Method for preventing harm from constrictions formed during coating with a protective coat in the cooling holes of gas-cooled parts

A method is disclosed for preventing harm caused by constrictions in the cooling holes of gas-cooled parts, in particular of gas turbines and/or burners. These constrictions form because the walls of the gas-cooled parts through which the cooling holes extend and which are covered on their outside with a protective, first coating are provided during the course of an overhaul with a second, repeat coating, and coating material is deposited in the cooling holes. Constant flow ratios in the cooling holes are achieved in a simple manner in that prior to the overhaul, the cooling holes are constricted with first means to an extent that corresponds to the later constrictions created during the course of the second coating. The first means are removed after the overhaul. The first means takes the form of an insert with holes that constrict the flow through the cooling holes.

FIELD OF THE INVENTION
 The invention relates to the field of technology of thermal machines, in
 particular gas turbines and/or burners, having walls of gas-cooled parts
 through which the cooling holes extend and which are covered on their
 outside with a protective coating.
 BACKGROUND OF THE INVENTION
 The coating of thermally stressed parts of gas turbines or burners equipped
 for the cooling of their walls with cooling holes for the air or gas
 cooling usually cannot be renewed easily by using overlay coating and/or
 thermal barrier coating, TBC, because the cooling holes are at least
 partially closed or constricted by the repeat coating, so that the cooling
 effect is changed and/or adversely affected. This is true, in particular,
 if the cooling holes have been provided in the wall after the original
 coating.
 Several solutions have been described for preventing harm from
 constrictions formed when coating parts provided with cooling holes. In
 one type of solution, measures are taken to prevent the constrictions from
 even forming. To accomplish this, the cooling holes are closed or filled
 prior to the coating. The filling of the cooling holes prevents coating
 material from penetrating into the holes. After the coating process is
 complete, the filling is again removed from the cooling holes, and the
 cooling holes are available with their original through-cross-section.
 Such techniques are known, for example, from U.S. Pat. No. 4,743,462 and
 U.S. Pat. No. 5,800,695.
 In another proposed solution, the cooling holes are left open, and the
 constrictions formed in the holes during coating are removed after the
 coating, by using a pluse UV laser beam as is known from U.S. Pat. No.
 5,216,808. It would also be conceivable, however, to remove the
 constrictions by pressing a grinding fluid through the cooling holes (U.S.
 Pat. No. 5,702,288) that abrades the constrictions in this manner.
 The disadvantage of this method is that the individual cooling holes must
 be carefully closed and then again exposed or freed from the constriction
 again carefully so that the originally intended cooling effect is fully
 preserved at all places on the cooled part, and no local overheating
 occurs as a result of insufficiently removed constrictions. In parts
 equipped with a large number of finely distributed cooling holes, this
 creates significant work during the processing.
 SUMMARY OF THE INVENTION
 It is therefore the objective of the invention to provide a method in which
 the through-cross-section of the individual cooling holes can, in a simple
 manner and without intensive filling or abrasion steps, practically be
 held constant during the repeat coating of the parts. This objective is
 accomplished by permitting constrictions of the cooling holes to be formed
 during the repeat coating. The constant through-cross-section of the
 cooling holes is achieved by using cooling holes with a larger
 through-cross-section than needed, and the desired through-cross-section
 is adjusted with an artificial constriction. If the cooling holes later
 are constricted during the re-coating due to a deposit of the coating
 material, the artificial constriction is removed. Since the artificial
 constriction is chosen so that it corresponds to the constriction forming
 during the re-coating, the through-cross-section of the cooling holes also
 is maintained after the re-coating.
 A preferred embodiment of the method according to the invention is
 characterized in that the first means include first inserts that rest in a
 covering manner against the insides of the walls and are provided with
 first openings, each of which is associated with the individual cooling
 holes and which constrict these cooling holes. Such prefabricated inserts
 make it possible to simultaneously and evenly constrict all cooling holes
 in a simple manner.
 In a preferred further development of this embodiment, the first inserts
 are taken away after the overhaul without any substitutions. This makes it
 especially simple to remove the artificial constrictions.
 If, however, the inserts have a permanent function in the part, it is,
 according to another further development of the embodiment, advantageous
 to replace the first inserts after the overhaul with second inserts that
 have second openings that are larger than those of the first inserts.

DETAILED DESCRIPTION OF THE INVENTION
 The method according to the invention involves a part (turbine blade,
 etc.), a section of which is shown in a generalized form in the
 cross-section in FIG. 1. The gas- or air-cooled part 10 comprises a wall
 11 with an inside 12 and an outside 13. The wall 11 is provided with a
 plurality of cooling holes 14, of which one is shown representatively in
 the figure. The cooling holes 14 may extend vertically through the wall 11
 or--as shown in FIG. 1--at an oblique angle to it. On the outside 13, the
 wall is provided with a first, original coating 15 created in the usual
 manner (for example by plasma spraying) and may consist of materials known
 per se (for example, an aluminide). The first coating 15 is provided in
 the area of the cooling holes 14 with one each opening 16 that is adapted
 in its cross-section to the cooling holes. This may be accomplished, for
 example, in that the cooling holes 14, are only drilled into the wall 11
 after the first coating 15 has been applied.
 The inside diameter of the cooling holes 14 or their through-cross-section
 has been chosen from the start larger than necessary for the intended
 cooling effect. In order to achieve the actually needed cross-section,
 FIG. 2 provides that on the inside 12 of the wall 11 an insert 17 is
 arranged that covers the inside 12 of the wall 11 and is only provided in
 the area of the cooling holes 14 with one each opening 18 that has been
 selected with smaller through-cross-section or diameter than the cooling
 hole 14, and in this way represents an artificial constriction for the
 cooling hole 14. The artificial constriction restricts the flow of the
 cooling medium through the cooling hole 14. Through-cross-section or
 diameter of the openings 16 are hereby selected so that the artificial
 constriction acts in the same manner as the constriction created later
 during the re-coating due to material deposition.
 If another coating 15' is applied on the outside of the wall 11--as shown
 in FIG. 3--during a re-coating, coating material is deposited in the
 spared area of the unprotected cooling hole 14 and forms a constriction
 19. If the first insert 17 of FIG. 2 with the constricting openings 18 is
 now removed (FIG. 3), the artificial constriction of the respective
 cooling hole created by the opening 18 is equivalently replaced by the
 constriction 19 so that the flow rations in the cooling hole 14 remain
 practically unchanged. This eliminates the necessity of in any way
 preventing the constriction 19 caused by deposition or the necessity of
 removing it after the fact. The use of the insert 17 has the advantage
 that all cooling holes are simultaneously and evenly constricted with a
 single, prefabricatable part, and the constriction later also can be
 eliminated again simultaneously. In principle, it would also be
 conceivable, however, to artificially constrict the cooling holes
 individually, which would require that additional expenditure be
 tolerated.
 If the insert 17 is a functional part of the part 10, it is useful that is
 replaced--as shown in FIG. 4--after the re-coating with a second insert
 17' in which the constricting holes 18 are replaced with larger,
 non-constricting holes 18'.