Failure in Firetube of Glycol Reboiler

[cespi82]

Good morning all,

We stripped down the Glycol Reboiler Firetube as a part of the scheduled Gas Deshydration Unit Turnaround.

We found the Firetube of the reboiler covered of a thick layer of deposits on the top half of the firetube. Once we proceeded removing the deposits we found a cracked area in the inner radius at the begining of the elbow. The material of the firetube is carbon steel. Please see the image attached.

My thoughts on this is that the layer of deposits insulated the top half of the firetube from cooling, or heat exchanging with the glycol, creating an overheating in that area. We are not sure about how long this deposits could have been there but it is not longer than 4 years when the last turnaround inspection took place. The damage morphology seems to me Creep like. We run lab test on the deposits and the result is as follows:

Appearance: Black / white / gritty solid

Chemical Analysis:

Hydrocarbons, soluble in Toluene = Less than 1 % ( from the condensate )

Sodium Chloride = 91.7 % m/m

Corrosion products & others = Not quantified , less than 5 %.

The operating temperature is around 400 F in normal conditions (Glycol Temperature), but with the insulating conditions, the reboiler is forced to increase the temperature to achieve the glycol optimal regeneration. It could happend that we reached or exceeded the creep range for CS locally in certain areas of the pipe, specially in that part of the elbow where we could have the highest temperature for its configuration.

There is one colleague suggesting that it could have been thermal fatigue due to the differential elongation/contraction of the tube in certain areas.

Do you guys have any clue on what could happened here?

Kind regards,

Cesar

 

[metengr]

This could be one of several possible damage mechanisms - from low temperature creep cracking (yes, this can occur in cold formed carbon steel bends) to some type of environmental-assisted cracking. What you need to do is to send this section in for metallurgical analysis, and another section of the u-bend for comparison.

Have you considered surface NDT, like wet fluorescent MT, to check for other locations of cracking?

 

[btrueblood]

Was an analysis of the glycol done, and what was its pH? What method was used to prevent oxidation breakdown of the glycol to glycolic acids?

 

[Ron]

Stress corrosion cracking

 

[Guest102023]

Ron, metengr

That is also my first impression, but some rudimentary laboratory examination should be performed.

"If you don't have time to do the job right the first time, when are you going to find time to repair it?"

 

[chicopee]

From you OP "My thoughts on this is that the layer of deposits insulated the top half of the firetube from cooling, or heat exchanging with the glycol, creating an overheating in that area." I think that you got right by the appearance of the cracks on the clean elbow, however, as noted above what caused the deposit? my question is: is it a regular practice to inspect such a unit once every four years? I can only speak of steam and hot water process boilers which require annual internal inspections mandated by jurisdictions and insurance companies. Do an analysis of the microstructure to determine the probable cause.

 

[chicopee]

I meant"... macrostructure of steel..."