To protect the substantial investment a utility has made in an underground water or wastewater system, it is prudent for staff to consider the risk of corrosion and how best to control it. Corrosion is recognized as a natural phenomenon which can cause premature deterioration of pipelines in certain soi conditions.
Soil test procedures to determine if a soil is potentially corrosive to ductile iron pipe are included in the standard known as ANSI/AWWA C105/A21.5--Polyethylene Encasement for Ductile Iron Pipe Systems, Appendix A (10 point system). And various methods are available to control corrosion so that a satisfactory service life for a selected pipeline product may be realized.
Corrosion Protection Options
One system commonly used for corrosion protection is cathodic protection. This method generally requires the presence of a bonded coating on the pipe. It requires also a joint bonding system for electrical continuity, and some means of applying an electrical current to the pipeline to make it play the role of a cathode. Current is supplied in one of two ways: by impressed current from an outside power source, or through the use of buried sacrificial anodes.
Installation cost of a cathodic protection system, while significant in itself, is not the only cost factor to be considered. The system itself is active, which means it must be inspected and maintained regularly. These after-installation costs can become substantially larger than the initial installation costs.
The ductile iron pipe industry recognizes that a properly designed, installed and maintained cathodic protection system can be an effective means of mitigating corrosion. However, the method most often recommended by this industry when the need for corrosion protection is indicated is loose-film polyethylene encasement, manufactured and installed in accordance with the ANSI/AWWA standard noted above. This is the only method for protecting ductile iron pipe in corrosive soils that has the endorsement of an AWWA Standard.
The encasement material is available in either 4 mil-thick cross-laminated high density polyethylene or 8 mil-thick low density polyethylene. It is usually supplied in tubular form in rolls. This tube is cut to length, slipped over each pipe section during installation, and secured in place with plastic adhesive tape or plastic strapping. It is a passive system that is easy to install, and has been shown to protect the pipe with no costly maintenance.
One hundred years generally is considered a useful service life for the products offered by the ductile iron pipe industry. To achieve this longevity in a corrosive environment, whichever method of corrosion control is specified at the time of installation must remain effective throughout that period.
What About the Costs?
To carry out a cost study of an actual project, the installation of approximately one mile of 30-in. TYTON JOINT Pipe, Thickness Class 52, ductile iron transmission main in an urban location was used as an example. The design engineer included detailed plans for a cathodic protection system as a bid alternate. Costs for materials were obtained from a ductile iron pipe manufacturer and a supplier of corrosion protection systems. Installation estimates were provided by a contractor with expertise in underground work. A life cycle cost analysis was used to compare the total cost to purchase, install, and maintain two different corrosion protection systems for the estimated service life of the pipeline. In this presentation, wherever a present cost is projected to a future cost, 3 percent is used consistently as the annual inflation rate. (For simplicity all numbers in the text and tables have been rounded out to the nearest $100 in most cases).
The bid cost for the polyethylene encasement material was $6,200, which included the adhesive tape required for joining. After reviewing the plans, a contractor reported there would be no additional installation costs. By way of explanation, he stated that on a low production job such as this (30-in. pipe or larger, city streets, many cross services, etc.), there is a substantial amount of crew waiting time. There is no need to add a man to handle the polyethylene encasement tasks, or to weld jumper cables to keep up with the laying schedule. That means the owner would get a mile of protection for the $6200 cost of the plastic tubing.
On the other hand, the cathodic protection design approach required a tape coating system consisting of a primer layer, an inner tape layer for corrosion protection, and an outer tape layer for mechanical protection, all in conformance with AWWA C214--Tape Coating System for the Exterior of Steel Water Pipelines. A coating contractor quoted over $57,400 to tapewrap the pipe for this job, or about nine times the cost of the polyethylene encasement.
The specifications called for double jumper cables, field-welded to bond all joints. Although joint bonding can increase the possibility of stray current pickup and stray current corrosion, it is essential in a cathodic protection system on ductile iron pipe. Nearly 300 joints were needed in this one mile of pipe. A corrosion protection firm quoted $10 per joint for bonding wires and accessories for a total of $2950. As mentioned earlier, the contractor would not add to the installation cost estimate, so attaching the jumpers on the 30-in. pipe would be done for no additional charge. The subtotal for tapewrap and joint bonds together would come to almost $60,400, or roughly ten times the cost of the polyethylene encasement system.
Other elements of the cathodic protection system required in the design were a test station, a zinc reference electrode, and ten 32-lb. magnesium anodes for each anode bed. Plans called for thirteen anode beds spaced along the pipeline. Each anode bed was quoted at $1,000.00, so the total would be $13,000.00. The anodes were to be installed five feet below the pipe invert and five feet on each side of the centerline of the pipeline. The underground contractor estimated the installation of these components would cost at $3,132 per anode bed, for a total of over $40, 700. To this point the cost to have the cathodic protection system installed had reached $114,100, which is more than eighteen times the cost of the encasement approach.
A consultant skilled in corrosion engineering quoted a cost of $1800 for annual inspection, based on one day in the field reading voltage potentials and one day preparing the report. The inflation-adjusted total for nineteen inspections, beginning a year after installation, was calculated to be just over $45,200 for the first 20 years. This brings the cost of the sacrificial anode cathodic protection system for this project to $159,300.
Costs Over the Pipeline's Projected Life
Life cycle cost analysis involves taking a realistic look at all costs associated with achieving the expected service life of a major investment like this pipeline under study. The method provides a comparison between alternative solutions to determine which is truly the more cost effective.
From over forty years of research and thirty-five years of field history, it has been established that polyethylene encasement does not deteriorate or wear out in service. Every time pipe is exhumed at a test site, a sample of the encasement is tested. In all examples so far, even on installations in service for over 30 years, the polyethylene film has been found to meet the requirements set for new material. Therefore it can be assumed there is no need for annual inspection, maintenance, or periodic replacement. After the initial purchase and installation of the polyethylene wrap, there should be no more cost. Consequently, the 100 year life cycle cost of the polyethylene encasement of this one mile of pipe is the original $6,200.
The life cycle cost calculation for the cathodic protection system assumed a design life of twenty years. This means it would have to be replaced every twenty years for the life of the pipeline. Earlier it was shown that the equipment cost for the anode beds at the time of installation was $13,000.00. Figure 1 shows the projected costs of purchasing new anodes every twenty years, with the first installation being made in 1994. The cost to install the anodes initially was $40, 700. Figure 2 lists how much the expense of installing new anode beds every twenty years would be.
Described already as active, a cathodic protection system must be inspected periodically. For the sacrificial anode scheme installed on this project, annual inspection was recommended. Figure 3 details inspection costs per score of years for the life of the pipeline. Considering the initial installation, the periodic replacement of anode beds, and annual inspection, the total cost to provide cathodic protection against corrosion throughout the projected service life was calculated to be just under $2,310,000.
The one mile of 30-in. ductile iron pipe (TYTON JOINT pipe, Thickness class 52, with standard asphaltic coating) was priced at just under $375,000. Figure 4 compares the cost of corrosion protection to the initial cost of the pipe.
This study showed the initial acquisition and installation costs of the cathodic protection system were approximately eighteen times the cost of purchasing and installing loose-film polyethylene encasement. In a life cycle cost analysis for an anticipated 100-year pipeline, a comparison of the two methods described revealed that the cathodic protection system could cost approximately 370 times the cost of the polyethylene encasement, and about six times the purchase price of the ductile iron pipe.
This article was adapted and edited by Ian Lisk from a paper authored by Gerry Craft which appeared in the Fall/Winter '95/'96 issue of the U.S. Piper, a publication of the U.S. Pipe and Foundry Co. In turn that article was a condensed version of a presentation he made at the Second International Conference, Pipeline Division of TCLEE/ASCE, held June 25 to 28, 1995, in Bellevue, Washington. The complete paper appeared in ADVANCES IN UNDERGROUND PIPELINE ENGINEERING.
Reprints are available upon request from U.S. Pipe at P.O. Box 10406, Birmingham, AL 35202; tel. 205-254-7442; fax. 205-254-7494. Gerry Craft is regional sales engineer in the company's Pacific Coast Region, Union City, California.