Thermocouples Attachment to PV

[Marc Stevenson]

When attaching thermocouples to pressure boundary via capacitance discharge welding the energy output should be limited to 125 W-S as per ASME Sec 8 Div. 2. Why is this limitation? Can it be lower than 125 W-S?

 

[metengr]

This power limitation was established by ASME B&PV Code Section I and Section VIII by previous experience with capacity discharge welding to avoid having a heat affected zone encroaching on the minimum design wall thickness of a pressure retaining item. Being a maximum power output, you can go lower.

 

[Marc Stevenson]

Thank you metengr for replying. I have a specification that limits it to 25 w-s. I need to understand the basis of that. I can't find any reference for the heat input value in any code aside from ASME. Can you suggest for me where to research?

 

[metengr]

The 125 value and probably your stated value of 25 were probably developed from past experience with coupons to measure the heat affected zone in the base material when attaching TC's. So, attempting to justify or re-create this data will be difficult unless to perform the testing yourself to verify the values. The heat affected zone is caused by local heat input that can significantly harden the PV steel (depending on the material) because of cooling rate.

The 25 W-S power limit was probably developed and specified to ensure you have no measurable heat affected zone on the surface of the pressure part when attaching TC's.

 

[Marc Stevenson]

It just came to my attention that the specification classifies it as "heat input must be less than 25 w-s" whereas the ASME classifies it as "energy output should be less than 125 w-s". Is there a difference? or a way to calculate the heat input?

 

[metengr]

The heat input for welding can be calculated.

http://www.amadamiyachi.com/servlet...tionalresources_articles&file=01530000000Jybm

 

[IRstuff]

Typically, in specifications "shall" and "must" are mandatory requirements, while "should" is normally taken to mean "strongly suggested"

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ASME won't offer advice on what is best for your process or what your specification requires. They just limit the energy, for reasons given by metengr.

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