UNDERGROUND PIPELINE PROTECTION FRENCH PRACTICE I T is before, and not after, the con defects thus found must be repaired A comprehensive monograph on struction of a pipeline that decisions on the spot. Particular care must be the topical problem of the pro should be made concerning the degree taken in zones with very humid or tection of underground pipelines of corrosion protection required. Such aggressive soil, or where strong stray against corrosion has been con a decision must, of course, be based currents are present. on the merits of the case. To find the tributed by J. Vincent-Genod, Cathodic protection most economic method in the given an engineer of the French circumstances, it may be necessary to However good a protective coating 'Societe des Transports Petro- carry out a preliminary survey, the may be, it can never be adequate to liers par Pipe-Line,' to the May cost of which would be negligible prevent corrosion entirely. But if it is 1955 issue of the French journal, compared with the capital values at combined with cathodic protection, stake. Travaux, which is mainly de the combined protection afforded can be very effective indeed. Generally speaking, a distinction voted to oil pipeline construc must be made between external and A considerable pool of experience tion. The following is a brief internal corrosion. Whilst the latter is now available on cathodic pro summary of that monograph. might be fought by adding special tection. Having selected the most inhibitors to the corrosive medium economic local current supplies, the concerned, external corrosion calls for next step is to choose the location of it is possible to estimate the rate of two types of protective measures which the protection points in accordance corrosion to be expected on each might be used jointly or separately: with their theoretical range and certain section of the proposed pipeline, and protective coatings (i.e. passive pro local conditions. thus to decide on the preventive tection), and cathodic protection (i.e. Where low-tension current is cheaply measures to be taken. active protection). available, the most economic method of cathodic protection is the use of Protective coatings Preliminary survey rectifiers. According to the quality of the insulation, the rectifiers must, The preliminary survey must in If circumstances call for the passive for a 12-in. pipeline, be spaced at clude all the potentially relevant protection of the pipeline by protective distances varying between 15 and 60 factors, i.e. the nature of the soil, the coating, a further choice must be made miles. An important part of the cost presence or absence of stray currents, between the several different types of of such a rectifier station is incurred etc. Survey methods include the coatings available for this specific in the earthing device which is also taking of soil samples, the investiga purpose. Most of them have a mere responsible for the highest energy tion of moisture conditions and the insulating function; others also act as consumption of the whole circuit. In measurement of soil resistance by inhibitors. recent American practice, metallic means of electrodes or by means of For economic reasons the coatings earthing devices have been replaced Schlumberger's quadripole method. most generally used for underground by carbon anodes which have a longer This method, which is generally pre pipelines are of the bituminous type service life. The rectifiers are com ferred by the specialist engineers of which have good dielectric properties bined with transformers from which the French company, consists of but may have to be specially treated the required voltage can be obtained. setting up a direct current between to obtain the necessary hardness and two electrodes situated at a given elasticity, important properties which Where a current supply is not distance from each other and measur must be regularly ascertained by available it may be possible to use ing the difference in the potential appropriate tests. These coatings magnesium anodes, a method which is thereby created between two other might be applied to the pipes prior to, also recommended where the section electrodes situated at given distances or at the time of, the laying of the of pipeline to be protected is so short between the two first. The survey pipeline, i.e. either in the workshop or that the installation of a rectifier also comprises the measurement of on the site. Given good workmanship, equipment would be uneconomic. pR values and the polarisation of the there is little to choose between these Special problems are encountered steel. two methods, but in situ application where a pipeline must be protected is usually more economic, as the choice against strong stray currents. In that In a study of possible sources of of the coating need, in this case, not case, special drainage devices may have corrosion, such as stray currents, be influenced by considerations con to be provided, e.g. in the vicinity of particular attention must be paid to cerning the mechanical protection of sub-stations of electrified railways. It direct current conductors with little the pipes during the transport. is sometimes difficult to determine the or no insulation against the soil (e.g. most suitable out of several alternative the rails of d.c. electrified railways; Where pipes are treated in advance, solutions because the electric potential electrolytic corrosion is particularly the coating must be carefully checked of the railway tracks may vary greatly active near railway sub-stations). A before the pipe is finally lowered into in terms of time and distance. The survey of this kind can be costly, but the trench. This can be done by drainage circuit may have to be con it may be simplified to a certain means of a special instrument, known trolled by automatically acting polarised extent by an expert interpretation of as the 'electric brush'; this is essen relays or equivalent devices to guard soil resistance data and geographical tially an electrode to which a high against the setting up of harmful data. voltage is applied which flashes over currents. where the coating is defective. Any With the aid of all this information, August 1955 CORROSION TECHNOLOGY 253
Anti-Corrosion Methods and Materials – Emerald Publishing
Published: Aug 1, 1955
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