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DUCTILE IRON PIPE VERSUS PVCO PIPE

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DUCTILE IRON PIPE VERSUS PVCO PIPE
By Richard W. Bonds, P.E., DIPRA Research/Technical Director

Introduction
Design engineers face a myriad of decisions when designing and specifying a potable water pipeline, including initial cost of the system, operating requirements, maintenance costs, dependability, and long-term performance. This brochurecompares the short- and long-term structural and performance attributes of Ductile Iron pipe and molecularly oriented polyvinyl chloride (pvco) pipe. It provides valid current information to engineers who must determine a basis for selecting piping materials. In addition to providing physical test data comparing the two pipe products, this article also compares applicable AWWA design standards foreach pipe, including ANSI/AWWA C150/A21.501 for Ductile Iron pipe and ANSI/AWWA C9092 for pvco pipe. The following data is drawn from several sources, including AWWA standards, published information from pipe manufacturers and associations, and physical testing from the Ductile Iron Pipe Research Association, Structural Composites Inc., and Plastics Engineering Laboratory.3 The tests reported in thisbrochure were conducted on 6-inch and 12-inch diameter Pressure Class 350 Ductile Iron pipe (the lowest Pressure Class available), and 6-inch and 12-inch diameter Pressure Class 150 pvco pipe. An attempt was made to obtain and test Pressure Class 200 pvco pipe (the highest rated in ANSI/AWWA C909); however, it was not available. This brochure presents sound engineering information that will provethat all materials are not equal.

DUCTILE IRON PIPE VERSUS PVCO PIPE

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Comparison Of Standards
The following table compares the requirements of ANSI/AWWA C150/A21.50 and ANSI/AWWA C151/A21.514 to ANSI/AWWA C909.

TABLE 1 Comparison Of Ductile Iron Pipe And pvco Pipe Standards
TOPIC Ductile Iron Pipe ANSI/AWWA C150/A21.50 ANSI/AWWA C151/A21.51
3”- 64” 18’, 20’ Rated up to 350 psi.Pressure Class 150, 200, 250, 300, & 350. Higher pressures may be designed. Designated as a flexible conduit. Separate design for internal pressure (hoop stress equation) and external load (bending stress and deflection). Casting tolerance and service allowance added to net thickness. Pressure Class: stress due to working pressure plus surge pressure cannot exceed the minimum yield strength of42,000 psi ÷ 2.0 safety factor. Nominal surge allowance is 100 psi (based on an instantaneous velocity change of approximately 2 fps), however, actual anticipated surge pressures should be used. Prism load + truck load. Ring bending stress limited to 48,000 psi, which is 1/2 the minimum ultimate bending strength. Deflection is limited to 3% of the outside diameter of the pipe, which is 1/2 of thedeflection that might damage the cement-mortar lining. The larger of these two thicknesses governs and is taken as the net thickness. AASHTO H20, assuming a single 16,000 lb. concentrated wheel load. Impact factor is 1.5 for all depths.

pvco Pipe ANSI/AWWA C909
4”- 12” 20’ Rated at 100, 150, & 200 psi at a service temperature of 73.4°F. For service temperatures greater than 73.4°F, the pressurerating must be appropriately reduced. Designed as a flexible conduit. Separate design for internal pressure (hoop stress equation) and external load (deflection) – external load design is not covered by a standard. No consideration for bending stress. Pressure Class: stress due to working pressure plus surge pressure cannot exceed the Hydrostatic Design Basis (7,100 psi) ÷ 2.5 safety factor(Hydrostatic Design Stress=2,840 psi). 23, 27, or 31 psi surge allowance for pressure class 100, 150, & 200 psi respectively. Based on an instantaneous velocity change of approximately 2 fps. Design not covered in the standard. Reference is made to AWWA M23 for design procedures. Prism load + truck load. Utilizes the Modified Iowa Deflection Equation, however, provides no deflection limits for design...
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