FIELD OF INVENTION
The present invention relates to a method of increasing strength and rigidity of
stay rings of large low head turbine by stiffening cylinders. More particularly, the
invention relates to a method of increasing strength and rigidity of stay ring of a
large low head turbine by stiffening cylinders without increasing the height of the
top deck of the stay ring.
BACKGROUND AND PRIOR ART OF THE INVENTION
As shown in Fig. 1 and 2 in a typical reaction type hydro turbine of conventional
design stay ring (1) is the basic foundation item on which the rest of the turbine
viz: the runner envelope and the water regulating mechanism are housed. Water
regulating mechanism in-turn houses the guide bearing and the shaft seal. The
stay ring (1) also transfers the superimposed load of generators and the
concrete super structure. This downward load is transferred from the upper
portion of stay ring (1) to the foundation below through stay vanes (2) which are
like columns/pillars. The upper portion of the stay ring (1) should be strong/rigid
enough so that it does not sag between the stay vanes (2).

The stay ring (1) has another function, it guides water to the turbine runner from
the spiral casing which is attached to it. Spiral casing is part of water conductor
which guides water from the reservoir to the turbine runner.
The stay ring (1) is thus subjected to the following loads:
a) Downward load due to generator.
b) Pull of the spiral casing due to water pressure in the spiral casing
In very low head Kaplan turbines spiral casing instead of being made of steel is
made of concrete in which case it is of a trapezoidal cross section instead of
circular cross section of a steel spiral casing.
Thus in case of low head Kaplan turbines with concrete spiral casing the load on
the spiral casing is predominantly vertically downward.
The vertical downward load includes the following:
a) Generators dead load of stator,
b) Generator dead load of rotor transferred through thrust bearing
and in-turn through generator bearing bracket.
c) Dead load of turbine runner & shaft transferred through generator
thrust bearing.

d) Hydraulic thrust of the turbine runner transferred through generator
thrust bearing.
e) Dead load concrete super structure above stay ring (1).
The spiral casing pressure of these type of low head turbines being very low the
upward spiral pull on stay ring(l) is low. Hence the stay ring (1) is to be
primarily designed for a predominantly very heavy downward load. This
downward load is transferred through the stay vanes (2) to the foundation. The
upper portion of the stay ring (1) is to be designed to provide sufficient rigidity
to prevent sagging of the larger span of upper portion between the stay vanes
(2).
Rigidity to the upper portion is provided in the conventional way by increasing
the height of the stay ring (1). Increasing strength/rigidity of the stay ring (1)
upper portion in this conventional way increases the height of top cover (5) and
guide vane stem (4) as shown in Fig.5, both of which have severe cost
repercussions. This invention (Fig. 3 & Fig.5) is about a novel way on increasing
strength/rigidity of stay ring (7) without increasing height of top cover (5) and
guide vane stem (4).
In the prior art design of stay ring (1) as shown in Fig. 2 strength/rigidity to the
stay ring (1) upper portion against very heavy downward load is provided by

increasing the height of the stay ring (1) upper portion as shown in Fig. 6. Since
top cover (5) of the water regulating mechanism is mounted on the stay ring (1)
the height of top cover (5) and the guide vane stem/trunnion (4) is increased
(Fig.5) which increase the cost of production.
OBJECTS OF THE INVENTION
Therefore it is an object of the invention to propose a method of increasing
strength and rigidity of stay ring of a large low head turbine by stiffening
cylinders which is capable of making the water regulating mechanism
economical.
Another object of the invention is to propose a method of increasing strength
and rigidity of stay ring of a large low head turbine by stiffening cylinders which
achieves result without increasing the mounting height of top cover (5).
Yet another object of the invention is to propose a method of increasing strength
and rigidity of stay ring of a large low head turbine by stiffening cylinders which
does not require to increase guide vane stem/trunnion height on account of a
taller top cover (5).

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS :-
Figure 1 :- Sectional view of turbine with stayring of conventional design.
Figure 2 :- Stay ring of conventional design.
Figure 3 :- Sectional view of turbine with stay ring of the present invention.
Figure 4a :- Stay ring with stiffening cylinders and ribs according to present
invention (Elevation View)
Figure 4b :- Stay ring with stiffening cylinders and ribs according to present
Invention (Plan View).
Figure 5 :- Comparison of the outer top cover height and guide vane
trunnion height of water regulating mechanism of the two
cases.
Figure 6 :- Comparison of the upper portion of the stay ring of conventional
design and the present invention.
Figure 7 :- Enlarged view of upper portion of the stay ring of the present
invention.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE
INVENTION
Fig. 1 shows sectional arrangement of Kaplan turbine with a conventional design
of stay ring (1). Top cover (5) of the guide apparatus (water regulating

mechanism) is bolted on the top plate of stay ring (1). The guide vanes (3)
which regulate the water are housed in the top cover (5). For regulating the
water the guide vanes (3) are rotated so as to vary the passage between guide
vane (3) and in turn to vary the discharge of water according to load. The guide
vanes (3) are rotated by levers assembled on guide vane trunnion (4) protruding
above the top cover (5).
Therefore height of guide vane stem/ trunnion (4) gets dictated by the height of
top cover (5). The height of top cover (5) gets dictated either from its strength
consideration or by the location of the top plate of stay ring (1). Location of top
plane of stay ring (1) is decided by the strength requirement of stay ring (1).
Stay ring (1) is designed for the vertical downward loads acting on the top
portion which are to be transferred to the foundations through stay vanes (2).
The simplest way to increase the rigidity/strength of stay ring (1) to bear the
vertical downward load is to increase the height of top portion (top deck) (8) of
stay ring (1).
Mounting height of top cover (5) beyond its strength requirement increases the
cost of top cover (5) as well as the guide vanes (3). Hence for economical design
of turbine top cover (5) mounting height should be minimum possible so as to
reduce guide vane stem/ trunnion (4) height.

In the present invention as shown in Fig. 5 lowering the mounting height of top
cover (5) by a height for example 'h' to invented stay ring (7), shorter would be
guide vane trunnion (4). Shorter guide vane trunnion (4) would amount to the
levers being closer to the guide vane (3). This in-turn would mean smaller
bending effect on the guide vane stem (4).
Smaller value of bending moment on guide vane stem (4) would lead to smaller
diameters being chosen for guide vane stems/trunnion (4). Smaller guide vane
stem (4) diameters would have lesser bearing friction torque and hence
requirement of a smaller guide vane servomotor. Shorter guide vane trunnion (4)
would mean more economical guide vanes (3) both in terms of material cost and
machining cost and less cost of guide vane servomotors and oil system of lesser
volume.
In the present invention the reduction in height (h) of top cover is 18-22%,
reduction in height of guide vane trunnion is 18-22% when the diameter of the
guide vane stem is reduced by 3-7% and size of the servomotor is reduced by 3-
7%. There is also a reduction in stay ring weight by 15-20% and in guide vane
weight by 22-27% for each guide vane.

Economical significance for a Kaplan turbine having diameter of 10
meters stay ring
a) Considering reduction of 20% in height (h) of top cover (S) amount of
material saving is 10-14T resulting a financial saving of Rs 10-14lakhs per
machine including fabrication cost.
b) A reduction in the stay ring (7) weight of 15% amounts to a saving of 10-
14T of material per turbine which makes a financial saving of Rs 10-14
lakhs per machine including fabrication cost.
c) A reduction in guide vane stem (4) diameter by 5% and in height by 20%
results a reduction in weight of guide vane (3). Considering 25%
reduction in weight for each guide vane, it amounts to 120 kg. There are
24 guide vanes per machine. Therefore the total saving including material
and machining cost is 10-12 lakhs per turbine.
d) Considering 5% reduction in size of guide vane servomotor the savings
per turbine is Rs 2-3 lakhs.
As shown in Fig. 7, in the present invention the invented stay ring (7) of a low
head Kaplan turbine with concrete spiral casing is designed in a novel way that

height of top portion of stay ring (7) from strength considerations does not affect
the mounting height of top cover (5) and in-turn the guide vane stems/ trunnion
(4) height of the water regulating mechanism. The required strength/rigidity of
stay ring (7) is obtained by welding stiffening cylinders (11, 13) and ribs (10,12,
14) on the top portions of stay ring (7). These stiffening cylinders (11, 13) are
outside the turbine pit liner and are embedded in concrete.
Referring to Fig. 7 stiffening cylinders (11,13) are welded on top-deck (8) of the
stay ring (7) to give the required strength and rigidity to the stay ring (7) without
increasing the height of the top cover (5) and guide vane stem/ trunnion (4) and
stiffening plate (9) are welded between top-deck (8) and stiffening cylinders
(11,13) to give extra strength at the location of stay vanes (2). Ribs (10,12,14)
are welded between stiffening plate (9)/ top plate (8) and stiffening cylinders
(11,13) to add strength and rigidity to stay ring (7).

WE CLAIM:
1. A method of increasing strength and rigidity of stay ring of a large low
head turbine by stiffening cylinders comprising:
reducing the mounting height (h) of the top-cover (5) by 18-22%;
reducing the height of guide vane trunnion (4) by 18-22%;
reducing the diameter of guide vane stems (4) by 3-7%; and
reducing the size of guide vane servomotor by 3-7%;
characterized in that stiffening cylinders (11,13) are welded on top deck
(8) of the stay ring (7) to give strength and rigidity to the stay ing (7)
without increasing the height of the top cover (5) and guide vane
stem/trunnion (4) and stiffening plate (9) are welded between top-deck
(8) and stiffening cylinders (11,13) to give extra strength at the location of
stay vanes (2) when ribs (10,12,14) are welded between stiffening plate
(9), top deck (8) and stiffening cylinder (11,13) to add strength and rigidity
to the stay ring (7).

A method of increasing strength and rigidity of stay ring of a large low head turbine by stiffening cylinders consists of reducing the mounting height of top cover (5), height of guide vane trunnion (4), diameter of guide vane stems (4) and size of guide vane servomotor wherein the stiffening cylinders (11,13) are
welded on top-deck (8) of stay ring (7) to give the required strength and rigidity to the stay ring (7) without increasing the height of the top cover (5) and guide vane stem/trunnion (4) and stiffening plate (9) are welded between top deck (8) and stiffening cylinders ( 11,13) to give extra strength at the location of stay
vanes (2) when ribs (10, 12, 14) are welded between stiffening plate (9), top deck (8) and stiffening cylinders (11,13) to add strength and rigidity to stay ring (7).