stress corrosion cracking damage 1

[gengi]

Dear all,

has someone already experienced exchanger tubes damage due to stress corrosion cracking mechanism which originates and propagates from the inner surfaces of tubes??

It seems that this phenomenon is due to the concomitant presence of 2 factors:

1) RESIDUAL EXPANDING STRESSES ON THE TUBES ;
2) PRESENCE OF AGGRESSIVE ELEMENTS SUCH AS CHLORIDES.

The additional data are:

- we are talking about of a shell & tube heat exchangers with fixed tubesheet and an expansions joint on the shell;

- tube material is SS 304
- shell side material is carbon steel

- tubeside fluid is REFORMED GAS

- tubeside working temperatures are Inlet 125°C / Outlet 125°C

- shell side fluid is PROCESS WATER

- shellside working temperatures are
Inlet 212/197°C
Outlet 138/136°C

- shellside working pressure 1,31 barg

- tubeside working pressure 18,49 barg

Any kind of help and suggestion will be highly appreciated.

many thanks

gengi

 

[metengr]

I have seen a similar problem with external scc in 304N tubing on the external surface in HP feedwater heater tubes.

 

[unclesyd]

Just a little more information needed.

Is the bundle vertical or horizontal?

Any chlorides at all in the water?

Is the tube sheet SS?

We experience SCC on 304L SS HXs' almost on routine basis due to chlorides on the shell side. It is not uncommon for this to happen.
Comeback with the info,

 

[DSB123]

gengi,
Just an observation. You state the inlet and outlet working temperatures on the tubeside to be the same 125 Deg C. This suggests the Hx is not too efficient.(i.e. where is the heat exchange??)

 

[etsen]

Assumption: If process water through shell side doesn't have chloride, reformed gas through tube side maybe have hydrogen sulfide and water, which would induce stress corrosion cracking (SCC) by residual expanding stress and temperature stress of the tubes. May it will help you.

 

[gengi]

Dear UNCLESYD,

- the exchanger is horizontal with fixed tubesheet and an expansions joint on the shell
- the chlorides may be in the tubeside
- the tubesheets are in carbon steel with SS weld deposit on the tubeside

rgds

gengi

 

[unclesyd]

Chloride SCC is normally caused by chlorides concentrating between the tube and tubesheet and cracking from the outside of the tube. This is especially true in vertical bundles where there is a possible of an air space at top tubesheet. It is not uncommon for this to happen in horizontal bundles if the tubes are not rolled to the back of the tubesheet. This or anything else that will cause a gap at tube to tubesheet interface can spell trouble. Higher temperatures will aggravate this condition as it allows the chlorides on the shell side to concentrate.
I've seen SCC from inside surface of a tube in horizontal bundles where temperatures have allowed condensation where any chlorides in the tubeside stream will be concentrated.

A little more information might help the possible responders understand the problem a little better

How deep were your tube rolled?

Were the tubes seal welded?

Was the tubesheet grooved?

Can you verify your temperatures?

 

[gengi]

Dear UNCLESYD,

first of all thanks for your reply.

The exchanger has the tube to tubesheet joint strength welded and light expanded (without grooves)for the entire tubesheet thickness up to 3 mm from the shellside face of tubesheet.

Please note that in my first post I mada a mistake.
The correct working temperatures are:

- tubeside fluid is REFORMED GAS
working temperatures:Inlet 212/197°C
Outlet 138/136°C

- shell side fluid is PROCESS WATER
shellside working temperatures:Inlet 125°C / Outlet 125°C

Could, in your opinion, the expansion joint (installed on the shell) be a matter of concern ??


rgds

gengi

 

[gr2vessels]

gengi,
I have some difficulties accepting the cracks opening in the tube from the inside, where you could expect only compressive stresses during normal operation or very small tensile stresses, compared with the much higher tensile stresses on the outer skin of the tube. The chlorides are in the water, isn't it? The Chloride attack is then expected, normally, on the outside of the tubes, not inside. Did you say dry chlorides suspended in the in the gas on the tubeside? The chlorides need to be dissolved in water in order to generate the "chloride attack on SS304"
Something isn't right here and we / you are missing some info, perhaps you could tell us where the cracks have developed, near the tubesheet or randomly over the length of tubes, how many tubes are affected (could be affected), any vibration of tubes experienced so far, also the huge pressure differential has to have a say...yes, the expansion joint could also be a wrong configuration forcing the tubes to buckle or push them near buckling limit.
More info please,
cheers,
gr2vessels

 

[gengi]

Dear gr2vessels,

the cracks have developed, randomly in the tubes thickness of the tubes part near the shell side face of the tubesheet.
I mean, where the tubes are in contact with the back face (shell side) of the tubesheet.

I have some doubts about the correct work of the flanged & flued expansion joint installed on the shell........

There has been a uncorrect use of the working temperatures ( mean metal temperature different from those considered during the design)???
or....
The expansion joint does not work correctly (wrong design)???
Now I would check the expansion joint calculation....

What do you suggent to do?????

rgds

gengi