Abstract
The series of tests discussed in this paper was made for the purpose of determining whether or not gate valves or globe valves could be used satisfactorily for regulating releases from irrigation-distribution systems under heads up to about 125 ft. Both types of valves proved unsuitable for making free-flow releases because of the wide dispersion of the jets under throttled conditions. Confinement of the jets to pipelines downstream induced cavitation in and downstream of the valves and a study was made of the pressures in the pipelines upstream and downstream of 6, 8, 10, and 12-in. gate valves, and a 6-in. globe valve to evolve a means of eliminating or minimizing the cavitation. A pressure relationship or “cavitation index” was found to be a useful means of determining the limits of differential head and back pressure to either control the location of, minimize, or eliminate cavitation damage. The design of the water passage immediately downstream from the valves and the pressure within this passage were the two main factors influencing the cavitation characteristics of an installation. One of the most effective means of eliminating cavitation damage below gate valves was the placing of sudden enlargements in the pipe sections immediately downstream. No damage could be detected in the concrete lining of the sudden enlargement under heads up to 150 ft with back pressures of approximately 15 ft, although noise similar to that which accompanies cavitation was present at heads in excess of about 90 ft.
A limited number of tests were made to study the resistance of rubber-like products to cavitation-erosion and their use in protecting the downstream pipe wall against this erosion. The conclusion was reached that such protection should not be used except in cases where cavitation will be mild.
Some consideration was given to venting the area in the top of the pipe just downstream of the valve, but this was considered objectionable because of the influence of entrained air on flow conditions and measuring devices, particularly propeller meters, placed downstream. Moreover, it was doubtful that air admitted to this region would reach all critical zones of local cavitation.
The information obtained from the valve investigation is very useful as design criteria for irrigation-distribution systems where valves are used for regulating the releases.
