Attaching Thermocouples to P5A Superheater Pendants

[JIPMKWA]

Good morning,

I am looking through project's scope of work and it says the removal and re-installation of thermocouples.

We are going to install a new secondary superheater ( 2 headers and 90 new elements ) and I was wondering how the thermocouples are normally attached and what type of NDE would be required for them ? It is my understanding that they are normally fillet/tack welded on to the tubes. I requirements in ASME Sec.1 in the PWHT section under P5A and it mentions capacitor discharge welding or electric resistance welding. I have never seen either of those processes performed before. Are there alternate NDE requirements / PWHT if you use SMAW or GTAW to weld them on ? It sounds like I need to get the design and location of these TC's from the engineer.

 

[davefitz]

If they are attached via spot welding in the vicinity of the final butt weld ( to the outlet header stubs ) then the same PWHT used to cure the final butt welds will also heat treat those spot welds. You can grind them off a few weeks after initial restart after all field test data is gathered. The same attachment process normally used to attach the T/C's that are used to monitor PWHT can be used for the field monitoring T/C's.

You have indicated pendants , which implies they are non-drainable. A good use for these T/C's is to confirm when all condensate has boiled out of the undrained inverts of the pendants during restarts. During initial firing you will see very high tube -to-tube temperature differences between the dry legs and the legs that are yet full of condensate. As soon as the temperature difference is less than 75 F then you can interpret that as final dry out of all legs and then you can increase the firing rate to higher values ( assuming all other permissives are met). As I recall , an old test at a Con-Ed boiler in 1961 indicated a 45 minute dryout period is typically needed at 10% heat input.

At normal load operations ( 30-100% MCR) you should record the temperature distribution of all of these outlet T/C's, to confirm the inlet orificing was correctly calculated and to confirm no tube is operating above is design metal temperature. If it is consistently a few tubes running hot then the header ligament region at those tube stubs would be the first header section to exhibit creep cracking.

"Whom the gods would destroy, they first make mad "