After each weld is completed, it is inspected. Photographic film is 

 placed on the outside of the weld and a radioactive source is placed in the 

 pipe to expose the film. The images on the developed film reveal any faults 

 in the welds. They can be repaired immediately. 



When all welds have been checked, the pipe is cleaned of all dirt, rust, 

 and foreign material. Often a special machine travels down the pipe with 

 rotating brushes and scrapers. Machines such as this may be self-propelled or 

 pulled by side- boom tractors. The pipe itself is lifted from its temporary 

 blocks for passage of the machinery. Several pipe-mounted machines may pass 

 any point, priming the pipe, coating it with asphalt or tar, and wrapping it 

 with fiberglass and heavy paper. Vehicles accompanying the passage of each 

 pipe-treating machine include at least two side-boom tractors, hot-tar vehicles, 

 and supply vehicles. 



Faults in the coating and wrapping process are detected by visual inspec- 

 tion and electronic detectors, and they are repaired by hand. Some pipe is 

 coated with cement before arriving at the field. Special molds are used to 

 create complete patches around the welded joint areas. 



The pipe is lowered into the hole. Sometimes weights or fasteners are 

 required to anchor the pipe in the hole so it does not float out. The pipe is 

 lowered by a series of side-boom tractors traveling along the route. They 

 lift the pipe and ease it into the hole in one continuous action as they 

 travel down the route. Pipe may be placed in difficult spots at this time, 

 such as under roads or under other pipelines. 



Just prior to the lowering and placement of pipe, excavation will take 

 place for the placement of block valves, bypass valves, and condensation 

 traps. The valves may be part of the automatic shut-off system of the pipeline. 

 A backhoe will dig an area on either side of the pipeline. Special valves are 

 trucked to the site and lowered into the widened area. The pipeline and 

 valves are joined and welded. 



Electrolytic devices are used to protect the pipe from electrolysis with 

 the soil. In moderately dry soils, sacrificial anodes such as pieces of zinc 

 are fastened to the pipe. The zinc is more easily oxidized than the iron in 

 the pipe, therefore protecting the pipe from electrolysis (by the natural 

 potential between materials). Cathodic protection involves the placing of a 

 potential between the pipe and a pile of scrap iron to counteract the natural 

 electrochemical potential in the soil. The scrap serves as an anode, and the 

 pipeline is a cathode when connected to a low-voltage negative potential. 



Scrap piles are buried every 1 .6 to 8 km (1 to 5 mi) (Davis and Cyrus, 

 1947), depending on the conductivity of the soil. Electrical connections to 

 the scrap and pipelines are made at these intervals. Power poles or buried 



114 



