Exhaust gas diffuser interface

An exhaust gas diffuser interface includes a stainless steel shell with varying levels of insulation. The varying levels of insulation concentrate thermal expansion and contraction at the turbine outlet end allowing it to be connected to the outlet by way of a seal weld. This construction allows expansion and contraction of the diffuser interface in unison with the attached turbine outlet, as well as the diffuser inlet.

BACKGROUND OF THE INVENTION 
The invention pertains to an interface between the exhaust gas outlet of a 
gas turbine and the inlet of an internally insulated exhaust gas diffuser. 
The gas turbine referred to in this application is of the type generally 
used in power generation. 
In constructing the power turbine system, the exhaust from the gas turbine 
would be routed through a diffuser on its way to being routed through 
other equipment by way of various valves in the exhaust stream. Exhaust 
diffusers are either internally insulated (i.e. the structural shell plate 
is protected from hot exhaust by internal insulation and liner system) or 
externally insulated (i.e. structural shell plate is exposed to hot 
exhaust.) The externally insulated equipment is considered to suffer from 
several deficiencies vis a vis internally insulated equipment. Namely, the 
internally insulated equipment is considered to be more efficient, to last 
longer in the field, and enjoy assembly benefits such as the ability to be 
shipped in at higher levels of assembly. The gas turbine is typically 
externally insulated while in many situations its remaining equipment is 
preferably internally insulated. 
The exhaust outlet from the gas turbine has to be connected to the inlet of 
the diffuser in such a way to provide a seal so that no exhaust leaks to 
the outside or ambient air through the connection. This sealing 
relationship must be maintained throughout the operating conditions of the 
power generation system. By way of example, the interface between the 
exhaust gas outlet and the diffuser can go from temperatures of subzero 
degrees F. to over 1200.degree. F. in the matter of a few minutes such as 
when a system located in a cold climate is started. Likewise, 
interruptions in operation of the gas turbine such as shut downs for 
maintenance, etc. will cause the interface to cool from elevated operating 
temperatures to the ambient temperature. These stresses of expanding and 
contracting must be repeatedly accommodated over the life of the 
interface. 
Prior interfaces between the gas turbine and diffuser relied on complicated 
gasketing arrangements to provide a gas tight seal that would survive over 
repeated cycling. The provision of this gasketed seal required precise 
machining, numerous parts and involved field assembly. As a result, the 
seal area was difficult to assemble, and expensive, both from a parts 
manufacture and assembly standpoint. Small deviations in the machining 
could also result in a unsatisfactory seal. 
Previously expanding bell type seals have been used experimentally in other 
connections in the exhaust path such as connecting exhaust plenums. Seals 
in these areas are not subject to the same stressful environment as 
immediately downstream of the turbine and before the diffuser. Results 
obtained from further downstream components are not always applicable to 
upstream situations. 
SUMMARY OF THE INVENTION 
The diffuser interface connection of the present invention reduces the 
possibility of exhaust gas leakage and is expected to provide a longer 
operating life than prior art diffuser interfaces. The diffuser interface 
is less expensive in that it does not need the costly machining of the 
prior art diffuser interfaces. The diffuser interface can also be either 
completely installed in the field or partially installed in the factory. 
The diffuser interface includes a tapered cone shaped fabrication that is 
welded between the exhaust diffuser and the gas turbine exhaust flange. 
The interface is made from steel and has flanges welded on each end. It is 
installed by bolting and/or clamping it to the exhaust diffuser intake 
flange and the gas turbine exhaust flange. The joints are then seal welded 
to prevent any leakage of exhaust gas. 
A varying level of insulation is installed in conjunction with the diffuser 
interface. By varying the level of internal insulation along the length of 
the diffuser interface, the location of where the diffuser interface 
undergoes its greatest expansion and contraction can be controlled. The 
insulation is installed so as to have that expansion and contraction take 
place in the region where the diffuser interface joins the exhaust gas 
outlet of the gas turbine. As a result, the welded joint between the 
diffuser interface and the gas turbine outlet can expand or contract as 
the gas turbine outlet expands or contracts, minimizing stresses between 
the interface and the exhaust gas outlet. The increased internal 
insulation downstream reduces the expansion and contraction rates for the 
outer surface of the diffuser interface so that when the interface is 
mated to the internally insulated diffuser inlet, that joint also expands 
and contracts with a minimum of thermal stress between the parts.

Normal gas flow direction in FIGS. 1-5 is left to right. 
DETAILED DESCRIPTION OF THE INVENTION 
In a typical set up for a power generation system, a gas turbine is 
exhausted into a diffuser. Such a typical set up is shown in FIG. 1, 
wherein a gas turbine 10 is exhausted into a diffuser 12. The diffuser in 
turn can exhaust either directly or by intermediate channels into a 
silencer 14 to acoustically quiet the exhaust. The exhaust is eventually 
released to the top of the stack 16. 
The diffuser itself is oftentimes of circular cross section. See FIG. 2. 
The diffuser 12 is also often conical, increasing in diameter from the 
turbine outlet end 18 towards the diffuser outlet end 20. The diffuser is 
supported on stands 22 which in turn are mounted to a concrete slab 24. 
The slab can be flat or stepped to correspond with the stands in achieving 
a horizontal orientation of the diffuser. Lifting lugs 26 are provided for 
positioning the diffuser into place during installation. The diffuser can 
be prefabricated in panels. The typical prefabrication may involve 
prefabricating the diffuser from four longitudinally extending panels, 
each panel representing approximately 90.degree. of the diffuser 
circumference. The panels are then joined at panel seam lines 28. 
Towards the turbine outlet end 18 of the diffuser 12 is mounted the 
diffuser interface 30, also sometimes referred to as the exhaust gas 
diffuser interface. 
FIG. 3 shows a diffuser interface according to the prior art. The gas 
turbine 10 ends at a gas turbine flange 32 which itself is connected to 
the gas turbine by way of convoluted expansion joint 34. The prior art 
diffuser interface 36 has a first interface flange 38 connected to the gas 
turbine flange 32 consisting of a plate or ring. The face of the second 
interface flange 40 bears against a bar of stainless steel 42 in which a 
groove is machined to hold a fiberglass gasket 44. The stainless steel bar 
42 and the fiberglass gasket 44 within it are clamped against the second 
interface flange 40 by a series of fasteners 46. The gasket performs the 
sealing function to make the joint gas tight. The retainer clip 50 helps 
hold the lining in place. The exterior of the diffuser 52 is likewise 
bolted to the prior art diffuser interface 36 by means of the fasteners 48 
securing a diffuser flange 54 to a stainless steel plate 56, which along 
with fasteners 46 seals the gasket 44 to the second interface flange 40. 
Turning to FIG. 4 an embodiment of the present invention is illustrated. 
The diffuser interface 58 bridges between the outlet of the gas turbine 59 
and the diffuser 12. First interface flange 38 is welded to the gas 
turbine flange 32. A thermally flexible region 60 is attached to the 
flange and extends downstream in a generally cylindrical form or a 
slightly tapering form with the circumference growing larger as the 
distance downstream increases to form a transition passage. The thermally 
flexible region is made of stainless steel material and is designed to 
have a thermal expansion rate equivalent to that of the gas turbine 
flange. As a result, when the temperature of the systems changes, the 
thermally flexible region will expand or contract at the same rate as the 
outlet flange. Due to this equivalent expansion and contraction, the 
stresses across the joint between the flanges 38 and 32 are minimized. As 
a result, the seal welding can survive the stresses and still provide a 
gas tight seal. A separate gasket is no longer required for making that 
seal. A tapered transition portion 62 is welded to the thermally flexible 
region 60. This transition portion 62 tapers outwardly in a downstream 
direction to match the periphery of the diffuser 12 at the transition 
portion-diffuser interface 64. The transition portion-diffuser interface 
64 can include a seal weld around the circumference. 
So as to localize the thermal expansion in the thermally flexible region 60 
of the interface 58, the tapered transition portion 62 is increasingly 
internally insulated in a downstream direction. Layers of insulation 66 
are added as the distance from the outlet end 18 increases. This 
additional insulation can be in the form of additional layers of 
insulation of a uniform resistivity to thermal transfer, or in the form of 
material of increasing resistivity to thermal transfer. At the end of the 
tapered transition portion 62 where the insulation joins to the diffuser 
12 at the tapered transition portion-diffuser interface 64, the internal 
insulation thickness and resistivity of the interface 58 should match the 
internal insulation of the diffuser 12. External insulation of the gas 
turbine and the diffuser interface may also be supplied as known in the 
prior art. Where the diffuser interface is internally insulated, the 
external insulation may be correspondingly decreased. 
To prolong the life of the insulation and to protect it against deleterious 
effects of the exhaust gas, an internal liner 68 covers the insulation. 
The liner is held in place by bolts 70 and washers 72 which hold the liner 
against the insulation. The bolts 70 can themselves be affixed to the 
interior of the diffuser interface itself shell, such as at 74 or studs 76 
can be affixed to the interior of the diffuser interface 58 by use of 
plates 78. The liner is allowed to float in response to thermal expansion 
and contraction by oversized holes through which the bolts 70 or studs 76 
protrude. 
FIG. 5 shows a portion of the liner assembly for the inside of diffuser 12. 
FIG. 5 is a flat layout of the liner assembly of approximately a 
90.degree. portion of the diffuser. FIG. 4 represents a section through 
section line 4--4 of FIG. 5. U-shaped channels 80 on top of liner 68 are 
also held down by nut and bolt combinations 70 or studs with nuts and 
washers 72. The liner 68 is provided in the form of plates or sheets which 
are lapped as shown in FIG. 6 to reduce the intrusion of gas against the 
insulation. By tying the floating liner to the thermally flexible region 
60, the floating liner flexes to conform with the changing contour of the 
outer cone as shown in FIG. 4A. FIG. 4A shows the interface 58 in a 
typical position assumed during operation when hot exhaust gases have 
heated up the components. The seal at the turbine outlet end 18 has been 
maintained because the welded seal has expanded at the same rate due to 
the expansion of the interface. The floating liner 68 has also moved with 
the thermally flexible region 60 to keep hot exhaust gases away from the 
insulation 66. The tapered transition portion-diffuser interface 64 is 
also maintained with minimum stress due to the like insulation and 
therefore, like expansion rates, due to the matching insulation. Due to 
the increasing insulation, less heat is transferred to the outside of the 
interface at the exit end and/or transferred at a slower rate than at the 
inlet end. Therefore, the interface cone tends to flex in the middle, 
rather than at the exit connection to the gas diffuser. As a result, gas 
tight seals are maintained. 
EXAMPLE 
By way of example, a diffuser having an inlet diameter of approximate ten 
feet at the inlet to the interface and an exit gas path diameter at the 
end of the diffuser of fifteen and one-half feet is approximately 
thirty-one feet long. The turbine exhaust outlet flange is constructed of 
a type 321 stainless steel with an expansion joint. The first interface 
flange is a plate 1".times.1/2" thick ASTM A 167 type 321 stainless steel. 
The thermally flexible region is approximately 10" long, with a tapered 
transition zone approximately 1', 8" long. The flexible liner is formed of 
11 gauge type 409 stainless steel. The insulation consists of an innermost 
(nearest the liner) layer of 2" insulation weighing 8 lbs per cubic ft. 
The insulating material is expanded ceramic fiber such as Kaowool. Three 
additional layers each 11/2" thick, weighing 8 lbs. per cubic. ft. of 
material are used at the tapered transition portion-diffuser interface. 
The entire diffuser interface is 2 ft. 6" in length. It is to be 
understood that the apparatus of the present will admit of other 
embodiments. The detailed description is given only to facilitate of the 
invention by those skilled in the art and should not be construed as 
limiting the invention.