A longer life for the condensers

Plastocor lining postpones tube replacement 

Its biggest-ever tubelining contract is giving ThyssenKrupp Xervon an opportunity to demonstrate its maintenance competence in the power plant sector. This industrial services provider recently applied Plastocor interior along the whole length of almost 40,000 tubes installed in the main condenser at the Cordemais heat-power plant in France. An additional part of the contract was the classical tube bottom coating covering an area of around 80 m². In opting for the solution, the power plant operator EDF (Électricité de France SA) is avoiding the very much more expensive re-tubing of the condenser and extending its working life by several years. 

Virtually all power plants worldwide feature Plastocor, this patented two-component epoxy resin coating material marketed exclusively by ThyssenKrupp Xervon. It is primarily used in condensers (old and new) where its cold-reactive properties provide years of protection against corrosion and erosion and also for heat exchangers, oil coolers, pump housings and cooling water pipes which when coated with Plastocor are resistant to cooling water, seawater, various chemical components, solid-content emulsions and liquids at temperatures of up to 80 °Celsius.

The experts at ThyssenKrupp Xervon have devised four options, which combine into a complete coating system for all the important cooling circuit components. Plastocor 400 K is a brushable substance configured specifically for the water chambers through which the cooling water flows and either manually rolled or mechanically sprayed in a thickness of between 500 to 600 µm. For the bottom section of the tubes, Plastocor 2000 has been developed; a 3 to 5 mm coat which is applied together with special plugs in several layers. Plastocor IL and Plastocor IL/Ceramic are three-layer varieties of lining (thickness: maximum 120 µm) that protect the entrance and exit areas of the tube up to a length of 400 mm against wear. Complete protection for the inside of the tubes is provided by the still relatively young "tubelining" technique. Together these Plastocor options allow the inside surfaces of small-caliber condenser tubes to be coated along their entire length.

Top marks for tubelining

Tubelining has proved most effective at the French nuclear power plant in Phénix. This power plant, which exercises pilot and prototype functions, dates back to 1973 and was scheduled for decom-missioning. As a result of the tubelining carried out on 20,000 condenser tubes around two years ago, it has been possible at relatively low cost to extend the life of the condensers by several years, with no observable loss in efficiency. At the same time, downtime losses were significantly reduced. This proved impressive and prompted EDF to likewise apply Plastocor as protection lining on the condenser at Cordemais (34 km to the west of Nantes). Eddy current tests had revealed on the main condenser's tubes built from high-strength brass (an alloy of copper, zinc, and nickel), such perva-sive wall thickness variations as to require, as an alternative option to relining, the complete re-placement of all tubes. This would, however, have cost ten times as much and taken up considera-bly more time (power plant outage).

As part of a general power plant overhaul, a 14-strong team from Xervon lined the tube plates and interior tubes between the start of April and the end of July. For this purpose, the plates were first sandblasted according to DIN EN ISO 12944 (part 4): roughness in excess of 60 µm, cleanliness degree SA 2,5. The 13 m long internal tubes were then jet treated in preparation for their lining. The fine abrasive was swirl blasted into the tube, and this process repeated along the entire length, with the result that all the surface deposits were removed, leaving behind a lining-friendly substrate. Then the 80 m² of tube bottoms were lined with Plastocor 2000. This latter process was carried out using a conical system of plugs devised for this purpose. The plugs acted as the female mold and before the lining was applied, inserted into the tube overhang or projection to allow for the desired lining thickness (normally 3 to 5 mm). After this, the lining was applied in several layers. After allow-ing eight to twelve hours for intermediate drying at a temperature of 20 °C, the lined surfaces were then ground, with the plugs subsequently removed to allow final sealing. The result was that the hardened lining modeled itself on the conical shape of the plugs for turbulence-free inflow into the cooling water tubes and their protection against corrosion.

This was followed by the tubelining process using Plasticor TL in the course of which the 37,150 tubes each measuring 13 meters were coated with a thin lining of Plastocor. The lining is always applied "on site" at the condenser, partly automatic with a specially developed tube lining machine. For this purpose, a hose fitted with a circular conical spraying nozzle penetrates the tube from one end to the other. The next step was the tubelining applied by the tubelining machine and while the hose retracted (outside of the tube, the spray nozzles were adjusted inside a second), the inside of the tube was coated along its entire wall. All the key parameters are computer controlled. The es-sential advantage of this method: the effect of the lining on condenser performance is no more than would result from the normal changes occurring in an unprotected tube. Quite the contrary: whereas fouling, corrosion and erosion normally after only a few months inhibit heat conductivity on a new, unlined tube, a Plastocor lined tube maintains a high rate of heat conductivity for a much longer time span. What's more, the hydrophilic surface is remarkably caking resistant and hence can be more readily cleaned and maintained.

The stiffest challenge posed by the Cordemais project was the very tight time-frame. Tubelining calls for vast expertise, a delicate touch, and is certainly not the type of work that can be rushed. What's more, the smaller net tube diameter of only 18 mm (20 mm or more is usual in Germany) didn't help accelerate the application of the lining, either. Despite all these obstacles, the Xervon specialists completed the job in good time.