Materials and production
Pipes can be made from a number of materials, including steel, cast iron, asbestos cement, aluminum, plastic or even prestressed concrete. Mechanical joints are normally used for most of these materials but steel and aluminum pipes can be welded together. Distribution systems for the handling of crude oil products or gas are invariably constructed from high-quality steel pipes with welded end joints.
Early steel pipes were produced by rolling from plate and joined by electric resistance welding; later a solid, drawn system without a seam was adopted. The increasing demand for large-diameter pipe led to the development of the submerged static head.
For a long line it may be necessary to install a number of pumping stations at intervals to provide the overall pressure requirements. Flow conditions for gas lines are more complicated because of the compressibility of gas; compressors are used to force the flow. On average, over flat terrain, pumping stations are 20 to 30 miles (32 to 48 km) apart.
The choice of drivers partly depends upon local conditions and the availability of fuel. They can be electric motors, steam turbines, gas engines, larger diesel engines or gas turbines. Electric motors usually prove to be the most acceptable.
Complicated pipeline systems are normally designed’ for failsafe, fully automatic operation. Control of a system, including all the principal equipment, can be done from one central room. All essential operating data are transmitted back to the control center continuously so that the controller is always fully informed of the operating conditions of the pipeline. Communication is normally by microwave radio or direct cable, although public utility circuits are sometimes used.
Many pipeline systems make use of computers to assist in hatching schedules, that is, the predetermined cycle of liquid products. Some of the latest systems are designed so that the control of all operations can be linked directly to a computer.
Buried steel pipelines are protected externally by a wrapped coating on the outer pipe, and it is usual for a cathodic protection system to be applied by means of an impressed current or sacrificial zinc anodes. Should a fault develop in the coating, corrosive action is diverted away from the pipe.
All cross-country steel pipelines are tested for tightness and resistance to pressure before they are put into operation. The testing medium is usually water and the test pressure is always higher than the actual maximum operating pressure. Before putting a pipeline into operation, it is also usual to pass a go devil or pig through its entire length, pushed through the pipe by air or water. A go devil consists of a short rod with circular steel plates attached at both ends, the front plate being made just a little smaller in diameter than the inside of the pipe. When passed through the whole line it clears the pipe of any obstructions and demonstrates that the pipe has been laid without damage. To enable easy location, a pig can be fitted with a short-life radioactive capsule for detection above ground.