Hope the attached note helps you
The combination of welding and expansion required on each exchanger is normally specified by the client. The following combinations of tube expansion and tube end welding may be adopted
depending on service conditions:
- expanded only;
- strength welded only;
- expanded and seal welded;
- strength welded and lightly expanded;
- strength welded and expanded;
- back face welded.
A strength weld is defined as a weld in which the minimum throat
thickness is not less than the tube wall thickness. A weld having a
smaller throat thickness than this is considered to be a seal weld and its
function is solely to seal the tube and the tubesheet.
For exchanger applications involving non-corrosive and nonpenetrative
services, expanded tube-to-tubesheet joints shall be used.
In many applications, for example condensers and low pressure reboilers, tube
expansion into grooves in the tubesheet without welding is satisfactory and economic
The maximum projection of the tube end shall be 3 mm. For vertical
thermosiphon heat exchangers, there shall be no projection of the tube
ends at the top tubesheet.
For services where total leak tightness is required, welded tube-totubesheet
joints shall be used. The type of welded joint required (i.e.
strength weld or seal weld) shall be specified by the purchaser.
With properly applied strength welds, tube expansion is frequently unnecessary as it
does not significantly add to the mechanical strength of the tube end fixing.
Where the crevice between tube and tubesheet must be minimised (e.g. to avoid
crevice corrosion, or when fretting is anticipated, or when good heat transfer must
be maintained between the tubes and the tubesheet), the tube may be expanded after
welding. This provides intimate contact between the outside diameter of the tubes
and the bore of the tubesheet holes and may be done after welding and leak testing,
but before final pressure testing.
Where light expansion after strength welding is specified (TEMA
RCB7.522), tube hole grooving is not required.
Where the crevice is to be eliminated (e.g. due to high thermal gradients), back face
welding shall be considered. This is an expensive technique because of the handling
of the bundle which is required and the complex welding equipment.
Where the additional security provided by strength welds in combination with tube
expansion into grooves is considered necessary, the sequence of operations and the
technique employed for tube location is important. Porosity can occur in the welds if
the tubes are fully expanded prior to welding and weld cracking may be encountered
with expansion after welding. Generally, expansion after welding is more
satisfactory.
Particular attention shall be given to tube end fixing where any of the following
could occur: thermal shock, thermal cycling or a large difference in thermal
expansion between the tubes and tubesheet.
Failure of tube-to-tubesheet attachments is extremely costly and by no means
uncommon. Selection of the optimum materials for both tubes and tubesheet together
with the correct combination of expansion and welding is essential to ensure
maximum integrity and service reliability
