There is a common misconception that fire hydrants can be used in place of combination air valves to provide the required air-release service. This method should be strongly discouraged for several reasons:

• The use of fire hydrants requires active user intervention, which creates a hidden maintenance cost. In order to be effective, the hydrants should be discharged once per day. At just five minutes per hydrant at a rate of $10.00 per hour, a small town with twenty hydrants would spend more on the labor used than it would have spent on twenty automatic air valves.
  
  
• Manual air release cannot accommodate surges in flow or demand. Only automatic air valves can provide the needed functions on demand in order to protect the water system.
  
  
• Hydrants cannot provide the vacuum-breaking function required to prevent contamination during negative-pressure events. Automatic air valves help protect the finished water at all hours, every day of the year.
  
  
• The pressures accumulated within the hydrant, even at a system pressure of 80 psi, can be extremely hazardous to maintenance workers. Accumulated pressures in some systems can reach more than ten times the rated system water pressure. Automatic air valves work without user intervention and prevent the accumulation of pressure.
  
  
• Hydrants are typically mounted to the sides of a main (at the 3 o'clock and 9 o'clock positions), rather than at the top. This can significantly reduce their effectiveness as air-release devices, even when flanged at the full diameter of the main. Automatic air valves are properly installed only at the top (12 o'clock) position, where they can provide complete discharge of accumulated air.

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