FAQ: Can cement-mortar linings be repaired in the field?
Repair is achieved by first cutting out the defective or damaged lining to the metal so that the edges of the lining not removed are reasonably perpendicular to the pipe wall or slightly undercut. A stiff mortar is then prepared, containing not less than one part of cement to two parts of sand, by volume. This mortar is applied to the cutout area and troweled smooth with adjoining lining. To provide for proper curing of patches by preventing too rapid of a moisture loss from the mortar, the patched area is normally seal-coated immediately after any surface water evaporates, or alternatively the area is kept moist (e.g. with wet rags or burlap over the area or with the ends of the pipe or fitting taped over with plastic film, etc.). Of course, in potable water-related applications, no patch or curing components should be used in the repair that would negatively affect health or water quality.
FAQ: What is the difference between Cast Iron and Ductile Iron?
Cast iron is a generic name for any high carbon molten iron poured as a casting. When used to refer to pipe, cast iron (sometimes called gray iron) is a specific type in which the free graphite (Carbon) is in the shape of flakes. Cast Iron pipe were introduced into the United States in 1817.
Ductile Iron is a specific type of cast iron in which the free graphite is in the shape of nodules or spheroids. (Other names for ductile iron are nodular iron or spheroidal graphite iron.) Ductile Iron Pipe were introduced to the market in 1955.
Although nearly identical chemically, the two irons are quite different metallurgically. The now obsolete standard for Cast Iron Pipe (ANSI/AWWA A21.6/C106) required an iron strength of 18/40 (18,000 psi Bursting Tensile Resistance and 40,000 psi Ring Modulus of Rupture.) Although tensile testing was not a requirement of this standard, a tensile test of gray cast iron pipe would give a test result of approximately 20,000 psi Ultimate Tensile Strength, with no measurable Yield Strength or Elongation.
The current standard for Ductile Iron Pipe (ANSI/AWWA A21.51/C151) requires a minimum grade of 60-42-10 (60,000 psi Ultimate Tensile Strength, 42,000 psi Yield Strength, and 10% Elongation.) In addition, Ductile Iron Pipe manufactured under this standard are required to meet a minimum of 7 ft lbs impact resistance by the Charpy test. (Compare Gray Iron Pipe with an impact resistance of approximately 2 ft lbs or less.)
The difference in the physical properties of these two materials is attributable almost entirely to the difference in the shape of the free graphite. The shape of the graphite is determined at the instant of solidification and is made nodular by the addition of magnesium to the molten iron bath. Although Cast Iron was the best engineering material available for pipe production for nearly five hundred years, the development of Ductile Iron Pipe provides a far superior product.
FAQ: What is the purpose of the "testing allowance" in ANSI/AWWA C600?
ANSI/AWWA C600 "Installation of Ductile-Iron Water Mains and Their Appurtenances" requires that newly installed Ductile Iron water mains be hydrostatically tested at not less than 1.25 times the working pressure at the highest point along the test section and not less than 1.5 times the working pressure at the lowest point of testing.
After the air has been expelled and the valve or valves segregating the part of the system under test have been closed, pressure is then normally applied with a hand pump, gasoline-powered pump, or fire department pumping equipment for large lines. After the main has been brought up to test pressure, it is held at least two hours and the make-up water measured with a displacement meter or by pumping the water from a vessel of known volume. The make-up water is called the "testing allowance," and the allowable amount is a function of length of pipe tested, nominal diameter of the pipe, and the average test pressure. The hydrostatic pressure test helps to identify damaged or defective pipe, fittings, joints, valves, or hydrants, and also the security of the thrust restraint system.
The "testing allowance" is not a "leakage allowance." Properly installed Ductile Iron pipelines with properly assembled joints are bottle-tight and do not leak. The "testing allowance" is, however, a practical measure used to maintain the pressure, which might actually drop because of factors other than leakage, including trapped air, absorption of water by the cement lining, extension of restrained joints and other small pipe-soil movements, temperature variations during testing, etc.