Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02979430 2017-09-12
Method for induction bend forming of a compression-resistant pipe
having a large wall thickness and a large diameter,
and induction pipe bending device
The invention relates to a method for induction bend forming of a compression-
resistant pipe with a
large wall thickness and a large diameter, in particular of a pipe in power
plants and pipelines, with
the features stated in the preamble of patent claim 1, as well as an induction
pipe bending device
suitable for carrying out same with the features stated in the preamble of
patent claim 8.
Pipes made of steel that have a large wall thickness in order to withstand the
stresses are needed for
transmitting liquid and gaseous media under pressure. Such requirements apply,
for example, to the
transport of hot steam in power plants, where pipe bends are necessary to
adapt the pipelines to
the structural conditions, or to transport crude oil or natural gas in
pipelines over long distances
where double bends are used at regular intervals to compensate for thermally
induced changes in
length. A large opening cross-section and correspondingly a large outer pipe
diameter is required to
enable a high throughput. Pipes referred to in this method typically have
nominal diameters greater
than 300 mm and a diameter to wall thickness ratio of 10:1 to 100:1, typically
of 20:1 to 70:1.
Such a method for induction bend forming has long been known, for example from
DE2513561 A1,
and has been continually improved in order to be able to produce very
dimensionally accurate pipe
bends despite the enormous dimensions and wall thicknesses of the pipes. While
precise adherence
to the specified bend angle is controlled for the pipe bending, two
disadvantageous shape deviations
remain in the region of the pipe bend. These are, on the one hand, the
ovality, i.e., a deviation of the
pipe cross-section from the desired ideal circular shape, and, on the other
hand, a weakening of the
wall thickness at the outer bend.
Round pipes with the above-mentioned size ratios are manufactured and
delivered with ovalities of
about 1%. A permissible out-of-roundness of the pipe bend after the induction
bending process is
4% according to European and North American standards. Larger deviations are
problematic
because locally different tensile stresses occur at the pipe wall due to the
internal pressure of the
media passing through the pipe bend. In the case of high-pressure
applications, for which these
thick-walled pipes are particularly intended, such additional stresses that
occur due to the out-of-
roundness are relevant. Thus, because of the geometric deviation, the wall
thickness often has to be
chosen larger than it would be required computationally based on the fluid
pressure alone.
The other disadvantageous effect on the pipe during induction bending is the
different wall
thickness distribution on the outer and inner bends. During bending around the
neutral zone, which
lies on the pipe's longitudinal axis, the pipe wall is subjected to tensile
stress in the region of the
outer bend to be formed. Since the outer bend is longer than the non-formed
pipe section, a
reduction in the wall thickness is inevitable. On the inner bend, on the other
hand, compressive
stresses are present during bending, and a wall thickness increase occurs
because of the necessary
shortening of the bend length. However, these unavoidable effects also lead to
the fact that the
strength calculation for the high-pressure application always has to be
applied to the wall that is
weakened the most, which is the wall on the outer bend. This is another reason
why the wall
thickness of the entire pipe must be selected significantly greater than on
the straight sections so
that sufficient strength is achieved in the pipe bend.
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The problem addressed by the invention is that of reducing the geometric
changes that weaken the
strength of the pipe bend, such as ovality and wall thickness reduction.
According to the invention, the problem is solved by providing a method for
induction bending with
the features of claim 1 and an induction bending device for carrying out the
method with the
features of claim 8.
The method according to the invention is based on the fact that an artificial
ovality is imposed on the
pipe before the forming begins, specifically in the form of a so-called lying
oval. Lying means that the
longer diameter axis of the oval, which corresponds to the shape of the pipe
cross-section, lies in the
bending plane. Since in practice induction bend forming can only be performed
in a horizontal plane
because of the large mass of the pipes and the required fixed arrangement of
the bending arm, the
long diameter axis is at the same time oriented horizontally.
In order to achieve the lying ovality, according to the invention, the tube is
vertically compressed in a
pressing device by means of a press punch and a counter support, or by two
press punches that
work against one another before heating and thus before entering the forming
zone, and is guided
laterally in the horizontal direction.
The compression occurs preferably by the same degree of out-of-roundness that
would occur in the
case of the induction bend forming process for a pipe bend with a certain bend
angle at the same
type of pipe. Particular preference is given to a continuous adaptation of the
degree of ovality during
the execution of the pipe bending process so that initially smaller pre-
ovalities are used that increase
toward the pipe bending center because the greatest ovality would occur there
without the
pretreatment process according to the invention.
Due to the forced cross-sectional shape of the pipe as a horizontal oval prior
to the inlet into the
inductor, all the ovalities at the beginning, in the center and also at the
end of the pipe bend are
compensated, with the beginning being defined as the front end viewed in the
feed direction. As a
result, a pipe with a circular cross-section, with very small tolerances
compared to conventional
forming, is achieved. The apparent paradox that, according to the invention, a
round cross-section at
the beginning of the pipe bend is obtained in spite of an earlier artificially
produced ovality before
the beginning of pipe bending lies in the internal distribution of compressive
and tensile stresses in
the pipe bend. While these stresses without the measure according to the
invention constitute the
cause for ovalities, under the effect of the pretreatment according to the
invention, all the effects
compensate each other.
The second measure according to the invention for optimizing the pipe geometry
during induction
bend forming is based on the approach of at least shifting the unavoidable,
different wall thickness
distribution on the inside and outside of the pipe bend. By moving the neutral
zone toward the
outside, the wall thickness in the inner bend increases even more due to the
natural volume
constancy. However, this has no negative effects on the strength and the
subsequent processability
of the pipe bend. It is essential that this measure can be used to reduce the
wall thickness reduction
on the outer side so that according to the invention a greater wall thickness
is obtained than was
previously possible with the use of a similar pipe.
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The wall thickness reduction in a 900 pipe bend produced according to the
conventional induction
bending method is up to 25% at a typical ratio of bending radius to pipe
diameter of, for example,
1.5 : 1. According to the invention, the wall thickness reduction can be
substantially reduced, in
particular, halved. This means that the wall thickness at the outer bend is
12.5% greater with the
method according to the invention than with the prior art. This also means
that either a higher
operating load is possible with the same wall thickness of the used pipe, or
even a lower initial wall
thickness can be selected under the same operating conditions. This, in turn,
results in a saving in
weight and costs.
The move of the neutral zone during pipe induction bend forming is achieved
according to the
invention in that the pipe cross-section is heated differently between the
outside and inside of the
pipe. In this case, the outer side of the bend is heated less than the inner
side of the bend. Due to
the higher temperature, the resistance to forming on the inside of the bend is
less than on the
outside of the bend, which results in the intended move of the neutral zone in
the bend toward the
outer side of the bend. The invention thus specifically utilizes the
deformation temperature interval
available for the material.
Forming with altered temperature profiles is performed according to the
invention in a subregion of
the bend angle. A transitional program takes place from the initial tangent
into this subregion, in
which the displacement from an initial position, where the move is gradually
shifted from an initial
position symmetrical to the pipe center toward the outside. A transitional
program is also applied
from the subregion into the end tangent, in which the temperature profile is
once again oriented
symmetrically.
Said partial region extends over approximately 80% - 90% of the provided bend
angle. In this case,
the partial region starts from the starting tangent at approximately 10 - 2
of the bend angle and
ends approximately 10 - 2 before the transition to the end tangent.
The move of the temperature profile provided according to the invention is
preferably based on an
adjustment of the annular inductor in the bending plane, in particular toward
the outside, preferably
coupled with an adaptation of the electrical power in the induction device,
i.e., a change in the
heating power. Due to the inductor adjustment toward the outside, the inductor
is closer to the pipe
wall on the inside of the pipe bend than on the outside, so that stronger
heating takes place here.
With approximately 5 - 50 mm, the adjustment range is very small in relation
to the used pipe
diameters of larger than 600 mm. In order to effect heating of large wall
thicknesses by induction,
the air gap, that is, the distance between the annular inductor as a current-
carrying conductor and
the pipe jacket, must not be too great. On the other hand, metallic contact
with the outer side of the
pipe must be avoided under all circumstances. The diameter of the inductor is
preferably set to 1.05
Dpipe plus 25mm. For a pipe with diameter Dpipe = 1000 mm, the resultant
theoretical adjustment
distance is 75mm, of which, however, practically only about 50mm can be used
for achieving a
laterally shifted temperature profile.
As an alternative or in addition to locally different heating, a targeted
energy removal can also occur
through local cooling.
Non-contact surface temperature measurements are taken on the inside and
outside of the bend,
and these values are provided to a control device. The temperature
distribution can be updated via
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the control device by increasing the cooling energy on the outside of the bend
and/or by increasing
the heating power at the inside of the bend and/or by changing the position of
the inductor in the
transverse direction.
In one preferred variant of the method according to the invention, a distance-
controlled and at the
same time, a performance-controlled method is provided.
This allows for a targeted influence of both the inner side of the bend and
the outer side of the
bend. The operator can select which side of the bend is to be primarily
controlled by distance, and
which side is to be controlled by energy and can specify the desired surface
temperatures, including
permissible tolerance ranges. The control device then automatically changes
the position of the
inductor in such a way that the desired relative distribution between the
inner and outer sides of the
pipe bend is reached and also adjusts the electrical power so that the
absolute forming
temperatures are reached.
Details of the invention are explained in more detail below with reference to
the drawings. The
figures show in detail:
Figure 1 a schematic view of an induction pipe bending device:
Figure 2 a top view of a pipe bend;
Figure 3 cross-sections according to the prior art in the cross-sectional
planes marked in Figure 2;
Figure 4 cross-sections according to the invention in the cross-sectional
planes marked in Figure 2;
Figure 5 a cross-section of the different wall thickness distribution in the
center of the pipe bend;
Figure 6 a longitudinal section of the different wall thickness distribution
in the center of the pipe
bend, and
Figure 7 a pressing device for the pre-ovalization.
Figure 1 shows an induction pipe bending device 100 comprising a stationary
machine bed 10 on
which a holding device 11 for a pipe 1 is arranged. The holding device 11
grips the pipe 1 at its rear
end and clamps it securely. In addition, the holding device 11 is movable in
relation to the machine
bed 10 in the direction of a pipe center axis 2, which at the same time
indicates the feed direction.
The feed is carried out via a hydraulic unit 12.
A bending arm 30 is pivotably mounted on a vertical bending axis 32, wherein
the distance of the
bending axis 32 can be adjusted perpendicular to the pipe center axis 2 in
order to set the desired
bending radius. A bending lock 31 with which the pipe 1 can be gripped and
clamped is arranged on
the bending arm 30.
Relatively close to the inductor 20 and to the heat-affected zone, a cooling
device (not shown here)
is arranged, with which, for example, cooling of the surface temperature is
effected using water as
soon as the corresponding length section has emerged from the forming zone.
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An induction device comprises an annular inductor 20, which is positioned with
its center in the
region of the pipe center axis 2.
While the aforementioned features are also a component of the known induction
pipe bending
devices, according to the invention, on the one hand, a transverse adjusting
device 21 is provided in
order to be able to move the inductor 20 transversely to the longitudinal axis
2 of the pipe 1 being
processed.
On the other hand, a pressing unit 50 is provided, of which a preferred
embodiment is illustrated in
Figure 7 in a view from the front, viewed from the machine bed 10 in the feed
direction. In a rack 51,
at least one hydraulic punch 52, 53 is arranged at the top and at the bottom,
each of which being
provided with a pressure roller 54, 55 in the form of a double cone or a
rotational hyperboloid or an
otherwise concave, rotationally symmetrical body. Through these forms, a load
distribution is
achieved with only one roller each on each side of the pipe 1 on two
sufficiently spaced apart lines
on the outer circumference of the pipe 1. This avoids running marks on the
pipe jacket due to
excessive surface pressure. The hydraulic punches 54, 55 are operated with the
same stroke after a
single adjustment to a center located on the tube center axis 2, such that the
pressure rollers 54, 55
simultaneously contact the pipe jacket and then effect the forming procedure
with equal forces. The
pipe thus remains centered in the vertical plane during the entire execution
of the bend forming
process.
Two further hydraulic punches 56, 57, each having at least one guide roller
58, 59 at their end, are
mounted on the right and left sides of the rack 51. In this way, the pipe 1 is
also centered in the
horizontal direction in such a way that it is compressed precisely with the
pressure rollers 54, 55 on
the middle axis 2 by means of the punches 52, 53 arranged above and below, and
no eccentricities
occur. By means of the hydraulic punches 56, 57 on the side, only the guide
rollers 58, 59 are
positioned and held, but no forming force is exerted by them on the pipe. The
lateral guide rollers
58, 59 are preferably convex-crowned or cylindrical, in order to prevent shape-
dependent securing
of the pipe 1 on the guide rollers in the vertical direction.
This arrangement on the horizontal and vertical axes applies to a pipe bend
that is carried out in a
horizontal plane.
As Figure 7 shows, the compression occurs exclusively in the vertical
direction, so that the cross-
section of the pipe 1 takes the form of an oval, i.e., the long diameter axis
extends horizontally. The
ovality is shown overemphasized for illustrative purposes in the presentation
according to Figure 7
as well as in Figure 3, which is explained below. In reality, the forced out-
of-roundness is only about
1% of the pipe diameter at the beginning, 1.5% at the end, and up to 4% of the
pipe diameter in the
center of the pipe bend so that it is barely visible to the naked eye.
The rack 51 of the press unit 50 is of annular design, in the sense that it is
closed in itself, i.e.,
unending. The outer shape is preferably diamond-shaped in the top view, with
one of the punches
52, 53, 55, 56 being arranged at each corner point.
Figure 2 shows a pipe bend 3 with a beginning tangent 2 and a tangent 4. Three
different section
planes A-A, B-B and C-C are marked in Figure 2, with the section plane B-B
being arranged in the
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center of the pipe bend 3 because the greatest deviations of the wall
thicknesses on the inner and
outer parts of the bend are present there.
The cross-sections at the locations marked in Figure 2 that would result in an
induction bending
process according to the prior art are shown in Figure 3. Accordingly, the
cross-section is circular
only in the area A-A, i.e., at the end tangent 4 on the non-formed pipe 1
being processed. As a result
of the forming process, a so-called standing ovality is obtained as the cross-
section B-B in the middle
of the bend 3, which also results in a lying ovality in the area C-C, that is,
at the transition to the
starting tangent 2.
By using the induction bending method according to the invention, on the other
hand, circular
shapes are formed for all three cross-sections A-A, B-B and C-C as shown in
Figure 4.
Figure 5 shows the different wall thickness distributions on the pipe bend 3
in a further cross-
sectional drawing in the plane B-B. The wall thickness is considerably thicker
on the inner pipe bend
3.2 than on the outer pipe bend 3.1. A vertical axis 3.3 that characterizes
the neutral zone is not at
the center of the pipe cross-section but instead, is offset toward the outside
of the pipe bend 3.1
according to the invention. According to the invention, this is achieved, for
example, by the following
asymmetrical temperature distribution in the forming zone:
Outside of the pipe bend 3.1 850 C
Inside of the pipe bend 3.2 1000 C
The inductor adjustment path at this point is only about 10 mm out of the
center. This small
adjustment path relative to the other geometrical dimensions is already
sufficient to achieve the
effects according to the invention.
Figure 6 shows the wall thickness distribution in a horizontal longitudinal
section through the pipe
bend 3. The dash-dotted line in the center represents the center axis 2 of the
pipe. The neutral zone
3.3 runs parallel to it. The dashed lines in the area of the inner pipe bend
3.2 and the outer pipe
bend 3.1 represent the wall thicknesses on the non-formed pipe 1. The solid
lines show the wall
thicknesses that arise after the bend forming is carried out. Again, the
deviations are shown over-
emphasized.
Examples of the wall thickness distribution for a processed pipe with a
nominal wall thickness of 10
mm are shown below:
a) Induction bend forming according to the prior art:
Outside of the bend 3.1 7.5 mm (-25%)
Inside of the bend 3.2 15.0 mm (+50%)
Change to the inner pipe diameter (constriction):-1.25 mm
b) Induction bend forming according to the invention:
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By suitably adapted temperatures, a shift of the neutral zone 3.3 inwards or
outwards can be
achieved. In general, an outward shift is aimed for according to the invention
in order to halve the
decrease:
Outside of the bend 3.1 8.75 mm (-12.5%)
Inside of the bend 3.2 17.50 mm (+ 75%)
Change of inner pipe
diameter (constriction): approx.-3.125 mm
Thus, the weakening of the outer side of the bend 3.1 has been reduced by
half. The simultaneous
increase in the wall thickness at the inner side of the bend 3.2 does,
however, lead to a slight
reduction in the inner diameter. The resulting reduction in the clear pipe
cross-section by about 2
mm is negligible in light of the large diameters of the pipes used.
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