26 BULLETIN 906, IT. S. DEPARTMENT OF AGRICULTURE. 



shows that a 14-inch pipe will carry 200 inches under these conditions. 

 (In taking levels the actual difference in elevation between the point 

 of entrance and the point where water is discharged must be taken. 

 If water is discharged over distributing stand, the elevation of top of 

 the stand must be taken.) 



Example 2. 



Assume an orchard lateral is run down a steep grade of 5 feet 

 per 100 feet, which is not uncommon in hillside tracts. It will be 

 seen from the table that a 6-inch pipe will carry 60 miner's inches of 

 water, but if a lateral is taken off this line that falls only 0.1 foot 

 per 100 feet, the table shows that a 12-inch pipe will be needed to 

 carry the same amount of water. If the lateral runs across a 10-acre 

 tract or 660 feet and the delivery stand carries a head of 6 feet, we 

 will have a total head available of 1 foot per 100 feet, requiring 

 only an 8-inch pipe. 



If grades vary, where open stands are frequent, allowing little or 

 no pressure in the pipe, the diameters of the pipe should vary with 

 the grade, especially where the variation is considerable. On the 

 other hand, if the pipe is under pressure from one end to the other 

 and there are no relief stands, the pipe should be the same diameter 

 throughout its entire length. 



Too many pipe contractors guess at the sizes of pipe to use and pay 

 little or no attention to grades. Usually however, they are on the 

 safe side where water runs down hill although the pipe may be several 

 sizes larger than necessary. On the other hand, it is common for 

 inexperienced men to install undersized pressure pipe that is used 

 for pumping, which means that the pump must be burdened with 

 additional lift in order to overcome the excessive frictional resistance 

 in the small pipe. If concrete pipe is directly connected to a pump, 

 the pipe may burst from excessive pressures, although the actual ele- 

 vation between the pump and outlet may be small, the total head 

 due to excessive friction causing failure. For example, it can be 

 seen from the table that if 250 miner's inches are pumped into a 10- 

 inch pipe that is laid on the level, friction will cause a head of 5 

 feet for every 100 feet length, or a total head at the pump of 50 feet, 

 if the pipe is 1,000 feet long. 



It is wise to keep concrete pipe under as low heads as possible and 

 where pressure is to be applied care should be taken to get the best 

 quality of pipe. It is often possible to control pressures on pipes 

 especially where there is a constant down grade. If the pipe is not 

 fitted with outlets open at the top, or other means of relieving pres- 

 sure such as overflow stands, open diversion boxes and so on, the 

 pressure on the lower end of the pipe will increase as the pipe fills, 

 and the lower sections may fail from excessive pressure. It is a 



