TITLE: AN IMPROVED EXHAUST HOOP FOR IMPULSE TURBINE
FIELD OF INVENTION:
This invention relates to steam turbines in general and more particularly to
downward flow exhaust hoods for such turbines. More specifically the present
invention relates to development of an improved exhaust hood with optimum
weight for operation under a wide range of frequencies and under adverse
conditions adaptable to steam turbine.
BACKGROUND ART
The steam leaving the last row of blades of steam turbines used in power
generation generally flows through a component called an "exhaust hood", from
which it discharges into a condenser. The most common type of exhaust hood is
one of "downward flow" design in which the condenser is located below the
exhaust hood. This arrangement saves floor space in a power station. The other
type is an "axial exhaust hood" in which the steam flow continues in the same
axial direction as in the turbine. This arrangement is generally used in barge-
mounted steam turbine based power units.
There was a specific requirement to develop an improved light-weight downward
flow exhaust hood for use with impulse steam turbine for operation under a wide
range of frequencies and under adverse conditions.

A prior art search was conducted to get information on various Patents in this
area of research. The information obtained is given in brief in the following.
An United States Patent 6971842: discloses an invention entitled "Low pressure
steam turbine exhaust hood" The abstract of which recites
An exhaust hood for a turbine includes a shell casing, an external support
structure, conical corner plates, and a butterfly plate. The shell casing includes
an inner surface and an outer surface. The external support structure is coupled
to the shell casing outer surface, and provides structural support to said shell
casing. The butterfly plate is coupled to the shell casing inner surface for
channeling flow into the exhaust hood and subsequently into the condenser. The
butterfly plate has a substantially elliptically-shaped cross-sectional profile that
facilitates reducing flow separation losses of steam flowing there through into
the exhaust hood.
United States Patent 4673426: Moisture pre-separator for a steam turbine
exhaust
Abstract:
A moisture pre-separator for the exhaust portion of a high pressure steam
turbine is disclosed. The pre-separator mounts in the nozzle and adjoining piping
of the exhaust portion and provides a first channel for capturing moisture caused
by secondary flow and a second channel for capturing moisture flowing on the
exhaust hood wall. Drains are provided for both channels.

US Patent No. 6419448: Flow by-pass system for use in steam turbine exhaust
hoods
Abstract
A flow by-pass system is provided in a downward-discharging exhaust hood of a
steam turbine to by-pass a small percentage of the total steam flow from the top
portion of the exhaust hood to the vicinity of the condenser and in that way
relieve excess pressure in the top portion incident to the more convoluted path
of the main portion of steam passing from the top to the bottom and to decrease
energy loss caused by friction and thus to improve turbine efficiency. The flow
by-pass system includes by-pass conduits within the front portion of the exhaust
hood extending from the top portion to the vicinity of the condenser. Such
conduits are formed by covering over the corners between the outer and end
walls of the exhaust hood and between the exhaust hood end wall and the
bearing cone outside surface with by-pass walls to form flow passages in the
corners behind the by-pass walls through which exhaust steam may pass from
the region of higher pressure in the top portion of the exhaust hood to the
region of lower pressure in vicinity of the condenser. Inlets and outlets are
provided in the by-pass walls for entrance and exit of the turbine exhaust steam
to the by-pass conduits.
United States Patent 6261055: Exhaust flow diffuser for a steam turbine

Abstract:
An annular diffuser having its inlet located at the exit of a last row of blades of a
steam turbine having initially very slowly increasing cross-sectional area with
distance to accommodate the diffusion produced by the decaying wakes in the
diffuser so as to prevent flow separation from diffuser walls and as a result to
foster the diffusion process and to increase the efficiency of the steam turbine.
The rate of increase of cross-sectional area, which is much smaller than that
appropriate in diffusers having uniform incompressible flow at their inlets, allows
wakes which form near the trailing edges of the last turbine blades to dissipate
while avoiding flow separation. In the diffuser of this invention, whether it is one
of fixed shape or one whose cross-sectional area can be changed by making use
of an adjustable guide vane which surrounds at least a portion of the bearing
cone, at a distance from inlet of one half of diffuser height at inlet, the cross-
sectional area increase is smaller than 5.0% of the inlet cross-sectional area.
This is equivalent to the corresponding two-dimensional straight-wall diffuser
angle of, approximately, 2.9 degrees. For the diffuser whose cross-sectional area
can be changed as required depending on its inlet flow conditions, the above
limit applies for preferably most of the travel path of the adjustable guide vane
but at least for the adjustable guide vane position closest to the turbine last
blades. The length of the diffuser of this invention, in its preferred embodiment,
measured along its mean line, is larger than or at least equal to 90% of the

length of last turbine blades. The outer flow guide which defines the outer wall
of the diffuser should have radius of curvature at its beginning larger than one
half of the length of turbine last blades and should have a horizontal tangent
there.
United States Patent 1,269,998: Steam Turbine
Abstract:
This discloses a steam turbine having a downwardly curved exhaust at the end of
the turbine. In order to better control the flow of steam to the condenser and
avoid backup of steam due to vortices and the like, the steam flow is divided up
into at least upper and lower streams by partitions or baffle plates. This avoids, it
is said, the steam from various portions of the turbine and particularly the top
portion and bottom portion from meeting each other at different angles, or from
different directions, causing eddies and the like which would interfere with rapid
exhaust of steam from the turbine. The uniformity of the travel passage of the
steam from the turbine when it enters the condenser or exhaust is thus
enhanced. This disclosure does not show an exhaust hood installation directly
over a condenser, but is an early example of the widespread continuing practice
of using guide vanes to aid in directing turbine exhaust flow.
United States Patent 3,791,759: Turbine Pressure Attenuation Plenum Chambers
Abstract:
This discloses in a gas turbine the use of pressure attenuation chambers
adjacent the exit from the turbine blades which are followed by stationary vanes.

Excess gas pressure causes leakage of flow through orifices into such chambers
when pressure rises excessively. The reference broadly illustrates the temporary
withdrawal of gas from the exhaust to equalize pressure with the intent of trying
to reduce circumferential pressure distortion in a turbine which is not provided
with a downwardly discharging exhaust hood.
United States Patent 3,149,470: Low Pressure Turbine Exhaust Hood
Abstract:
This discloses a hollow, substantially frusto-conical, flow dividing member
disposed inside an exhaust hood co-axial with the turbine rotor which divides the
flow from the turbine casing outlet into radially inner and radially outer annular
portions which are further sub-divided by additional substantially radial flow
guiding walls which form a number of parallel passages leading toward the
exhaust hood outlet. One of the two flow annuli is formed between the circular
opening of the flow-dividing member and the outer flow guide extending from
turbine casing. The other annulus is formed between the circular opening of the
flow dividing member and the bearing cone. This prior invention has little if any
relation to the present invention in which only a small fraction of the total turbine
exhaust flow is by-passed from the top quadrant of the exhaust hood to the
bottom portion. There is no physical similarity between these two inventions. In
the present invention the space within the exhaust: hood remains essentially
unchanged by the introduction of the flow by-pass system of the invention. In
this invention the turbine exhaust flow annulus is sub-divided, in proximity of the
turbine last stage blades, into two portions.

United States Patent 4,013,378: Axial Flow Turbine Exhaust Hood
Abstract:
This discloses the use of a plurality of curved guide vanes of varying curvatures
spaced about a circumferential guide ring in the exhaust from a turbine to direct
exhaust steam from a generally axial to radial direction, and secondary guide
vanes directing steam flow toward the discharge opening of the exhaust hood
toward the condenser. There is no by-passing or draining off of some of the
steam from the upper or top portion of the exhaust hood and directing it to the
lower or bottom portion to decrease the pressure and the flow velocity in the top
portion of the exhaust hood.
United States Patent 4,214,452: Exhaust Device for a Condensable-Fluid Axial-
Flow Turbine
Abstract:
This discloses in an annular diffuser an extraction-type suction slot or slots
through which a fraction of flowing fluid is removed at the outer flow guide so as
to make the pressure gradient there either negative or zero. The removed fluid is
directed to a lower portion of a two-zone condenser and the remainder to a
higher-pressure portion of the two-zone condenser. The location of the removal
of fluid at the outer flow guide of the annular exit flow diffuser in the vicinity of
the turbine last stage blades distinguishes this invention from the present
invention in which fluid is removed in the exhaust hood proper. In addition, the

object of this invention is to prevent flow separation from the outer flow guide or
"break down of the fluid flow" there and not to decrease the pressure and flow
velocity in the top portion of the exhaust hood which is object of the present
invention. Also, since in a Low-pressure (LP) condensing turbine the pressure at
the location of the suction slot on the convex side of the diffuser is usually as low
or even lower than the pressure in the condenser itself, this invention can only
be used in either a High-pressure (HP) section of the turbine or in the
Intermediate-pressure (IP) section of a condensing turbine, or only with a
specially-designed condenser having multiple zones.
United States Patent 4,326,832: Exhaust Outer Casing
Abstract:
This discloses a side discharge steam exhaust including the use of a guide plate
or vane that separates the top discharge steam from lower discharge steam in
the exhaust hood and uses curved guide vanes in the hood. However, there is no
siphoning off of a portion of the steam exhaust from the upper portion of the
exhaust hood and transferring it to the.lower portion of the hood.
United States Patent 5,174,120: Turbine Exhaust Arrangement for Improved
Efficiency
Abstract:
This discloses a turbine arrangement in which a condenser on the bottom of
such arrangement is divided by a plate in the centre of the condenser which
separates the inlet sections of the tubes of the condenser from the outlet

sections of the tubes of the condenser creating a low pressure chamber and a
higher pressure chamber, or section, forming thereby a zoned condenser. At the
same time the exhaust hood above has partition plates dividing the steam exiting
the turbine into left and right half portions. An increased efficiency is claimed.
Although in this arrangement the turbine exhaust is divided into separate zones,
in the arrangement of the present invention siphoning off of elevated pressure
steam from the top of an exhaust hood and transferring it to the bottom of the
exhaust hood is not shown.
United States Patent 5,257,906: Exhaust System for a Turbo machine
Abstract:
This discloses a steam turbine arrangement in which the outer edge of the outer
flow guide approaches a vertical orientation and is overall more extended on the
bottom and in which an upper baffle within the exhaust hood dissipates the
horseshoe-shaped vortex in the upper section and sides of the exhaust hood by
"crowding" the vortex against such baffle, thus apparently minimizing formation
of such vortex and preventing it from expanding and growing. There is no
disclosure of a means for siphoning of fluid from the top portion of the exhaust
hood to the bottom portion.
OBJCTECTS OF THE INVENTION
It is therefore an object of the present invention to propose an improved Exhaust
hood for impulse turbine which eliminate the disadvantage of prior art

Another object of the present invention is to propose an improved Exhaust hood
for impulse turbine which is light weight.
A further object of the present invention is to propose an improved Exhaust hood
for impulse turbine which saves space.
A still further object of the present invention is to propose an improved Exhaust
hood for impulse turbine which is operatable in multiple range of frequencies.
An yet further object of the present invention is to propose an improved Exhaust
hood for impulse turbine which is operable under adverse conditions.
A still another object of the present invention is to propose an improved Exhaust
hood for impulse turbine which is less costly.
BRIEF DESCRIPTION OF DRAWINGS
Figure 1 (Sheet No. 1) shows the isometric view of the exhaust hood indicating
various parts.
Figure 2 (Sheet No. 2) shows the central vertical section of the exhaust hood, as
viewed normal to the direction of steam flow.
Figure 3 (Sheet No. 3) shows the end view of the exhaust hood, as viewed in the
opposite direction of steam flow.

Figure 4 (Sheet No. 4) shows the normal and improved exhaust hoods in
assembly with turbine.
Figure 5 (Sheet No. 5) shows the end views of the normal and improved exhaust
hoods.
Figure 6 (Sheet No. 6) shows the front views of the normal and improved
exhaust hoods.
Figure 7 (Sheet No. 7) shows the natural frequencies of the normal and
improved exhaust hoods and the normal running frequency of the turbine and its
2X multiple.
DETAILED DESCRIPTION OF A PREFERED EMBODIMENT OF THE
Though prior art covered various important aspects of the exhaust hood design,
no single one was found to be adequate to achieve the desired objective. Hence,
an improved light-weight downward flow exhaust hood was designed from
fundamentals for use with the impulse steam turbine for operation under a wide
range of frequencies and under adverse conditions.
The exhaust hood is made in two parts, the lower part (1) and the upper part
(2). The lower part (2) consists of the portion below the parting plane (3) up to
the condenser flange (4). The bearing is also located in this part. Two vertical
internal stiffeners are provided in the axial direction in the lower portion. On the
outer surface also suitable stiffeners (5) are provided to achieve required
stiffness in both vertical and horizontal directions. A support plate (9) is provided
for resting the exhaust hood on

the foundation. The thickness of the support plate (9) and parting plane flange
(3) are suitably chosen. Suitable lifting lugs (6) are provided for handling
purpose.
The internal portion of the upper part is designed without any stiffeners etc. to
ensure smooth flow of exhaust steam from the turbine. The external portion is
provided with suitable stiffening ribs (5) in addition to the parting plane flange
(3) and lifting lugs (6) to give the necessary stiffness to the exhaust hood.
Figures 1 to 3 show the details of the exhaust hood.
Finite Element Analyses (FEA) have been carried out to ascertain the suitability
of the improved exhaust hood from stiffness, stress, deformation and natural
frequency points of view and also adequacy of parting plane flange bolt size and
extent of bolt tightening. By carrying out FEA at different stages, a compact &
light-weight exhaust hood has been obtained. The modal analysis results have
shown that the exhaust hood can operate under a wide frequency range and the
contact stress analysis results have shown that it can work under adverse
conditions also.
Figure 4 shows the normal and improved exhaust hoods in assembly with
turbine. As can be seen from this Figure, there is considerable reduction in size
of the exhaust hood. Figures 5 and 6 show the end views and front views of the
normal and improved exhaust hood highlighting the reduction in size of the
improved exhaust hood as compared to the normal one. The weight of the
improved exhaust hood is 3244 Kg compared to 6590 Kg of the normal one.

From the results of the stiffness analyses it was observed that the vertical and
horizontal stiffness of the improved exhaust hood were 2.1xlO6 N/mm and
1.65xlO6 N/mm respectively compared to 1.81xlO6 N/mm and 1.82 xlO6 N/mm
of the normal one. The desired minimum values are 1.8 xlO6 N/mm and 1.2 x
106 N/mm respectively. It may be noted that in the improved exhaust hood the
vertical stiffness has a better margin and also the horizontal stiffness is better
optimised.
Figure 7 shows the natural frequencies of the normal and improved exhaust
hoods and the normal running frequency of the turbine and its 2X multiple. It
can be seen that the improved exhaust hood gives optimised values of natural
frequencies with sufficient margins from normal running speed. Particularly, it
may be noted that the 4th natural frequency of the normal exhaust hood is quite
close to the normal running speed, whereas the same is better placed in the
improved exhaust hood. The natural frequencies are also quite low compared to
2X multiple of normal running speed as desired. Hence, it may be surmised that
the improved exhaust hood can operate under a wide frequency range.
From the contact analysis results of the parting plane flange, it was observed
that the contact status over the entire surface is sticking as desired. Also, there

is good contact pressure of the order of 12 to 14 N/mm2, which is very much
higher than the differential pressure in the exhaust hood and hence the
improved exhaust hood can work under adverse loading conditions also.
From the stress analysis results it was seen that the maximum vonMises stress is
about 154 N/mm2, which is highly localised and the overall stress is less than 17
N/mm2. Hence the exhaust hood is safe from stress point of view. It was also
seen that the maximum total deformation is about 0.087 mm, which occurs in a
small portion of the exhaust hood surrounded by some more portion where it is
in the range of 0.07 to 0.08 mm and in the major part of the exhaust hood it is
less than 0.06 mm. Hence, the exhaust hood is safe from deformation point of
view also.


WE CLAIM
1. An improved configuration of a light-weight downward flow exhaust
hood for impulse turbine for operation under a wide range of
frequencies and under adverse conditions has been arrived at by
carrying out FEA at various stages to ensure adherence to desired
stiffness, stresses, deformations and natural frequencies and also
adequacy of parting plane flange bolt size and extent of bolt tightening.

An improved downward flow exhaust hood for impulse steam turbine for
operation under a wide range of frequencies and under adverse conditions has
been obtained. This has been possible due to elaborate Finite Element Analyses
carried out at different stages. Adequate stiffeners are provided at appropriate
locations to obtain desirable structural integrity. FE analyses are carried out to
ascertain the stiffness in the vertical and horizontal directions, natural
frequencies through modal analysis and to determine the stresses and
deformations of the exhaust hood under all the loads coming in actual operation.
Nonlinear contact stress analysis is also carried out to ascertain adequacy of bolt
size and bolt tightening.