22 



BULLETIN 906, U. S. DEPARTMENT OF AGRICULTURE. 



Saugus, Calif., is 80 feet high and was built to protect a 16-inch rein- 

 forced concrete pipe that runs gradually up grade for over 2,000 feet. 

 The standpipe is made of 12-inch riveted steel pipe, supported by a 

 steel windmill tower. 



Open boxes of concrete are built to about 15 feet in height and act 

 as relief stands and at the same time are used as diversion boxes or 

 for measuring water over weirs. 



Where pressures are too great to permit the installation of a stand- 

 pipe, it is common to use air drums to relieve back pressure. Air 

 drums are chambers that are partly full of air, the elasticity of the 

 air preventing excessive pressures on the pipe. Air chambers have 

 been used successfully in some installations, but there are so many 

 cases of the air chamber being waterlogged at a critical time that a 



-PAPER RING SET INS/DE 

 THIMBLE TO PREVENT 

 ADHERING 



^K.COPRER THIMBLE SET 

 IN PIPE 



Fig. 



-Expansion joints for high pressure. 



number have been replaced by relief stands. Some air drums are 

 equipped with an automatic air pump that keeps the chamber filled 

 with the proper proportion of air, but as any such apparatus is liable 

 to get out of order, the air drum may be worthless when needed the 

 most. There are some instances on record where air pumps have de- 

 livered an excess of air which has caused large bubbles of air to col- 

 lect in the main. 



Relief valves have also been used. These are so adjusted that an 

 excess of pressure will open them and let out enough water to bring 

 down the pressure in the pipe. Relief valves are not in great favor, 

 however, as they may stick under high pressures or blow off under 

 small variations of pressure which occur when a pump is directly 

 connected to a main. 



Check valves have been installed at intervals along a main leading 

 from a pump but have met with little success. Unless provision is 

 made to allow some water to escape at the pump when it is suddenly 



