Page images
PDF
EPUB

Sinks...

Lavatories.

Bath tubs..

Water-closet tanks.

SEC. 75. Water supply control. A main shut-off on the water supply line shall be provided near the curb. Accessible shut-offs shall be provided on the main supply line just inside the foundation wall for each flat or apartment of a building, for each lawn sprinkler, for supply to each hot water tank, and for each water closet.

SEC. 76. Water-supply pipes and fittings: Material. All water-supply pipes for a plumbing system shall be of lead, galvanized wrought-iron, or steel, brass, or cast iron, with brass or galvanized cast iron or galvanized malleable iron fittings. No pipe or fittings that have been used for other purposes shall be used for distributing water.

SEC. 77. Water supply, protection. All concealed water pipes, storage tanks, flushing cisterns, and all exposed pipes or tanks subject to freezing temperatures shall be efficiently protected against freezing.

SEC. 78. Relief valves. Wherever a check valve is installed on the coldwater supply pipe between the street main and the hot-water tank there shall be installed on the hot-water distributing system a suitable relief valve.

SEC. 79. Pumps and hydrants. All pumps and hydrants shall be protected from surface water and contamination.

Bibliography

(1) Recommended Minimum Requirements for Plumbing in Dwellings and Similar Buildings. United States Department of Commerce. Elimination of Waste Series. Washington. Government Printing Office, 1924.

(2) Report of the Special Plumbing Board of the Massachusetts Department of Public Health. Boston, 1920.

(3) The Need of Uniformity in Plumbing Regulations. George C. Whipple. Jour. N. E. Water Works Assn., 35, 2, 1921, p. 117-122.

(4) The Flush Valve. Gordon M. Fair. Jour. N. E. Water Works Assn., 35, 2, 1921, p. 137-150.

(5) Principles and Practice of Plumbing. J. J. Cosgrove. 3d edition. Technical Book Publishing Co., Scranton. 1922.

(6) Standard Practical Plumbing. R. M. Starbuck. Norman W. Henly Publishing Co., New York.

(7) The Prevention of Corrosion in Hot Water Supply Systems. C. R. Texter. Jour. Amer. Water Works Assn., 10, 1923, p. 764–772.

(8) The Water Supply, Sewerage, and Plumbing of Modern City Buildings. W. P. Gerhard. John Wiley & Sons. (1910?)

(9) Water Hammer. O. Simin. Proc. Amer. Water Works Assn., 1904, p. 341.

1 Lead-lined, cement-lined, tin-lined and copper pipe may also be included in this list.

CHAPTER XVIII

CORROSION OF PIPES

Corrosion is a process or combination of processes by which metals wear away. Two causal factors are requisite to corrosion: moisture or water, and air or oxygen. In the absence of moisture metals do not corrode; in the absence of oxygen they are attacked but slightly. The corrosion of metals usually covers them with deposits or incrustations of the metal in altered form (rust).

Water pipes are subject both to internal and external corrosion. The latter is sometimes the more serious of the two. Corrosion, more than any other factor or combination of factors, determines the efficient life of water pipe. In metal pipes corrosion disirtegrates the metal and reduces the wall thickness; in wood pipes it rusts the metal bands; in reinforced cement or concrete pipes it attacks the reinforcing wherever it is insufficiently protected.

The efficiency of water pipes varies with their carrying capacity and tightness. New smooth pipes convey more water under a given head than old rough ones. In time, corrosion "eats" through the pipes in places. They begin to leak and water is wasted.

The quality of pipe service depends upon the ability of the pipe to deliver the water to the consumer in the condition received. In flowing through a long pipe, water comes into contact with many metals and other materials. The metallic compounds worn away from the pipe surfaces by corrosion may dissolve in the water or be carried away in suspension.

Causes of corrosion

Corrosion is a surface phenomenon. Its causes are electrochemical in nature: chemical because corrosion is due to reactions of the metal with different elements or compounds in the water; electrical, because these reactions require a concurrent interchange of electrical charges. The theories by which corrosion may be reasonably explained are discussed below. Three main types of corrosion occur in water pipes: self-corrosion, galvanic corrosion, and electrolytic corrosion.

Self-corrosion. The most general type of corrosion is known as self-corrosion and takes place when a metal is brought in contact with water containing oxygen.

The tendency of metals and other solids to go into solution varies greatly. This is illustrated in the following table which shows the more common metals arrayed according to their "solution pressure." Each metal has a greater tendency to dissolve than the succeeding ones. Zinc, for example, has a higher solution pressure than iron, iron than lead, and lead than copper. The order of the elements in this table is not alone the order of their solution pressure, but, since, as we shall see later, solution of metals is the result of electro-chemical activity, it is also the order of electrical and chemical activity of the elements. Each metal is said to be more active, or electro-positive, than the succeeding ones.

[blocks in formation]

When a single metal, such as iron, is immersed in water free from impurities, including oxygen, it exerts its solution pressure by discharging into the liquid a certain number of electrically charged particles called ions. These metal ions carry a charge of positive electricity and therefore increase the positive electricity of the water while decreasing that of the metal which becomes negatively charged. The electrical balance is upset, the water has too much positive electricity, the metal too little. Equilibrium is regained, however, by the displacement from the water of an equivalent number of positively charged ions which migrate to the metal. These ions are hydrogen iors, the only positive ions that are present in pure water. Pure water is not an inert substance consisting of molecules of H2O. It has the power of dissociation and contains in addition to molecules of H2O a number of unassociated hydrogen ions (H) and

1 Solution pressure is tendency to dissolve.

hydroxyl ions (OH). The hydrogen ions carry a charge of positive electricity; the hydroxyl ions, an equivalent charge of negative electricity, so that an electrical balance is maintained. The hydrogen ions that have been displaced from the water and have migrated to the metal lose their positive charge to the metal, neutralize its negative charge, and themselves "plate out" on the metal as molecules of hydrogen in the form of a film of hydrogen gas, protecting it against further solution. In water free from oxygen this is as far as self-corrosion can proceed. The electro-chemical nature of the process of solution thus makes it self-inhibitory and the amount of metal dissolved would not ordinarily be detected analytically.

When oxygen is present in the water, however, conditions become very different. A chemical reaction occurs between the oxygen and the hydrogen film which combine to form water. The protecting film is thereby removed, the metal once more becomes exposed to contact with the water and as a result re-exerts its solution pressure. More metal ions go into solution, and self-corrosion continues as long as there are enough hydrogen ions to neutralize the metal and sufficient oxygen to remove the hydrogen film. It is seen that two elements are essential to self-corrosion, hydrogen ions and oxygen. The greater the number of hydrogen ions in water the more potentially acid it is; the fewer, the more potentially alkaline.

In pure water the positive metal ions discharged into the water do not exist long as such, but combine with the negative hydroxyl ions of the water and often with the oxygen to form an alkaline chemical compound which in the case of iron is the well-known rust. In water containing impurities the process of self-corrosion may be carried on with slight modifications.

Galvanic corrosion. Galvanic corrosion, sometimes called contact corrosion, occurs when two dissimilar metals in contact with each other are immersed in water. The mechanism of this second type of corrosion may be likened to that of an electric battery or galvanic cell. When two dissimilar metals are immersed in water, either in contact with each other or connected externally with a wire, only one of them discharges metallic ions into solution; the other receives the hydrogen ions which "plate out" on it.

2 Some writers claim that in natural waters oxygen acts more directly by interacting with the water and liberating positive electricity which neutralizes the negative charge of the metal.

When, for example, a strip of zinc and a strip of copper, that are in contact with each other, are immersed in water, a current of electricity flows from the zinc through the water into the copper because the zinc is the more electro-positive metal and possesses the greater solution pressure. The zinc becomes negatively charged, but is neutralized by the return current from the copper so that it is enabled to exert its solution pressure again and again; it thus corrodes. The hydrogen, on the other hand, plates out on the copper and forms a film which, if not removed by combining with the oxygen of the water, will prevent the current from reaching the copper. Corrosion will then be retarded and finally inhibited. This is called polarization of the copper. In extreme cases the evolution of hydrogen is so rapid that bubbles escape, thus depolarizing the copper and permitting galvanic action to continue.

It is seen that the galvanic current accelerates the electro-chemical activity of the more electro-positive metal and its consequent corrosion. The greater the conductivity of the water, the less its obstruction to the flow of the galvanic current and the more rapid the corrosion.

Galvanic corrosion plays an important part in the disintegration of water pipes where it may be due to any of the following factors: contact between pipes or fittings of different materials; impurities in the metal itself; mill scale on the pipe surface; segregation of the constituents of the pipe by improper manufacturing methods; poor application of metallic pipe coatings; use of unsatisfactory materials for back-filling pipe trenches. In each case galvanic currents are generated between the different metals or substances and the more electro-positive one of any couple is corroded rapidly. The action under galvanic corrosion is not necessarily confined closely to the point of contact. It may extend to some distance on either side.

Electrolytic corrosion. Electrolytic corrosion occurs in water pipes whenever a current of electricity which has found its way into a pipe flows from the pipe into the moist earth or into a less resistant conductor. Corrosion is confined to the spot where the current leaves the pipe and differs from galvanic corrosion in that the current is not generated in the pipe itself but is obtained from an outside source, such as stray currents from the return circuit of a street railway system.

« PreviousContinue »