Coating processes – Interior of hollow article coating – Metal base
Reexamination Certificate
1998-06-22
2001-07-03
Beck, Shrive (Department: 1762)
Coating processes
Interior of hollow article coating
Metal base
C427S230000, C427S292000, C427S388100, C427S435000, C165S110000
Reexamination Certificate
active
06254930
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a coating for tube sheets (also referred to herein as “tubesheets” or “tube beds”) and heat exchanger coolant tubes extending from them, especially steam condensers, based on hardening plastic mixtures that can be obtained by cleaning the surfaces provided for coating using an abrasive; closing the tube inlets and outlets with removable plugs; applying at least one layer of a hardening plastic coating or mixture on the tube sheet; allowing the coating to harden so that additional mechanical processing can ensue, and processing the surface; removing the plugs from the tube inlets and outlets, as well as applying at least one layer of a hardening plastic coating at least in the inlet area of the coolant tube, and allowing it to harden, as well as a process for coating the tube sheet and heat exchanger coolant tubes extending from these. In an alternative embodiment only the tube inlets are closed with removable plugs, after the surfaces are cleaned with an abrasive, and the plugs are removed only from the tube inlets after the surface is processed.
2. Summary of the Related Art
How to provide tube sheets having heat exchangers, as they are for example employed in facilities for production of electrical energy, with a coat of plastic to counteract the effects of corrosion is known. Tube sheets and the coolant tubes extending from them are subject to a variety of external influences, especially mechanical, chemical, and elector-magnetic stresses. Mechanical stresses occur as a result of solid particles carried along by the coolant, sand, for example. In addition, enlargements in the roll in section, an area of the tube of the coolant tubes on the tube sheet occur as a result of the difference in temperature between the coolant and the steam to be condensed, which can exceed 100° C. Chemical stresses result from the nature of the coolant, for example, from its loading with salts or acid substances. In particular, remark should be made in this regard about the known corrosive effects of sea water or heavily-loaded river water employed for coolant purposes. The electro-chemical or galvanic corrosion that should be mentioned is that which occurs as a result of development of galvanic elements on metallic border surfaces, especially at the transitions from the tube sheets to coolant tube, and which is strongly promoted by electrically conductive liquids like sea water. In addition, there are limitations on the functionality of the tube sheet as a result of deposits of undesirable materials, formation of algae, etc., on its surface, which is particularly promoted by surface roughness resulting from the effects of corrosion. This has as its result that the effects of corrosion and deposits accelerate with the age of the tube sheet because they increasingly form new locations for corrosion and deposits to take hold.
From very early on, therefore, steps have been taken to provide tube sheets with a coating of plastic material that reduces corrosion. In particular, thick coats of epoxy resin were used for this purpose, these being adapted to the tubing inlets and outlets using certain techniques, for example, by using formed plugs during application. In this way coating of the tube sheets can initially be adapted seamlessly at the tubing inlets and outlets, interior coating of the mostly non-corrosive materials remaining at the ends of the tubes or in the area of the coating generally being dispensed with. But even in such solutions, coolant water could penetrate over time through microcracks and therefore could certainly not prevent development of galvanic elements; this having as its result an increasing incidence of corrosion after formation of the first crack. Even including the coolant tubes in the coated surface, at least in the area of its inlet and outlet, achieved only limited improvements, since the prevailing extreme thermal and mechanical stresses in this area lead to formation of hair-cracks in exactly the sensitive area that transitions from tube sheet to coolant tube. If, however, the bond between the tube sheet and the tube coating is broken even once at these locations, the protective effect of the coating is increasingly affected.
Measures of the type just described are known, for example, from GB-A-1 175 157, DE-U-1 939 665, DE-U-7 702 562, and EP-A-O236 388.
SUMMARY OF THE INVENTION
Considering the previously described problems, the task of the invention is based on providing the tube sheet (which, as indicated above may also be referred to herein as a “tubesheet” or “tube bed”) and the coolant tube inlets and outlets adjacent to the tube sheet an integrated coating for both, which coating offers long-term resistance to the mechanical stresses at the transition points and which at the same time is suitable for resisting chemical stresses resulting from the coolant.
This task is solved using a coating of the type described at the beginning, in which the coolant tubing (or cooling tube) coating is affixed reactively to the tube sheet coating by timed application and in which the coolant tube coating exhibits in comparison to the tube sheet coating a greater elasticity having an elongation at break at least 2% greater in accordance with ASTM Standard D522, “Standard Test Methods for Mandrel Test of Attached Organic Coatings” (November 1993, believed to be identified as D522-93-A) with respect to the elongation at break (or elongation at tear) of the tube sheet coating.
Timing the coating processes on the tube sheet and in the coolant tubes allows cross linking between the coating edges of the coating in the tubes and the tube sheet coating to occur, so that there is a chemical bond especially capable of bearing. At the same time and additionally, the relatively greater elasticity of the coolant tubing coating effects better resistance to mechanical stress in the inlet and outlet areas of the tube at those locations that experience galvanic corrosion. It has been demonstrated that an increase of 2% in the elongation at tear in accordance with ASTM Standard D522 (or “ASTM D552”) is in general sufficient to effect the improvement in the coating bond, an elongation at tear in the tube sheet coating of less than 5% and in the coolant tube coating of less than 10% being assumed, in order to provide the hardness, resistance to abrasion, and compressive resistance necessary for the durability of the coating. On the other hand, for the tube sheet coating, elongation at tear should not fall below 2% in order to avoid brittleness. Materials having elongation at tear in accordance with ASTM D522 of 2 to 4% have proved particularly suitable for the tube sheet, and 4 to 9% for the coolant tubes. Of particular advantage are coatings having elongations at tear of more than 3% for the tube sheet and more than 5% for the coolant tubes.
In order to apply the layers of coating necessary for lasting operation over several years and at the same time to ensure quality relative to adhesion and freedom from pore and hairline tears, it is useful to apply the coating in accordance with the invention in multiple layers, each layer being applied to the still-reactive surface of the layer underneath, in order to achieve chemical cross linkage. For purposes of utility, two or three layers are applied both to the tube sheet and to the coolant tubes; these may be differently colored in order to allow coloration to be used to inspect remaining thickness of the coating from time to time. The minimum layer thickness of the entire coating for the interior coat of the tubes is at least about 80 &mgr;m and for the tube sheet is at least 2000 &mgr;m. Layer thicknesses of 20 mm and more are easily possible without suffering losses in fastness. This is a particular advantage when working with coating tube sheets that are already heavily corroded and that exhibit deep scars from corrosion.
It has proved to be very useful to provide the cleaned surfaces of the tube sheet and the coolant tubes with a primer prior to applying the actual coa
Beck Shrive
Calcagni Jennifer
Hunton & Williams
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