Waste heat boiler economizer tubes

[kkp3456]

We have a waste heat boiler in our plant. The economizer tubes are heavily leaking & boiler needs shut down for the same reason.
We are suspecting leak at the joints but it could also be in the tube itself

There are two questions -

1) Economizer tubes are CS A192 while there are u bends which is P5 (2 1/4Cr Mo ). Does it require PWHT for these joints? We referred ASME code & it says it is not mandatory.
Also can we replace a part of tube as a repair option or is it advisable to replace complete portion of the tube? Is there any repair code for such applications?

2) Now the economizer is bypassed & the flue gas temperature at its inlet is around 300 degC but there is no heat transfer as there is no water inside. Will it affect the life of tube?

Your valuable inputs are highly appreciated.

 

[davefitz]

not sure, but if the return bends are bent 180 degree and made of 2 1/4 Cr, I would think that those bends need stress releiving , although the weld might not.

Where do the leaks originate?

The 300 C stack will cause trouble with the stack CEMS , stack paint, workers exposure, etc.

 

[kkp3456]

Thank you.

Yes the return bends are 180 deg.
But i need to know, after welding do the joints need stress relieving as the code is not giving it a mandatory requirement.

Usually the leaks are observed on the return bend joints, but mostly due to corrosion & not cracks. This is what the history/ records show.

 

[metengr]

Please refer to ASME Section I, Table PW 39-4 the thickness for PWHT of the P-No 5A side of the weld joint is greater than 5/8" wall thickness. The stress relieving of the u-bends should not be done in the field and depends on the supplier. Bends of this type would be formed either warm or hot, and in the shop.

 

[metengr]

2) Now the economizer is bypassed & the flue gas temperature at its inlet is around 300 degC but there is no heat transfer as there is no water inside. Will it affect the life of tube?

No. Regarding the tubing you are not above the temperature for time dependent properties (creep deformation or stress rupture). Locations of tube support and terminal tube attachment welds could be subjected to high than design bending stresses caused by higher than anticipated thermal stresses. Do not remain by-passed any longer than necessary.

 

[kkp3456]

Thank you very much. This is very useful information.

 

[Robert40]

The welding on the tubes does not require stress relieving. We do not do stress relieving in the field when we weld in new tubes or repair tubes

On larger boiler systems with bent tube designs we typically will repair the leaking section only. This is normally done due to access and the amount of effort to tear done a unit to have access for a full replacement. Keep in mind that with replacing a section only you will typically have to due some window welding

Hope this helps