A Basic InfoSewer Wet Well, Pump and Force Main System

A Basic InfoSewer Wet Well, Pump, and Force Main System

InfoSewer Hydraulic Modeling: Components and Behavior

In the realm of sanitary sewer systems, effective wastewater management relies on the intricate interplay of various components. These components include manholes, outlets, wet wells, pipes, pumps, and informational objects like loads, curves, patterns, and controls. InfoSewer, a powerful hydraulic modeling software, provides a comprehensive framework for simulating the behavior of these elements. This document delves into the specifics of how InfoSewer models each component.

1. Manholes

Manholes serve as crucial junctions within the sewer network. They are points where:

• Multiple sewer lines (links) converge.
• Wastewater loads enter the system.
• Pipe characteristics, such as diameter, slope, or material, change.

1.1 Input Data

The fundamental input data required for manholes in InfoSewer include:

• Rim Elevation: The elevation of the top of the manhole structure. This is essential for determining overflow conditions.
• Invert Elevation: The elevation of the lowest point inside the manhole where the pipe connects. This is crucial to determine the flow path of the sewer.
• Manhole Diameter: The diameter of the manhole structure itself. This, along with the rim elevation, helps calculate surcharge (when the manhole is full and under pressure) and potential flooding.
• Baseline Load: The initial wastewater flow entering the manhole.

1.2 Load Characteristics

• Positive Loads: Loads in InfoSewer are assumed to be positive, representing the inflow of wastewater into the system.
• Time-Varying Loads: Manholes can have loads that fluctuate over time, reflecting the dynamic nature of wastewater generation.
• Load Types: InfoSewer allows the assignment of different load types to manholes, reflecting the source and characteristics of the wastewater. Examples include:
  • Low-Density Residential
  • Medium-Density Residential
  • High-Density Residential
  • Industrial
  • Commercial
• No Loading: A manhole can be defined with no load, indicating a junction point without any direct wastewater input.

1.3 Computed Results

InfoSewer calculates the following results for manholes during a simulation:

• Base Load: The initial or constant load assigned to the manhole.
• Total Load: The cumulative wastewater flow entering the manhole, considering all contributing sources and time variations.
• Overload Storage (Surcharge Volume): The volume of wastewater that a manhole can temporarily hold when its capacity is exceeded before overflow occurs. This represents the surcharge condition.
• Hydraulic Grade (HGL): The elevation of the water surface within the manhole. It is a sum of the elevation head and pressure head.

1.4 Special Manhole Types

• Junction Chambers: These represent nodes in pressurized sewer systems (force mains) where pumps connect to pipes.
• Outlets: These designate discharge points where wastewater exits the modeled system.

1.5 Flow Splits (Bifurcations)

Flow splits model points where a single incoming flow is divided among multiple outgoing pipes. InfoSewer provides four methods for allocating flows at these junctions:

• Fixed Flow Split Percentage: The user defines a fixed percentage of the total inflow to be directed to each downstream pipe.
• Variable Flow Split Percentage: A user-defined curve dictates the percentage of flow directed to a specific downstream pipe as a function of the total inflow to the manhole.
• Inflow-Outflow Flow Split: For each downstream pipe, a curve defines the outflow as a function of the total inflow to the manhole. This is useful for modeling hydraulic structures like weirs or orifices that control downstream flow.
• Automatic Flow Split: InfoSewer automatically calculates the flow distribution to downstream pipes based on their invert elevations, diameters, and hydraulic characteristics (e.g. roughness). This method simulates the natural flow distribution based on the principles of energy and continuity.

2. Wet Wells

Wet wells are essential components that provide temporary storage within the sewer system. They are typically used in conjunction with pumping stations.

2.1 Input Data

• Bottom Elevation: The elevation of the lowest point of the wet well.
• Minimum Level: The lowest permissible water level above the bottom elevation.
• Maximum Level: The highest permissible water level above the bottom elevation.
• Initial Level: The starting water level at the beginning of the simulation.
• Diameter (for Cylindrical Wet Wells): The diameter of a cylindrical wet well.

2.2 Variable Geometry

• Representative Diameter: For wet wells with relatively constant cross-sectional areas, a representative diameter can be used to simplify the model.
• Volume-Depth Curve: For wet wells with irregular shapes, InfoSewer allows the user to define a curve that specifies the stored volume as a function of water depth. This enables accurate modeling of complex geometries.

2.3 Operational Logic

• Level Control: InfoSewer simulates the filling and emptying of wet wells, ensuring that the water level remains within the defined minimum and maximum limits.
• Pump Control: When the water level in a wet well drops below the minimum level, InfoSewer automatically shuts off the associated pumps to prevent dry running.

3. Pipes/Open Channels

Pipes and open channels are the conduits that transport wastewater between different points in the network.

3.1 Pipe Characteristics

• Closed Conduits (Pipes): Typically used in sanitary and combined sewer systems.
• Open Channels: Open to the atmosphere; more common in storm sewer systems.
• Shapes: InfoSewer supports various pipe and channel shapes:
  • Circular
  • Rectangular
  • Trapezoidal
  • Triangular
  • Parabolic
  • Irregular (defined by a shape curve)

3.2 Unidirectional Flow

InfoSewer assumes unidirectional flow within pipes and channels. Reverse flow is not directly modeled, although the solver will flag this condition as an error or warning.

3.3 Input Data

• Inlet and Outlet Node: The manholes or wet-wells that the pipe connects.
• Length: The physical length of the pipe.
• Diameter/Shape: The cross-sectional geometry of the pipe.
• Roughness Coefficient: A parameter that accounts for the frictional resistance of the pipe walls (e.g., Manning's n, Hazen-Williams C).
• Slope: The elevation difference between the inlet and outlet, which drives gravity flow.

3.4 Computed Results

• Flow Rate: The volume of wastewater flowing through the pipe per unit time.
• Velocity: The average speed of the wastewater flow.
• Depth: The depth of flow within the pipe or channel.
• Hydraulic Grade Line (HGL): The elevation of the water surface along the pipe.

4. Pumps

Pumps add energy to the wastewater, increasing its hydraulic head and enabling it to flow against gravity or overcome significant elevation differences.

4.1 Pump Curves

InfoSewer uses pump curves to define the relationship between the flow rate a pump can deliver and the head it can generate. The type of pump curve depends on the available data.

• Fixed Capacity: A constant flow rate is specified, regardless of head. This can be used to represent a flow source or a simplified pump model where flow is assumed constant over the operational range.
• Single-Point Curve: Defined by a single design point (desired flow and head). InfoSewer extrapolates a three-point curve based on this point, assuming:
  • Shutoff Head (zero flow) = 133% of design head
  • Maximum Flow (zero head) = 200% of design flow
• Three-Point Curve: Defined by three operating points. InfoSewer fits a curve of the form: H = H0 - aQ^b
  • H = Head
  • H0 = Shutoff head (head at zero flow)
  • Q = Flow rate
  • a and b are coefficients derived from the three data points
• Multi-Point Curve: Defined by four or more operating points. InfoSewer will generate an equation for the pump curve to model the pump's performance over a wider range of flows and heads.
• Variable Speed Pumps: For pumps with variable speed drives, InfoSewer can model the shifting of the pump curve as the speed changes. The affinity laws are used to adjust the pump curve based on the speed ratio.

4.2 Computed Results

• Flow Rate: The actual flow rate delivered by the pump.
• Head Gain: The increase in hydraulic head imparted by the pump.
• Pump Speed (for variable speed pumps): The operating speed of the pump.
• Power Consumption: The electrical power used by the pump.

5. Informational Objects

• Loads: Represent the inflow of wastewater into the system at specific locations (e.g., manholes).
• Curves: Define relationships between two variables. Examples include:
  • Pump Curves (Head vs. Flow)
  • Wet Well Volume-Depth Curves
  • Flow Split Curves (Inflow vs. Outflow)
  • Rating Curves (Stage vs. Discharge)
• Patterns: Describe how loads or other parameters vary over time.
• Controls: Define rules for operating pumps, valves, and other control elements within the system.

Conclusion

InfoSewer provides a robust and versatile platform for modeling the complex hydraulic behavior of sanitary sewer systems. By accurately representing the characteristics and interactions of manholes, wet wells, pipes, pumps, and other components, engineers can analyze system performance, identify potential problems, optimize designs, and plan for future growth. Understanding the details of how InfoSewer models these elements is crucial for effective and reliable sewer system analysis.