InfoSewer FM Split Issue

InfoSewer FM Split Issue

A common challenge in InfoSewer (and other sewer modeling software) when dealing with force main splits: how to accurately model the flow distribution at the point where a single force main diverges into two or more downstream force mains. The method you described, using a duplicate wet well, pump, and force main, is a valid and often effective workaround. Let's explore this solution and discuss other potential approaches.

The Problem: Force Main Splits in InfoSewer

• Single Upstream Link: InfoSewer, by its nature, typically allows only one upstream link connected to a wet well. This represents the physical reality that a pump draws from a single wet well.
• Flow Splitting at Junctions: When a force main discharges into a gravity sewer system at a junction (manhole), InfoSewer can handle the flow split using the standard junction logic (based on hydraulic grade lines and downstream pipe capacities).
• Force Main to Force Main Split: However, when a force main needs to split into two or more force mains, the standard junction logic is not directly applicable because the flow is entirely driven by the upstream pump, not by gravity or downstream hydraulic conditions.

Solution: Duplicate Wet Well, Pump, and Force Main

The method you described effectively addresses this issue:

1. Duplicate Wet Well: Create a new wet well node immediately downstream of the original force main's discharge point (which is likely a chamber or a manhole).
2. Duplicate Pump: Create a new pump connected to this new wet well.
   • Pump Curve: This new pump should have a fixed capacity (as you mentioned) equal to the capacity of the original pump that feeds the split. This ensures that the total flow entering the split is maintained.
   • Pump Type: It is best to have the pump type be a constant flow pump.
3. Short Force Main: Connect the new wet well to the original discharge manhole with a short force main. This link will carry the entire flow from the original force main. The purpose of this force main is to transition the flow from the new wet well to the node where you want the split to occur. This short force main can be a very small diameter pipe. The purpose of the small diameter is to keep the flow as pressurized flow.
4. Split at Junction: Now, the original discharge manhole has two inflows:
   • The short force main from the duplicate wet well.
   • One or more gravity or pressure pipes carrying flow from other parts of the system. This manhole will now act as the flow splitting point. You can connect your two (or more) downstream force mains to this manhole, along with any other gravity or pressure pipes that need to be connected.
5. 50/50 Split (or Other Ratios): In your example, you want a 50/50 split to links 25 and 35. You can achieve this by:
   • Equal Pipe Characteristics: Make sure that links 25 and 35 (and their downstream networks) have similar hydraulic characteristics (diameter, roughness, slope) so that they offer roughly equal resistance to flow.
   • Weir Splitting at the Junction: The most accurate way to force a 50/50 split is to model a weir inside the manhole to divide the flow. The weir crest length will control the split of the flow.

Advantages of this Method:

• Maintains Flow Continuity: The total flow from the original force main is preserved.
• Accurate Flow Splitting: The flow split can be controlled by adjusting the downstream pipe characteristics or, more precisely, by modeling weirs.
• Flexibility: You can model different split ratios by adjusting the downstream pipe characteristics or weir geometry.
• Relatively Simple: It uses standard InfoSewer elements (wet wells, pumps, force mains, junctions).

Other Potential Approaches:

1. Control Rules:

   • You could potentially use control rules to manipulate the flows in the downstream force mains based on the flow in the upstream force main. For instance, you could set the flow in one downstream force main to be a fraction of the upstream flow and let InfoSewer calculate the flow in the other downstream force main to satisfy continuity.
   • Complexity: This approach can become complex to set up and maintain, especially if the split ratio needs to vary dynamically.

2. Specialized Split Node (if available):

   • Some sewer modeling software packages might offer specialized node types designed to handle force main splits directly. It's worth checking if InfoSewer has such a feature, although it's not a standard element.

Important Considerations:

• Headloss: Make sure to account for headlosses in the short force main connecting the duplicate wet well and the original discharge manhole.
• Numerical Stability: Introducing short, small-diameter pipes can sometimes affect numerical stability. Monitor your model for any instabilities and consider adjusting time steps or other solver settings if necessary.
• Real-world Behavior: Keep in mind that real-world flow splits at force main junctions can be complex and influenced by factors like turbulence and momentum. The chosen modeling approach should be a reasonable approximation of the actual physical behavior.
• Verification: If possible, validate your model results against field measurements or more detailed CFD (Computational Fluid Dynamics) simulations to ensure that the flow split is being accurately represented.

In conclusion, the duplicate wet well, pump, and force main method is a practical and effective way to model force main splits in InfoSewer. It provides a good balance of simplicity, accuracy, and flexibility. While other approaches might be possible, this method is generally recommended for its robustness and ease of implementation.