Metal duct subjected to steep temperature gradients

The present invention relates to a metal duct, the inside of which is subjected to steep temperature gradients, and which includes a sheet metal tube of rectangular crosssection with rounded corners and reinforced with outer stiffeners constituted by members which are disposed in planes perpendicular to the axis of symmetry of the tube and which are connected thereto. The stiffening members are constituted by rectilinear first members welded to the sides of the tube and interconnected at each corner via a respective second member which is secured via its ends to two adjacent first members, the second member being spaced apart from the duct. The duct is designed, in particular, to form a gas turbine exhaust duct.

The present invention relates to a metal duct subjected to steep 
temperature gradients, and concerns in particular a gas turbine exhaust 
duct. 
BACKGROUND OF THE INVENTION 
More precisely, it concerns a metal duct, the inside of which is subjected 
to steep temperature gradients, and which comprises a sheet metal tube of 
rectangular cross-section with rounded corners and reinforced with outer 
stiffeners constituted by members which are disposed in planes 
perpendicular to the axis of symmetry of the tube and which are connected 
thereto. 
Conventionally, the tube is rectangular, or preferably square, in 
cross-section, with rounded corners so as to limit the stresses where they 
are at their maximum levels. The stiffening members which provide 
mechanical strength are constituted by members welded around the entire 
periphery of the sheet metal tube. Since the duct is subjected to steep 
temperature gradients, those faces of the stiffening members which are 
welded to the tube heat up very quickly while their outside portions are 
still at low temperatures. The differences in temperature on either side 
of the web of each of the members generate thermal stresses linked to the 
resulting differential expansion. At the corners, where the values of 
conventional mechanical stresses are at their maximum levels, the addition 
of the differential thermal stresses often causes the performance levels 
of the materials to be exceeded. As a result the stiffening members crack 
and more seriously the tube tears, and there is then a danger that leaks 
of the very hot medium being conveyed will occur, and that the damage will 
be made worse. 
A known solution consists in lagging the outside of the stiffening members 
so as to limit the flow of heat through the duct sections and therefore 
the thermal differentials, but the inertia of the masses of the ducts 
considerably limits the performance levels of that solution. 
Another known solution is to limit the height of the stiffening members so 
as to reduce the temperature difference between their bases and their 
tops, thereby requiring the stiffening pitch to be increased so as to 
retain the mechanical characteristics. This is not economically 
advantageous, and limits only the problem of fixing of the stiffening 
members, without guaranteeing the sealing strength of the ducts. 
OBJECTS AND SUMMARY OF THE INVENTION 
An object of the present invention is to solve those problems by means of a 
particularly simple system of stiffening members. 
To this end, according to the invention, the stiffening members are 
constituted by rectilinear first members welded to the sides of the tube 
and interconnected at each corner via a respective second member which is 
secured via its ends to two adjacent first members, the second member 
being spaced apart from the duct. 
The invention therefore aims to dissociate the stiffening members from the 
tube at the corners of the duct. Both faces of the second member, and in 
particular the face facing the tube are insulated by respective layers of 
air, thereby limiting the flow of heat to a much greater extent than any 
external insulation. In this way, the maximum temperature difference 
between the bases and tops is considerably reduced. 
If the residual constraints are still too great relative to the performance 
levels of the materials, the cracks in the corner stiffening members will 
not be able to propagate and reach the tube. In this way, the tube will 
remain fluid-tight, thereby avoiding any worsening of the damage, such as 
the lagging being destroyed, the stiffeners being at too high a 
temperature, the stiffeners breaking, etc. 
Preferably, each second member is welded to the adjacent first members. 
Advantageously, each second member has a rounded shape that matches the 
shape of each of the corners of the tube, the clearance between the second 
member and the tube being substantially constant. 
The invention also concerns a gas turbine exhaust duct constituted by a 
duct such as the duct described above. 
Such ducts are, in general, made of refractory steel and are subjected to 
sudden variations in temperature due to the gas turbine being started up 
quickly. The gastightness of the tube is essential, regardless of whether 
or not the tube is insulated from the gas.

MORE DETAILED DESCRIPTION 
The duct is constituted by a sheet metal tube 1 of rectangular or 
preferably square cross-section having rounded corners. Stiffeners are 
disposed around the periphery of the tube, and at a pitch determined by 
its mechanical strength. The stiffeners comprise first members 2 that are 
preferably channel-section and that are welded to the sides of the tube 1, 
and second members 3 that are preferably channel-section, but that may be 
I-section, and that are connected via their ends to two adjacent first 
members 2. Preferably, the second members 3 are welded to the first 
members 2 via respective plates 5 welded to the ends of the first members 
2. 
There is a substantially constant amount of clearance 4 between the second 
members 3 and the tube 1, the shape of second members being rounded to 
match the shape of the corners of the tube 1. The clearance 4 enables an 
insulating layer of air to be created, and the absence of contact between 
the tube 1 and the second members 3 prevents any cracks in the members 3 
subjected to the thermal stresses from being transmitted to the tube 1.