream pressure = 65 then valve opens
Upstream pressure = 55 and downstream pressure = 45 than valve controls

How does the Pressure Reducing valve (PRV) work?

A pressure reducing valve (PRV) will throttle the flow to prevent the downstream pressure or hydraulic grade from exceeding a user-defined pre-set value. In order to achieve its pressure reducing ability, a specific headloss will be induced through the PRV, such that the resulting downstream pressure obeys the setting.

The valve can be in one of three states:

Valve is CLOSED if downstream pressure exceeds the pressure setting or is greater than the upstream pressure (to prevent reverse flow).
Valve is OPEN if upstream pressure is less than setting and downstream pressure is less than upsteam pressure.
Valve CONTROLS if upstream pressure is greater than setting and downstream pressure equals setting.

Example: PRV setting is 55psi
Downstream pressure = 65 then valve closes
Upstream Pressure = 45 and downstream pressure = 50 then valve closes
Upstream pressure = 45 and downstream pressure = 40 then valve opens
Upstream pressure = 70 and downstream pressure = 55 than valve controls

What happens when a tank becomes empty or full?

When the water level in the tank reaches the minimum or the maximum (as specified in the properties), a built-in altitude valve will close the adjacent pipe. An empty tank will close the downstream pipe, since it cannot drain any more and a full tank will close the upstream pipe, since it can't fill any more. Once the opposite conditions occur in the system, the pipe(s) will open back up.

This can present a problem in some systems, because for example, as soon as the pipe closes for an empty tank, it may instantly be able to fill again from another pipe, triggering the pipe to reopen. As soon as the pipe reopens, it drains to empty, closing the pipe again. This can cause excessive intermediate timesteps and rapid oscillations in the system. So, it is suggested that you configure your controls (typically pump controls) such that the tanks never become full or empty. (Example:  If your tank fills at an elevation of 90 ft. you might want to set your pump to shut down at 89.9 ft or if your tank empties at a level of 2 ft. you might set your pump control to shut down at 2.1 ft.)

How can I model a connection to an existing water main? 

It is always best to model all the way back to the water source, at least using skeletonized data. If you cannot do this, the best approximation would be to conduct static and residual hydrant tests at the connection location and enter that information in the model as a reservoir and pump. For more detailed information, see the "modeling tips" section of the User Guide, which you can download at Docs.Bentley.com > Geospatial > Bentley WaterGEMS > XM Edition user guide.

How can I model a Top Feed/Bottom Gravity Discharge Tank? 

You can do this by placing a Pressure Sustaining valve (PSV) upstream of the tank, with the hydraulic grade setting equal to the elevation of the top of the tank. For more information, see the "modeling tips" section of the user guide. 

Why does my pump system head curve look strange when I only have demands downstream? (no tanks or reservoirs) 

The system head curve option is invalid for this condition, because a range of flows cannot be tested. This is because the pump can only operate at one point: the flow value equal to the sum of all demands, with the corresponding head value. The scale exaggeration throws off the graph; really what you're seeing plotted is a near-vertical line for the system head curve.  

How does the gas law model work for a hydropneumatic tank? 

Given the initial level/HGL and liquid volume, the gas law model keeps the "nrt" part of the equation constant (called K) and is thus able to compute HGL for a given volume, or vice versa, as the conditions in the model change over the course of an EPS simulation. One should still use controls to turn the pump on/off based on the minimum/maximum level/HGL in the hydropneumatic tank. 

The atmospheric pressure needs to be specified because the gas law equation needs to work with absolute pressure, NOT gauge pressure. So, in the pv=nrt equation, the atmospheric pressure head is added to the gauged pressure head in the tank, to acheive the absolute pressure. 

Why is the HGL in my hydropneumatic tank sometimes very large and beyond the pump control range? 

Hydropneumatic tanks have a very short cycle time compared with large tanks. Therefore, when hydropneumatic tanks are used in a model, a very short hydraulic time step may be needed or the tank may overshoot its on and off levels. If this occurs, the hydraulic time step in the calculation options should be reduced. 

What does it mean when a node reports a negative pressure?

A negative pressure occurs when the hydraulic grade is below the physical elevation of a node. If WaterCAD/GEMS says the pressures will be negative, then in all likelyhood you will have problems. Assuming all data input has been checked, there are usually two general causes of negative pressure:

1. Trying to serve a customer at too high of an elevation. This will show up as low/negative pressure at any demand. You need to increase pump head or adjust pressure zone boundaries.

2. Some restriction in the system. This will show up as good pressure during low demand and poor pressure at high demand. You need to look at the model results and see if the pipes are too small causing excess head loss or the pumps are inadequate such that they are running far off to the right of the curve. You need to upsize the pipes or pumps accordingly.

You should also check your demands for errors. Since your demands are likely based on historic averages, a significant decrease in pressure may skew the results, since the demands would likely be decreased in that condition. You may consider using pressure dependent demands or flow emitters. You may also consider conducting a transient analysis using Bentley HAMMER, if the problem occurs at a transmission main.

Why do I get a negative pressure at a high point in my system? Shouldn't the pump add enough head to push the water over the hill?

By default, pumps only consider the boundary conditions (reservoirs and tank elevations) in your system. So, the pump will add enough head to lift the water to the downstream known hydraulic grade. It does not consider junction elevations inbetween. If you are using WaterCAD or WaterGEMS V8i, you should add an Air Valve element at the high point to properly model this situation. By placing an air valve at the high point, the pump sees the air valve elevation as its downstream boundary condition for instances in which pressure would have otherwise been negative at the high point. For any air valve that is expected to be open in this way, ensure that you select "false" for the "Treat air valve as junction?" attribute.

See Also

General WaterGEMS V8 FAQ

Product TechNotes and FAQs

Haestad Methods Product Tech Notes And FAQs

External Links

Bentley Technical Support KnowledgeBase

Bentley LEARN Server