=193. Flow in Old and New Cast-iron Pipes—Tubercles.=—The velocity of flow through cast-iron mains or conduits or through the cast-iron pipes of a distribution system of public water-supply depends largely upon the condition of the interior surface of the pipes as affected by age. All cast-iron pipes before being shipped from the foundry where they are manufactured are immersed in a hot bath of suitable coal-tar pitch composition in order to protect them from corrosion. After having been in use a few years this coating on the interior of the pipes is worn off in spots and corrosion at once begins. The iron oxide produced under these circumstances forms projections, or tubercles as they are called, of greatly exaggerated volume and out of all proportion to the actual weight of oxide of iron. When the pipes are emptied these tubercles are readily removed by scraping, but before their removal they greatly obstruct the flow of water through the pipes. Indeed this obstruction is so great that the discharging capacity of cast-iron mains must be treated in view of its depreciation from this source. Table XVII exhibits the value of the coefficient _c_ to be used in Chezy’s formula for all cast-iron pipes having been in use for the periods shown. TABLE XVII. TABLE OF VALUES OF _f_ AND _c_. LEGEND: (A) = Diameter, Inches. (B) = Hydraulic Radius _r_, Inches. (C) = Velocity, Feet per Second. (D) = Coefficient _c_. (E) = Coefficient _f_. ---------+-------------------------+-----+----+-----+-----+------ | | | | | | Authority.| Pipe-line. | | | | | | | (A) |(B) | (C) | (D) | (E) | | | | | | ---------+-------------------------+-----+----+-----+-----+------ Darcy |New Pipe | 3.22| .8 | 0.29| 78.5| .0418 | | | |10.71|100.0| .0257 ---------+-------------------------+-----+----+-----+-----+------ Darcy |Old cast-iron pipe lined | 9.63|2.41| 1.00| 72.5| .0489 | with deposit | | |12.42| 74.0| .0468 ---------+-------------------------+-----+----+-----+-----+------ Darcy |Pipe above cleaned | 9.63|2.41| 0.91| 90.0| .0316 | | | |14.75| 98.0| .0269 ---------+-------------------------+-----+----+-----+-----+------ Brush |Cast-iron pipe tar-coated|20 |5 | 2.00|114.0| .0197 | and in service 5 years. | | | 3.00|110.0| .0214 ---------+-------------------------+-----+----+-----+-----+------ Darrach |Cast-iron pipe in service|20 |5 | 2.71| 67.5| .0568 | 11 years | | | 5.11| 83.0| .0376 ---------+-------------------------+-----+----+-----+-----+------