Model Approach to Pressure Mains

By analyzing the characteristics of a wastewater collection system through the use of modeling software, consultants were able to identify the problems and then develop a comprehensive solution

• By Douglas Maitland
• Mar 01, 2004

The use of conventional gravity flow sewers for the collection and transport of wastewater from residences and commercial establishments continues to be popular, especially in areas where the density of development is low. In connection with this type of wastewater collection system, pressure sewers are frequently used. In pressure sewer systems, wastewater is collected and discharged into a holding tank and them pumped to a pressure or gravity flow collector sewer.

According to Metcalf & Eddy's Wastewater Engineering, a standard reference manual in the wastewater treatment field, one of the advantages of using a pressure system design is that it eliminates the need for small pumping stations and makes it possible to use a small-diameter plastic pipe put in at shallow depths in the place of a much larger diameter conventional pipe put in at greater depts. However, when problems related to frequent wet weather events occur with this type of system, it can be challenging to determine the actual problem areas because of the variability of the heads, which are the heights of the column of wastewater surface that are supported by the standard pressure at given points. This happens because the number of pumps in operation impacts the heads in the system. In order to figure out the cause of such problems with pressure sewers and find solutions to correct them, wastewater plant operators and consultants are increasingly using software-modeling programs.

Modeling in Practice
During wet weather events, Anne Arundel County, (Annapolis, Md.), experiences frequent alarms and costly pump-outs of the main pumping stations at Carr's Ridge, Mayo, Pa. Lombardo Associates Inc. (LAI) was commissioned by the county to investigate the cause of the problem and develop a remedial strategy.

The Carr's Ridge region is low lying, with most of the residential area at sea level, and therefore requires individual house pumps to feed the collectors and interceptor. These pumps are either Gould 1/2 horsepower (hp) 0.37 kilowatt (kW) or 1/3 hp (0.25 kW), depending on the expected system head at the connection.

The main interceptor runs east to west through Carr's Ridge and varies from 2 inches 50 millimeters (mm) to 4 inches (100 mm) in diameter with bolted inspection holes and air release valves. A section along this interceptor acts as a variable grade gravity sewer during average dry-weather conditions. During wet-weather conditions, however, the entire interceptor acts as a low-pressure force main, (also known as a rising main) with the exception of a small length just prior to the interceptor draining into a pump station.

Modeling low-pressure force mains is a difficult task because heads are so dependent on the number of pumps operating. LAI turned to Haestad Methods' SewerCAD, sanitary sewer modeling software to complete the task.

According to Pio Lombardo, president of LAI, superior pumping algorithms and robust scenario management were needed for modeling this type of situation. It was necessary to analyze a plethora of operating scenarios in order to determine the precise problem areas and to develop the most effective remedial strategy.

SewerCAD was used to model three major scenarios that were identified as dry weather, base wet weather and flooding wet weather, which correspond roughly to average daily flow (ADF), two times the ADF and four times the ADF, respectively. Analysis using SewerCAD identified that sections of the main interceptor operate as a gravity main under dry and base wet weather conditions, and as a low-pressure force main under flooding wet weather conditions. A more detailed study of the flooding wet weather condition revealed that heads in the main could exceed shutoff head for all except five of the 36 pumps.

By dropping the loading in seven incremental stages from the maximum flooding wet weather flow, LAI determined a hierarchy for pump operation. The first tier represented those pumps that were able to operate even under the flooding wet weather condition. Subsequent tiers progressively stepped down to the base wet weather condition, where all pumps were able to operate. By focusing on the pumps in the more critical locations, LAI was able to develop a comprehensive plan for the replacement of their pumps.

Modeling in Theory
Pressure sewers can be modeled at several levels of detail, depending on whether the modeler is interested in the overall sizing of mains or pump cycling and the unsteady nature of the flows. In the first case, the system loads are modeled by placing them at junction nodes, much as users are placed at junction nodes in a water distribution model. Nodes only need to be placed at intersections, changes in diameter, and high points, such as air-release valves.

At the other extreme, each pump and user can be modeled with an extended-period simulation. Runs that simulate each pump provide insight into the range of conditions that the system will experience, including fluctuations of pressure and flow in the system. This type of model run requires additional information, such as individual pump curves, a junction node and pipe for each service connection, and the volume and elevation of the storage tank at each house. At an intermediate level of detail, the modeler can represent each house connection as a known inflow with a dimensionless flow pattern for extended-period simulation runs. This setup is based on the assumption that the pumps' discharges are not limited by the pressurized network. It provides a good approximation for positive-displacement pumps, but is less accurate for centrifugal pumps.

The design of a pressure sewer requires a layout of the piping, which should preferably be done on a computer aided design (CAD) or geographical information system (GIS) based map, much like any other sewer design. This makes it much easier to import the geometry and distances into the model for hydraulic calculations. The design can be based on paper maps or drawings, but then the representation of the system in the model will not be exactly to scale; therefore, the modeler must manually measure all lengths. The loading of individual nodes is based on the number of houses or other customers near each node.

While a manual or spreadsheet hydraulic analysis may be sufficient for a system with only a few customers, a hydraulic analysis model becomes necessary as the number of customers and the complexity of the system increases. Most design work can be done with a steady-state model. The key design parameter for pipe sizing is the velocity. For the system to work effectively, the velocity at peak flow should be kept below 5 feet per second (f/s) 1.5 meter per second (m/s), although there are some situations, such as short runs of pressure pipe, in which higher velocities can be tolerated. More importantly, velocity should be greater than 2 ft/s (0.6 m/s) in all pipes, for at least some part of the day, to prevent solids deposition.

With positive-displacement pumps, line pressure can be as high as 60 pounds per square inch (psi) (475 kPa) at low points and standard pumps will still operate. However, the pressure in the main must be considered when selecting centrifugal pumps. Higher-horsepower pumps with larger impellers may be required at low points. Model nodes should be placed at high and low points, in addition to intersections, to check pressures. There is no minimum pressure that must be maintained in the system. If the model predicts negative pressures, this is an indication that an air-release or vacuum-breaker valve may be needed at that location to prevent air blockage or pipe collapse. These valves can be difficult to model, since they act as a pressure node when pressure is positive. However, if the pressure is negative, they act as a reservoir open to atmospheric pressure on the upstream side and as an inflow node on the downstream side. Many of these situations are a result of pumping downhill and are avoided if the pressure sewer terminates at a high point.

It is usually desirable to draw a profile of the ground and pipe. The profile views from models can provide insight into pressures along the line. It may be better to use gravity sewers in areas with long stretches of downhill slope and only use pressure sewers in the portions of the system where the terrain undulates. While the gravity section may have higher construction costs, it should be easier to operate and doesn't require replacement and repair of pumps. For example, a mixed gravity and pressure system uses pressure sewers on one side of a drainage divide, but switches to a gravity sewer on the other side to flow down to a pump station wet well, where it is pumped through a force main to the plant.

Another application for modeling the system design is to identify the head required at any service connection. This is not a major issue for positive-displacement pumps if the maximum pressure is below the pump's threshold, but estimating the head the pump must work against can be important for centrifugal pumps. This can be modeled by inserting a node in the model for each service line and giving that node a fixed inflow. By varying the number of pumps running at one time, the modeler can determine the pressure in the system for any number of pumps and can, therefore, select the needed head and horsepower for the pump. The sizing of individual pumps is based primarily on the peak flow from that customer or group of customers. Individual home pumps are usually sized for 10 to 15 gallons per minute (gpm) 0.7 to 1.0 liters per second (L/s). The fixture unit method can be used to determine peak flow in commercial and industrial buildings.

The success of modeling software in the analysis of wastewater collection systems is based on its versatility. By modeling characteristics of the systems ranging from sizing of the mains and pumps to quantities of flows the systems typically handle, the software gives wastewater treatment operators and consultants insight into the range of conditions the systems experience. Such information is critical in developing a comprehensive plan for managing these complex wastewater collection systems.

Additional information on this topic can be found in Wastewater Collection System Modeling & Design, which is expected to be released by Haestad Press in the summer of 2004. For more information on this textbook, visit www.haestad.com/library.

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Haestad Methods' SewerCAD Sanitary Sewer Modeling Software
SewerCAD is a program for the design and analysis of wastewater collection systems. According to the company, it is the only sanitary sewer modeling software that offers true enterprise-wide data-sharing, with multiple graphical user-interfaces (GUI's) and database, spreadsheet, CAD, and GIS faculties. The program's GUIs include Haestad Methods' Stand-Alone Windows editor, AutoCAD, ArcInfo and ArcView.

SewerCAD's GUI's and data exchange capabilities allow users to develop and load complex models of combined gravity and pressure networks. The gravity portion of a system can be designed and analyzed according to either a gradually varied flow calculation or a standard capacity analysis.

With SewerCAD's automatic design capabilities, engineers can design a system based on user-defined constraints for velocity, cover and slope. SewerCAD can be used to design for partially full pipes, multiple parallel sections, maximum section size, invert/crown-matching criteria and the allowance of drop structures. Engineers have the option of disabling SewerCAD's design feature on a pipe-by-pipe basis, enabling them to design all or a potion of their system.

SewerCAD's loading model provides complete support for all types of sanitary and wet-weather loads. The unit sanitary load library is customizable and supports population, area, discharge, and count based sanitary loads, as well as hydrographs and load patterns. Wet-weather inflow can be added to the model as instantaneous loads, hydrographs, or base loads with associated patterns. Infiltration can be computed based on pie length, pipe diameter/length, pipe surface area or unit count. Loads are peaked according to a peaking factor or a customizable extreme flow factor library.

With SewerCAD's graphical editor and scenario management features, engineers can design new systems and analyze the performance on a current system. These capabilities facilitate the process of analyzing many design alternatives and locate potential problems in an existing system. For more information on Haestad Methods' SewerCAD, visit

For more information, visit www.haestad.com/sewercad.

This article originally appeared in the 03/01/2004 issue of Environmental Protection.