WATERHAMMER AND SURGE TANKS 143 



interposed between tank and conduit, the level of the stored 

 water is quite independent of the acceleration, and does not affect 

 the waterwheel governor directly. The water in the standpipe 

 takes care of these things, and acts like a simple tank of small 

 dimensions which is supplemented by the steadying action of the 

 stored water, fed into the system in an independent, non-syn- 

 chronous manner, meeting all demands for water without causing 

 the unstable pendulum-like behavior which is so characteristic 

 of the simple surge tank. 



Mr. R. D. Johnson 1 has derived the following equation for 

 determining the maximum surge in simple surge tanks: 



A/ 



+h 2 

 \ ag 



where 



* Q max =- maximum surge up or down, in feet, measured in starting, 

 from reservoir or head-water level, and, jn stopping, 

 from a distance below this equal to the friction head, h f \ 

 P = cross-sectional area of pipe line, in square feet; 

 L = length of pipe line in feet; 

 v = velocity of water in pipe, in feet per second; 

 A = cross-section area of surge tank, in square feet; 

 g = acceleration of gravity; 



/?/= total feet of head lost due to friction in pipe between res- 

 ervoir and surge tank. 



Fig. 68 illustrates the pressure variations with simple surge 

 tanks, the upper curve to the right illustrating the rise in pressure 

 with the closing of the gates and the lower curve, the drop in 

 pressure due to the opening thereof. 



Figs. 69 and 27 show the design and arrangement of a large 

 differential surge tank. This particular tank consists of a cylin- 

 drical shell, 50 feet in diameter and 80 feet high, with a hemi- 

 spherical bottom which adds 25 feet to the height, and its capacity 

 is, therefore, 1,400,000 gallons. The tank is supported on ten 

 columns with heavy concrete footings. It and the riser are housed 

 in with a frame wooden structure providing a surrounding air 

 space which can be heated when necessary from a small house 

 below. The top of the roof of this structure is 205 feet above the 



1 American Society of Civil Engineers, Vol. 79, 1915, p. 265. 



