H_Exchenger,NACE0175,PWHT!!! 1

[MicroEng]

Hi all
we have some H_Exchenger in NACE MR0175 condition.
Tube is S.S. 316L & Tube Sheet is C.S.(A-105).
Tubes double expanded & seal weld to tube sheet.
electrode is E-309 & W.P.S. is `nt content PWHT.
but impacted & 200 brinel is it`t hardness.
in start up seal weld is cracked & leak from them.
Please explain me What is the matter?is PWHT mandatory?

 

[metengr]

No, PWHT must not be performed on expanded tubes in this application. The PWHT will disturb the expanded roll joint(s) and result in leakage. The seal weld is not required to be PWHT for this type of application because the weld deposit thickness, which is the nominal thickness as described in UCS-56), is below nominal thickness requirement for PWHT of P1 material, per ASME B&PV code, Section VIII, Div 1.

Leaks sometimes do occur at tube to tubesheet seal welds. Locally remove the cracked seal weld depsoit, and weld repair using E309 with a 250 deg F preheat. After welding is performed , so cool and verify the seal weld repair using Liquid penetrant (PT).

 

[tickle]

Sounds as though it could be a differential expansion problem between the shell and the tubes. If the tubes are at different temperature to the shell then they will expand differently. If the shell is carbon steel (I am guessing it is) and the tubes stainless, these have different coefficients of thermal expansion. Both of these will put stress on to the seal welds that are not strong enough. Strength welds can (I think from memory) take about 1mm of differentil expansion, but it depends on lots of factors such as tube wall thickness, tube length. Otherwise you need to use expansion bellows on the shell.

I think that you need to look at the shell and tube side temperatures and calculate the differential expansion. During start up it may actually be the worse case. Then get the manufacturer of the HE to confirm that he has designed the HE for this (and other) case.

I think that the requirement for PWHT is concerned with the thickness of the welds and not the requirement for NACE. Therefore please can you provide more information about the HE to allow others to answer this part of the question.

 

[GenB]

If you must perform PWHT, do it before tubing, the tubes and seal welds are not to be pwht.
you may be able to reroll and reweld the tubes to repair the problem, I hope you are still on time
ER

 

[MicroEng]

Thanx all
my H_Exchenger is floating head type & differential expansion don`t make stress.
sure service is in Tube .
i think NACE have somethig about weld with different
base metal but i don`t fond it.

 

[roca]

Microeng
Why is the tubesheet straight carbon steel?
Should this not be carbon Steel (A105) with a stainless steel cladding face on it?

 

[tickle]

Roca

Why should it be a clad tube sheet? There are numerous reasons for having a straight CS one:- cannot add corrosion allowance to the tubes, availability of st st tubes.

I have made S&T HE's with all CS construction apart from the tubes.

Microeng
What type of floating head does the exchanger have? Is there sufficient room for differential expansion?

 

[MicroEng]

roca

Why the tubesheet must be S.S.?
is it mandatory in NACE?(for H2S content)
the corrosion is`nt my problem becose all of piping is C.S.
--------
tickle

the room is sufficient for expantion.