PWHT thermocouples

[GenB]

Can any one share information on the best location of thermocouples and quantity needed for PWHT of PV or boiler drum to assure complete PWHT of the heads and longseam welds on small PV 30 to 60in OD x 60 to 120in Long.
I currently order 2 thermocouples per vessel but it may not be enough (note that some PV mfrs get away with only 1 thermocouple.
I have an issue to resolve with the AI.
The PWHT-sub can always suggest but they are not experts in the ASME Code
genb

 

[metengr]

GenB;
If you are going to PWHT multiple pressure retaining components in a single operation (furnace charge), ASME B&PV provides guidance on the placement of thermocouples (TC’s) as you are probably well aware (see ASME Section VIII, Part UW-40). The TC’s need to be placed at the bottom, center and top of the charge to assure adequate PWHT.

If you are going to PWHT only a single vessel at a time, there is no ASME B&PV Code guidance regarding optimum number of TC's. I believe the two TC’s - one for the weld and one for the base metal would be sufficient for a furnace. More TC's may be required if you are using resistance heating elements at different weld locations.

Since PWHT requires a procedure per ASME Code, the application of heat, method of attachment and placement (location of soak band) of TC’s are usually specified. I would use the guidance of the sub to convince the AI.

 

[SJones]

There are also requirements to prevent excessive thermal differentials per unit length of the item. If the items in question exceed this unit length then you will need thermocouples located to verify that this requirement is complied with.

Steve Jones
Materials & Corrosion Engineer

http://www.pdo.co.om/pdo/

 

[GenB]

Thanks for the insight,
the issue is a complicated one,
I am aware that even when we send one item
it is put in large furnace along with other items

I will keep studying for a good solution,
I will keep you posted.
Thanks again and
Ha Happy and Prosperous 2006
GenB