Water utility managers are always looking for proven methods to improve operations, enhance the water system and improve service standards. Precipitation, microbiological activity or corrosion can all form deposits on the pipe walls of the water distribution system over time. These accumulations can cause unwanted tastes, odors or discolorations of the water, reduce chlorine residual and may limit the pipe?s conveyance capacity.
Water main flushing is an effective method to remove these accumulations. There are two main flushing methods that are generally used: conventional flushing and unidirectional flushing (UDF). This paper presents the benefits of UDF, a discussion of how to establish a UDF program and operational considerations for the program.
Conventional Flushing
The conventional flushing method consists of opening hydrants in the different targeted areas and discharging the water until the accumulations are removed and the water becomes clear. This method of flushing is easy to conduct by maintenance and operations crews, but requires a large amount of water and may not completely clean the pipe network. The graphic in Figure 1 shows how the water flows to an open hydrant using the conventional flushing method. The water moves freely from all directions to an open hydrant. Since there is less flow in a given pipe, velocities may be too low to adequately clean, or scour, the pipes.
Unidirectional Flushing
Unidirectional isolates each pipeline to create flow in a single direction to quickly and efficiently clean the pipe. The cleaned mains may have improved water clarity or color, reduced turbidity and improved chlorine residual. The graphic in Figure 2 shows how the water flows through an isolated pipeline in a single direction by closing valves and using specific hydrants. By concentrating flow, UDF creates higher velocities that are better able to clean the pipe. Major advantages of this method are:
- Improved cleaning of accumulated deposits on pipes;
- Requires less water than conventional flushing; and
- Reduces the impact on customers.
Sand, gravel, plastic, biofilms and other accumulated materials can be removed by flushing. Flushing times of a half hour or less are typical, which can equate to substantial water savings. Therefore, UDF can be an important component of a water use efficiency or conservation program. The reduced flushing time also limits the impact on the utility?s customers. An additional benefit of UDF is that it incorporates the use of a large number of valves and hydrants providing for a simultaneous valve exercising and hydrant testing program. The major disadvantage of UDF is the planning to develop the flushing program and additional crew time to inspect the required valves and hydrants prior to the flushing. It may also require more hydrants to be flushed than in a conventional program.
Creating a UDF Program
A UDF program should be well planned prior to implementation to ensure an efficient program and that safe conditions for operators and the public are provided. There are a number of reliable resources available for designing a UDF program, including the American Water Works (AWWA) Unidirectional Flushing DVD (2002), AWWA Research Foundation reports (Ellison, D. (2003), Friedman, Melinda and Holt, David (2003), and Hasit, Yakir J., et al. (2004)), journal articles and conference proceedings, and helpful guides (British Columbia Water & Waste Association (2004), Brand (Undated), and others). An outline of the steps and key criteria are summarized in the following section.
UDF Plan Creation
A UDF plan is typically a series of maps that document the flushing sequences that are needed to clean the distribution system. Each flush targets a specific pipeline by opening a hydrant or hydrants and opening or closing system valves to isolate the pipeline of interest. UDF sequences are created using a series of simple heuristic rules. These parameters are not absolute, but do provide general limitations on the extent of flushing to be performed.
- Begin from a clean water source;
- Flush from larger to smaller pipes;
- Limit flushing lengths to an average of 1,500 ft;
- Maintain minimum pressures within the system;
- Consider stormwater infrastructure, traffic control and other constraints to ensure efficient and safe execution.
To ensure that clean source water is used, flushing series typically radiate out from a treatment plant, well, or water reservoir/tank. The flushing series are commonly grouped, called a flushing zone to divide the system by pressure zone, represent different clean sources, or for operational considerations (public notification, scheduling crews, etc.).
Electronic tools and hydraulic modeling are not required to create a UDF plan; however, CAD, GIS and hydraulic models can aid in the efficient creation of the plan. Commercial water distribution modeling software has created UDF specific tools to aid in the setup of UDF sequences and produce ?field-ready? output. To use these tools, specific numerical criteria should be set based on the literature, as described in the next section.
Numerical Flushing Criteria
Flushing criteria aim to ensure that there is sufficient velocity to scour each pipe, while maintaining safe conditions for operators and the public. Numerical flushing criteria are typically used for velocity, minimum pressure and flushing volume.
Velocity
The AWWA and other literature sources recommend that velocities of greater than 5 ft per second be achieved, with a minimum velocity of approximately 2 ft per second. However, Melinda Friedman and David Holt, in their report, ?Establishing Site Specific Flushing Velocities,? found that velocities between 2 ft per second and 4 ft per second may provide similar benefit to higher velocities in pipes that had previously been flushed with the UDF method.
According to Friedman and Holt, a utility should consider if there are ?self-cleaning? mains that routinely reach the 2 feet to 4 feet per second pipes, as these mains may be excluded from the UDF program. Large diameter mains may also warrant special consideration, as they may be unable to achieve the necessary flushing velocities. Additionally, other cleaning methods may prove to be more effective on large diameter mains or those with substantial build-up/corrosion.
Pressure
Minimum service pressures should be maintained at all times, as set by the policies or criteria of the utility, regulating agency or fire marshal. Field measurement may be used to confirm that there are sufficient pressures at or near the flushing hydrant. A hydraulic model may be used to determine the minimum service pressures throughout the system during flushing.
Volume
Flushing volumes should be large enough to allow sufficient scouring and water turnover. Literature recommends flushing a minimum of two pipe volumes. Additionally, a maximum flushing volume may be established to accommodate operational constraints. However, visual inspection or measurements, such as turbidity, should be always be used to confirm that adequate water quality has been restored before ending the flushing sequence.
Operational Considerations for a UDF Plan
To conduct safe and effective flushing, the UDF plan must take into account operational considerations. Since a large number of valves and hydrants are used, it is likely that the cooperation of multiple departments, agencies and customers will be needed. Major operational considerations include:
- Manage discharge of flushed water to avoid localized flooding, adverse water quality, and other site-specific considerations (de-chlorination may also be required);
- Establish traffic control for the safety of operators and drivers;
- Public notification of flushing activities to nearby customers who will or may be impacted;
- Addressing inaccurate system information, such as missing valves and hydrants; and
- Recording data to improve future flushing efficiency.
Critical customers may require advanced planning and additional public notification. These customers may include hospitals, medical and dental offices, industrial sites and customers with special needs. Flushing may be required at night or on weekends to meet the needs of these customers.
It is recommended that flushing crews maintain records of the flushing activities. The information may be used to update system maps where inaccurate system information was found, identify infrastructure for maintenance, such as a stuck valve and improve future flushing efficiency. Recording the actual flushing time, hydrant flow or velocity and pressure can also provide valuable information for planning future flushing.
Unidirectional flushing quickly and efficiently cleans the pipes to improved water clarity or color, reduced turbidity and improved chlorine residual. System valves are opened and closed to isolate a pipeline during flushing, which improves cleaning and reduces water use. Creating a UDF plan allows the systematic planning needed for an efficient program and safe conditions for operators and the public. The UDF plan may consider multiple criteria, including velocity, pressure, length and volume. Operational considerations should also be included in the planning and execution of the UDF program.
Lara Kammereck, P.E., PMP, is a civil engineer with Carollo Engineers.
Dan Reisinger, P.E., is a lead water resources engineer with Carollo Engineers.
