A recent water pipe project at a major university involved installing dual 36-inch DI pipelines underneath a critical building on campus. The lines were installed to transport water to a chiller system to cool other buildings.
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The piping material selected for this project was Ductile iron because of its ability to withstand ground shift and heavy pressures from the structure that rests above it. PVC, being weak in nature, tends to burst into small pieces due to heavy pressures versus stronger materials such as DI that will not obliterate.
PVC failures also have a history of being catastrophic. That is, not just a loss of some water or water pressure but a TOTAL LOSS of water. This instability is a severe issue for seismic prone areas. Not only is there a threat of damage from earthquakes, but also vulnerability to destruction from fires that typically follow the earthquakes. If there’s no water, there’s no fire protection.
PVC pipe can also be adversely affected in areas prone to wildfires. For example, the City of Santa Rosa, California estimates the repair of the contaminated Fountain Grove water system could take up to two years. Test results conducted on PVC pipelines indicated they absorbed benzene and other chemicals, resulting in an advisory to residents to not drink or bathe in the water. Replacement costs are estimated at more than $43 million dollars.
Permeation
DI pipe is impermeable and protects the water supply from toxic infiltration. The C909-16 Molecularly Oriented Polyvinyl Chloride Standard states that polyvinyl chloride may be subject to permeation by low molecular weight organic solvents or petroleum products. Why take a chance with a permeable material?
A brief review of the older specifications in chronological order may help define their usefulness, as well as help in the appreciation of the improved modern standards.
The basis for design in almost all specifications to date is the Barlow formula, or "Hoop Stress" formula. It embodies the basic principle for design of a cylinder for internal pressure. The formula may be stated as t= PD/2S in which t is the thickness of the pipe in inches; P is the internal pressure in pounds per square inch (psi); D is the outside diameter in inches; and S is the allowable working stress of the metal in pounds per square inch.
In the development of the design of cast iron pipe, this formula has been modified in several ways by prominent water works engineers such as Allen Hazen, Thomas H. Wiggin, James T. Fanning, Dexter Brackett, I. J. Fairchild and James P. Kirkwood. Mr. Kirkwood, as chief engineer for the Brooklyn Water Works, developed a design for cast iron pipe that was a variant of the Barlow formula. Kirkwood’s calculations took into consideration casting imperfection, strength of the metal and other factors affecting the life of the pipe. In the late 1880s, a formula by Dexter Brackett, distribution engineer for the City of Boston, was adopted by the New England Water Works Association as its standard.
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Although the 1902 NEWWA standards did not provide a formula for pipe thicknesses, the Brackett formula was used in determining the thicknesses recommended.
In 1908, AWWA adopted a standard covering bell and spigot pipe produced in 12-foot laying lengths by the pit casting method. Prior to 1908, at least two unofficial documents dealing with pipe design were acknowledged by AWWA. The first of these used thicknesses for pipe determined by averaging the thicknesses used in a large number of American cities. The second dealt with actual design of pipe based on Brackett’s method with variations.
The 1908 AWWA standards employed a system of class designations applied to specific wall thicknesses in diameters 4" to 84" inclusive for a range of hydraulic heads. The most common of these classes were A, B, C and D for 100, 200, 300 and 400 feet hydraulic head, respectively. The design was based on a variation of the Brackett formula by J. T. Fanning and included a variation in the outside diameter for the different classes of pipe. The basic design of pipe with a different outside diameter for each class was followed in modern specifications until the 1961 revisions. The general acceptance by the water works industry of the standardized mechanical joint necessitated a universal outside diameter for cast iron pipe.
AWWA revised its standards in 1939 to incorporate a new method of designing cast iron pressure pipe. This new method was published as ANSI A21.1. The A21.1 method of determining the required thickness of cast iron pipe took into consideration trench load and internal pressure in combination. Trench load consists of the earth load on the pipe plus any transient load resulting from traffic over the trench; internal pressure consists of the design working pressure plus an additional allowance for surge pressure. Laying conditions and properties of the iron in the pipe are also factors involved in the design. Additions for casting tolerance are included in the design thickness. With the advent of ductile iron pipe and its flexibility, this additive method of design became obsolete. As noted in the following paragraph, ductile iron design employs flexible conduit principles since the internal pressure relieves the external load.
Actually, the first standard covering centrifugally cast pipe was issued by the U.S. government in 1927, and was known as Federal Specification No. 537. In July 1931, the specification was revised to include pipe cast centrifugally in sand-lined molds, pipe cast centrifugally in metal molds and pit cast pipe. This specification has been modified several times and is now basically the same as ANSI/AWWA Standards.
Development of ductile iron in the 1950s initiated research into design of ductile iron pipe to take advantage of the superior flexibility, strength, toughness, impact resistance and corrosion resistance of this new metal. The A21 Committee issued the ANSI A21.50 (AWWA H3-65) and ANSI A21.51 (AWWA C151) Standards for ductile iron pipe in 1965. The work of M. G. Spangler and others at Iowa State University on flexible conduit is the basis for principles that have been applied extensively by the designers of flexible underground pipe. The design principles and procedures for ductile iron pipe that were included in the ANSI Standard A21.50 (AWWA C150) were verified by trench tests at AMERICAN and tests conducted by various researchers. AMERICAN’s former Technical Director Dr. Ed Sears was instrumental in these developments.
Continued research on ductile iron pipe reflects through these updated standards the advancements in metallurgical technology and manufacturing skills. Furthermore, the quality of AMERICAN’s products and conformance to appropriate specifications are assured by the British Standards Institute’s certification that AMERICAN’s quality system complies with ISO 9001 Quality Management System Standard.
AMERICAN also subscribes to NSF’s listing program for products under ANSI/NSF Standard 61 — Drinking Water System Components — Health Effects. Check with AMERICAN for current listing of our products.
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