Repair of gas turbine diaphragm

A method of refurbishing worn diaphragm rails for turbo machines. This method comprises machining the worn part of the diaphragm rails such that a clean and geometrically exact machined surface is achieved. Welding one or more layers on these machined surfaces builds up a cladding that overtops the nominal dimensions of new diaphragm. The method further comprises machining the cladding such that it has the nominal dimensions of a new diaphragm.

BACKGROUND OF THE INVENTION

The subject matter disclosed herein relates to the art of turbomachines and, more particularly, to a turbomachine diaphragm and a method of repairing a turbomachine diaphragm.

In a turbomachine, air is passed into an inlet of a compressor. The air is passed through various stages of the compressor to form a compressed airflow. A portion of the compressed airflow is passed to a combustion assembly and another portion of the compressed airflow is passed to a turbine portion and used for cooling. In the combustion assembly, the compressed airflow is mixed with fuel and combusted to form a high temperature gas stream and exhaust gases. The high temperature gas stream is channeled to the turbine portion via a transition piece. The transition piece guides the high temperature gas stream toward a hot gas path of the turbine portion. The high temperature gas stream expands through various stages of the turbine portion converting thermal energy to mechanical energy that rotates a turbine shaft. The turbine portion may be used in a variety of applications including providing power to a pump, an electrical generator, a vehicle, or the like.

From U.S. Pat. No. 9,303,512 B2 a method for repairing a turbomachine diaphragm is known. It includes removing a worn area from a diaphragm rail member by machining and forming a repair coupon mounting element in the diaphragm rail member. Further a repair coupon is machined and bonded to the machined surface at the repair coupon mounting element.

In US 2010/0290902 A1 a technique for refurbishing nozzle diaphragm sections of a gas turbine is described. This technique replaces an eroded or worn section of the nozzle diaphragm with a replacement part designed to be positively locked in a slot machined in the nozzle diaphragm. The replacement part is formed of a material having a similar coefficient of expansion as the material used for manufacturing the original nozzle diaphragm. The combination of the nozzle diaphragm and the replacement part conform to the original manufacturer's dimensional specifications for the nozzle diaphragm.

BRIEF DESCRIPTION OF THE INVENTION

It is an object of the invention to further improve and enhance the possibilities of repairing a turbomachine diaphragm. Further it is an object of the invention that the repaired diaphragm has improved abilities and extended service life compared to a new diaphragm.

These objects are achieved using a method of repairing a turbomachine diaphragm according to claim1.

The claimed method is rather efficient since it comprises only three steps. The first and second step consist of machining the worn or eroded coupon of the diaphragm rail such that a clean, not corroded and geometrically exact machined surface is generated. Of course, the machined surface has reduced dimensions compared to the nominal dimensions of the diaphragm rail member. The difference in size between the machined surface and the nominal dimensions of a new diaphragm are filled by a filler material, which forms the claimed cladding (third step).

The claimed method has been successfully executed repairing diaphragm cast of a nickel iron alloy, so-called Ni-resist, and a cladding consisting of a filler material from austenitic stainless steel, such as LS 309LSI, 300 series fillers and 312. The claimed method is flexible since only the worn parts of the diaphragm rail member are machined and cladded. Those parts of the rail member that are not worn or eroded, need not be cladded. This means that solely the worn parts of the diaphragm are repaired, reducing costs for machining and cladding of the diaphragm.

In case the distance between the machined surfaces and the nominal dimensions of the diaphragm are greater than the thickness of one layer of the cladding, several layers up to ten or even more layers can be welded to the machined surface of the diaphragm such that the whole affected feature or coupon is being restored. At the end of the welding process the surface of the cladding overtops the nominal dimensions of a new diaphragm. The last step of the claimed method consists of machining the cladding to the nominal dimensions of a new diaphragm according to the manufacturer's specification. Machining may be a milling process or any other suitable process.

In a further advantageous embodiment each welding layer includes several weld passes welded in close proximity to each other such that a compact cladding is achieved.

The claimed method may be further improved if the main welding parameters, such as weld speed, current and voltage, wire speed, and other parameters, well-known to the man skilled in the art, are adapted accordingly. It is possible to adapt these parameters for each pass of a layer, each layer or only once for welding a complete cladding.

To avoid or at least reduce mechanical stress due to the welding process it is preferred that in case the machined surfaces have a symmetric cross-sectional area the passes are welded alternating on each side of the axis of symmetry.

It has been proven advantageous if the welding process is a MIG/MAG-process. Further, it has been proven advantageous if the welding process is supported by an inert gas, wherein the inert gas consists of more than 90% argon and the rest CO2. A TIG process may also be used for the repair process however different parameters are used in this case.

After the welding process is finished, the claimed method comprises the step of machining the cladding until it has the nominal dimensions of a new diaphragm according to the manufacturer's specification.

The above-mentioned objects are also achieved using a turbine machined diaphragm according to one of the claims11to13.

DETAILED DESCRIPTION

Referring toFIG. 1, a turbomachine, in accordance with an exemplary embodiment, is indicated generally at2. Turbomachine2includes a turbine portion3having a housing4that defines, at least in part, a hot gas path10. Turbine portion3includes a first stage12, having a plurality of first stage vanes or nozzles14, and first stage buckets or blades16; a second stage17having a plurality of second stage vanes or nozzles18and second stage buckets or blades20; and a third stage21having a plurality of third stage vanes or nozzles22and third stage buckets or blades24. Of course it should be understood that turbine portion3could also include additional stages (not shown).

Hot combustion gases flow axially along hot gas path10through nozzles14,18, and22, impact and rotate blades16,20, and24. In addition, a cooling airflow is guided into a wheelspace (not separately labeled) of turbine portion3. The cooling airflow, typically from a compressor portion (not shown) is directed through various components of turbine portion3to reduce localized hot spots, improve wear, and increase an overall component life. Each nozzle14,18, and22includes a corresponding diaphragm, one of which is shown at30, that provides a seal which prevents hot gases from passing from hot gas path10into the wheelspace. Diaphragm30cooperates with additional structure, (not shown), to limit ingestion of hot gases into the wheelspace. Loss of hot gases from hot gas path10into the wheelspace reduces operational efficiency of turbine portion3. Over time, portions of diaphragm30may become worn and require localized repair as will be discussed more fully below.

Reference will now follow toFIG. 2in describing a new, not worn diaphragm30. Diaphragm30is often cast from nickel-iron alloy and includes a body34having a sealing section36, a first rail member38and a second rail member39. Sealing section36includes a first end portion44that extends to a second end portion45through an intermediate portion46that defines an outer surface portion48and an inner surface portion49. Outer surface portion48is provided with a plurality of seal elements50that cooperate with additional structure (not shown) arranged in the wheel space of turbine portion3. First rail member38extends from first end portion44and second rail member39extends from second end portion45. First rail member38includes a first end section54that extends to a second end section55through an intermediate section56that defines an inner surface section58and an outer surface section59. Outer surface section59includes a discourager seal mounting section51that supports a discourager seal (not separately labeled). Second end section55supports a coupon63being an integral part of the first rail member38.

Similarly, second rail member39includes a first end section74that extends to a second end section75through an intermediate section76that defines an inner surface section78and an outer surface section79. Outer surface section79includes a discourager seal mounting section81that supports a discourager seal (not separately labeled). Second end section75supports a coupon83being an integral part of the second rail member39. Over time the original coupons63,83wear. Worn coupons63,83may allow hot gasses to flow from hot gas path10into the wheel space or other regions of the turbomachine. The loss of gases from the hot gas path10reduces turbine efficiency. Accordingly, diaphragms30are either repaired or replaced during a maintenance interval. In accordance with the exemplary embodiment, instead of a labor intensive repair of the original coupon, the exemplary embodiment discloses various techniques for replacing the original coupon with a repair coupon.

Reference will now be made toFIG. 3illustrating in more detail the contour of the coupon83with an understanding that coupon63has a rather similar contour.

Second end section75includes a surface90and the coupon83. The surface90, the coupon83and a discourager seal mounting section81among others constitute the main dimensions of the second rail member39.

More specifically, coupon83includes an end104joined by first and second opposing sides105and106forming a substantially rectangular cross-sectional area defined by104,104and106. This cross-sectional area complies with the nominal dimensions of a new diaphragm30according to the manufacturer's specification.

A hatched line103illustrates an exemplary contour of a worn diaphragm30. By comparing the cross-sectional area and the cross-sectional area103of a worn diaphragm30, it becomes apparent that due to the reduced contour103of a worn diaphragm30the loss of hot gases increases significantly and repair of the diaphragm30is required.

Before cladding the worn part(s) of the diaphragm30the worn part(s) have to be removed in part such that the claimed welding process start on a clean machined surface of the diaphragm30. The at least on machined surface can be manufactured for example by milling or the like.

In the embodiment illustrated inFIG. 3there are five (5) machined surfaces110,111,112,113and114. As can be seen by the comparison of the machined surfaces110to114and the cross-sectional area of a new diaphragm30according to manufacturer's specification (c. f. the reference numerals90,104,105and106) it can be seen that the contour of the machined surfaces110to114is smaller than the contour of a new diaphragm. The volume between the machined surfaces110to114and nominal dimensions (c. f.90,104,105and106) are filled by a filler material.

Welding this filler material to the machined surfaces110to114creates a compact cladding comprising one or more layers which fills the a. m. volume. Of course, the cladding has to overtop the contour of a new diaphragm30since welding is a process that does not produce geometrically exact surfaces. This means that a part of the cladding has to be machined after the cladding process to bring the cladding in conformity with the nominal dimensions following the surface90, the sides105and106as well as the end104of a new diaphragm30.

FIG. 4, wherein like reference numbers represent corresponding parts in the respective views, illustrates a perspective view of the second rail member39.

FIG. 5, wherein like reference numbers represent corresponding parts in the respective views, illustrates a perspective view of the contour103of the second rail member39that is worn over an angle alpha in a tangential direction.

The angle alpha illustrates the length of the worn part of a diaphragm rail member39. Of course, only the worn parts of the diaphragm have to be repaired. It is in most cases not necessary to machine the diaphragm rail member39over its entire length. This reduces the machining time and further reduces the time and expenses for welding a cladding on the machined surfaces.

Of course, if necessary it is possible to machine the diaphragm over the entire length of its rail members38,39and weld a cladding over the entire length of the rail member38,39. This is necessary, if the whole rail member is worn or if the material of the diaphragm that is exposed to the hot gases should be replaced by a cladding material that better withstands the hot gases, such as austenitic stainless steel compared to cast nickel iron, which is in most cases the material of the diaphragm30.

TheFIGS. 6A, 6B and 6Cillustrate the cross-sectional area of a worn diaphragm that has been machined according toFIG. 3. As can be seen fromFIG. 6athe machined surfaces110to114make a more or less symmetric cross-sectional area. To reduce the thermal tensions to the diaphragm it is preferred if the weld passes are alternatingly welded on both sides of the axis of symmetry.

In this particular case, a first weld pass1is welded on the machined surface112, which is the end of the machined contour. A second weld pass2is welded in the corner between the machined surfaces114and113. A third weld pass3is welded in the corner between the machined surfaces110and111. The sequence of the weld passes1to22can be seen fromFIGS. 6ato 6c. Each weld pass has a number and this number describes the sequence of the welding passes welded to the diaphragm.

The most important welding parameters have been listed in the subsequent tables that are linked to each of the figures.

Very good results have been achieved using these welding parameters if the diaphragm is cast of nickel iron and the filler-material for welding the passes is an austenitic stainless steel. Appropriate stainless steel alloys are known under the tradenames 300 series and 312.

FIGS. 6A, 6B and 6Cillustrate the process of cladding a machined surface of a diaphragm rail member. In this case a weld robot or a weld automat is used. The welding method is MIG/MAG. A TIG process may also be used for the repair process however different parameters are used in this case.

Cladding is achieved by welding several passes side by side. If necessary several layers of passes are welded to achieve the desired contour of the cladding. Up to ten layers have been welded in several applications.

In theFIGS. 6 and 7the passes have been numbered and in the respective tables listed below the most important welding parameters (welding speed and welding angle) have been noted.

Passwelding angle10°245° (0 arc length correction)345° (0 arc length correction)467.5°567.5°622.5°722.5°890°990°1022.5°1122.5°
The weld passes1to11results in a compact first layer of the cladding.

Passwelding angle1245° (0 arc length correction)1345° (0 arc length correction)1467.5°1567.5°1622.5°1722.5°1890°1990°
The weld passes12to19results in a compact second layer of the cladding.

Passwelding angle200°210°220°
The weld passes20to22results in a compact second layer of the cladding. The entirety of welding passes1to22forms the cladding116.

FIG. 7to G illustrate a further embodiment of the claimed method. The cladding generated in this embodiment comprises ten (10) layers.

The entirety of weld passes1to51forms the cladding116.