Tube-to-tubesheet joints 3

[johnnymist2003]

Hi all,

does anybody know of any specific document reference where guidance is given as to when strength welded tube-to-tubesheet joints should be considered (eg. for toxic service, etc.) I have looked in ASME VIII, Div.1, API, & ISO 16812, & I can find no specific requirements. I'd appreciate specific document references.
Thank you.
John

 

[metengr]

You might want to check ASME B&PV code, Section VIII, Div 1, Subsection C, Part UHX. This section contains information on tube-to-tubesheet weld design.

 

[johnnymist2003]

Hi metengr,
I have checked Part UHX, this gives guidelines on joint design, but not on when to apply a particular joint configuration.
Thank you
John

 

[metengr]

The option of using partial or full penetration welds or even seal welds for tube to tubesheet joints is up to you, the designer. For example, if you decide to roll tubes or explosively bond the tubes to the tubesheet, one can take credit for strength and simply use a seal weld to assure leak tightness.

 

[rmw]

In the power industry where welded tube to tubesheet joints are common, the weld is considered a seal, and does not contribute to the tube pull out strength. These range from very high pressure Hx's to vacuum condensers. The goal of the seal weld is to prevent leakage past the strength (expanded joint.)

In that industry, Hx's in identical service can be found to exist without seal welded joints, although I can't ever remember seeing a HP FWH (first point) in a supercritical plant without seal welded tube joints, while many of the intermediate and LP FWH's in the same type of plant are commonly found as rolled only joints.

That said, I was once aware that a field retubing crew retubing a first point heater in a supercritical plant found a complete row of tubes that had failed to be expanded in any form, and they had given good as good of service as the rolled joints. The tubesheets on this Hx were 27" thick.

Seal welding is to prevent leakage if your application is such that leakage would be detrimental. In the case of the 27" thick tubesheet example above, leakage would cause tubesheet wormhole damage to a very expensive piece of equipment.

I can think of a couple of other areas, and I am sure that there must be more, where seal welding is advisable. One would be to prevent product mixing, and another would be lethal (and as you mention, toxic) service. Such would be the reason in the case of condenser tube seal welding applications. The cooling water can have some stuff in it that you don't want leaking into your condensate/BFW streams.

As Metengr said, it is your call, but plan to expand the tubes as required to obtain your tube pull out strength.

rmw

 

[stanweld]

Strength welds are often mandated when the design is for fixed tube sheets, stayed by the tubes. Tube sheets are relatively thin and flex with tube expansion, resulting in high shear loads at the tube to tubesheet interface. It is even more important to strength weld these joints when thermal/pressure cycling is anticipated. As metengr and rmw have indicated, the design is up to the designer based on service conditions and the need to assure non-mixing of the two fluids should a leak occur.

 

[EdStainless]

Another common service that is seal welded is acid coolers. Pure acid is almost non-corrosive, but dilute it just a bit and all hell breaks loose.
Often you find strength welds on Hx with light gage tubing, where the rolled only joints don't generate enough strength.
You also need to consider the alloys involved. Sometimes you do more damage in terms of local corrosion resistance than you gain in strength. I know of a large set of Hx that use superferritic tubes and alloy 59 overlayed tubesheets. They have very nasty cooling ater tubeside and they get a little bit of etching in the HAZ of the strength welds (made with 59 filler)on the inside of the tubes. In this case the tubes have pleanty of excess corrosion resistance so it isn't a problem.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.

http://www.trenttube.com/Trent/tech_form.htm

 

[unclesyd]

Another advantage to seal welding tubes/tubesheets is that if there is any type corrosion to the face or tubes it will facilitate the reuse of the tubesheets if you have to. This was great help in our case as most of our processes liked crevices no matter how small.
Every Hx except the Cu alloy ones on site was rolled, . and welded with a minimum of 2 serrations.

Prior to the availability some of the better alloys we were in a continuos process of rebuilding SS Hx for our processes. The tubesheets were originally made thicker than required to allow a skin cut if needed. Nearly all the bundles in the very corrosive services had the fixed tubesheet symmetrical, we could flip it. If no work was required on the sheets a 240 tube bundle could be turned around in 14 or so hours.

 

[roca]

John
Hope the attached note help 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

 

[GenB]

You are talking toxic then Lethal design will be the answer so you have to look at not only seal welding but welded tubes . all depends on the x contamination of the process.
er