Some Notes on Electrolysis of Water Pipes
Little Done to Remedy Trouble-Pipes Damaged Where Current Leaves—Pipe Drainage Partial Alleviation—Insulated Return Rest
ONE of the troubles with which the water works superintendent has to deal is that of the wasting and corrosion of the pipes of his distribution system by the passage of stray electric currents through them. Electrolysis caused by such currents from nearby electric railroads is so common a problem that the following article will be found of interest by in any heads of water works departments:
The flow of electricity through an electric circuit may be roughly compared with the flow of water through a hydraulic circuit. For instance, to maintain a uniform current or flow of water through a pipe and to at the same time elevate the water from a sump “a” to a point “b,” from which it may be supposed to flow back continuously to the sump, requires a certain amount of power applied at the pump. The static head (a to b), plus friction and mechanical losses, may he termed the difference of potential. To maintain a steady electric current of given amperage through an electric circuit and to raise the electricity through a difference of potential of so many volts, requires the application of a certain amount of power at the electric generator.
If no pipe, or a pipe of insufficient size, is provided to convey the water back to the sump, all or a portion of it will return over and through the ground, choosing the path ot’ least resistance. If no suitable wire is provided for the return flow of electric current to the generator, to complete the circuit, the current will return through the ground, through buried pipes or any other conductors it may find, choosing the path of least resistance.
Little Done to Insure Proper Negative Return
Taking an electric car line, the current is discharged by the generator into the trolley wire, from whence it passes down the trolley, through the motor of the car to the rails. It has then in some manner to return to the power house to complete the circuit. A suitable insulated wire, connected at frequent intervals to the rails at once suggests itself as the most effective means of completing the circuit and stringent regulations along such lines were adopted in Great Britain soon after the introduction of the electric tram. Many cities of continental Europe followed suit. In our own country, however, nothing really effective was done to insure a proper negative return. A portion of the current will return through the steel rails of the tracks, if these are carefully bonded, hut the imperfect conductor so provided does not prevent the return of annoying stray currents through the earth, through buried steel and iron pipes, etc.
Pipes Damaged Where Current Leaves Them
One of the methods of improving rail return, adopted in this country, is “pipe drainage.” It pretends to nothing more than partial alleviation and consists in connecting buried water and gas pipes, etc., to the rails by copper cables, in places where the electric current tends to leave the former. It is evident that this transforms the pipes into negative returns, to a large extent, causing damage near joints which may happen to offer a high resistance and facilitating the passage of current from one pipe line to another where the “drainage” may happen to he unequal. This is explained more fully further on. It is at those places where the returning current. having entered into and traveled along the pipes, leaves them, that the pipes are damaged.
“The insulated negative feeder wire affords the only fundamental means for the prevention of electrolysis. Insulated negative returns have been installed in some American cities, but no general action in this direction has been taken by street railway companies. The reason for this is, of course, the expense involved.”
The action is somewhat similar to that occurring in an electric battery, where the cathode is corroded as the current leaves it. The battery analogy must not he carried too far. however, as the local formation of a true galvanic battery occurs only under special conditions, where the galvanic action produces “selfcorrosion” of the pipes, apart from electrolysis caused bv stray currents from railway systems.
The point is that a buried water pipe in wet soil will be deeply pitted where the current leaves it. if there be sufficient amperage. The amount of corrosion has been estimated for iron pipes at about 15 to 25 pounds per ampere per year. There is a marked tendency for the current to be discharged locally, which produces pitting instead of uniform corrosion, and a small hole may be eaten entirely through the pipe.
“Pipe Drainage” Does Not Prevent Discharge
At outlying points of the trolley system, the current tends to leave the rails and return through the soil and buried pipes, flowing in the general direction of the generating station. As the station is approached. the currents in the ground and pipes tend to return to the rails. Theoretically, it is in this positive area near the station, where the currents flow from the pipes, that damage is done to the pipes. Actually, however, the trouble is not confined to the so-called positive area and many places will be found where, owing to high resistance at a pipe joint, the current leaves the pipe for the earth and returns to the pipes beyond the joint. Also, there may be two systems of pipes, as gas and water, both in the negative territory and negative to the railway return, but one more strongly negative than the other. Currents will then be discharged from the pipes having the greatest resistance to those offering less, causing corrosion of the former. “Pipe drainage,” unless equal, which is impractical, will not prevent this; in fact, pipe drainage may facilitate corrosion in the negative area through this interchange of current by pipes.
On the part of the street railway companies, little has been done to prevent electrolysis, although there are some gratifying exceptions. Attention has been given mainly to more effective rail bonding and pipe drainage. Troubles from electrolysis have grown with the years and agitation for some decisive action, suspended during the war, is now acute.
Some Methods of Partial Prevention
Among the methods of partial prevention which have been investigated or used are (1) covering the pipes completely with an insulating coating, (2) chemical protection, (5) cement coatings, (4) cathodic protection, so called, (5) conducting coatings, (6) electric screens and (7) insulating joints in pipes.
It may be fairly said that insulating paints and coverings for pipes have proven of little value. Tests by the 1′. S. Bureau of Standards, covering paints, dips and wrappings, showed early breakdowns for paints, with somewhat better results for dips and wrappings. The conclusion arrived at was. “as far as paints are concerned, however, there can be no question but that as they are at present applied they are not only of no value when applied only in positive areas, but they may do actual harm by concentrating the current discharge on a comparatively small portion of the pipe surface, thus giving rise to rapid pitting. On the other hand, such coatings are unquestionably of great value in preventing selfcorrosion in the case of pipes not subject to electrolysis from stray currents.”
Chemical protection consists in saturating the soil around the pipes with lime or some other chemical which will tend to alter the chemical composition of the soil and so prevent the passage of current to and from the pipes. Unfortunately, the chemicals, as lime for instance, must be and remain practically pure. In practice, conversion into sulphates, carbonates, chlorides, etc., takes place rapidly and the protection is then gone.
Cement coatings have been shown by test to possess little value. Where cement coatings have been apparently protective, cement joints have been used, and comparative tests of cement-coated pipes laid with lead and cement joints respectively, have shown that it was the cement joints and not the cement coating which afforded the protection. After two years of service, excavations showed the cementcoated pipes laid with lead joints to have suffered considerably.
Cathodic protection is attempted by devices calculated to maintain the pipes at all times negative to the earth. One method is the discharging of an alternating current into the pipes (street railway currents are direct) to produce a strong hydroxide solution in the soil, tending to render the iron passive. Another method consists in connecting a low-voltage generator between the pipes and the rails. These schemes, as well as most others hereinbefore described, are more interesting than practical.
(Continued on page 436)
(Continued from page 408)
Conducting coatings are pipe coverings which are intended to receive the current from the pipe by metallic conduction and transfer it to the earth without injuring the pipes. The cost of any effective noncorrodable covering is sufficient to remove conducting coverings from further consideration.
Electric screens are masses of metal to be placed between pipes and track, connected by wires to the pipes. The current leaves the pipes by way of the wire and enters the earth from the screen. It is the screen which is then corroded, instead of the pipe. This is successful, hut the prohibitive cost prevents the extensive use of screens.
Insulating Pipes Offer Most Practical Treatment
Insulating joints in pipes are generally believed to offer the most practical treatment, aside from the definite prevention of electrolysis by the provision of suitable negative returns by the railway companies. Where insulating joints have been used by gas companies, cement joints have been largely resorted to. Lead joints offer considerable resistance, but long experience has shown that the resistance is not sufficient to reduce the current to a safe value. Gas mains carry low pressures and carefully made cement joints, appear to be effective. Cement joints for water mains are a very different matter. If joint insulation is to be generally effective, and not localized in action, such joints must be placed at frequent intervals. The writer has used Leadite, partly for the purpose of insulating joints. Tests by the Bureau of Standards on parallel mains, one jointed with lead (16 inches diameter) and the other with Leadite (12 inches diameter), showed a current in the lead-joint main one hundred times that in the main jointed with Leadite. Another test of a 12-inch Leadite main and a 16-inch lead main, showed one hundred and twenty times more current in the main having lead joints. A comparison between a 12-inch Leadite main and a 4-inch lead main, showed over six times more current in the lead-jointed main, despite the great disparity of size. The mains so tested had been in service for periods varying from two to ten years and those jointed with Leadite had given no trouble during that time.
Taken by themselves, these tests would indicate that a cheap and effective remedy is available for water works men. Personally, my experience leads me to believe that while some jointing materials and special couplings are effective when first installed, the effectiveness of most of them does not last long.
From this I would except the special jointing length of wood staves, built into the line. Its makeup limits it to occasional joints, and there may be objection to the use of wood, but it appears to offer lastin, effective resistance and the insulation provided is by no means localized. Too occasional insulation means, of course, that considerable current may be picked up between joints, which will leave the pipe at the insulated joint, to the detriment of the pipe.
Insulated Negative Return Best Method
From what has preceded, it will he evident that the insulated negative feeder wire affords the only fundamental means for the prevention of electrolysis. I disregard three-wire systems, etc., on the score of cost. Insulated negative returns have been installed in some American cities, but no general action in this direction has been taken by street railway companies. The reason for this is of course the expense involved. It is claimed by some investigators that the mitigation of electrolysis may be so undertaken by a railway company as to effect operating economies which will more than suffice to justify the expense involved, and that in very few cases would financial hardship result. As to this the writer professes no knowledge. If it is a fact, then the railway companies should be forced to provide adequate returns. If it is not, it would seem that some fair arrangement might be arrived at whereby parties benefiting would share the expense.