Quick Closing Valves for Isolating Breaks in Large Mains

We usually designate as large water mains those that are from 20 to 48 inches in diameter, the 48-inch diameter main being generally the largest size cast iron pipe installed in a distribution system. As the use of cast iron pipe for distribution purposes is almost universal, both in the United States and Canada, the breaks that occur are virtually limited to pipes of this material. A break in cast iron pipe is generally caused by the metal being stressed to the point where a crack will be started, and due to the lack of elasticity of the metal, the excessive stressing of the metal continues at the end of the crack, causing the crack to extend until finally it is of sufficient length to permit the pressure of the water to tear out a section of the pipe. The portion of the pipe wall that is the last to give away is generally found to show a fracture that makes an angle of 45 degrees to the adjoining pipe surface, instead of 90 degrees, as is the case for the major portion of the crack, the metal being actually torn asunder by the pressure of the water. The opening in the pipe resulting from the break is often equal to the cross sectional area of the pipe itself, and therefore the water escapes at a very rapid rate. The writer has known of a break in a 48-inch main in New York City permitting a flow to take place at the rate of about 150 million gallons a day, whereas the usual flow through a 48-inch main is at the rate of approximately 20 million gallons a day. Such an enormous volume of water coming to the surface of a city street quickly fills the sewers, floods cellars, breaks up pavement, stops traffic, and where there are rapid transit subways, may flood such a subway to a depth that prevents the operation of trains. Pressures in the distribution system may be lowered to such an extent that there may be no water on higher ground, and an insufficient pressure for domestic