For more than 50 years, ductile iron

Transcription

1 For more than 50 years, ductile iron (DI) pipe has been the most common material for water transmission and distribution systems. Strength, durability, and reliability make DI pipe the industry standard. A ferrous material, DI pipe is subject to corrosion when installed in aggressive environments. When corrosive environments are encountered and identified, the iron pipe industry recommends appropriate corrosion control. The most commonly used form of corrosion control for DI pipe involves the use of polyethylene (PE) encasement. 1-4 An American Water Works Association (AWWA) Engineering and Construction Division survey reported 95% of the utilities polled used PE encasement for their corrosion protection of DI pipe. 5 Many tools and procedures are available to help in the identification of corrosive soil conditions and their subsequent consequences. Parameters such as soil resistivity, ph, moisture content, oxidation-reduction potential, sulfi des, chlorides, sulfates, etc., can all be measured and evaluated by the experienced corrosion engineer. 6-8 After pipelines are installed, there are also tools to evaluate the condition of the pipe. Water utility break records and maintenance reports can give valuable insight concerning the condition of a pipe system. Survey methods and techniques such as pipe-to-earth potential measurements, close interval surveys, cell-to-cell surveys, side-drain technique, etc., have also been used DI pipelines generally are not electrically continuous because of their rubber-gasketed, bell-and-spigot installation design. For this reason, test stations generally are not installed for DI pipeline systems. These are important considerations in choosing and evaluating a condition survey procedure for DI pipelines. Presented are case histories of condition assessment investigations at three CASE HISTORY Investigating Polyethylene- Encased Ductile Iron Pipelines DANIEL W. CRABTREE AND MARK R. BRESLIN, Ductile Iron Pipe Research Association, Birmingham, Alabama Tis article presents several case histories where ductile iron (DI) pipeline sections have been investigated to ascertain the corrosion control benefits of polyethylene (PE) encasement. Te procedures and results of the investigations are evaluated and discussed. Tese investigations demonstrate the overall effectiveness of PE encasement as a corrosion control system for DI pipe. October 2008 MATERIALS PERFORMANCE 49

2 Investigating Polyethylene-Encased Ductile Iron Pipelines FIGURE 1 Typical cell-to-cell potential survey graph. Theoretical anodic areas of the positive-to-negative shift were chosen as excavation/inspection sites for all of the investigations. FIGURE 2 Twenty-year-old PE-encased 16-in DI pipe at an anodic location from the cell-tocell survey in San Antonio, Texas. prominent water utilities that practice corrosion control of their DI pipe systems: BexarMet Water District, San Antonio, Texas; Charleston Water System, South Carolina; and Onondaga County Water Authority, Syracuse, New York. The predominant system of corrosion control utilized by these three utilities is PE encasement in accordance with the ANSI/ AWWA C105/A21.5 Standard. 6 Procedures At each of the investigation sites, there were minimal traffic control issues and reasonable excavation access. Utility personnel had determined the location of the pipeline at each site and fl agged it accordingly. All other underground utilities in the vicinity of the investigation route were also located and identifi ed. Discussions with other utilities personnel and field observations determined that no sources of potential stray current were in the general area. Since these pipelines, like most DI pipelines, were not installed with jointcontinuity bonds or test stations, cell-tocell potential surveys and side-drain techniques were performed to search for corrosion. The cell-to-cell survey intervals were 5 ft (1.5 m) directly over the pipeline, with a perpendicular distance of 10 ft (3 m) on both sides of the pipeline for side-drain measurements. A high impedance voltmeter and two matching copper/copper sulfate (Cu/CuSO 4 ) half cells were used for the survey. Data from the survey were directly entered into a notebook computer and used to generate a fi nished graph for evaluation and to select locations for excavation. In theory, the cell-to-cell potential survey is intended to identify areas of active corrosion that would correspond to locations where the PE encasement was damaged. On the survey graph, such areas should be located where the cell-tocell potential shifts from positive to negative. All excavation/inspection sites were generally from locations on the survey graphs with the greatest magnitude of the positive-to-negative shift (Figure 1). In the area of specific condition assessment locations, the in situ soil resistivity was measured. In situ four-pin soil resistivities have limited use when determining the specifi c corrosivity of soils around buried pipelines. These types of measurements cannot be taken closely parallel to the pipeline under investigation or the results can be skewed. Proper procedures for these resistivity measurements require 50 MATERIALS PERFORMANCE October 2008

3 FIGURE 3 A21.5 standard, which was in effect at the time of its installation. After inspection and removal of the installed PE encasement material, the pipe surface was cleaned and examined. The examination procedures included wire brushing, probing with a pointed hammer, measuring pipe-to-earth potentials, and recording the depth of any corrosion-related pitting. At the conclusion of the inspections, new PE encasement was installed and the pipe inspection site backfilled. Charleston, South Carolina: Twenty-two-year-old PE-encased 12-in DI pipe at a location where the cell-to-cell survey indicated an anodic area. FIGURE 4 The 18-year-old PE encasement in Syracuse, New York, was cleaned to allow for close inspection of any damage that may have occurred during its installation. that the pins be perpendicular to the pipeline and no closer than 15 ft (4.6 m) from the pipe for a 10-ft pin spacing. 9 Representative soil samples were obtained from each excavation site and tested for resistivity, ph, and oxidation-reduction potential, along with qualitative tests for sulfides, chlorides, and moisture. At each of the excavation sites, the soil was carefully removed from around the pipe to evaluate the condition of the PE encasement. A representative sample of the exhumed PE material was tested for thickness, elongation, and tensile strength to determine if it conformed to the requirements of the ANSI/AWWA C105/ Results BexarMet Water District San Antonio, Texas The district personnel made available ~1 mi (1.6 km) of right of way containing a PE-encased 16-in (406-mm) diameter DI water main for the condition assessment work. The pipeline was installed in Three specific sites were identified for excavation and inspection using the cell-to-cell survey and side drain measurements. No damage was observed to the installed PE encasement at these inspection locations. The soil test results indicated an aggressive environment, but the exposed 16-in DI pipe was found to be in excellent condition beneath the PE wrap. The pipe surface was moist and clean, with only minor surface oxidation present (Figure 2). Upon sounding and probing of the pipe surface, it was revealed that no pitting or graphitization had occurred. Charleston Water System Charleston, South Carolina System personnel made available approximately a half-mile (0.8-km) section of PE-encased 12-in (305-mm) DI water main. The pipeline was installed in Two specific sites were identified for excavation and inspection. No damage to the installed PE encasement was observed at either of the two October 2008 MATERIALS PERFORMANCE 51

4 Investigating Polyethylene-Encased Ductile Iron Pipelines FIGURE 5 Syracuse, New York: Eighteen-year-old PE-encased 6-in DI pipe at a location where the cell-to-cell survey indicated an anodic area. locations. The soils were found to be aggressive, but the exposed 12-in diameter DI pipe was found to be in excellent condition beneath the PE wrap (Figure 3). The pipe surface displayed only minor surface oxidation. Sounding and probing of the pipe revealed no pitting or graphitization. At the second excavation site, an abandoned 3-in (76-mm) cast iron pipeline was discovered ~4 ft (1.2 m) north of the 12-in DI pipe. Records indicated that the 3-in pipeline was installed in the 1950s and was, therefore, ~55 years old. The 3-in pipe did not have any corrosion protection and had experienced corrosion-related pitting. At a small area cleaned for inspection, the deepest corrosion pit measured 0.18 in (4.57 mm), which would equate to a corrosion rate of 3.3 mpy. Onondaga County Water Authority Syracuse, New York Authority personnel made available approximately one mile of right of way containing a PE-encased, 8-in (203-mm) DI water main. The pipeline was installed in Three specific sites were identified for excavation and inspection. No damage was observed to the installed PE encasement at each of the identified inspection locations (Figure 4). The soil samples were found to exhibit corrosive characteristics, but the exposed 8-in diameter DI pipe was found to be in excellent condition beneath the PE wrap. The pipe surface displayed only minor surface oxidation (Figure 5). Sounding and probing of the pipe revealed no pitting or graphitization. Additional Results from the Investigations All tests of the PE samples procured from the excavations/inspections exceeded the requirements of the ANSI/AWWA C105/A21.5 Standard, which was in effect at the time of the installations of the pipelines. In situ four-pin resistivity values in accordance with ASTM G57 Standard 11 (including Barnes Method layering results) ranged from 2,700 to 4,690 Ω-cm at the San Antonio sites, 3,925 to 4,120 Ω-cm at Charleston, and 1,200 to 8,500 Ω- cm in Syracuse. Soil box resistivity results in accordance with ASTM G187 Standard 12 (from soil samples obtained at all excavation sites and representative of the soil environment in contact with the pipelines) ranged from 1,400 to 1,480 Ω-cm at San Antonio, 1,440 to 1,480 Ω-cm at Charleston, and 680 to 980 Ω-cm at Syracuse. For all specific results of these investigations see NACE International paper number 127, Investigating DI Pipelines, which was presented at the Pipeline Integrity Symposium during CORROSION/2007 in Nashville, Tennessee. Conclusions These investigations were performed at eight inspection sites with the cooperation of three utilities. At each site, PE encasement had provided corrosion protection to DI pipe in corrosive soil conditions. These results mirror numerous reports, publications, and tests that indicate PE encasement has provided viable protection for millions of feet of gray and DI pipe since its first use in ,3-4,13-17 As with any method of corrosion protection, proper material specifi cations and installation procedures must be maintained to ensure the system s integrity. Appropriate national and international standards are readily available to achieve this result. 6,18-21 Specifi c conclusions from these 20- year-old case histories are as follows: Cell-to-cell potential surveys and side-drain technique measurements are not reliable in locating corrosion activity on PE-encased DI pipe. At eight different locations from all three water systems, the potential tests indicated active corrosion where none was found. In situ four-pin soil resistivity measurements consistently provide higher values (less corrosive) than 52 MATERIALS PERFORMANCE October 2008

5 those of representative soil box resistivities. Whenever soil resistivity values are presented, the method used in their determination must be referenced. Soil corrosivity tools such as the 10- point Soil Evaluation System 6 and The Design Decision Model 7 determined that the soils at each of the investigation sites were aggressive to DI pipe. These methods, when used correctly, have provided and continue to provide beneficial assistance in the determination of corrosion control for DI pipe. At several of the inspection locations, moisture under the PE encasement was encountered but no corrosionrelated pitting was discovered. It has been demonstrated at these three investigation locations that PE encasement can be an effective system of corrosion protection for DI pipe. Acknowledgments The authors wish to thank Johnnie Terrazas, P.E. (BexarMet Water District), Chris Sordelet (Charleston Water System), John Van Deusen (Onondaga County Water Authority), Dale Lindemuth, P.E. (Corrpro), and Bill Foulds, P.E. (NACE Corrosion Specialist retired) for their contributions and cooperation with this work. References 1 T.F. Stroud, Corrosion Control Measures for Ductile Iron Pipe, CORRO- SION/89, paper no. 585 (Houston, TX: NACE, 1989). 2 E.C. Bell, A.E. Romer, Making Baggies Work for Ductile Iron Pipe, Corrosion and Corrosion Control Research of Iron Pipe, Pipelines 2004, ASCE Annual Conference, August 1-4, 2004, San Diego, CA. Trade name. 3 Ductile Iron Pipe Research Association, Cast Iron Pipe Century and Sesquicentury Club Records and Correspondence, ongoing. 4 Corrosion Protection for Ductile Iron Pipe by Polyethylene Sleeves, Kubota Ltd. Report, October American Water Works Association, AWWA Engineering and Construction Division Survey, October 2000 Mainstream, Denver, CO. 6 ANSI/AWWA C105/A21.5 (latest revision), Polyethylene Encasement for Ductile Iron Pipe Systems (New York, NY: ANSI and Denver, CO: AWWA). 7 DIPRA, The Design Decision Model for Corrosion Control of Ductile Iron Pipelines, December 2004, Ductile Iron Pipe Research Association, Birmingham, AL 8 D.H. Kroon, D. Lindemuth, S. Sampson, T. Vincenzo, Corrosion Protection of Ductile Iron Pipe, CORRO- SION/2004, paper no. 46 (Houston, TX: NACE, 2004). 9 A.W. Peabody, Control of Pipeline Corrosion (Houston, TX: NACE, 1967). 10 Appalachian Underground Short Course, Advanced Course, West Virginia University, Morgantown, WV, ASTM G57, Standard Test Method for Field Measurement of Soil Resistivity Using the Wenner Four-Electrode Method (West Conshohocken, PA: ASTM). 12 ASTM G187, Standard Test Method for Measurement of Soil Using the Two-Electrode Soil Box Method (West Conshohocken, PA: ASTM). 13 A.M. Horton, Protecting Pipe with Polyethylene Encasement, , AWWA Water World News 4 (1988): pp R.W. Bonds, L.M. Barnard, A.M. Horton, G.L. Oliver, Corrosion and Corrosion Control of Iron Pipe 75 Years of Research, Journal AWWA 6 (2005). 15 DIPRA, Inspection Report Cathodically Protected Ductile Iron Pipe Encased in Loose Polyethylene Film Dickinson, ND, April 21, 2004 (Birmingham, AL). 16 DIPRA, Inspection Report Cathodically Protected Ductile Iron Pipe Encased in Loose Polyethylene Film Lafourche Parish, Louisiana, May 28, 2003 (Birmingham, AL). 17 T.F. Stroud, Infrastructure: Is the Problem Being Blown Out of Proportion? Ductile Iron Pipe News, Fall/Winter (1985): p ASTM A 674 (latest revision), Polyethylene Encasement of Ductile Iron Pipe Systems (West Conshohocken, PA: ASTM). 19 ISO 8180 (latest revision), Ductile Iron Pipes Polyethylene Sleeving (Geneva, Switzerland: ISO). 20 JDPA Z 2005 (latest revision), Polyethylene Sleeves for Corrosion Protection of Ductile Iron Pipes (Tokyo, Japan: JDPA). 21 BS 6076 (latest revision), Tubular Polyethylene Film for Use as Protective Sleeving for Buried Iron Pipes and Fittings (London, U.K.: BSI). This article is based on CORROSION/2007 paper no. 127, presented in Nashville, Tennessee. DANIEL W. CRABTREE is the research coordinator for the Ductile Iron Pipe Research Association (DIPRA), 245 Riverchase Pkwy. E., Ste. O, Birmingham, AL 35244, where he has been employed for 30 years. He is a 25- year NACE International member and a NACE-certified Corrosion Specialist and Cathodic Protection Specialist. Currently, he is the chairman of the ASTM G01.10 Subcommittee on Corrosion in Soils. MARK BRESLIN is a registered professional engineer in the state of Alabama. He is currently the staff engineer for DIPRA, where he has been employed for 17 years. He has been a member of NACE International for 17 years and is certified as both a Corrosion Specialist and Cathodic Protection Specialist. He has a B.S. degree in mechanical engineering. Need reprints of MP ads, articles, or covers? REPRINTS ARE A GREAT INVESTMENT! Professionally printed reprints and photocopied reprints of all MP ads, articles, and covers are available for purchase. Reprints can be customized with your company s logo, additional product information, or the magazine cover with no limits on creativity! Order your reprints today; it simply makes good business sense! For reprint information and rates, call October 2008 MATERIALS PERFORMANCE 53