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Stress relieved on API 650 tanks after shell course erection

Stress relieved on API 650 tanks after shell course erection

Stress relieved on API 650 tanks after shell course erection

As per API 650, When the shell material is Group IV, IVA, V, or VI, all opening connections requiring reinforcement in a shell
plate or thickened insert plate more than 13 mm (1/2 in.) thick shall be prefabricated into the shell plate or thickened
insert plate, and the prefabricated assembly shall be thermally stress relieved within a temperature range of 600 °C to
650 °C (1100 °F to 1200 °F) for 1 hour per 25 mm (1 in.) of thickness prior to installation

Question: -
Material used - ASTM A 516 Gr 70 N - Group V of thickness 14 mm
PHWT not performed in prefabricated condition due to orientation problem
Bottom shell course erected and PWHT planned.
But in the tank foundation Bitucell (Bitumen impregnated fiber board for expansion joint filler on precast concrete components)
There may be possibility of fire or burn out on Bitucell in concrete during shell course nozzle PWHT.

Can any one suggest the way to skip PWHT at this situation, since the cost & time of rework is more.

RE: Stress relieved on API 650 tanks after shell course erection

You already skipped it- it's required to be done prior to erection of the shell.

RE: Stress relieved on API 650 tanks after shell course erection

Hi Stephen,

Thanks for your reply.
Yes I know that is skipped, and PWHT is required.
I am seeking for the better solution, since our subcontractor mess up the activity because of scope change.
Even if we plan to perform PWHT on erected shell course, bottom Bitucell layer will get damage.
The most important thing is the tanks are for Fire water storage and not for Hydrocarbon storage.

Any other way as per API 650 to skip PWHT.

RE: Stress relieved on API 650 tanks after shell course erection

Probably not the preferred option of a new build tank (and may not even be permitted by API 650) but would a fracture mechanics assessment to waive PWHT be an option?

This assessment would require information such as (not an exhaustive list):

maximum defect size and orientation (from inspection);
toughness data (Charpy test then apply correlation or/preferably results from a CTOD test);
stresses for all design conditions (hand calculations if sufficient or FEA then linearisation etc.);
process data (design conditions and is it in cyclic service where the defects can propagate?);
residual stresses;
etc...oh, and most importantly...a good engineer/consultant!

Depending on the complexity of the problem this is not necessarily a quick assessment to complete.

RE: Stress relieved on API 650 tanks after shell course erection

You could cut the shell section out, stress-relieve it, and put it back in.

You've got two different issues here.
One is compliance with API-650. That requires it be stress-relieved prior to installation in the tank, so no matter what you do when it's in place, you still don't comply with that.
The other issue is, will the tank fall down. Probably not, and you could come up with all kinds of reasoning, testing, or procedures to try to show that, but it doesn't fix item 1 there, either.

RE: Stress relieved on API 650 tanks after shell course erection

Install and PWHT the nozzles as close as you can and adjust the piping as needed to match the tank nozzles.

Install and PWHT each nozzle in a single or multiple insert or thickened insert plates. Observe all weld spacing requirements. Build the tank with no nozzles. Cut the holes and weld in the PWHT insert nozzle assemblies so they match the piping.

Build the lower shell ring from Group III material so PWHT is not needed, cut the nozzles in to match the piping.

If you and the owner are convinced the risk is acceptable or can be reasonably made acceptable and technically justifiable by additional PMI, weld prep, NDe, etc, note the variance on the name plate. Note: there is simply no valid discussion, rationalization or justification that protection of the environment, public health and safety are less important than cost. Make sure your priorities are straight on this. Note: picture yourself and your boss in a courtroom defending your decision in front of your children.

RE: Stress relieved on API 650 tanks after shell course erection

Since you are building a water tank you could run your current design through AWWA to see if you could pass it through that standard. I don't believe PWHT is required. However the design stress for A516-70 goes down so you would have to reduce your liquid height. Probably going to be painful if you try to change your design standard after you have your tank built...

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