Understanding the case for flushing

The deterioration of pipes in a water distribution system is a major challenge for water utilities. Unidirectional flushing (UDF) can help to improve water quality.

Pipe Deposits:

• In some pipes, depending on material and age, deposits can form on inside walls.
• Can cause hydraulic capacity to decrease over time.
• Can result from chemical precipitation, microbiological activity, and in the case of cast iron pipes, corrosion buildup.

• Deposits can increase pipe roughness and decrease effective pipe diameter, possibly resulting in:
  • Inability to deliver fire flows
  • Increased disinfectant demand
  • Chlorine degradation
  • Complaints about water taste, odor, and color
  • Potential for public health issues
  • Increased pumping costs
  • Reduced hydraulic capacity
  • Lower residual pressure
• Certain pipe materials, such as cast iron, are more susceptible to issues.

Unidirectional Flushing:

• Can prolong the life expectancy of a distribution system and reduce the potential for waterborne diseases.
• Can help remove biofilm growth and particulates that lead to chlorine loss, as well as taste, odor, and color concerns.
• In InfoWater Pro UDF, unidirectional flushing is performed by first isolating a portion of the distribution system, then opening fire hydrants or flushing valves.
• Can ideally create enough velocity to scour pipes and remove unwanted materials.
• Flushed water is then discharged through a hydrant.

• Generally begins in portions of system closest to water sources, proceeding to outward extents.
• Important to understand distinction among three flushing methods: UDF, conventional flushing, and continuous blow-off.

UDF Method:

• Most efficient flushing method.
• Involves systematic plan to direct flushed water in single direction along pipe to maximize velocity—can close valves and open hydrants to create one-way flow in water mains.
• Linear flow results in higher velocities, typically 5 or 6 feet per second.

• Maximizes linear velocity near pipe wall—ideal for removing biofilm and hardened corrosion materials.
• Flush begins at clean water source and ends at hydrant, so only clean water is used.
• Eliminates chances of dirty water and debris being inadvertently transported around the system.
• Not feasible in emergency situations, as it takes proper planning and time to carry out.

Conventional Flushing:

• No control of flow direction—for example, opening several hydrants at once in same area until water clears up.
• Effective for removing loose sediment, but less effective at raising pipeline velocities.
• Yields velocities of less than 2 feet per second, so not effective at removing deposits or biofilm.
• Can cause dirty water and debris to be inadvertently transported around the system.
• With lower velocities, often requires larger amounts of water, decreasing efficiency.
• Often used reactively to fix areas of poor water quality or in response to aesthetic complaints.

Continuous Blow-Off:

• Consists of a small, continuous or intermittent water usage to refresh the line.
• Usually seen as a short-term preventive strategy.
• Often used to solve stagnation problems.
• Comparatively low velocity of less than 1 foot per second, and can use large amount of water, resulting in poor efficiency.