FIELD OF THE INVENTION
The present invention generally relates to discharge elbow of a single floor
supported cooling water pump (CWP), used in thermal power plants. More
particularly, the invention relates to an improved discharge elbow assembly to
reduce structural resonance phenomenon on single floor supported cooling water
pump assembly.
BACKGROUND OF THE INVENTION
Cooling water pump or Circulating Water Pump (CWP) is mostly adapted in
Thermal Power Plants, refineries and waste water treatment plants. The function
of the pump is to send cooling water to the condenser and remove the latent
heat from exhaust steam of steam turbine.
CWP is a vertically mounted mixed flow pump having a closed type impeller. The
complete assembly of a CWP basically comprises a main assemblies Suction
Casing (1), an Impeller Casing (2), a Pump Casing (3), Element Assemblies I and
II (4-5), an Intermediate Foundation Ring (6), a Discharge Elbow Assembly (7)
and a Motor Stool Assembly (8) as shown in figure 1.
The CWP per se constitutes a rotor inside the casing. Vibration problems are
often associated with these types of rotating machines. Every mechanical System
has resonant frequencies. If the system is exposed to vibration at these
frequencies, then its vibration amplifies. The excessive vibration may lead to
chronic premature failure of the motor bearings, the seal of the discharge head
stuffing box and may also lead to a complete failure of the CWP assembly.
Considering the complexity of the situation, it is vital to identify the resonant
frequencies of the system. The present inventors through experimentations and
analyses for example, Finite Element Analysis (FEM), recognized that the natural
frequencies of the resonance are near to the operating frequency of the CWP,
which need determination. Accordingly, a complete CWP model has been
simulated in static and dynamic environment to study the possible pump
behaviour during actual operation. It was realized that deflection values and
stresses on the pump assembly are within limits but dynamic analysis shows that
the natural frequency of the pump (say, 8.1 HZ) is very close to the motor
operating frequency, leading to excessive vibrations. Since, there is no possibility
to change the operating speed of the in-built rotor, the only possible technical
solution could be to stiffen the structure of the complete CWP assembly. As the
resonant frequency is related to the mass and stiffness of the system i.e, f=Vk/m
where k = stiffness and m = mass. Adding inertia or stiffeners to the CWP
assembly, in particular at low stiffened components of the assembly, would be a
viable solution to the persisting resonance problem.
OBJECTS OF THE INVENTION
It is therefore an object of present invention to propose an improved discharge
elbow assembly to reduce structural resonance phenomenon on single floor
supported water pump assembly, which eliminates the disadvantages of prior
art.
Another object of the invention is to propose an improved discharge elbow
assembly to reduce structural resonance phenomenon on single floor supported
cooling water pump assembly, which is capable to shift natural frequency, of the
pump 10% away from operating speed of the pump rotor to avoid resonance
phenomenon.
A still another object of the invention is to propose an improved discharge elbow
assembly to reduce structural resonance phenomenon on single floor supported
water pump assembly, which delimits the structural deflections and stresses to
acceptable levels.
A further object of the present invention is to propose an improved discharge
elbow assembly to reduce structural resonance phenomenon on single floor
supported water pump assembly, which minimizes physical deterioration of
complete pump structure from material fatigue.
A still further object of the invention is to propose an improved discharge elbow
assembly to reduce structural resonance phenomenon on single floor supported
water pump assembly, which allows an efficient performance of the pump.
SUMMARY OF THE INVENTION
According to the invention, several iterations of a complete CWP module have
been carried out to achieve a configuration of the pump assembly, which could
be easily implemented to raise the resonant frequency i.e. 8.1 Hz, above the
operating speed (490 RPM, i.e. 8.16 Hz) by identifying the flexible parts through
simulation and then stiffen them.
In an analysis of the mode shapes of several known CWP assembly, it is
identified that the most flexible part is the discharge elbow because of its
peculiar shape and large cut-outs, which are required from functional point of
view of the CWP. The most flexible direction is found along the direction of
discharge. The components of the CWP assembly disposed above the elbow, i.e.
stool, motor and cooler each merely acts as an extension of the elbow and does
not significantly contribute to flexural stiffness. Their contribution is more
towards the tip mass. Hence, a first improvement was made in the movement
along the direction of discharge. This became possible by welding first ends of
two supports (ISMB Beam) to the bottom stool flange, and supporting second
ends of both beams at the existing support wall available behind the elbow.
Additionally, the elbow and discharge opening areas are stiffened with additional
plates and gussets. Selection of the stiffeners and supports are optimized to
maintain deflection and stresses within an allowable limit.
According to the invention, the improvement is characterized by the modification
of the discharge elbow assembly as indicated in the Table-I below :-
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 - shows an isometric view of a CWP structure accompanied provided
with supports and stiffeners according to the invention.
Figure 2 - Shows an enlarged view of a Discharge Elbow Assembly of prior art
showing its subcomponents.
Figure 3 - is an enlarged view of a Discharge Elbow Assembly showing supports
and gussets added around the outer periphery of elbow near the opening area
according to the invention.
Figure 4 - shows view of the discharge supporting shell with circular plates,
Vertical plates added as stiffeners according to the invention.
Figure 5 - shows an isometric view of Elbow Opening Area being a part of
Discharge Elbow with gussets of the invention.
DETAILED DESCRIPTION OF A PREFFERED EMBODIMENT OF THE
INVENTION
Discharge Elbow Assembly (7) component of the CWP, as modified is shown in
figure 2, which comprises :
Motor stool Bottom Flange (12);
Discharge elbow (13);
Discharge Supporting Shell (14);
Discharge opening area (15);
Thrust opening area (16); and
Discharge elbow bottom flange (17).
The newly added members as shown in figures 3 and 4 comprises:
-Support Members: Pair of ISMB Beam (18);
-Three Circular Stiffener near thrust block opening area (19);
- Two vertical stiffeners over supporting shell (20);
- Four vertical stiffeners over supporting shell near bend provided at discharge
elbow (21); and
- Six Gussets (22) around periphery of discharge opening area.
The complete assembly of Cooling Water Pump (CWP) along with incorporated
supporting members (18), plurality of vertical stiffeners (21) and gussets (22) is
shown in figure 1. The assembly is supported on an Intermediate Foundation
ring (6). Figure 1 shows that a motor mass (10) is lumped at C.G location above
a motor stool (8). A cooler mass (9) is lumped at a location at least one meter
away from the motor C.G from both the sides. In addition to these masses (8,9),
a rotor mass (11) is lumped at upper portion of the discharge elbow assembly
(7) as shown in figure 2.
From functional point of view, the Discharge elbow assembly has a peculiar kind
of shape having inadequate stiffness due to large cut-outs provided at the top of
the Discharge elbow assembly. It causes two flexural modes in orthogonal
directions below the Intermediate foundation support (6). The most flexible
direction is found to be one along the direction of discharge.
In order to restraint the movement along the direction of discharge, two ISMB
200 with angular spacing 65° are provided as support members (18) as shown in
figure 3. These support members (18) are welded at the bottom stool flange
(12) above the thrust block opening area (16) at one end while other ends are
supported to the existing support wall available behind the elbow. These support
members (18) add additional mass to the assembly, thereby enhancing stiffness
to the assembly structure. This has resulted in shifting of the natural frequency
from operating frequency.
As shown in Figure 3, a plurality of stiffeners has been incorporated at a location
based on study of the mode shapes pattern while simulation of the model in
dynamic environment. To increase the stiffness of the discharge supporting shell
(14) at thrust block opening (16), three circular plates (19) of thickness 32 mm
are welded around the periphery of opening area. Two vertical stiffeners (20) of
32 mm thickness starting from mid of the end circular plates (19) are provided
over top portion of the supporting shell (14), as shown in figure 4.
At least four discontinuous vertical stiffeners of 32 mm (21) have been welded
over the supporting shell (14) near the bend portion of the discharge elbow (13)
from both the sides as shown in figure 4.
Near the discharge elbow opening (15), six gussets (22) are provided (see figure
5). These gussets enhance the stiffness along the flow direction. These gussets
(22) are identical in thickness of 50mm.
According to the members selected as tabulated in Table-I, the consolidated
modal results achieved according to the invention, are shown in Table-II. The
modal results show that the assembly structure is free from resonance problem,
with stresses and deflection being maintained under the allowable limit.
Table-II shows the natural frequencies after the introduction of the support
members, vertical stiffeners and gussets of first five mode shapes. Table III
indicates the resonant frequency i.e. 8.1 Hz (as the operating speed of rotor is
490 rpm) causing resonance before stiffening. After enhancing the stiffness of
the discharge elbow, it has been brought up to 10.4 Hz from 8.1 Hz. Thus, the
introduction of stiffeers and gussets around the elbow assembly adequately
improves the assembly structure from modal considerations, and the CWP
complete assembly shall safely operate being free from any vibrations in long
run
WE CLAIM :
1. An improved discharge elbow assembly to reduce structural resonance
phenomenon on single floor supported water pump assembly, the
improvement is characterized in that:
- at least two ISMB beam with angular spacing of 65° are welded at the
bottom stool flange above a thrust block opening area at a first end, the
second ends being supported on the existing support wall available behind
the elbow;
- at least three circular plates of identical size welded around the periphery
of the opening area over a supporting shell of the discharge assembly;
- a plurality of first vertical stiffeners of identical thickness starting from
middle of the end circular plates over top portion of the supporting shell;
- at least four second vertical stiffeners welded over the supporting shell
near the bend portion of the discharge elbow from both the sides; and
- at least six gussets of identical thickness welded over the discharge elbow
opening.
2. The improved discharge elbow assembly as claimed in claim 1, wherein the
support beams, circular plates, stiffeners, and gussets are formed of identical
materials.
3. The improved discharge elbow assembly as claimed in claim 1, wherein the
resonant frequency of the complete CWP assembly structure is enhanced by
about 25%.

ABSTRACT

The invention relates to an improved discharge elbow assembly to reduce
structural resonance phenomenon on single floor supported water pump
assembly, the improvement is characterized in that : at least two ISMB
beam with angular spacing of 65° are welded at the bottom stool flange
above a thrust block opening area at a first end, the second ends being
supported on the existing support wall available behind the elbow; at least
three circular plates of identical size welded around the periphery of the
opening area over a supporting shell of the discharge assembly; a plurality
of first vertical stiffeners of identical thickness starting from middle of the
end circular plates over top portion of the supporting shell; at least four
second vertical stiffeners welded over the supporting shell near the bend
portion of the discharge elbow from both the sides; and at least six
gussets of identical thickness welded over the discharge elbow opening.