Tube expansion details (Mechanical versus Hydraulic
expansion
During the last step, the tubes can be expanded. Two types of
expansion can be distinguished:Mechanical expansion is executed by
the dual axis CNC machine mentioned under figure 2.3. This CNC
machine willcontinuously control the tool wear and tube diameters
beforeand after rolling. Expansion percentages from 3 to 5 % are
typical. The normal expansion length as prescribed by TEMA is 50
mm. Longer expansion lengths will be executed in different
steps.Fluid expansion or hydraulic expansion is used in case of
tubes with high strength and/or high ticknesses. In case of fluid
expansion, the expansion is normally executed over the total length
of the tubesheet excluding 6 mm on the tubesheet surface. (Is
depending on the fact that clad material is used, orsolid material)
For a schematic overview see Fig. 2.7
Fig. 2.7
Fluid Expansion procedure
In Fig. 2.8 an overview is given of the different steps
executedduring the fluid expansion procedure:a) Inserting the probe
into the tubeb) Loading the expanding zone with water at low
pressurec) Reaching high pressure (4000 - 5000 bar)d)
Recuperating the watere) Probe pulling outf) Expanded tube
Before and after the expansion, the inside diameter of each tube
will be measuredand will result in a printed mapping over-view of
all tubes basedon their dimensionalcharacteristics.
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Fluid expansion has the advantage that no contamination of the
tubes can occur due to the sliding of the rollers on the inside of
the tubes. An expansion pressure of 4.500 bar for tubes with a
thickness of 2.4 mm is typical for normal contact expansion.
(Expansion to optimize contact between tube and tubesheet.)
Fig. 2.9.
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For fluid expansion, a dedicated probe is made for
eachparticular expansion project. (See fig 2.9 and fig 2.10 )This
probe is produced taking into account the inner diameter of the
tube holes and the expansion length in the tube sheet. Due to the
construction of the probe, the relative expansionzone within the
tubesheet is always correct.It is standard procedure to measure the
inner diameter of the tube before and after the expansion with an
electronicmicrometer connected to a computer, indicating the
resultsper tubeposition.After expansion, an eddy-current test can
be executed, todetermine diminution of wall thickness, and to check
oncracks in the tube. (See also fig. 2.6)
Fig. 2.10
Conclusions
The tube to tubesheet connection as described above has proven
its high quality result over the years, and is highly recommended.
It has become part of the standard Coek Engineering working
procedure, and guarantees a low defect percentage on tube to tube
sheet connections.
