Flushing of hydrants is necessary to promote water quality and test hydrants for pressure and flow. Chlorine residuals in the water distribution system can drop drastically at certain times during the year. Maintaining good chlorine residual levels is essential to ensuring proper system disinfection and is required by law. Flushing the system helps to maintain these residuals, particularly in areas with long residence times (i.e. water that spends a long time in the pipe before being used).
In addition, fire hydrants can be flushed to analyze the flow and pressure in an area. These tests are often performed by Fire Department personnel and are typically done for insurance rating purposes.