Steam turbine bell seals

An improved bell seal arrangement for steam turbines is provided that allows the bell to expand thermally without damage to the opposing seal surface of the inner shell by the addition of components that allow yielding of the seal surface without causing permanent distortion or surface damage.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
Steam turbines whose design includes double shell construction require 
devices that allow the two shells to expand and contract differentially, 
without allowing significant leakage out of the steam pipes that carry 
steam from the outer shell to the inner shell. 
2. Description of the Prior Art 
A common system employed by turbine manufacturers is called a bell seal. 
The bell slides into a tube held by the inner shell providing a minimum 
radial clearance with the tube, yet allowing vertical differential motion 
of the inner and outer shells. The bell is also secured to a tube held by 
the outer shell in such a way that it can slide, permitting differential 
motion in either the lateral or axial directions relative to the shaft, 
yet maintaining a small clearance that keeps leakage to a minimum. 
This bell seal system is commonly found, after service, to have a clearance 
between the bell and the inner shell tube of about 0.010 inches. This 
allows significant leakage and loss of turbine output. Replacement of the 
bell seal is usually ineffective, with the clearance and leakage 
recurring. 
The major problem is that the bell seal itself is of very powerful 
construction and when it becomes hot during starting procedures, while the 
inner shell tube is still relatively cool, its thermal growth can stretch 
and crush the opposing surfaces on the inner shell tube. Even during 
steady state operation, the bell may be hotter than the inner shell tube. 
The described problem is especially apparent on larger turbines where the 
bell diameter is greater. 
An improvement to the bell seal system that prevents crushing the mating 
surfaces or stretching them beyond the elastic limit would provide 
significant improvement in turbine efficiency. 
SUMMARY OF THE INVENTION 
It is the object of the present invention to provide a bell seal system 
that achieves small leakage clearance in spite of unavoidable thermal 
gradients. 
The invention is practiced by providing opposing seal surfaces and 
materials that permit differential expansion caused by thermal gradients 
without causing either surface damage or permanent stretching of the walls 
and sealing components. 
This is accomplished by modifying the bell itself to provide a "deflection" 
slot, or by interposing an additional component between the inner shell 
and bell, the component opposing the bell seal. This component is thin 
enough and of satisfactory material to be stretched by the bell seal 
during a start-up without exceeding its elastic limit, thus maintaining a 
small clearance at all operating conditions.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 1, some of the key elements of the high pressure inlets 
to a prior art turbine are shown, illustrating current practice. A shaft 
11 carries buckets or rotating blades (not shown) that pass 
circumferentially downstream of nozzles 12. Steam is admitted to the 
nozzles through a passageway 30 by means of an external pipe 15, connected 
to an outer shell 16 and to an inner shell 21. Passageway 30 continues to 
the entrance of the nozzles 12. 
The drawing shows only one inlet. In actual practice, a full circle of 
inlets would include six to eight such inlet passages, each providing 
steam to a separate section of nozzles. 
Inner shell 21 includes nozzles 12, flow passages 30, flange bolts 18, a 
flange 31 and a cylindrical surface 19 to provide a small clearance seal 
with a bell seal 13 that is held in an extension of outer shell 16. There 
are, of course, many other components in the inner shell, but they are not 
significant to this invention. 
Outer shell 16 includes cylindrical tubes or pipes 33 that conduct steam 
from the outer shell to the inner shell through passage 30. Tubes or pipes 
33 also locate bell seal 13 which prevents or minimizes steam leakage from 
passage 30 into the space between the inner and outer shells. The bell 
seal is free to move sideways by sliding of a contact surface 34 to 
facilitate differential motion of the inner and outer shells. The bell 
seal can slide vertically along seal surfaces 19 to also accommodate 
differential expansion of the inner and outer shells. 
A nut 14 holds the bell in proper vertical alignment with the outer shell 
inlet pipes 33 allowing any necessary side motion of contact surface 34. 
The outer shell also includes flanges 32 and bolts 17, and is connected to 
the main steam pipes 15. Other components are also present, but not 
necessary for this discussion. 
A high pressure zone in the area below bell seal 13 is identified by 35; 
and a low pressure zone in the area above bell seal 13 is identified by 
36. 
FIG. 2 is an enlarged, fragmentary, cross sectional view of a portion of 
the prior art bell seal of FIG. 1 and the adjacent locating and seal 
surfaces. Bell 13 is secured to outer shell tube 33 by nut 14. The nut is 
tightened so as to position the bell vertically yet not prevent sideways 
motion at surface 34. The bell makes small clearance contact with inner 
shell 21 at surface 19. 
FIG. 3 is a fragmentary cross sectional view of the turbine of FIG. 1, with 
the bell seal area incorporating improvements in accordance with a 
preferred form of the invention. 
In the turbine of FIG. 3, shaft 11 carries buckets or rotating blades (not 
shown) that pass circumferentially downstream of nozzles 12. Steam is 
admitted to the nozzles through passageway 30 by means of external pipe 
15, connected to outer shell 16 and to inner shell 21. Passageway 30 
continues to the entrance of the nozzles 12. 
The drawing shows only one inlet. In actual practice, a full circle of 
inlets would include six to eight such inlet passages, each providing 
steam to a separate section of nozzles. 
Inner shell 21 includes nozzles 12, flow passages 30, flange bolts 18, 
flange 31 and a cylindrical surface 119 to provide a small clearance seal 
with a bell seal 113 that is held in an extension of outer shell 16. 
Outer shell 16 includes cylindrical tubes or pipes 33 that conduct steam 
from the outer shell to the inner shell through passage 30. Tubes or pipes 
33 also locate bell seal 113 which prevents or minimizes steam leakage 
from passage 30 into the space between the inner and outer shells. The 
bell seal is free to move sideways by sliding of a contact surface 134 to 
facilitate differential motion of the inner and outer shells. Bell seal 
113 can slide vertically along seal surface 119 to also accommodate 
differential expansion of the inner and outer shells. 
A nut 14 holds the bell in proper vertical alignment with the outer shell 
inlet pipes 33 allowing any necessary side motion of contact surface 134. 
The outer shell also includes flanges 32 and bolts 17, and is connected to 
the main steam pipes 15. Other components are also present, but not 
necessary for this discussion. 
As with the turbine of FIG. 1, the high pressure zone in the area below 
bell seal 13 is identified by 35; and the low pressure zone in the area 
above bell seal 13 is identified by 36. 
As best seen in FIG. 3A, which is an enlarged, fragmentary cross sectional 
view of the bell seal area of FIG. 3 with improvements in accordance with 
the invention, bell 113 has been machined to provide radial space for a 
flexing ring seal 122. The flexing ring seal is secured to inner shell 21 
by a weld 125 and has a clearance gap 127 with inner shell 21 permitting 
radial motion when the bell is enlarged relative to the inner shell due to 
the hotter condition of the bell. Radial seal surface 119 between flexing 
seal 122 and bell 113 minimizes leakage past the bell. 
FIG. 4 is an enlarged fragmentary cross sectional view of the bell seal 
area with a first alternative improvement in accordance with the 
invention. A bell 213 is machined to provide radial space for a two-part 
flexing ring seal 222 comprising first and second parts 222A and 222B 
respectively. Flexing ring seal 222 can be stretched when the bell is 
enlarged. Flexing ring seal first part 222A is vertically secured by a 
combination of a shoulder 226 on an inner shell 221 and flexing ring 
second part 222B secured to inner shell 221 by a weld 225. A vertical seal 
surface 223 between flexing ring first part 222A and flexing ring second 
part 222B and a radial seal surface 219 between flexing ring first part 
222A and bell 213 minimize leakage past the bell seal. A clearance gap 227 
between flexing ring first part 222A and inner shell 221 permits radial 
motion when the bell is enlarged relative to the inner shell due to its 
hotter condition. 
Bell 213 is secured to outer shell tube 33 by nut 14. 
FIG. 5 is an enlarged, fragmentary cross sectional view of the bell seal 
area with a second alternative improvement in accordance with the 
invention. A bell 313 is machined to provide radial space for a two-part 
flexing ring seal 322 comprising first and second parts 322A and 322B 
respectively, with second part 322B acting as a locating ring for first 
part 322A. The two parts are provided with a breech lock fit that allows 
them to be engaged. The lock is secured by a dowel pin 329 and the second 
part 322B is vertically secured to an inner shell 321 by a weld 325. Dowel 
pin 329 must be secured after assembly by such as a weld 330. 
A seal surface 323 between flexing ring first part 322A and second part 
322B, and a seal surface 319 between flexing ring first part 322A and bell 
313 minimize leakage around the bell. 
A clearance gap 327 between flexing ring first part 322A and inner shell 
321 permits radial motion when the bell is enlarged relative to the inner 
shell due to its hotter condition. 
Bell 313 is secured to outer shell tube 33 by nut 14. 
FIG. 6 is an enlarged, fragmentary cross sectional view of the bell seal 
area and showing a third alternative improvement in accordance with the 
invention. 
All parts are the same as those shown in FIGS. 1 and 2 except for the 
addition of a vertically disposed slot 437 in a bell seal 413. 
Slot 437 extends inwardly into bell seal 413 from a lower face 418 of the 
bell and is disposed inwardly of and in spaced parallelism to an outer 
peripheral seal surface 419 of the bell. 
Slot 437 allows seal surface 419 of the bell to yield when thermal growth 
of the bell causes it to be larger than inner shell 21. The stress caused 
by the worst expected yielding should not be high enough to result in 
creep or permanent distortion of the bell. 
FIG. 7 is an enlarged, fragmentary, cross sectional view of the bell seal 
area showing a fourth alternative improvement in accordance with the 
invention. The parts are the same as those shown in FIG. 6 except for the 
addition of means for limiting the yielding of the bell to a preselected 
value. 
A vertically-disposed slot 537 in a bell seal 513 extends inwardly into the 
bell seal from a lower face 518 of the bell and is disposed inwardly of 
and in spaced parallelism to an outer peripheral seal surface 519 of the 
bell. 
Slot 537 allows seal surface 519 of the bell to yield when thermal growth 
of the bell causes it to be larger than inner shell 21. The stress caused 
by the worst expected yielding should not be high enough to result in 
creep or permanent distortion of the bell. 
A substantially flat annular ring 538 is disposed in a complementary 
opening 539 in lower face 518 of the bell and has a horizontally-extending 
lower planar face 540 disposed on a plane with lower face 518 and a 
vertically extending peripheral outer face 541 disposed in spaced 
parallelism to an inwardly facing wall 542 of slot 537 to provide a gap 
543 between ring face 541 and the bell. The magnitude of yielding is 
limited to the gap 543 to assure that the stresses caused by yielding are 
small enough to prevent bending or creep. 
Ring 538 is secured in opening 539 by a weld 544 and by screws 545 (only 
one of which is shown), which extend through the ring and are threaded at 
their inner ends in the bell seal. 
Screws 545 are further enclosed in place as by welds 546. 
It should be noted that the materials of the flexing rings as well as the 
bell seal flexing surfaces 419 and 519 shown in FIGS. 6 and 7, 
respectively, must tolerate the combination of stress and temperature 
without exceeding the elastic limit or creeping. The forces imposed on the 
opposing surface of the inner shell must likewise not be so high as to 
cause permanent distortion or wear on that surface. 
As mentioned previously, existing bell seals are especially vulnerable to 
rapid heating during cold starts. They are directly exposed to the hot 
incoming steam and get hot quicker than the portion of the inner shell 
which surrounds them. That portion of the inner shell not only is not 
directly exposed to the high velocity steam, it also has much cooler steam 
on the opposite side of the wall from that which faces the bell at seal 
surface 19. In addition, during light load steady state operation, the 
temperature difference of the bell and shell will be somewhat greater than 
when operating at full load. Since the bell seal is very strong in 
construction, it tends to force the opposing wall to be stretched, leading 
to enlargement caused by creep as well as surface damage. 
Beyond the temperature effects, this area of the turbine is subjected to 
high frequency pressure fluctuations which tend to vibrate any components 
which have freedom of motion. Split piston rings, which are sometimes used 
to provide seal surfaces for the bell seal, have shown obvious troubles 
due to vibration. Even bell seals have the capability to vibrate and 
batter the inner shell seal surface once some clearance has been created. 
The invention hereof resolves these problems. In FIGS. 3 and 3A, flexing 
ring 122 is a relatively thin ring that can be expanded by a growing bell 
seal without requiring either very large surface forces or internal ring 
stress. This reduces the tendency for creep and surface damage. Further, 
the ring is strongly secured and not vulnerable to vibration. By keeping a 
neat fit against the bell seal, it also tends to restrict vibration of the 
bell itself. 
In the two-part flexing rings 222 and 322 of FIGS. 4 and 5 respectively, 
additional seal surfaces 223 and 323 respectively will create a friction 
force if vibrational motion of the bell is encountered, thus reducing the 
tendency for vibration. 
The sealing action in FIGS. 4 and 5 is effected by the pressure force that 
results from high pressure at area 35 below the bell seals 213, 313, and 
low pressure at the area 36 above the bell seals 213, 313. 
It should also be noted that the FIGS. 4 and 5 flexing rings 222 and 322 
can heat and cool rapidly at similar rates to the bell without being 
restrained by the adjacent inner shell walls. 
It is pointed out that the flexing ring second part 222B of FIG. 4 and the 
breech lock flexing ring second part 322B of FIG. 5 could be secured to 
the inner shell by machined threads rather than by the welds 225 and 325, 
or by a combination of threads and a weld. 
The radial clearance spaces 127, 227, 327, 437 and 543 of FIGS. 3, 4, 5, 6 
and 7 respectively, should be large enough to permit thermal growth of the 
bell for the greatest temperature difference expected between the shell 
wall and the bell. A 500.degree. F. difference, for example, could require 
about 0.017" of space, depending upon the size of the bell. 
Other modifications not shown or discussed will be obvious to those skilled 
in the art.