Temperature difference between thermocouples in PWHT device 2

[zibaeng]

Hi.
How much difference in temperature between thermocouples in PWHT device is allowable?
Thank you.

 

[austsa]

Check the code that you're working to.

 

[zibaeng]

Would you mention some usual example?

 

[GenB]

I will double check this. But I think is not more than 50F diff. At holding time. My procedure calls for min. two t.c.

 

[DrEqt]

zibaeng,
If you are working acc to ASME VIII, it is 250F (140C) durng holding, for CS and LAS. See UCS-56(d).

 

[zibaeng]

Thank you GenB.
Thank you DrEqt, but UCS-56(d) is for vessel, I need procedure for piping.

 

[racookpe1978]

Well, you have two (or three or four) possible problems here.

one. If you have one TC, you know what the temperature is at that TC during the PWHT cycle.
If you have two TC's, you are no longer sure which one is correct (or =- since both might be wrong - which one is closer to the real temperature).

two. If you have a TC on the surface, you are only estimating what the real temperature is inside the steel, several inches (CM's) away from the measurement point. The longer the soak time, the slower the heatup rate and cooldown rates are, the better the insulation is around the TC, the further the TC is from the heat source and any cool zones, the better your assumption will be that the actual center-of-steel-temperature is compared to the surface-of-steel-temperature-under-the-TC.

three. You can probably assume that the surface temperature between any two or three TC will be close to the distance weighted average of the average of the two (or three) TC's. But steel-inside-the-PV-wall temperature will never be the "perfect average" of any two or three surface temperatures.

 

[SJones]

Consult AWS D10.10.

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/pub/8/83b/b04

All answers are personal opinions only and are in no way connected with any employer.

 

[davefitz]

Some questions yet need to be asked.
1. are the t/c's spot welded to the metal or just surface contacted? The spot welded config would likely give more accurate and consistent results. If only surface contact is used , the conduction heat transfer from metal to t/c can be overwhelmed by the heat lost along the length of the sheat to the environment and lead to a large error.

2. is the error a steady state error or only during heat-up? During heat up the t/c closest to the induction heating coil will be hotter. Also, to avoid exceeding the 1st critical temp during heat up, a t/c should also be placed directly under the heating coil.


as mentioned earlier, see AWS D10.10

"Nobody expects the Spanish Inquisition! "