492 WATER SUPPLY AND SEWAGE DISPOSAL 



What must be the height of a standpipe which will produce 

 100 Ib. of pressure to the sq. in. upon the city mains? 



576. Pressure Low at the Top of Tall Buildings. Let us 

 suppose that the top floor of an eight-story building is 100 ft. 

 above the level of the city mains. What pressure will a 

 144-ft. standpipe produce in the faucets on that eighth floor? 

 It is evident that 100 ft. of water in the house pipes will just 

 balance 100 ft. in the standpipe. A head of only 44 ft. is then 

 available. There will be a pressure of about 19 Ib. to the 

 sq. in. on the eighth floor, whereas the faucets in the basement 

 of the building will have a pressure of about 62J^ Ib. to the 

 sq. in. Calculate this. 



577. The Loss of Pressure by Friction. Thus far we have 

 not considered any loss of pressure due to the friction of the 

 water flowing through the pipes. At times of large fires in 

 cities when many large streams of water are being used, the 

 flow of the water through the city mains is so hindered by 

 friction that often the pressure is greatly reduced. In such 

 cases fire engines, which are merely steam force-pumps, are 

 used to produce the necessary pressure to throw the stream 

 of water to the top of the building. 



Some modern cities maintain a high-pressure water system 

 through the business portion for the special purpose of fighting 

 fire (Fig. 301). With the construction of twenty-story build- 

 ings, 300 ft. or more in height, it has been found necessary to 

 maintain a pressure of 200 Ib., sometimes even 300 Ib. to the 

 sq. in. in order to fight fire effectively. 



678. The Fire Hydrant. In cities, fire hydrants are attached 

 to the city main at frequent intervals. Fire hose can be at- 

 tached quickly to the fire hydrant at the connection near the 

 top of the hydrant (Fig. 302). By opening the cutoff valve 

 the full pressure on the city main may then be utilized in 

 fighting fire. A, Fig. 302, shows the position of the valve when 

 the city pressure is cut off the hydrant; B, Fig. 302, shows the 

 position of the valve when it is open and the city pressure is 

 on the hydrant. The valve is controlled by a rod extending 



