Pressure Vessel PWHT Distortion
Pressure Vessel PWHT Distortion
(OP)
We have just finished repairing a pressure vessel that was distorted due to PWHT. The direct cause of the distorition is not clear (support, overheat, or flame impingement problem) and I was wondering if there is a someone who has dealt with this (I'm sure there are a few) and what kind of 'expertise' they could provide? It was an internally fired vessel that was only externally supported. the diameter is 14ft with a tan length of ~100ft (1-3/4" wall).
Any thoughts??
Any thoughts??





RE: Pressure Vessel PWHT Distortion
RE: Pressure Vessel PWHT Distortion
RE: Pressure Vessel PWHT Distortion
RE: Pressure Vessel PWHT Distortion
Where was the distortion and how much was it?
What is the material of construction?
RE: Pressure Vessel PWHT Distortion
RE: Pressure Vessel PWHT Distortion
When the firing first began, the vessel 'bananad' (for lack of a better term) from the top portion heating first and it eventually evened out. They then had difficulty getting up to temperature so the upper limit was increased to 1250F to get the coolest point to 1150F min, the whole process took about 52 hrs. The charts did not show a greater than 250F temperature differential between TC, but it also did not show the initial firing (started at around 300-400F).
We of course now realize that they were not experienced enough on a vessel of this size, and we paid for it. We need to find the exact cause and where the scope of responsibility is.
Was the distortion due to the flame impingement (no deflection) or just the prolonged temperature (creep as cb4 pointed out)?
Was the temperture not even and was it higher in areas away from the TC due to thermal pockets?
Was there lack of support under the vessel or would it have still sagged with additional saddles?
To answer your questions unclesyd, The material was SA516 GR70N. There was a distortion of max-min diameter of 4" at the worst spot which was directly where the one of the two firings occured and it was very 'egg shaped' as cb4 asked. The other firing was on the transition cone which was not affected.
As I said, we did manage to fix this successfully, but it was costly (not nearly as much as replacing cans). Any help on this problem would be very much appreciated. Thanks
RE: Pressure Vessel PWHT Distortion
RE: Pressure Vessel PWHT Distortion
The use and placement of support is another area that is based on experience. The more you support a vessel such as yours the harder it is to heat in place because the supports get in the way. It is not as easy as some say as you just can’t throw in few supports.
If fuel was still cheap one might do it the old fashioned way, heat treat the cans, then do the girth welds after assembly..
As you have learned that this type PWHT is best left to one well versed in the art. If you a considering doing more vessels of similar size I would try to get some assistance from an experienced heat treater. Another possibility would be a burner manufacturer who could be of considerable help if approached right.
RE: Pressure Vessel PWHT Distortion
RE: Pressure Vessel PWHT Distortion
Regarding proper set up, there is some art to this, and there is no substitute for using an experienced heat treating company.
RE: Pressure Vessel PWHT Distortion
1. P Number& Group Number of the material
2. Heat treatment method
3. Maximum charge temperature for furnace heat treatment
4. Nominal thickness
5. Heat up rate (MAX) 400 Deg F divided by metal thickness
6. Minimum & maximum hold temperature and time
7. Cool down rate (MAX) 500 Deg F divided by metal thickness
8. Cool down method
RE: Pressure Vessel PWHT Distortion
RE: Pressure Vessel PWHT Distortion
This is a question for cb4 or anyone else: Can you give some additional information about Larson Miller parameter? I was thinking that you could extrapolate the stress value for the shell material and run Zick calculations for the temporary saddles to detect potential local distortion.
RE: Pressure Vessel PWHT Distortion
RE: Pressure Vessel PWHT Distortion
Your vessel was overheated which violates the PWHT max temp.
now is is annealed and possible some areas were re-normalized.
you fixed the vessel after PWHT.
Now you will have to stress relieve the complete ass'y again... 'm sorry,
ER
RE: Pressure Vessel PWHT Distortion
But there are two points I would like to stress:
i) I agree with generalblr: you solved the mechanical problem, however if you have ASME Code Stamped Vessels you are probably required to heat treat the vessels again since you over heated it. Talk to your A.I.
ii) I do not concur with carljpme about running a Zick Calculation. It is not appropriate in this case, we are dealing with a vessel supported in three saddles. L.P. Zick method was specifically developed for vessels supported in TWO saddles. I would advise to use FEM.
Mac10.
RE: Pressure Vessel PWHT Distortion
I think CB4 has hit on the real problem, which is the creep value at an elevated temperature for such an extended cycle duration. The head and the transition will provide exceptional stiffness and the shell in the middle can oval.
I have never seen a vessel that would take 2 days to PWHT. We have performed PWHT on 7.25 inch thick steam drums using internal firing and it never took more than 30 hours.
Westcoast: Did you externally insulate the outside of the vessel before internally firing? You could take Hardness readings at many different locations along the vessel. This would give you a good indication if you overheated localized areas.