By Peter G. Berthelsen
Within five years after commissioning of the 500-mw Wivenhoe pumped-storage plant, staff began experiencing problems related to corrosion in the cooling water pipes. To solve the problem, the staff applied a variety of innovative polymer-based materials. Operating results indicate success with these materials for combating corrosion.
All the original cooling water pipes at the 500-mw Wivenhoe pumped-storage plant in Australia are made of galvanized steel. Within five years after the facility began operating in 1984, the galvanization had seriously deteriorated. In small-diameter pipes, severe corrosion resulted in leakage from pinholes that developed in the welds. In addition, biofouling and overgrowth due to corrosion constricted flow through the pipes. The corrosion also appeared in other galvanized steel cooling water components — valves, flexible pipe connectors, and heat exchanger water boxes.
To deal with the situation, Tarong Energy Corporation used polymer pipes and coating products. Although these were not “new” materials, the use of polymers in the power industry is unusual. As benefits were achieved, company personnel gained higher levels of confidence in this decision. The result has been a significant decrease in corrosion and related water losses, as well as decreased biofouling in the pipes.
Problem with the cooling water system
Galvanized steel pipe was used at Wivenhoe for both exterior and embedded pipes. The rare exceptions include embedded stainless steel pipe penetrations through the concrete silo walls that provide intake of cooling water and discharge it back into the tailrace. Several other specially selected components are not galvanized steel, such as pipe sections in the vicinity of neutral links for the generators, where non-magnetic material is required.
The thickness of the coating used to galvanize steel pipes varies in proportion to the steel thickness. At Wivenhoe, the pipes with the smallest bores were the first to be affected by loss of the galvanizing coating. These pipes were mainly those 40 to 50 millimeters in diameter but up to 100 millimeters, which had the thinnest walls and thus the thinnest galvanizing. In the early 1980s, galvanizing coatings averaged 55 microns for steel 1.5 to 3 millimeters thick, 70 microns for steel 3 to 6 millimeters thick, and 85 microns for steel more than 6 millimeters thick.
Another problem at Wivenhoe was some misfits of the pre-fabricated pipe sections during construction. This situation was dealt with through cutting and re-welding at the site. Consequently, the galvanized coatings on these pipes were compromised. Corrosion commenced as soon as these pipes were put into service, quickly leading to leakage problems.
Starting in 1987, Tarong Energy performed corrosion investigations, which identified that electrolysis attracted bacteria to the corrosion sites and accelerated the metal damage. The first investigation was carried out as a result of pinhole failures of the dewatering air compressor (DWAC) cooling water pipes. Other investigations followed when observations during unit overhauls indicated that corrosion in pipes was become more widespread. Specialized surface inspections were carried out in situ. In several cases, samples of pipe components and corrosion products were taken for laboratory analysis. Lab reports indicated that biofouling appeared to form more readily once corrosion had commenced. The corrosion tubercles and biofouling compounded to create dramatic flow restrictions. For example, nominal flows of 200 liters per minute to main shaft glands were falling to alarm conditions of 120 liters per minute within two years of returning to service after unit overhauls.
Small bore cooling water pipes embedded in the station structural concrete can be as long as 26 meters between entry and exit flanges. Hundreds of meters exist in total. Corrosion, biofouling, and flow problems in this vast network of pipes led to low flow alarms, plant control sequence interruptions, and unit trips.
Work undertaken to deal with this situation involved cleaning the pipes every three years. Personnel performed the cleaning during unit overhauls, using high-pressure water blasting. This restored correct flow conditions, but corrosion and biofouling began again as soon as the plant was returned to service. Dismantling the pipes, removing them from the site, cleaning them, and repairing leakage defects is very labor-intensive and expensive.
Until a solution could be found, Tarong Energy designed a “back-wash” system, for rapid response to low flow alarms. This system, implemented in 1990, blasted compressed air through the pipe in the reverse flow direction to dislodge biofouling and improve flow rates.
In 1992, Tarong Energy began a trial to test the use of copolymer coating as an alternative to epoxy painting on the water boxes in the unit heat exchangers. Based on favorable results, the company moved forward with use of this material on all water boxes.
As the investigation into ways to deal with corrosion and biofouling in the cooling water pipes continued, Tarong Energy incorporated such other innovative materials as polyethylene pipe, an epoxy resin/polyester liner, polymer coatings, and a flexible pipe connector with a polytetrafluorethylene (PTFE) sleeve. The company’s use of these materials has provided satisfactory solutions for the problems with corrosion and biofouling at Wivenhoe.
The following sections of the article describe experiences with the use of each of these materials.
Copolymers are an alternative to the use of polyesters, epoxies, and polyvinyl chlorides (PVCs). These coatings developed porosity, micro-cracking, and embrittlement, resulting in limited service life.
Copolymer coatings containing a combination of zinc/epoxy/polyurethanes provided ideal corrosion resistance through to excellent gloss retention, a pre-requisite for biofouling resistance. However, volatile organic compounds (VOCs) associated with these coatings raised environmental concerns. These earlier copolymers evolved to result in ethylene methyl acrylic acid (EMAA), which provides highly desirable corrosion protection and biofouling resistance.
Polyethylene does not adhere readily to metals. This can be rectified by modifying ethylene with methyl acrylic acid to create EMAA. The chemical interaction between the acid and the metal surface forms a strong bond that does not delaminate. Galvanizing the steel components is optional but is an inexpensive method of providing a substrate of uniform material and surface finish, ideally suitable for the EMAA coating.
The exposed surface of EMAA sets to a glassy non-adhesive finish, ideal for resisting biofouling. EMAA is a thermoplastic, providing toughness, flexibility, non-conductivity, and chemical resistance. Its can be remelted for repairs after the initial application.
The recommended coating thickness is 250 µm to 400 µm. It is expected to provide 30 to 50 years of service life.
In 1992, Tarong Energy’s corrosion specialist introduced ethylene-based copolymer coatings at Wivenhoe as an alternative to epoxy painting on heat exchanger water boxes. Epoxy coatings endure reasonably well on new components but quickly begin to break down after being applied to corrosion-damaged surfaces, even with weld repairs and grit blasting preparation. Typical life of these coating on new components is ten years, compared with three to four years at Wivenhoe.
In 1992, Tarong Energy obtained samples of the copolymer from Stoneman’s Engineering, a coating supplier in Queensland, to trial at Wivenhoe. This trial provided favorable results, and two water boxes were treated with the ethylene-based copolymer coating in 1994. Although the coating is more expensive than epoxy painting, the overall refurbishment cost for the water boxes — including cleaning, weld repairing, grit blasting, and coating — is only marginally higher. A formalized cost-benefit analysis was not carried out.
Based on success with the heat exchanger water boxes, Tarong Energy chose EMAA to coat the steel pipe components being fabricated for the cooling water system.
Other uses of polymer coatings
Metal components of many shapes and sizes are suitable for polymer coatings. Provided the surfaces are correctly prepared and the components are pre-heated, polymer coatings may be successfully applied. The polymer is supplied in powder form and may be applied by way of:
- Fluidized bed, a technique to be performed in a workshop. Compressed air is used to agitate polymer powder in a containment vessel, creating a fluidized bed. The article to be coated is immersed into the vessel after surface preparation and pre-heating. Polymer particles “melt” onto the surfaces and bond together, forming a uniform coating.
- Static electric “spraying,” again predominately a workshop-based technique. Compressed air delivers polymer powder through a nozzle where it receives a static charge. The static electricity ensures uniform distribution onto the metal surface.
- Flame spray, which may be used in a workshop or in the field. An applicator with gas fuel heats the metal, and the applicator delivers the polymer powder to the surfaces using compressed air.
When applied to galvanized steel, the ethylene methacrylic acid (EMAA) copolymer coating provides very desirable corrosion protection and biofouling resistance.
Corrosion protection provided by the polymer coatings applied at Wivenhoe has been excellent. Minor failures have been observed on several occasions. Due to the fact that corrosion re-commences on any exposed metal, it is difficult to identify the mechanism of failure with certainty. Likely causes range from:
- Improper surface preparation and cleanliness resulting in partial delamination of the coating. Ideal surface preparation can be extremely difficult with components that have been in service for some time and incurred severe pitting damage;
- Geometry that creates difficulties with surface preparation and the distribution of the polymer powder uniformly to all surfaces. Components too large for the fluidized bed process are coated with one of the spray techniques, and this can lead to inadequate coating on surfaces concealed by their geometry; and
- Operator error or inexperience resulting in non-uniformity. Especially with the spray techniques, operator skills are very important in achieving ideal results.
The cooling water system for the dewatering air compressors (DWACs) at Wivenhoe is independent from the cooling system system for the generating units. But the same corrosion and biofouling problems were occurring. In 1999, Tarong Energy hired a technical assistant to assess the replacement of the cooling water pipes for the DWACs. This employee had experience with polyethylene pipe and so included this material as a substitute for galvanized steel pipes. Polyethylene pipe and fittings are available in straight lengths or coils, and in sizes and pressure ratings to suit the requirements of the Wivenhoe cooling water systems. Other material options considered but not chosen were stainless steel (high cost) and rubber flexible hose pipes (would require numerous metal fabricated manifolds for distribution and extensive support frames).
Corrosion of the cooling water pipes at the 500-mw Wivenhoe pumped-storage facility resulted in significant flow restrictions. This flange is nearly completely choked with corrosion tubercles and biofouling.
The net present value (NPV) technique was used to prepare a cost-benefit analysis. This resulted in an about 40 percent saving in fabricating and supplying polyethylene pipe components compared with galvanized steel.
The decision to replace the DWAC pipes with polyethylene was undertaken with some degree of caution. The DWAC pipes consisted of numerous elbows, tees, and branches and were subject to significant vibration. However, the DWAC cooling water pipe system is not extensive, and one machine (of four) could be changed initially as a trial.
After the polyethylene pipe was installed in 1999, one minor defect occurred, in the form of a cracked branch stub weld. This was easily repaired.
Tarong Energy engaged Central Plumbing Services Pty Ltd., a contractor with extensive experience and expertise with polyethylene pipe, to provide advice on the design and selection of the pipe, fittings, components, and support bracketing. The utility also retained this contractor to handle the installation. Polyethylene pipes were installed on the remaining three compressors later that same year.
After six years of successful operation, Tarong Energy determined that polyethylene pipe was acceptable in eliminating corrosion and resisting biofouling. It became the preferred material for replacing small bore cooling water pipes on the systems for the main generating plant.
Exterior pipes of main cooling water system
In early 2005, Tarong Energy carried out an appraisal of all small-bore exterior pipes, for the purpose of replacing them with polyethylene. The project was approved on the basis of capitalizing on the practical benefits, and by extrapolating the NPV cost-benefit analysis undertaken for the DWACs in 1999.
Flexible pipe connectors with a polytetrafluoroethylene (PTFE) sleeve encased in braiding are corrosion resistant and eliminate biofouling.
For various components of the cooling water system, such as distribution manifolds and items that attach directly to embedded pipe flanges, it was decided that steel still should be used to provide strength and stiffness and support for valves. To ensure adequate corrosion protection and biofouling resistance for these steel components, they were first galvanized and then coated with a modified polymer material. The polymer-coated components were delivered in March 2005, in conjunction with deliveries of polyethylene pipe and fittings. All small bore cooling water pipes were replaced on Unit 1 with polyethylene pipe and fittings and polymer-coated steel components during an overhaul in May 2005.
Since these polyethylene pipes were installed, no defects or leaks have occurred. However, just two weeks after commission in June 2005, a few leaks were noted where polyethylene pipe was connected to valves. These defects were related to valve components and were easily rectified. Repairs were completed by March 2006.
Epoxy resin/polyester liner
Finding a solution for corrosion and biofouling problems in pipes embedded in concrete presented a unique challenge. Plant reliability was maintained through the program of high-pressure water blasting every three years. However, dislodged corrosion debris was carried in the water flow, occasionally blocking heat exchanger tubes or causing scoring damage to journal surfaces in shaft glands and draft tube bearings. If the problem continued untreated, eventually the embedded pipe wall would perforate, releasing pressurized water into the structural concrete. Water permeating through cracks in concrete can emerge in unexpected and undesirable locations.
Tarong Energy considered and discounted several options:
- Bore holes through the structural concrete to provide tunnels for new cooling water pipes. There would be a need for several tunnels through about 8 meters of heavily reinforced concrete. Severe constraints existed in siting concrete boring machines at the particular locations.
- Re-arrange the cooling water distribution systems and manifolds to enable alternative exterior paths to the delivery points. This would require massive redesign of the cooling water mains and re-routing numerous pipes, causing interference with several auxiliary plant installations (high-pressure oil pumps, filters, and heat exchangers).
- Adapt mechanical chipping tools to a rotary drain cleaning device that could be inserted through theembedded pipes, to prepare the inner surfaces for coating with surface-tolerant paints. This option would require real frontier approaches. Applying a paint coating through a 50-millimeter embedded pipe would be a challenge, let alone inspecting the finished coating for quality acceptance.
In 2000, Tarong Energy learned about the Sideliner system from Sideliner Pty Ltd. Sideliner was developed for urban reticulated water services. This system inserts an epoxy resin/polyester fabric composite into the bore of a pipe, creating an impervious lining the entire length of the pipe between the entry and exit flanges. After bonding the ends of the liner to the original pipe ends, the result is an epoxy/polyester composite tube, impervious and self supporting inside the original pipe.
The liner protects the pipe against corrosion and greatly reduces biofouling. The service life of the liner in water piping is anticipated to be indefinite, not being subject to abrasive flow conditions, aggressive chemicals, ultraviolet radiation, or heat degradation.
The polyester liner is manufactured for the specific pipe diameter and length. Specialized equipment and techniques are required to insert the liner and cure the epoxy.
The system is non-reversible and the cured liner is non-removable with mechanical or chemical means. In the event of a malfunction during insertion, the liner can be withdrawn before the epoxy begins to cure.
Testing before installation
The Sideliner system is theoretically capable of lining pipes as small as 50 millimeters in diameter. However, Brisbane Water, the only Queensland contractor licensed to use the system, had no previous experience with pipes of this size. It appeared that Tarong Energy would need to create some individual history with this system at Wivenhoe. To this end, 50-millimeter exterior cooling water pipe sections totaling about 20 meters in length were dismantled and removed to the contractor’s work yard for a trial. The pipe sections contained numerous short radius elbows, enabling reasonable simulation of embedded pipework.
The procedure and equipment are highly specialized. This specialization extends to the manufacture of the polyester liner, design of the device for deploying the liner into the pipe, technique for curing the epoxy, and training of the personnel carrying out the work.
After insertion of a liner into the sample pipe, a section of pipe that included an elbow was split for internal examination. Observations indicated:
- The liner was pressed tightly against the pipe bore and was self-supporting;
- Typical thickness was 1.5 millimeters;
- The liner was impervious;
- Adhesion to the pipe bore was not 100 percent, but this was not considered highly important. As long as water could not permeate the liner, further corrosion of the pipe would not occur;
- The liner formed itself over discontinuities in the pipe, such as weld beads. In situations where the discontinuities were severe, surplus epoxy resin filled the gaps;
- The liner formed well around the elbow, with only minor creasing around the inner radius;
- The inner surface of the liner was smooth with minor texturing from the polyester fabric surface. The hard, glassy finish of the cured epoxy is satisfactory for resisting bio-fouling; and
- The pipe bore is reduced marginally, about 11 percent based on 50-millimeter-diameter pipe and liner thickness of 1.5 millimeters. This would increase the velocity of a typical flow rate of 200 liters per minute to 1.7 meters per second from 1.5 meters per second.
The outcome of the trial was considered sufficiently encouraging to undertake the lining of one embedded cooling pipe within the station. The liner was successfully inserted and cured in 2002.
The function of the liner is to contain the cooling water flow and eliminate any water from contact with the original pipe surface. To fulfil this purpose, special attention is required to the bores of entry and exit flanges. Small grinding tools were used to thoroughly clean the flanges around the liner to bare metal. An epoxy resin compound was then applied to bond to the metal and the protruding ends of the liner. The epoxy and liner were then dressed flat to reconstitute the flange surface.
After three years of service, the pipe was dismantled, cleaned, and inspected. The liner was intact, as was the bonding at the entry and exit flanges. Based on the success of these results, action was taken to proceed with further lining of embedded pipes. Three more 50-millimeter-diameter embedded pipes were lined during the overhaul of Unit 1 in 2005.
Flexible pipe connectors for the cooling water supply and return lines for the generator air coolers at Wivenhoe have a disappointing history of failures, in the form of pinhole leakage from aggressive corrosion. Several leaking flexible components occurred during normal operation within the first two years after commissioning. Each event was evident from a high level alarm at the drain collection tank for the generator enclosure. To replace the offending flexible component, the unit had to be stopped and isolated, the cooling water system drained, the component replaced, and the unit returned to service.
Tarong Energy recently learned of an alternative, a flexible pipe connector with a polytetrafluoroethylene (PTFE) sleeve encased in braiding. Another contractor, Safetyright Pty Ltd., became aware of this type of connector while assessing a redesign and replacement of all cooling water pipes within the generator enclosures in 2005. The connectors are available from Pacific Hoseflex Pty Ltd., a well-established company fabricating specialized pipe components in Queensland.
Although this connector is more expensive meter for meter than the convoluted stainless steel components, the material becomes cost-effective when consideration is given to the arrangement from the supply ring main to the air cooler flanges. The original arrangement consisted of pipe sections made up of bends and elbows and a short flexible connector. Fabrication of numerous parts is expensive, and removal and re-installation is labor-intensive. By combining the multiple components into a single full-length item that includes the flexible connector, bends, and elbows, the arrangement becomes cost-effective. In addition, labor for installation is reduced.
Pacific Hoseflex fabricated one full-length flexible component, which was installed in October 2006. An internal inspection of this component is scheduled for January 2009. The cost to purchase one PTFE flexible component was about 15 percent higher than the aggregate cost of the multi-component assembly, but the time to install this flexible component was only half. These PTFE flexible components will be cleaned and reinstalled every three years when the generator air cooler tubes are cleaned. Labor will be reduced due to the single item configuration, handling will be easier because of the lower weight, and unexpected outages due to water leakage will be eliminated.
The PTFE lining of the connectors is corrosion resistant and eliminates biofouling. The pressure rating and temperature rating of the PTFE is more than adequate for the conditions within the generator enclosure. Apart from the PTFE lining, the materials used in the fabrication are stainless steel.
Using polymer materials and technology within the Wivenhoe cooling water system is providing satisfactory solutions for problems with corrosion and biofouling.
Embedded pipes were the most challenging, ranging from the consequences of deterioration of the original pipe, the search for a suitable process to combat corrosion, the adaptation of this process to the small diameter and long lengths of pipe within the confines of the power station, the implementation of the process, through to the level of confidence in the life expectancy of the system and material used.
Historically, the endurance of epoxy coatings is less than that of polyethylenes. In the case of the Sideliner system, the finished liner is a reinforced epoxy composite compared with an epoxy paint system. Any negative aspects of epoxies, such as micro cracking and embrittlement, are offset by the reinforcing provided by the polyester fabric.
Other opportunities exist at Wivenhoe for combating corrosion with polymers and non-metals. For example, station drainage pipes and domestic water pipes are being evaluated for partial replacement with polyethylene pipe.
Mr. Berthelsen may be reached at Tarong Energy Corporation, Wivenhoe Power Station, P.O. Box 800, Brisbane, Queensland 4306 Australia; (61) 7-32284321; E-mail: peter.berthelsen@ tarongenergy.com.au.
Peter Berthelsen is senior mechanical engineer with Tarong Energy Corporation in Australia. He is responsible for mechanical work at the 500-mw Wivenhoe pumped-storage plant.