At first glance, it might appear that ?virtually any type of shutoff valve would work, but a closer look at the operating conditions provides a broader picture. These outlet Valves, which are releasing water, are doing so at huge volumes, ?relatively high head pressures and at a resulting high velocity. The combination of output flow conditions can result in severe damage from cavitation at the valve outlet. This phenomenon can ?rapidly destroy many types of valves, so measures to abate the cavitation must be taken either in valve selection or ?piping design.

During the early part of the 20th century, standard Valve types such as gate and globe valves were given this flow control job. But the effects of cavitation destroyed them at an alarming rate. As a result, engineers looked at new valve designs, such as the Larner-Johnson needle valve, to solve the problem. Other solutions were also tried, included forcing air into the downstream flow of standard valve types to keep cavitation from occurring.

The needle valves, however, have done a good job for many decades, yet newer designs have now replaced many of those work horses. Both fixed-cone and jet-flow gate valve designs are now used in such applications and are ?currently being retrofit into existing structures as funds permit.

The fixed-cone, or Howell-Bunger Valve, is a simple balanced design that requires little energy to open or close. The outlet of a fixed-cone valve is very similar to the outlet of a fireman