[MW:14947] backing vs back weld

Dear expert's,
 
anybody can give me a calrification regarding backing & back weld
 
as per my knowledge ,welding with back strip,retainers or weld metal (double bevel groove) can considered as backing
 
single bevel groove ,welding filled from groove side then gouged from inside root and finish with one single layer or more called back weld
 
qustns
 
1) is anywhere in the code given any t limitation or how many layers can weld from inside root (single bevel groove)  for back weld ? (more layers can consider as double bevel is it rit)
2) one wleder qualified in 16 mm plate single bevel groove with backing, can he weld a double bevel groove of 30mm plate or not ?
3) one welder qualified for 16 mm plate single bevel groove with backing,can he wled a single bevel groove of 8 mm plate with back weld ?
 
 
thanks in adavnce
 
praveen

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