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Gonzaga · **Joined:** Sat Dec 15, 2001 1:00 am **Posts:** 610

akonrad wrote:

Jugno: can you tell whether you can see thru all the tubes in all your heat exchangers so we can verify our hypotheses on bulging? Otherwise it's all close to chiromancy. My concern is whether you have high residual stresses in welded tubes beacause that might be much bigger problem to you than the shortened tubes. Can you measue length of the shell after PWHT and compare to before PWHT?

Mauro: what do you think about my fears? Am I crazy?
Konrad

The only explanation about tubes gone short is because they are bent inside the bundle. I would not have concern on residual stress: short tubes are compressed not tensioned and they bent without any other damage if not at the fillet weld. The "reconstruction" by welding is actually a build up of the tubesheet hole and doesn't add any resistance to the tube attachment weld. Groove, girth welding of tube shall be full RT, there is no prohibition in Code. Mauro

akonrad · **Posted:** Tue Mar 06, 2007 9:47 am

What if the welded tubes got stretched beyond Yield Point during heating, and they are supported by dense baffles, so they didn't buckle? They are pushing the tubesheet now with a force, inducing stresses in welds.

Juergen Fleischfresser · **Posted:** Wed Mar 07, 2007 11:25 pm

Quote:

Gonzaga wrote:
"Another point to consider is: Is it required to PWHT tubes to tubesheet welds in L service? or it is exempted per Fig UCS-56 (for PNo.1) Note (2)(c)(2)? .."

PWHT acc. to UCS-56 and UW-2(a) are applicable only for carbon and low alloy steel. For the tube-to-tubsheet welding of the above mentioned vessel is PWHT not required, because they are type 304 steel [see table UHA-32, P-No. 8].

If austenic steels type 304 was heat treated at temperature and times as required for carbon or low alloy steels, they are sensible for intergranular corrosion. That means it is not the right solution for the manufacturing sequence that this austenitic steel will be heat treated with the cabon steel portion of the vessel. Especially for a chemical pressure vessel this is not acceptable regardless to code requiremnets.

Please see also UHA-102 with reference to ASME IID, A-320 and UHA-105 with reference to ASME IID, A-350

Code:

ASME IID

A-320 INTERGRANULAR CORROSION
When austenitic steels are held for a sufficient time
between 425°C and 870°C, chromium carbide tends to
precipitate preferentially at austenite grain boundaries.
This type of precipitation is referred to as sensitization.
Intergranular corrosion takes place when a sensitized
material is exposed to a sufficiently strong corrosive
medium for a long enough time.
Methods for combating intergranular corrosion include
the following:
(a) An anneal at 1 010°C to 1 120°C followed by rapid
cooling through the sensitization range. Subsequent operation
of a vessel in the sensitization range may resensitize
the material.
(b) Stabilizing with columbium, titanium, or tantalum.
These elements have a strong affinity for carbon and, in
sufficient quantities, prevent the precipitation of chromium
carbides.
(c) Use of types of steel having a low enough carbon
content that the chromium depletion at the grain boundaries
is kept within safe limits.

A-350 HEAT TREATMENT OF AUSTENITIC CHROMIUM–NICKEL STEELS
In recognition of controversial opinion relative to the
effects of postweld heat treatment of austenitic stainless
steels, mandatory requirements for such have been omitted.
Service experience is too limited to permit comparison
between the relative safety of as-welded and postweld
heat treated austenitic steel weldments, particularly in
thick sections. It is recognized that the stability of austenitic
steels and their optimum behavior in service are
influenced by the mechanical and thermal treatment they
have received; however, it is a basic principle that the
Code rules are intended to provide minimum safety
requirements for new construction, not to cover deterioration
which may occur in service as a result of corrosion,
instability of the material, or unusual operating conditions
such as fatigue or shock loading.
Where maximum corrosion resistance is required, it is
advisable to heat treat in such a fashion as to place all
chromium carbides in solution. For such service it is
recommended that the following procedure be followed:
hold the vessel within the solution temperature range
prescribed by the governing material specifications for
not less than 1 hr /25.4 mm of thickness. Quench all
parts of the vessel uniformly and as rapidly as possible.
Material not stabilized with columbium or titanium
should be cooled through the range from 925°C to 540°C
in not more than 3 min. The rapid cooling should be
continued to below 425°C. Slower cooling rates may be
just as satisfactory for some compositions of the material
and conditions of service.

A possibility to avoid the heat treatment of the austenitic steel is:
-buttering of groove face of the carbon steel portion with Ni-base alloy
-PWHT as required for the carbon steel portion (including the buttering)
-welding of the SS-portion to the buttered groove face of the cabon steel portion - and PWHT is not required

Gonzaga · **Joined:** Sat Dec 15, 2001 1:00 am **Posts:** 610

A possibility to avoid the heat treatment of the austenitic steel is:
-buttering of groove face of the carbon steel portion with Ni-base alloy
-PWHT as required for the carbon steel portion (including the buttering)
-welding of the SS-portion to the buttered groove face of the cabon steel portion - and PWHT is not required.

Of course this is the correct procedure. But the work and the damage is done. Our friend wanted a solution to his problem. Apparently this solution was not possible to be given, I am afraid it means that he has to scrap his tube bundle. Mauro

Jugno Lal · **Joined:** Fri Feb 23, 2007 3:37 pm **Posts:** 9

Dear Konrad Anikiel,

We can not see through the tubes which are at centre of tubesheet only. Tubes at outer periphery of tube sheet are very clear to see through. We have carried out ball test and rod test with rod dia 5mm. Both the tests are ok. With rod test, we analysed that tube bending problem with tube is only within 500 mm distance from face of tubesheet. This tube bending /shortening may be a result of weld distortion in tube sheet welding with shell (being fixed tube sheet design). One other thing we have observed is that, tube is bended in horizontal direction not in vertical (means there is no sag). PWHT for P8 material is not required, we carried out this just beacuase of shell (Of SA 516-70). Now with issue of tube to tubesheet joint, can we go for appendix-A, Fig-2 (5) or (7).

Regards,

Jugno Lal

akonrad · **Posted:** Wed Mar 14, 2007 8:09 pm

Jugno,
Mauro's explanation gets quite clear now. The tubes in the center got buckled because of too much compression during cooling, after being pulled back from lightly expanded connection during heatup. Too fast process. I don't know how did you calculate your tubesheet, but if it was UHX- you cannot leave the tubes as they are, because your calculations assume that the tubesheet is supported by tubes. If they are buckled- no support comes and stresses are generated in the tubesheet, perhaps exceeding allowable values. You really need to find some possibility to remove these tubes, straighten them up and inserting them back. Maybe pushing a tight rod inside the tubes will cause them to straighten and jump back to their intended position, without removing them.
Otherwise you may perform FEA (will be a piece of hard job) and check whether you can leave the tubes buckled. I suppose that contact pressure between tubes and tubesheet or baffles is big enough to cause stress corrosion. I've seen such tubes after a quite short operation- they looked like cut by a Jedi sword in the back plane of the tubesheet. Just imagine what happens if ammonia at that high pressure finds its way to the tubeside, and how much pressure will your heads stand.

Jugno Lal · **Joined:** Fri Feb 23, 2007 3:37 pm **Posts:** 9

Dear Konrad Anikiel,

How you conclude this all as a final remedial action.

Regards,

Jugno Lal

akonrad · **Posted:** Thu Mar 15, 2007 1:18 pm

Jugno,
How do you think anyone can give you solutions, knowing almost nothing and having lethal service vessels, that can kill many people quickly? You didn't tell how your tubesheets were calculated and suppose me to come to any conclusion? I try to be helpful, but I'm not the Wizard of Oz. Just look how many people wrote really helpfull things in this topic, but knowing too few things- they just couldn't bring you any solution.

Gonzaga · **Joined:** Sat Dec 15, 2001 1:00 am **Posts:** 610

Jugno Lal wrote:

Dear Konrad Anikiel,

How you conclude this all as a final remedial action.

Regards,

Jugno Lal

For the tubes bent and short:
As the User I would not accept: I would be afraid for my own life
As Your AI I would not accept because I want to sleep in peace
As QC Mgr. I would not accept for same reason above
As Owner of your Company I would try to find somebody who may ensure me that there is no risk and try to save my money and my conscience, apparently it what you are looking for.
There is no money to pay human life..... listen to me. Don't take risk and don't hesitate. If you are in doubt it is just enough. To leave as it is you must be fully convinced, no doubt. Otherwise CUT.
Mauro

akonrad · **Posted:** Fri Mar 16, 2007 10:03 am

To be precise: I too think that there is too many reasons not to leave it as it is. There are of course ways to go; it can be done the easy way, it can be done the hard way- depends on what is really screwed up and how badly. Anyway I hope the heat exchangers won't be installed in my neighbourhood.

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