Note: Descriptions are shown in the official language in which they were submitted.
~079C~1
The invention relates to an apparatus for the pressure-
tight hydraulic securing of tubes in the bores of tube endplates,
more particularly in the case of heat exchangers in which the
tubes are inserted with clearance into the bores, are subsequent-
ly expanded by hydraulic pressure means and are pressed against
the wall of the bore.
m e method of mechanical roll expansion of tubes has
been employed for a long time for the mounting of tubes in the
bores of tube endplates for heat exchangers. In the construc-
tion of heat exchangers, as well as other similarly related
equipment, the connection between tube and tube endplate repre-
sents a manufacturing process of great significance in the area
of safety and process technology. Generally, the requirements on
the safety and service life of such a tube mounting have substan-
tially increased in recent years, particularly in view of the
use of heat exchangers in nuclear power plants. These increased
quality requirements cannot be adequately met by utilizing the
method of mechanical roll expansion, quite apart from the fact
that the roll expansion process is relatively time-consuming.
According to another known method for mounting tubes
in bores, the tube is expanded by the use of hydraulic pressure
means and is pressed against the wall of the bore, the material ~ -
of the tube endplate surrounding the wall being also partially
deformed. Apparatus by means of which tubes can be mounted in
bores by hydraulic expansion is disclosed in the German
Offenlegungsschrift 24 00 148.
m e method of hydraulic expansion of tubes is superior -
to mechanical rolling both in terms of handling and efficiency.
However, as regards the reliability of sealing-tightness under
all operating conditions which can be expected, the results of
hydraulic expansion methods disclosed hitherto have not yet been
satisfactory.
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1~79041
The present invention proposes to develop the known
process of the pressure-tight mounting of tubes in bores by
hydraulic expansion so that desired stressing of the tube in the
bore, i.e., a pre-definable adhesion pressure, can be achieved
with due reference to the materials and the geometrical dimen-
sions. The adhesion pressure in this case is a measure of the
force by means of which the tube is retained in the tube end-
plate against axial withdrawal from the bore.
According to the present invention, there is provided
an apparatus for the pressure-tight hydraulic securing of tubes
in tube endplates, particularly for heat exchangers, comprising
a cylindrical probe which in the securing process is inserted
into a tube to be secured, leaving an expansion space which is
sealed at the ends and into which a hydraulic pressure medium is
introduced through a channel in the probe, while at each end of
the expansion space a groove extends around the outer wall of
the probe and contains a sealing ring of an elastic material of
high strength, while the probe, axially outside the expansion
space defined by the sealing rings, has cylindrical sections,
20 characterized in that the probe channel is connected to the
high pressure side of a pressure multiplier and a hydraulic pump
is provided together with means for connecting the hydraulic
pump to the low pressure side of the pressure multiplier. This
apparatus enables the pressure of the hydraulic pressure medium
to be increased until it is above the "limiting pressure" as
hereinafter defined, so that there is plastic deformation of the
tube after pressure relief, the diameter of the tube is larger
than in the initial state, and therefore a permanent cramping of
the tube is produced by the wall of the bore.
An embodiment of the invention comprises a control
valve ~or the pressure medium, the control valve having three
operating positions, viz. for filing, expanding and relief, by
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1~790~1
means of which the pump can be connected with the low pressure
side or with the high pressure side of the pressure multiplier,
or the high pressure side of the multiplier can be connected to
a storage vessel. Preferably, a relief valve which can be set
to a certain preselected pressure valve is connected to the low
pressure side of the pressure multiplier.
In a preferred method of using the apparatus of the
invention, the pressure of the hydraulic pressure medium is
continuously increased. It is necessary to increase the expan-
sion pressure beyond a pre-defined "limiting pressure" to achieve
a reliable mounting of the tube so that the tube is subjected to
a residual stressing force in the bore after the expansion pro-
cess. The said "limiting pressure" is defined as the pressure at
which the unobstructed resilient recovery of bore and tube are of
equal magnitude, i.e., their difference is equal to zero. The
physical conditions will be explained hereinbelow.
An important advantage of this method is that the
required expansion pressure can be accurately adjusted on the
basis of the measurable working pressure of the liquid pressure
medium so that a high degree of accuracy can be achieved in
obtaining the desired stressing of the tube. The accurately
measurable working pressure also ensures a high degree of
uniformity for the plurality of repeated tube mountings in a
single tube endplate. Furthermore, when using the apparatus
according to the invention, the expansion zones can be accurately
defined. In this way, it is possible to reliably avoid a gap
between the tube and the tube endplate, even at the end of the
bore on the inside of the tube endplate. The expansion zone in
this case is located to the edge of the bore. Hitherto it was
not possible to perform roll expansion to the edge region without
endangering the tubes. However, it is known that any gaps in
the tube mounting are the causes of nests of corrosion.
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10~7~C~41
One embodiment of the invention is described herein-
below by reference to the accompanying drawings, in which:
Figure 1 is a diagrammatic view of one embodiment of
the apparatus for the hydraulic expansion of
tubes by means of a probe; and
Figure 2 is a diagram showing the characteristic of
deformation of the tube and endplate during
the expansion process.
The apparatus for hydraulic expansion of tubes illus-
trated in Figure 1 comprises a probe 1, a pump 2, a pressurebooster or multiplier 3, a pressure measuring apparatus 4, a
relief valve 5, a storage vessel 6 for pressure fluid, and a
control valve 7.
The tube 8 which is to be expanded is inserted into a
bore of a tube endplate 9. The probe 1 contains a cylindrical
mandrel on which two ring seals 10 are supported in circumfer-
ential grooves and seal the ends of an annular expanding space
situated between them.
The method of operation of the expanding device is as
follows:
Prior to the beginning of the expansion operation, the
secondary region is filled with fluid and the stepped piston
of the multiplier 3 is thrust into the starting position illus-
trated in the drawing. This state is achieved by moving the
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1079041
control valve 7 into the "fill" position. The control valve is
subsequently moved to the "expanding" position. The pressure
produced by the high pressure pump 2 acts on the large cross-
sectional surface area of the stepped piston associated with the
multiplier 3 so that, in accordance with the cross-sectional
ratio of the stepped piston, a correspondingly high pressure is
produced in the secondary region. The working pressure in the
secondary region is monitored by means of the pressure measuring
apparatus 4 in the primary region. The primary pressure is limi-
ted by the relief valve 5. As soon as the desired maximum
pressure is reached, the fluid is discharged via the relief valve
from the primary region into the storage vessel 6,
On completion of the expanding operation the control
valve is moved into the "relief" position. The probe 1 can then
be withdrawn from the expanded tube 8 without difficulty.
Figure 2 shows the deformation characteristics of the
tube 8 and the tube endplate 9 during the expanding operation.
In the region a the tube 8 is resiliently stressed to the yield
limit with the constantly rising pressure of the pressure medium.
In the region _ the tube 8 flows until it has bridged the gap
between the external wall of the tube and the wall of the bore.
This procedure takes place without any substantial increase of
pressure of the pressure medium. In region c,the bore wall in
the tube endplate 9 is elastically deformed and the tube 8 is
simultaneously subjected to further plastic deformation.
The final pressure is calculated and adjusted on the
relief valve 5 with due reference to the materials, the geometry
of the tube 8, and the tube endplate 9, and with reference to
the desired adhesion pressure.
The distances a and c which are plotted in Figure 2
on the radial coordinate are a measure of the elastic deformation
of the tube and of the tube endplate. They also represent the
10`79~4~
unobstructed elastic recovery of these two components. As can
be seen by reference to the graph, the distance c is greater
than the distance a, i.e. the difference of the unobstructed
elastic recovery of the endplate bore and of the tube is positive.
The expanding pressure at which the unobstructed elas-
tic recoveries of the endplate bore and the tube are of equal
magnitude, i.e. c' - a = o, is referred to as the limiting pres-
sure. When the working pressure reaches the said limiting value
during the expanding operation, the tube will bear upon the
wall of the bore after pressure is relieved but there will
be practically no clamping action and therefore no adhesion
pressure. It will therefore be recognized that in order to achieve
a firm tube connection, it is necessaty to increase the working
pressure during the expanding operation beyond the above
mentioned limiting pressure.
Numerical Example:
External tube diameter 22 mm
Tube wall thickness 2.9 mm
Tube material Incoloy 800
Tube endplate material 22 NiMoCr 37
Triangular pitch 30 mm
results obtained:
Yield pressure of the tube 1995 bar
Limiting pressure 3106 bar
Final pressure 4700 bar
Adhesion pressure, calculated 728 bar
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