Tube U Bend Stress Relief 1

[fortmceng]

I am looking for some information on Stress Relieving requirements of Carbon Steel Tubes U Bends for Heat Exchanger (Material: SA179, Code API 660, ASME Sec VIII D1):
API 660 is not very clear and leaves it for the customer to decide. What should be the best approach in the absence of clear guidelines from code.
1) Is Electric Resistant method acceptable.
2) What should be the cooling and heating rate.
3) How much hold time should be sufficient.
The intent is to relieve residual stresses due to cold forming. I have come across some manufacturer's who thinks ER is sufficient whereas others prefer Furnace method for SR. There is substantial difference in cost between these two methods. I would like to know what should be the basis for selecting SR methods. Application is in a refinery.

Thanks in advance.

 

[metengr]

fortmceng;
If you need to stress relieve u-bends, I would use the PWHT guidelines for P-No 1 base material in Table UCS 56, of Section VIII, Div 1 of the ASME B&PV Code. In addition, you can use the required heat up and cool down rates in UCS-56 for PWHT, as well. Resistance heat or furnace heat are fine as well as induction heat with a procedure.

 

[fortmceng]

Thanks metengr. Initially I was thinking to use UCS-56 but talking to one of the fabricator, he told me that they do SR for 15 sec and then just leave it to cool in ambient air. He claimed that they have done thousands and never had any issues or complaints.

 

[metengr]

fortmceng;
I have been to many tube fabrication shops and the time for temperature for heat treatment is based on wall thickness. Before anything else, I would ask for their heat treatment procedure specific to tube bends and review it. Most fab shops for boiler and pressure parts will have a process procedure that describes the method of heat, and measurement of temperature. If the heat is applied by furnace or electric resistance methods, 15 seconds is a bit short. If induction heat coils are used, this might near the stated time of 15 seconds.

 

[GenB]

15 secs seems like anealing and not stress relieving, a bit confusing but I would like to learn about it.

 

[fortmceng]

GenB/Metengr,

Sorry couldn't get back earlier with the procedure.

Contractor has mentioned that the Heat treatment method is using Resistance heating. Heating rate not to exceed 100 deg F/sec. For P1 final temperature is 1200 to 1250 F, therefore it would mean that the final temperature could be achieved in 12.5 seconds. Cooling is done using ambient air. I am not sure if this method would be enough to remove residual stresses.. The requirement of SR is due to process (sulfide stress corrosion).

Thanks

 

[metengr]

fortmceng;
From my experience, the reduction in residual stress during thermal treatment has more to do with exposure temperature than time. The temperature mentioned is acceptable.

I would probably use hardness testing as my guide for adequacy of the post bending thermal treatment. In other words, use the stated procedure and have some random hardness testing performed on tube samples before and after thermal treatment. If the hardness of the post bend thermally treated tubes is near the original tube hardness, I would be satisfied that the thermal treatment procedure is successful. The only variable that I cold see needing change would be slightly longer time at 1200 deg F to assure the necessary knockdown in residual stress from cold bending.

He claimed that they have done thousands and never had any issues or complaints.

...Trust but verify.