ELECTROLYSIS —TROUBLES CAUSED THEREBY AND REMEDIES WHICH MAY BE APPLIED
(Continued from page 187)
The current survey consists of measurements of current flow on pipes at selected locations in the piping system. To determine the current flowing on a pipe, the voltage drop between two points on a continuous length of pipe is measured by means of a millivoltmeter. From this voltage drop and from the resistance of the included pipe, which may be obtained from published tables, the strength of the current is computed. To measure this drop it is necessary to expose the pipe and to make good electrical contact between the millivoltmeter leads and the pipe. The best contact for these current measurements is obtained by soldering the connecting wires directly to the pipe or to two brass plugs screwed into the pipe. This method is particularly advantageous when readings are to be taken over a considerable time, and it is customary to use rubber-covered wires and bring them to the surface of the street, leaving the ends in service boxes which then form permanent stations for electrical measurements. This is exceedingly convenient because it is then possible to make current measurements on the pipe without again making an excavation. Such permanent contact wires are illustrated in Fig. 3. It should be noted that small potential differences, such as 0.1 millivolt or less, may be caused by local galvanic or thermal action. Where such small values are found in a test for drop on a pipe a careful investigation should be made to ascertain whether the observed potential difference is actually drop due to current flow.
Since current destroys metal only where it leaves for soil, it is important to know where the current leaves a pipe. Current measurements on pipes are, therefore, frequently made at two or more stations simultaneously in order to determine the change, in current on the pipe between the stations. For these current measurements indicating or recording millivoltmeters are employed. By comparing the characteristic variations of currents and voltages measured for twenty-four-hour periods at selected locations with the characteristics of the neighboring electric railways, it is often possible to identify the source of the stray current flowing on a pipe. From a study of the results of the potential and current surveys it can be determined where current is leaving the piping, and at a number of such points excavations should then be made and the exposed pipe examined with a test hammer for corrosion of the ion. Where corrosion and pitting is found at points where current is leaving the pipe, it may be taken as evidence that at least part if not all of the corrosion has been caused by electrolysis.
Remedial Measures Applied to Pipes.
Attempts have been made to protect underground pipes from electrolysis by insulating them from earth by paints or dips. Practical experience as well as a large number of tests have, however, shown that no dip or paint will permanently protect a pipe against electrolysis in wet soil. It has been found that in most cases the applied coatings have either been completely destroyed by the effects of the wet soil and the electric currents, or defects in the coating have developed, causing concentrated corrosion at such defective spots. Where it is attempted to apply a heated material like pitch or asphaltum to a cold pipe, it is impossible to completely cover the pipe. Pitch and similar compounds have been applied to pipes with wrappings of jute or of some similar material. A number of layers can be applied in this way so as to build up any desired thickness of insulating covering. Such covering if sufficiently thick will afford protection against electrolysis, provided that it is mechanically perfect. The great difficulty in practice is to apply such a covering without leaving defective spots through which moisture will have access to the metal of the pipe.
Pipes which are covered with imperfect insulating coatings, or coverings exposing bare spots of metal, are in much greater danger from electrolysis where positive to earth than are bare pipes, for the reason that the stray currents will leave only from these bare spots and here produce concentrated corrosion. One form of insulating covering which appears to afford complete protection is a layer of from one to two inches of a material like coal tar pitch, parolite or asphaltum, of such a grade that it is not brittle, and so will not crack, but yet is hard enough to remain in place. Embedding a pipe in cement or concrete, even if this is several inches in thickness, will not protect it from electrolysis, because damp cement or concrete is an electrolytic conductor. Insulating covering, even if imperfect, is useful on pipes in negative districts where current flows from earth to the pipes, because this covering will increase the resistance from earth to the pipes and thereby correspondingly reduce the amount of current reaching the pipes. Current flow on metallic pipe lines can be practically prevented by using a sufficient number of insulating joints. Insulating joints in pipe lines should not be confined to the positive areas, but should be installed in all places along the pipe line where there is any considerable potential gradients in the earth parallel to the pipe. The frequency with which insulating joints must be installed in a pipe line in order to assure reasonable protection from electrolysis depends upon the potential gradient through earth and upon the electrical resistivity of the earth. Where service pipes are endangered by current which flows to them cither from the main or from house piping, such current flow can be prevented and the service pipes protected by placing insulating joints in them at the main or in the building, or at both locations. Insulating covering and insulating joints can be applied in special cases to individual pipes, but cannot ordinarily be applied in an extensive manner to a piping network.
A method of mitigating electrolysis which has frequently been employed in this country is pipe drainage. This consists of connecting the pipes to the railway return circuit at a sufficient number of points to render the pipes at all points negative to the electric railway tracks. Once electrical drainage is applied, it is usually found that thereafter everything is left to take care of itself, and no attention is paid to the railway return circuit. When electrical drainage is applied to a single system of underground pipes, without making a complete investigation of the effects of possible high resistance joints, etc., the installation may be made at relatively small cost, and when so applied it usually relieves the acute danger from electrolysis in the immediate neighborhood where the drainage connections are made. Both of these considerations have served to favor the electrical drainage system. However, a single drained underground piping system becomes a source of serious danger to other systems. If electrical drainage is applied comprehensively to all underground metallic systems, it will not only be found very expensive to install, but, likewise, expensive to maintain. A further and perhaps the most serious objection to the drainage system is that sufficiently complete tests cannot be. practically made to determine whether the drained system is safe or is still in danger from electrolysis.
In future installations of underground piping systems in the neighborhood of electric railways, precautions should be taken to minimize the flow of stray current to the pipes. To this end the pipes should be laid as far from the electric railway tracks as practicable. Metallic contacts with the tracks, .such as may exist at the iron gate boxes used in water piping systems, must be carefully avoided. Where the pipes cross steel bridges carrying electric railway tracks in metallic contact with the bridge structure, the pipes should be supported on wooden blocks or otherwise insulated from the metal of the bridge structure. Insulating joints should be installed at the entrance of pipes to car barns.
Remedial Measures Applied to Electric Railways.
The only way to entirely prevent electrolysis from stray railway currents is to prevent leakage of currents to earth from electric railway systems by the use of a separate and completely insulated return conductor, instead of using the running tracks as part of the return circuit. This is accomplished with the double underground trolley system, and with the double overhead trolley system. These systems, while entirely effective in preventing electrolysis, have, however, not been generally adopted, probably because of the added expense and of the added complication involved over the ordinary single-trolley system, While leakage of current from singletrolley electric railways cannot be entirely Drevented by any methods that can be applied to these railways, the amount of stray current produced by a single-trolley railway can by adequate measures be reduced to any desired minimum values. The direct cause of stray currents front electric railways is voltage drop in the running tracks. It is, therefore, clear that by reducing this voltage drop the stray currents leaking from such tracks will be correspondingly reduced. The reduction of stray currents through earth can best be accomplished by the following means, given in the order of their importance:
- By increasing the number of directcurrent supply stations in systems extending over large areas, so as to reduce the radius to which any one station supplies current, and also by supplying all of the railways in any locality from one supply station in this locality.
- By increasing the electrical conductance of the tracks, through the use of heavy rails, through the use of low-resistance rail joint bonds and cross bonds, and through the interconnection of the electric railway tracks of all systems, where these come close together.
- By removing current from the tracks by insulated return feeders, and by maintaining the negative bus-bar insulated from ground at the supply station, in all cases where the voltage drop in the tracks would otherwise be excessive. This arrangement is known as the insulated track feeder system, or the insulated return feeder system.
- By increasing the resistance between tracks and earth as much as practicable, through draining the roadbed, and, on private right-of-way, through maintaining the rails out of contact with earth.
Where a road operates on a private rightof-way, the rails can often be practically insulated from ground and the escape of current from the tracks prevented. For surface roads this can be accomplished by placing the rails on wooden ties above ground and using broken stone for ballast and keeping the rails out of contact with ground. In the case of railway lines operating on elevated structures, the rails can be fastened to wooden ties and kept out of contact with the structures. These rails, supplemented where necessary with negative feeder cables, also insulated from the structure, can then be used for the return conductor.
It has been proposed to employ a 3-wire system for distributing current to an electric railway. The practical effect of the 3-wire system is to very greatly reduce the track voltage drop and correspondingly reduce the total amount of corrosion from electrolysis. In addition to this the remaining corrosion from electrolysis is distributed over widely scattered areas.
The insulated track feeder system, in conjunction with proper track bonding, usually affords the most feasible means for reducing track voltage drop and thereby reducing stray currents through earth in an existing electric railway. In this system feeders insulated from earth are connected from the negative bus-bar to selected points on the track network. The stray currents which leak from the rails to earth concentrate in earth and on the underground piping in the neighboring of the railway power station, where they must return to the rails to get back to the negative bus-bar. If connection between the negative bus-bar and the rails at the power station is removed, and the currents collected from the rails at points near the center of each railway line by means of insulated track feeders, the concentration of current in the neighborhood of the power station is entirely removed. With this arrangement the current used by each individual line tends to flow away from the rails at both ends of this line, and toward the rails near the center of the line. The insulated track feeder system is frequently confused with the system of paralleling the tracks with return feeders, which has been most commonly used in American electric railways. From the standpoint of reducing track voltage drop the two systems are, however, totally different. With copper feeders paralleling the tracks, the voltage drop in the tracks ⅛ reduced only in the proportion that the conductance of the track circuit is increased. With the insulated track feeder system the voltage drop in the insulated feeders does not occur in the tracks nor in the earth, and therefore may be made as high as economy dictates.
Experience shows that an increasing amount of damage by electrolysis is occurring on underground piping systems in many localities throughout the country where adequate measures have not been taken to reduce this damage. The principal and generally the sole sources of stray electric currents causing this damage are the single-trolley direct-current electric railways employing the running tracks in contact with earth as part of the return circuit. Experience extending over many years in foreign countries and over ten years in this country has shown that practicable and economical methods of construction can be applied to such electric railway systems which will remove acute dangers from stray currents to underground piping systems and which will greatly reduce the electrolysis danger in all cases, and in most cases will make this danger negligible. Mitigating methods applied to underground pipes fail to attack the source of the trouble and should be applied only in special cases, if at all, and then only after adequate methods of minimizing the production of stray currents have been applied to the railway system. Metallic connections from underground water pipes to the railway return circuit which cause these pipes to become a substantial part of this return circuit are inadequate for the protection of the pipes and are frequently dangerous. Such connections greatly increase current flow on pipes, and while they may afford local protection, they generally distribute electrolysis troubles to other localities where they are more difficult to find, and in this way frequently give a false impression of immunity. Metallic connections from water pipes to the railway return circuit should generally not be permitted and in no case unless a careful study of conditions has shown that no serious danger will be produced. Such connections should never be applied to an underground piping system as the principal means of electrolysis mitigation.
In view of the fact that the railway companies in common with the pipe-owning companies are public utilities operating under public franchises and utilizing city streets, it is the duty of both of these utilities to co-operate in order that the causes and extent of any danger from stray currents can be more readily ascertained. Further, the satisfactory solution of the electrolysis problem is one which requires the co-operation of all of the interests concerned. Electrolysis is an engineering problem and can be handled by engineering methods in such a manner that no hardship need be imposed nor should be imposed on any one. There is no reason why the negative feeder system should not be laid out along the same engineering lines as the positive feeder system. I think that if the electric railway companies would realize this and the owners of underground properties would co-operate in a practical way, we could obtain a satisfactory and practical solution of the electrolysis problem. For instance, it often happens that the judicious installation of a few insulating joints will save a lot of money in railway track feeders, and in such cases such joints should be installed.
A most important step towards securing the co-operation, which is absolutely necessary in order to obtain adequate and permanent relief from electrolysis, has been made by the formation of the American Committee on Electrolysis. This committee includes representatives of the electric railway, water, gas, electric light and telephone interests. This committee was organized in 1913 and has completed a preliminary report setting forth the facts regarding electrolysis, upon which the representatives of all of the varied interests have agreed. The committee has already accomplished a great deal towards producing a closer co-operation between the interests owning the electric railways and those owning the underground structures, and it is to be hoped that the future work of this committee will result in the unanimous adoption of recommendations which will reasonably safeguard underground piping systems against electrolysis.