FIELD OF THE INVENTION
This invention generally relates to Francis type hydro turbines with
facility for bottom removal of a runner. More particularly the invention
relates to a device for reducing deflection of a pivot ring including the
weight of the pivot ring in a Francis type hydro turbine.
BACKGROUND OF THE INVENTION
In a typical Francis type hydroelectric plant, a pivot ring constitutes an
essential feature of a guide apparatus and the pivot ring performs the
following function:-
1. It houses the pivot of the guide vane about which the guide vane
rotates that is the lower bush of the guide vane.
2. It houses the lower labyrinth in which rotates the runner in a fine
clearance.
The pivot ring is attached to a stay ring on its upper outer end. The
pivot ring at its lower inner end is connected to a draft tube cone. In
the typical turbines not having facility for bottom removal of runner,
the pivot ring and the draft tube cone are embedded in concrete.
However in Francis turbines which have a feature for bottom removal
of the runner, the pivot ring including the draft tube cone are not
embedded in concrete, but are dismantable that is they can be
removed from the bottom of the turbine. The pivot ring and the draft
tube cone are not shielded by the concrete but exposed. In a
conventional turbine, a part
of the load on the pivot ring is shared by the concrete. However for a
pivot ring of a turbine having facility for bottom removal of the runner,
the pivot ring suspends from the stay ring. Due to the hydraulic load
acting on the pivot ring, the pivot ring deflects downwards which in-
turn consumes the clearances of the guide vane stem and the bush. To
reduce the deflection of the pivot ring within the permissible
clearances of the stem/bush of the guide vane, the pivot ring has to be
configured as a substantially rigid component which in turn increases
the weight and cost of the pivot ring exorbitantly. Further, the pivot
ring suspended form the stay ring being free at its lower end is prone
to vibration leading to failure of the bolts joining pivot ring to the stay
ing prior art turbine, in which the pivot ring is suspended form the
stay ring with a dismantling gap at the bottom end of the draft tube
cone, the pivot ring is bolted/suspended from the stay ring. The draft
tube cone which in-turn bolted to the pivot ring, is suspended from the
pivot ring. The dismantling joint is at the bottom end of the draft tube
cone where it joins with the draft tube elbow
This arrangement has the following two problems:-
i) the downward hydraulic load on the pivot ring, deflects the
pivot ring downward and consumes the bearing clearances of
a guide vane pivot stem and its bush. Thus unless the rigidity
of the pivot ring is greatly enhanced, the guide vanes may
seize in the pivot ring.
ii) the flow induced vibrations of the draft tube cone get
transferred to the pivot ring to a significant extent on account
of the long overhang from the bolting of the pivot ring-stay
ring to the bottom free end of the draft tube cone.
In another prior art turbine in which the pivot ring suspended from the
stay ring and the draft tube cone anchored at the bottom end with a
dismantling gap at a top end of the draft tube cone, the pivot ring is
bolted to the stay ring. The draft tube cone is anchored at its bottom
end that is the draft tube cone Is bolted to the flange on the draft tube
elbow. The dismantling gap is at the top end that is at the joint of the
draft tube to the pivot ring.
As compared to the earlier prior art, the arrangement of the presently
discussed prior art, has the advantage that the flow induced vibrations
of the draft tube cone do not get significantly transferred to the pivot
ring as the draft tube cone gets reasonably isolated form the pivot ring
on account of the dismantling gap being at the top of the draft tube
cone.
However this arrangement still suffers from the drawback of the
downeard deflection of the pivot ring. Thus, the rigidity of the pivot
ring needs to be greatly enhanced in order to arrest the downward
deflection of the pivot ring, which in turn increases the weight of the
pivot ring too.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose a device for
reducing deflection of a pivot ring including the weight of the pivot ring
in a Francis type hydro turbine which prevents seizure of the guide
vane pivot stems in the bottom bushes.
Another object of the invention is to propose a device for reducing
deflection of a pivot ring including the weight of the pivot ring in a
Francis type hydro turbine which reduces the inertia requirement of
the pivot ring enabling reduction of the weight of the pivot ring.
A further object of the invention is to propose a device for reducing
deflection of a pivot ring including the weight of the pivot ring in a
Francis type hydro turbine which minimizes the vibration of the pivot
ring by providing a support at its bottom end.
SUMMARY OF THE INVENTION
Accordingly, there is provided a device for reducing deflection of a
pivot ring including the weight of the pivot ring in a Francis type hydro
turbine, the device comprising:- a pivot ring releasably attached to a
stay ring; a draft tube cone having a top portion and a bottom portion,
the top portion comprising a plurality of ribs configured to
accommodate a pad; a free end of the pivot ring provided with a
support means form the top portion of the draft tube cone; and a nut
portion of a support screw provided on the top of the pad, the support
screw resting against a hardened disc which is fixed to a bottom of the
pivot ring by a fixing screw, the supporting screw when tightened on
the pivot ring being locked by a lock-nut thereby transferring a
significant portion of the hydraulic load to a concrete structure of the
turbine.
In the present invention, the pivot ring for a Francis type hydro turbine having
facility for bottom ramovai of tha runnar is provided with a support on the top of
the draft tube cone. Such a support disposed balow the suspended pivot ring
reduces the deflection of the pivot ring. The arrangement is analogous to
providing a prop at the free overhang end of a cantilever beam. This
prop/support provided at the bottom end of the pivot ring substantially reduces
the inertia/strength requirement of the pivot ring. Reduced inertia requirement
in-turn reduces the malarial content in the pivot ring and hence the cost of the
pivot ring. Further, the provision of a support below the pivot ring significantly
reduces the vibration level of the pivot ring.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 - Shows a pivot ring suspended from a stay ring having a dismantling
gap at the bottom end of a draft tube cone according to prior art.
Figure 2 - Shows a pivot ring suspended from a stay ring, a draft tube cone
being anchored at a bottom end with a dismantling gap at a top end of the draft
tube cone according to the prior art.
Figure 3 - Shows a pivot ring with a prop/support on a draft tube cone according
to the present invention.
Figure 4 - Shows a comparison of the deflection pattern between a pivot ring of
prior art and a pivot ring of the invention.
Figure 5- Shows a view of a prop/support arrangement on an upper
draft tube cone according to the invention.
Figure 6- A Close up view of a prop/support for a pivot ring according
to the invention.
Figure 7- Exploded view showing details of a pivot ring and a
prop/support arrangement as of the invention.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE
INVENTION
Figure 1- Shows a pivot ring (1) and a draft tube cone (2a,2b) in-turn
suspended form a stay ring (3), and a dismantling gap (DG) is
maintained at a bottom end (2b). This arrangement suffers from
deflection and vibration problems as already described herein above. A
guide vane (4) having a guide vane pivot means with a bush (5) is
provided within pivot ring (1). A runner (6) having a labyrinth seal (7)
is provided to the turbine with or without removal facility. A spiral
casing (10) of the turbine is arranged above the concrete casing.
Figure 2- Shows a pivot ring (1) which is suspended form a stay ring
(3). A draft tube cone (2a,2b) is anchored at the bottom end (2b) and a
dismantling gap (DG) is maintained at a top end (2a) that is, at the
junction of the draft tube cone (2a,2b) to the pivot ring (1). In this
arrangement though the vibration problems are minimized to a
reasonable extent, the deflection problem still persists for which
substantial strengthening of the pivot ring (1) is required.
Figure 3 - Shows a support moans (11) for a pivot ring (1) which arrests the
defioction of the pivot ring (1). In this configuration, the pivot ring (1) is bolted
to a stay ring (3). The free overhung end of the pivot ring (1) instead of being
left free is given the support (11) from the top of a drift tube cone (2a). This
support (11) significantly arrests the deflection of the pivot ring (1), thus
reducing the inertia that is the strength requirement of the pivot ring (1).
For the support arrangement, the configuration of a plurality of ribs (12)
disposed on a top draft tube cone (2a) is modified to accommodate a pad (13).
On the top of the pad (13), a nut portion (16) of a support srew (15) is
disposed. The support screw (15) bears against a hardened disc (14) which is
fixed to a bottom of the pivot ring (1) by a fixing screw (17). Alter the support
screw (15) is tightened against the pivot ring (1), the support screw (17) is
locked by a locknut (18). Thus a significant portion of the hydraulic load on the
overhwig portion of the pivot ring (1) gets transferred through the support
screws (15) and the ribs (12) of the draft tube cone (2a, 2b) to the concreta thus
improving rigidity of the whole device. Further, the vibration levels of the pivot
ring (1) and the draft tube cone (2a, 2b) reduces significantly.
WE CLAIM
1. A novel supporting device for removable overhanging pivot ring
of a Francis type hydro turbine to reduce deflection & weight, the
device comprising:-
- a pivot ring (1) releasably attached to a stay ring (3);
- a draft tube cone (2a,2b) having a top portion (2a) and a bottom
portion (2b), the top portion (2a) comprising a plurality of ribs
(12) configured to accommodate a pad (13);
- a free end of the pivot ring (1) provided with a support means
(11) from the top portion of the draft tube cone (2a); and
- a nut portion (16) of a support screw (15) provided on the top of
the pad (13), the support screw (15) resting against a hardened
disc (10) which is fixed to a bottom of the pivot ring (1) by a
fixing screw (17), the supporting screw (17) when tightened on
the pivot ring (1) being locked by a lock-nut (18) thereby
transferring a significant portion of the hydraulic load to a
concrete structure of the turbine.
Characterised in that the said support means (11) transferring a
significant portion of the hydraulic load to a concrete structure of
the turbine which in-turn prevents deflection of the overhanging
pivot ring.
2. The device as claimed in claim 1, wherein the support means (11)
is provided at a bottom end of the overhanging pivot ring (1).
3. A device for reducing deflection of overhanging pivot ring
including tfie weight of the pivot ring in a Francis type hydro
turbine as herein substantially described and illustrated with
reference to the accompanying drawings.

The invention relates to a novel supporting device to reduce deflection and weight of removable, overhanging pivot ring of Francis turbines, the device comprises a pivot ring (1) releasably attached to a stay ring (3); a draft tube cone (2a,2b) having a top portion (2a) and a bottom portion (2b), the top portion (2a) comprising a plurality of ribs (12) configured to accommodate a pad (13); a free end of the pivot ring (1) provided with a support means (11) from the top portion of the draft tube cone (2a) and a nut portion (16) of a support screw (15) provided on the top of the pad (13), the support screw (15) resting against a hardened disc (10) which is fixed to a bottom of the pivot ring (1) by a fixing screw (17), the supporting screw (17) when tightened on the
pivot ring (1) being locked by a lock-nut (18) thereby transferring a significant portion of the hydraulic load to a concrete structure of the
turbine characterised in that the said support means (11) transferring a significant portion of the hydraulic load to a concrete structure of the
turbine which in-turn prevents deflection of the overhanging pivot ring.