steam coil RT requirement

[vesselguy]

hi all,

I'm currently specifying the radiographic requirements for the A106-C steam coils inside a steam boiler. The boiler feed water/steam is inside the pipe and the flame is on the outside of the pipe, inside a refractory lined radiant section. The last boiler our company purchased called for 100%RT of the radiant and convection coils. I think 100% was called for previously may be due to thinking that what if the coil leaked then it may ruin the refractory and need to shut down the boiler for repair... and so on.

I think we can save a bunch of money by calling out only 10% of welds, and 10% of each welder's work. My thinking is this is in steam service and even if it leaked, no one will be hurt as the coil is inside the radiant box. With a well qualified welder and a good QA/QC in the shop, there shouldn't be bad welds for these carbon steel shop welded pipe coils. Hence, I feel justify that 10% RT should be good enough. What does other experienced guys here thinks about this issue? Thanks.

 

[metengr]

vesselguy;
What is the diameter of the steam coil? The reason for my asking is that Section I Table PW-11 would apply. Did you review this table?

 

[Mr168]

I'll second metengr as far as checking against PW-11 first, along with any client requirements. If your material is under NPS 4 or 1/2", then evaluate the guys in your shop.

In theory, every "qualified" welder should never turn out any defects. Unfortunately, there is often far less overlap between being a good welder and a qualified welder than one would hope for, and a "good" QC guy is often just as rare. You can take RT records from prior jobs of the same type of component and use those as a reference to make your decision, or perform a small study on your next run. Start out with 100% RT, then break it down welder by welder and compare reject rates to determine if the idea if feasible, and who/where improvements need to be made.

 

[vesselguy]

metengr and Mr168,
Thank you for your input. Yes, I did checked PW-11 and it does not apply to my case because the coils are 3"-Sch80. Hence, RT is up to the user to specify.

I just found a communicate between the previous fabricator and the previous team that bought the boilers. the vendor said "in the past 100%RT was specified to look for "grapes" on the inside of the pipe welds. Since our weld root are done with TIG this issue is no longer valid..."

I have never heard of the term "grapes" but I imagine it must mean some lumps of weldments projecting on the inside surfaces of the weld.

 

[Mr168]

Grapes is a term typically associated with burn though, particularly on stick welded root passes. If you run it too hot, the keyhole opens up, and you get a glob on the backside of the weld where the filler/molten base metal has collapsed inwards.