FIELD OF INVENTION
The present invention relates to a method of Internal cooling of balanced piston
of a steam turbine with welded rotor configuration. More particularly, the invention
relates to a cooling arrangement in balancing piston area where cooling steam is guided
inside the balancing piston of rotor to cool the metal in high temperature zone of
turbine rotor which otherwise necessitates using of Nickel based alloys in the zones of
high temperature. For this purpose, a turbine welded rotor configuration is implemented
where a balancing piston is having double weld joint consisting of a bimetallic weld and
adjacent to it a similar metal weld.
BACKGROUND OF THE INVENTION
In India, fossil fuel is the mainstay of power sector industry and it is expected to
remain so in future also. To utilize the fossil fuel more efficiently in power plants and to
achieve the goal of reduction in global warming, indigenous development of more
efficient fossil fuel power plant technology operating at Advanced Ultra Super Critical
(AUSC) parameters (Temperature more than 7000C) has already been initiated in India.

To increase the efficiency of steam power plants the classical method is to
increase the operating parameters. But higher the temperature of operation, lesser is
the resistance of material to creep (a time-dependent failure of materials at elevated
temperature under applied stresses). The utility of materials, exposed to elevated
temperature and pressure, is limited by their capacity to sustain elevated temperatures
without undergoing loss of strength over prolonged exposure. Chrome steels (9-10%
Chrome) currently used in steam turbines operating at supercritical (SC)/Ultra Super
Critical (USC) steam parameters exhaust their load carrying capability at around 6000C.
Steam turbine rotors operating at Advance Ultra Super Critical (AUSC) parameters are
expected to have a Rotor Inlet Temperature of greater than 7000C. Hence, Nickel based
alloy Alloy617(M) is identified for the rotor zone operating above the capability range of
chrome steels. At the location where the temperature is high enough to mitigate the
use of Chrome steels a bimetallic weld between Alloy617(M) and Chrome steel is
envisaged. Also creep capability of material improves sharply even with a small
decrease in temperature, hence cooling arrangement is required for the material in high
temperature turbine inlet zone, where stresses are high.
OBJECTS OF THE INVENTION
Therefore, it is an object of the invention to propose a method of Internal
cooling of balanced piston of a steam turbine with welded rotor configuration, which is
capable of providing a cooling arrangement interior to a balancing piston through a

number of drilled radial holes made on chrome steel portion of balanced piston and
thereby cooling its highest temperature zone.
Another object of the invention is to propose a method of Internal cooling of
balanced piston of a steam turbine with welded rotor configuration, which is able to
provide a balancing piston with double weld, joint consisting of a bimetallic weld and
adjacent to it a similar metal weld required to withstand high stress and high
temperature.
SUMMARY OF THE INVENTION
A steam turbine with a casing, comprising a bimetallic welded steam turbine
rotor (4) arranged inside an inner casing (3) along the rotational axis (5) of turbine.
Inner casing (3) is assembled inside the outer casing (2). Turbine rotor (4) is having a
balancing piston (10) to compensate the axial thrust. Balancing piston (10) is having
double weld joint consisting of a bimetallic weld and adjacent to it a similar metal weld.
A dissimilar metal weld (14 & 17) is created by welding a small length hollow
circular metal piece (15 & 18) of one metal with the portion of turbine rotor that is
made of another metal and extensive nondestructive testing of dissimilar metal weld is
carried out with this configuration;

A similar metal weld (16 & 19) is created adjacent to dissimilar metal weld by
welding the free end of small length metal piece and remaining portion of turbine rotor.
To reduce the cost of turbine components, use of expensive Nickel based alloys
is planned only in the zones of high temperatures. For this purpose, a turbine rotor (4)
configuration with dissimilar weld configuration (Alloy617 to Chrome steels) is planned.
As dissimilar metal weld between these two materials is not a field proven technology in
steam turbine rotors, a more stringent inspection of these dissimilar metal welds is
implemented. To have a better access to these dissimilar welds from both directions,
current configuration of rotor with manufacturing plan of rotor given in Figure 4 is
envisaged.
In the beginning, hollow pieces of small length of Chrome steel (15 & 18) are to
be welded on both the ends of Alloy 617(M) portion of the rotor. These dissimilar metal
welds (14 & 17) are inspected through several nondestructive techniques to the
required level of satisfaction in this condition itself. Subsequently portion of the rotor
made of Chrome steel is welded to the respective sides of the rotor at both the ends,
forming similar metal welds (16 & 19). As these later welds are similar metal welds (16
& 19) and there is huge experience of such similar metal welding, inspection
requirements are less stringent for these welds compare to dissimilar metal weld.

The seals (not shown in figure) provided between balancing piston (10) and
inner casing (3), minimizes the leakage of steam (11) from this gap. As shown in Fig. 2,
this leak-off steam (11) expands and cools down while passing through these seals. It
is proposed to cool it further by mixing it with high pressure low temperature steam
(12) filled between inner and outer casing. This steam (24; mixture of 11 & 12) is
allowed to expand further in seals. The invention is based upon the aspect that during
operation this steam (21) at relatively low temperature is guided to interior of rotor in
balancing piston (10) area through circumferentially spaced radial holes (25) in chrome
steel portion of rotor. Inside the rotor (4) it is first guided towards high temperature
inlet zone for cooling it. The steam is further guided through an axial hole (22) at rotor
center to gland seal area which is connected with gland seal condenser. With the
proposed cooling configuration balancing piston and inlet zone, both can be cooled.
It is further proposed to have blade grooves on the metal piece in between
dissimilar metal weld and similar metal weld in the flow path area.
A further aspect of invention is that all the features that are source of stress
concentration i.e. radial holes to guide the steam towards interior of the rotor and
contours forming the steam guiding path are made in more proven material (Chrome
steel).

A further aspect of invention is, if required a pressure test of the cavity interior
to balancing piston of rotor could be carried out, which will be useful for accessing the
soundness of both the welds on balancing piston.
A further aspect of invention is that the allowable stress limit of the material
which is used for steam turbine rotor is increased because of cooling arrangement in
balancing piston area.
Welded configuration allows the use of high strength high cost material only in
the high temperature zone. As temperature of steam beyond this zone is in the
capability range of Chrome steel, which is relatively low cost material, a dissimilar metal
weld is created which allows the use of Chrome steel from dissimilar weld location
onwards. This results in a cost saving and optimal utilization of materials.
A cooling arrangement in which steam flows interior to balance piston through
radial holes (25) and subsequently directed towards the steam inlet side of turbine rotor
for cooling its highest temperature zone. This steam (21) subsequently moves towards
rotor center and from there it moves along a hole (22), which is in the direction of
turbine axis. Finally this steam comes out of turbine rotor through radial holes (23)
which open in a zone connected with gland steam condenser.

Radial holes (25), from which steam flows interior to balancing piston are drilled
in Chrome steel portion of balancing piston (10). Location of these holes is after similar
metal weld in direction opposite of steam flow in flow path.
Radial holes (23), from which steam comes out of turbine rotor, are placed in a
zone that is connected with gland steam condenser.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 represents a cross sectional view of a high pressure steam turbine as per the
proposed invention.
Figure 2 represents the cooling arrangement as per the proposed invention.
Figure 3 represents the configuration of weld falling in flow path area.
Figure 4 represents manufacturing sequence of proposed configuration of rotor.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE
INVENTION
In figure 1 a section through a high pressure steam turbine is shown. The high
pressure steam turbine section (1), as an embodiment of a steam turbine, comprises of
outer casing (2), inner casing (3) and turbine rotor (4). Inside the inner casing (3), a

turbine rotor (4) is assembled in such a way that it can rotate about a rotational axis
(5). The turbine rotor (4) comprises of moving blades (6), which are assembled in the
grooves on the surface of turbine rotor (4). The inner casing (3) comprises of fixed
blades (7), which are assembled in grooves on the inner surface of inner casing (3).
The zone comprising moving blades (6) and fixed blades (7) is called flow path (20).
The fixed and moving blades are arranged in such a way that steam flows in direction
(8). The high pressure turbine section (1) has an inlet zone (9) through which steam
enters between inner casing (3) and turbine rotor (4) and subsequently passes through
fixed blades (7) and moving blades (6) alternatively. During passing through fixed
blades (7) and moving blades (6) steam expands and it exerts a force on moving blades
(6), loses its internal energy, which is converted into rotation of turbine rotor (4). The
force, which steam exerts on moving blade (6), is having a component along the
rotational axis (5) of turbine rotor (4) in the direction of steam flow (8). To compensate
this thrust and to have resultant thrust opposite to the direction of steam flow (8) as
per design requirement, a balancing piston (10) is provided in the design of turbine
rotor (4). Balancing piston (10) is a projected portion on turbine rotor (4). A pressure
difference across the balancing piston is created in such a way that turbine rotor (4)
has a net residual force in axial direction opposite to direction (8), which is taken care
by thrust bearing (not represented in figure).

To reduce the cost of turbine rotor (4), use of expensive alloys is planned only in
the zones of high temperatures and hence turbine rotor (4) is a dissimilar metal welded
configuration. Manufacturing sequence of turbine rotor (4) is shown in Figure 4 and
final configuration of turbine rotor (4) without assembling moving blades (6) is shown in
the step-3 of same figure. One of these dissimilar metal weld (14) is in balancing piston
(10) area and the second dissimilar metal weld (17) is in flow path (20) region. To
ensure a better inspection of these dissimilar metal welds (14) and (17), in the
beginning only small length hollow circular pieces (15) and (18) of another metal are
welded at balance piston (10) side and flow path (20) side respectively (Figure 4 step
1). After successful nondestructive examination of these dissimilar metal welds (14) and
(17) at this stage, respective portions of rotor made of Chrome steel are welded to
these small hollow circular pieces (15) and (18) with similar metal welds (16) and (19)
respectively. Hence by placing a similar metal weld adjacent to dissimilar metal weld,
better inspection of dissimilar metal weld is ensured. Small hollow circular metal piece
(18) that is in flow path zone (20), may also have assembled moving blade(s) (6). The
dissimilar metal weld (17) and similar metal weld (19) in flow path (20) zone are shown
in Figure 1 and Figure 3.
The proposed cooling arrangement is shown in Figure 2. A small portion (11) of
inlet steam leaks through the inlet zone (9) to the seals (not shown in figure), which
are assembled in the gap between balancing piston (10) of turbine rotor (4) and inner

casing (3). This leak-off steam (11) expands and cools along its path in seals. High
pressure low temperature steam (12) filled between outer casing (2) and inner casing
(3) enters in the balancing piston area through radial holes (13) in the inner casing (3)
and mixes with the leak-off steam (11). This mixed steam (24) expands and cools
further along its path as it passes through seals. A portion of this mixed steam (21) is
guided interior to balancing piston (10) of turbine rotor (4) through radial holes (25)
while remaining portion of cooled steam (27) further expands in the seals. With the
help of deflector (26), the steam in interior (21) of balancing piston (10) is directed
towards inlet zone (9) of turbine rotor (4) for cooling of high temperature zone of
turbine rotor (4). Subsequently the flow of interior steam (21) is directed towards
center of rotor, where it flows through an axial hole (22) at the center of turbine rotor
(4) and finally it flows out of the turbine rotor (4) through radial holes (23) and mixes
with remaining steam (27). This zone is connected with gland steam condenser (not
shown in figure) and the mixed steam goes to gland steam condenser from here.
It is important to note that in addition to providing a welded rotor that provides
excellent cooling arrangement of balancing piston, it is apparent that the embodiment
of the invention, as described hereinabove, shall provide an extremely critical technique
for welding the dissimilar metals for manufacturing more reliable welded rotors,
adaptable for different ratings of turbines, particularly steam and gas, providing an
even further versatile scope of use.

An internal pressure testing of cavity formed inside turbine rotor, as a
supplement to nondestructive testing is carried out to ensure the integrity of welds on
turbine rotor.
In the manufacturing sequence of turbine rotor similar (16 & 19) and dissimilar
welds (14 & 17) are placed close to each other and dissimilar metal weld (14 & 17) is
created first and inspected before welding the similar metals.

WE CLAIM
1. A method of Internal cooling of balanced piston of a steam turbine with welded
rotor configuration, the said method comprising;
disposing small length hollow circular pieces (15, 18) welded at balance piston
(10) side and flow path (20) side;
welding dissimilar metals (14, 17) with said circular pieces (15, 18);
inspecting dissimilar metal weld thoroughly;
welding similar metals (16, 19) with circular pieces (15, 18);
guiding a portion of steam (21) interior to balancing piston (10) through radial
holes (25);
directing the steam in interior (21) of balancing piston (10) with a deflector (26)
towards inlet zone (9) of turbine rotor (4) to cool high temperature zone of turbine
rotor (4);
directing the flow of interior steam (21) towards center of rotor;
wherein the said steam (21) flows through an axial hole (22) at the center of
turbine rotor (4) and finally flows out of the turbine rotor through radial holes (23) and
mixes with remaining steam (27) and the mixed steam goes to gland steam condenser.

2. The method as claimed in claim 1, wherein similar (16 & 19) and dissimilar welds
(14 & 17) are placed close to each other and dissimilar metal weld (14 & 17) is created
first and inspected before welding the similar metals.