Use of deformed tubesheet

[pve5]

We have two tube fixed flat circular tubesheets of boiler which are deformed due to excessive differential expension during operation. Maximum deflection of about 2 inches is around central flue gas pipe.The material of tubesheets is carbon steel and per NDE report the condition of material after deformation is unchanged i.e hardness and thickness are intact.

We intend to repalce the damaged central flue gas pipe and replace the damaged tubes and use the tubesheets as it is. The tube holes in the tubesheet shall confirm to TEMA standards and the tube to tube sheet joints shall be strength welded with light expansion. After completion of all welding, Boiler will be PWHT and pressure tested.

Could any one with similar problem share his experience and thoughts.

 

[metengr]

Based on your reported deflection, have you also looked at re-drilling the existing tubesheet holes? Re-using the deformed tubesheet may pose a problem with re-tubing using existing sized tubes.

 

[pve5]

metengr ( Materials )

Tube holes size will be to suit the tubes. Tube holes with over tolerances will be machined after weld build up. As specified before all holes will be within TEMA tolerances.

 

[unclesyd]

As mentioned above by metengr I would certainly try to retube one or two tubes in the area of the greatest deformation. You may have trouble inserting new tubes into the existing holes and the possibility of over/under rolling the new tubes. This is dependent on the tube gauge, tubesheet thickness, and baffle spacing.

 

[JohnGP]

Yes, the tubeholes will no longer be in direct alignment with such deformation of the tubesheets, so might create some difficulty for new tubes. Also, was the "excessive differential expansion" due to a one-off significant event, or just normal operation? I'm wondering can the same occur again such that you end up with 4" maximum deflection of the tubesheets by starting off with 2" in the "cold" condition.

 

[unclesyd]

pve5,

Can you comeback with you dimensions; tube size, tubesheet thickness, tubesheet material , etc.

You probably already have a plan of attack however I think a little discussion would benefit a lot of members.

 

[pve5]

unclesyd (Materials);

We would try retube the holes at highest deflection.In order to minimize problem in of alignment of holes, we are also planning to straighten the tubesheet by applying hydraluic jacks with end plates to distribute the load and reduce the deflection.

The thickness of tubesheet is 1.25 inch x 94" dia with 42 " opening at centre to weld the flue gas pipe.The material is SA 516 GR.70N. The tube sheet is with radiused hub butt welded to shell and bevel welded to channel side. Excessive deflection was due to malfunction of instrument and not due to normal operation.

 

[waskillywabbit]

I'm confused, why would you re-use deformed tubesheets in the first place and then go to what seems a lot of trouble to re-use them? If they have already deformed 2", what keeps them from deforming more? Seems risky at best, catastrophic at worst.

Why not just replace them?

Brian

 

[unclesyd]

As stated by waskillywabbit what you are proposing to do will more than likely be extremely difficult to do and could be catastrophic.

Do I understand you in that you are going to jack against the end (cover) plates?

Have you checked the tubeside of the tubesheet, especially for cracked ligaments?

Is the flue gas pipe removed?

How many tubes will you be jacking against?
The tubes will bow as they have to take up 4" if they didn't pull out of the tubesheet when it deformed.

 

[pve5]

Unclesyd ( Materials )

Jacking will be done against the dished head cover. Tube sheet is checked all over with UT and meets code requirements.

Flue gas pipe and all tubes in upper portion are removed and there will be no jacking against the tubes.

 

[unclesyd]

How is the dished head cover attached to the tubesheet?

 

[rjoaks]

Replace the Boiler.

 

[pve5]

uncesyd ( Materials )

Diddhed head with channel is butt welded to tube sheet

 

[unclesyd]

Like everyone else I recommend that the exchanger be replaced, but also realizing that things are not always immediately possible.

Your problem is getting more complicated with each post. I'm not seeing how you can push against the upper half of the donut with enough force to move the tubesheet.

Can you post a picture or two?

http://www.tinypic.com/

or

http://www.flickr.com/