Buried pipe

Cathodic protection (CP) is an electrochemical technique of corrosion control in which the potential of a metal surface is moved in a cathodic direction to reduce the thermodynamic tendency for corrosion. CP requires that the item to be protected be in contact with an electrolyte. Only those parts of the item that are electrically coupled to the anode and to which the CP current can flow are protected. Thus, the inside of a buried pipe is not capable of cathodic protection unless a suitable anode is placed inside the pipe. The electrolyte through which the CP current flows is usually seawater or soil. Fresh waters generally have inadequate conductivity (but the interiors of galvanized hot water tanks are sometimes protected by a sacrificial magnesium anode) and the conductivity... 

Since buried pipes for water, sewage and gas are a major use of cast iron, the corrosion of buried iron structures needs special consideration in any study of the corrosion properties of cast iron. It is also a very complex topic that is not fully understood. 

A characteristic of the corrosion on buried ferrous metals is that the attack is usually mostly in the form of pitting, especially with the cast irons. This raises a problem in measuring the extent of corrosion in burial trials. Usually both the weight loss, measuring the average loss of section, and the deepest pit, measuring the maximum loss of section, are reported. For assessing the severity of the attack on buried pipes, the second parameter is clearly the most important. 

Fig. 10.23 Groundbeds for buried pipes using impressed current...

A substantial reduction in the amount of stray current picked up by nearby buried pipes or cables may be achieved by interrupting the longitudinal conductivity of the structure by means of insulating gaps or joints. Care must be taken in siting the gaps, and they should preferably be placed in localities where the current tends to enter the structure and at points on each side of the track where the pipe or cable crosses under the rails. 

If the electrical continuity of a buried pipe or cable is broken at a point where a.c. is liable to flow owing to the presence of a bond, the gap should be bridged by means of a continuity bond. This will prevent the appearance of a dangerous voltage between the two sections of pipe or cable. 

Nevertheless, special consideration should be given to any measured small positive changes in structure/soil potential on a nearby buried pipe or cable if there is reason to believe that the secondary structure is already corroding because of local soil conditions, or as a result of stray currents from another source. 

Thermoplastic resins, such as vinyl chlorides, vinyl acetates and polyamides are employed, particularly in the water industry, on buried pipes and fittings. To provide both internal and external coating, application may be by one of these principle techniques dipping in a plastisol, fluidised beds or electrostatic spray. 

In terms of dead loads, the shape of the trench in which the pipe will be buried is also a factor. Generally speaking, a narrow trench with vertical sidewalls will impose less of a load on the pipe than will a wider trench with sloping side walls. It is necessary also to know the modulus of soil reaction (E), which is dependent on the type or classification of the native soil, the backfill material that is contemplated, and the desired consolidation of the backfill material. Soil consolidation is important, because it contributes to the strength of a flexible conduit in a buried pipe system. 

Since a buried pipe movement is resisted by the surrounding soil, a tensile load is produced within the pipe. The internal longitudinal pressure load in the pipe is independent of the length of the pipe. Thus, Poisson s effect must be considered when designing any length of pipe, whether long or short that is part of a buried pipe system. Buried pipes are influenced by friction with their surrounding media. 

Hydrostatic test pressure (TP) shall be limited to the pressure calculated per para. IP-10.7.2. The maximum testing interval shall be 10 years. Inline examination is normally applied to pipelines or buried piping specifically designed for this type of assessment and should be a requirement for piping systems only when the piping is specifically designed for inline examination. 

PL-2.8 ANCHORAGE FOR BURIED PIPING PL-2.8.1 Pipe Bends or Offsets... 

Bends or offsets in buried pipe cause longitudinal forces that must be resisted by anchorage at the bend,... 

Several recovery scenarios were considered for remediation. Initially, construction of a narrow, permeable trench parallel to the canal appeared to be an appropriate interception system. The construction technique considered was use of a specially designed deep trenching unit. This type of trench would have included a tile drain leading to a single two-pump recovery well. However, a review of the subsurface site plans and interviews with long-term employees determined that an unknown number of buried pipes traverse the area intended for the trench construction. Disruption of refining operations and safety considerations resulted in rejection of this option. 

For any activated carbon system, the capital costs will be dependent on contaminant types and concentration as well as treatment goals. Capital costs will also increase in cold-weather climates, since systems may require buried piping, heating, and housing units. The major contributors to operations and maintenance (O M) costs are treatment/replacement of spent carbon, disposal of residuals, and monitoring effluent concentrations (D11022L, p. 11-100). Also, activated carbon systems are usually part of a treatment train. Treatment costs will be highly dependent on the other systems used to deliver the contaminants to the activated carbon unit. 

Coimecting iron objects to a more active metal is called cathodic protection. Cathodic protection is widely used to protect underground storage tanks, ship hulls, bridges, and buried pipes. One of the most common forms of cathodic protection is to connect the object to magnesium. When magnesium is coimected to an iron object, magnesium rather than iron becomes the anode in the oxidation process. In cathodic... 

Buried pipes Externally lined with protective tapes and coating with epoxy ... 

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