Mike Tucker, Furmanite International, UK
Power plant refurbishment may take a variety of different forms, most of them involving shutdown to allow the necessary work to be carried out. The latest advanced carbon fibre composite repair technology is one exception, enabling full structural strength and pressure integrity to be permanently restored to corroded, eroded or otherwise weakened or damaged plant, prolonging asset life without requiring shutdown or disrupting production.
Preparing filler over a flange to receive carbon composites, in order to complete the repair to a section of pipework
The technology was considered the domain of industries such as aerospace and motor racing, but today sees far more widespread application in industrial plant repair and rehabilitation across a range of industries, including the power industry, from oil, gas and coal fired power plants to nuclear. Avoiding the need for cutting and the replacement of damaged sections of plant, with associated downtime, cost and logistical issues, as well as the ability to be applied while the plant continues to operate as normal, has a particular value where mature assets may require refurbishment work, but downtime comes at a price.
What is a composite?
A composite is defined as a material made of two or more distinct materials, and for industrial applications these are generally a fibre and a resin the choice of which fibre and which resin is critical, and the balance of the two will affect the final characteristics, performance and strength of the repair. Carbon fibre and epoxy resin composites can restore full strength and integrity to plant from pipes of 1-60 inches (2.5-152.4 cm) or more in diameter, to tanks and pressure vessels, including complex geometries and configurations such as tees, elbows, Ys, flanges, nozzles and so on. Typically, in the power industry on water or high pressure steam plant in situations where coming off line is not the preferred option, and the cost of dismantling would be high.
These repairs have ten times the strength of steel and twice the stiffness, yet are light in weight (at less than a quarter the density of steel), and can be designed for a life of 20 years or more, and to accommodate high pressures (to over 200 barg) and high temperatures (to 200 °C), as required. In situations where external corrosion has occurred, a composite repair can prevent this from continuing. Furthermore, the repairs themselves are corrosion-free, and extremely low maintenance throughout their design life.
High strength repair
After establishing the details of the problem and requirements, and engineering a composite repair designed to take into account all the variables and meet the need, the repair process itself involves application of layers of carbon fibre impregnated with epoxy resin, following thorough surface preparation, in accordance with the thickness, taper, area of overlap onto good metal, and orientation of the fibres, as specified in the repair design.
The installation process looks simple, but is in fact a specialist skill. Surface preparation (typically using grit or water blasting) is critical, removing oxidation from the metal and ensuring a good bond for the fibre and resin. This is followed by a glass fibre tie-coat, which provides a high quality interface and degree of electrical insulation to guard against galvanic reaction between the substrate and carbon fibre. The resin-impregnated carbon fibre layers are then applied, followed by a final, sacrificial peel ply layer which removes any excess resin and provides a surface finish ready to accept paint or any other desired finish, once the resin has cured.
Importantly, no hotwork is required and no prefabrication is involved, since the materials are applied in situ to almost any configuration. Moreover, the materials are light and easy to handle valuable advantages where there are issues of awkward or confined access. The fact that the repair itself is also low profile is beneficial in such cases. Often a repair of just 5 mm thickness will be sufficient to restore full structural strength and pressure integrity.
The repair design takes a number of critical factors into account, from the size of the repair, the geometry, internal pressure and temperature, and pipeline material, to whether there is sufficient good metal onto which to overlap the repair, and the load-bearing contribution of the substrate. Whether or not there is a through-wall defect will have implications for the repair design, for example, where there is no through-wall damage the repair material will be largely subjected to membrane forces, and the thickness reduces to one of load share between the repair laminate and the underlying substrate, with the laminate thickness determined accordingly. Equally, the shape of any defect has further implications. A crack, for instance, will require different design calculations to a hole, while a defect extending round the circumference of a pipe involves different design considerations again.
A further consideration, where internal corrosion has caused a problem, is the need to design the repair to allow for this to continue, potentially leading to total loss of the original substrate underneath the composite, while still retaining full integrity for the lifetime of the repair.
Where initially the industry was slow to accept this ‘new’ repair technology, its use is becoming more widespread, and confidence has been gained as proof in application has demonstrated what can be achieved.
Repairs in the field
In an example at one power station, a material fatigue problem was discovered during a routine inspection on a tee connection on a 24-inch recirculation cooling water line. Shutdown was not desirable, and vibration in the pipe because of its location meant a hot tap and bypass around the damaged section was not viable. Instead, composites technology avoided the need to drain or bypass and replace the damaged section, and after full validation of the repair design, a 26 mm thick repair was applied, designed to limit the cyclical stress to the steel to below 48 MPa, restoring full structural strength and pressure containment for a 25 year lifetime.
A very different scenario saw composites used at a hydropower station, where four ductile iron lines largely encased in rock within a service tunnel in the mountainside were found to be suffering from severe corrosion and erosion. Given the location of the lines, replacement was impractical. The flexibility of the composite repair materials, and the avoidance of hot work and of any prefabrication requirements, meant that this repair method could be successfully applied, on-line. Carbon fibre and epoxy resin layers were applied to a thickness of 5.6 mm to restore strength and pressure containment. Specialist clamps were used to strengthen the section of pipe at the point of entry into the rock of the tunnel, with the composite repair extended to encompass the clamp to ensure pressure containment across the interface and structural integrity of the whole section of pipe.
Serving the nuclear industry
In another case, composite repairs were undertaken at British Energy’s Hunterston B site on a non-isolatable section of cooling water pipework, while the power station remained fully operational. The cast iron pipework making up one of the cooling systems needed reinforcement, as replacement would have involved considerable disruption.
The project involved numerous challenges. The system consisted of three vertically mounted seawater coolers with 10-inch, 12-inch and 18-inch diameter feed and return headers, with associated valves and strainers. In addition to the total extent of the pipework, which incorporated bends and multiple flanges, it also included valves that had to remain operable, and sections of pipework passing through concrete which had to be incorporated into the repair, while all pipe supports had to be replaced without disturbing or displacing the pipework.
Furmanite worked with British Energy to review a number of potential options and establish the optimum method and design that would meet all the requirements, and allow the repairs and strengthening to be carried out.
Composite repairs can accommodate complex geometries such as elbows, shown here, restoring full strength and pressure integrity
Ultimately the solution developed involved its carbon fibre and epoxy resin composites technology, backed by proven leak sealing technology for the areas that composite repairs alone could not accommodate. These included restraints to tie the repair to the concrete floors where the pipes passed through, and extension spindles to keep the valve operable. The composite repair and leak sealing techniques were adapted to suit the specific needs of the application.
Designed to accommodate both hoop and axial load, against a design temperature of 38 °C and pressure of 2.8 barg, the composite repair not only provided pressure containment, but also structural strengthening to restore full integrity.
Two different carbon fabrics were used to allow the repair thickness to be kept to a minimum, with up to seven layers applied (approximately 7 mm), depending on the pipe diameter. The numerous tees and other complex geometries were incorporated into the repair, requiring up to 24 carbon plies to achieve the necessary strength. The thickness of the composite repairs had to be kept to a minimum because of the cramped access of the workspace, which was classed as ‘confined space’ with associated restrictions applying. A bespoke space saving design was also developed to allow the necessary repairs to be undertaken to the extremely tight access blanked flanges at the ends of the two headers.
With all challenges addressed (including the working conditions of high humidity and temperatures of up to 40 °C, as well as the cramped access), the project was successfully completed with no lost time accidents or disruption to plant operation.
Why Carbon composites make sense
Since the 1970s engineers have hailed composites as a solution that could change the face of industry. But only more recently, as pioneers have proved the benefits and reliability in action, has widespread acceptance of the capabilities of this technology for industrial plant repair and refurbishment been realised, considerably supported by moves in the late 1990s and early 2000s to standardize design and qualification systems.
Confidence in the technology continues to grow, and its application in the power industry continues to extend more widely, while developments in the technology itself are on-going to introduce further capability improvements and benefits.
Certainly, as a technology that facilitates permanent repair and strengthening of damaged or weakened plant with no disruption to operation, its value in power plant refurbishment to prolong asset life where shutdown is not desirable is high. Little wonder, perhaps, that composites have been claimed to be the greatest materials development since steel.