ashfaq is talking about an oil refinery fired heater, quite different than a boiler. If a leak develops in a boiler tube some water or steam will spill out. If a leak develops in a refinery heater, an oil product will spill out and the whole heater will catch on fire.
My suggestion may seem unusual. If you measured the tube surface hardness is because you shut the heater down, waited until it was cool and someone got into the heater through the manhole to take that measurement.
This is my suggestion: fire the heater if it isn't already in operation, get it up to nearly 600 ÂșC, maintain the temperature for half an hour and switch the burners down. Close all the points where air could squeeze in, including the burners. Fired heaters use to have a butterfly valve into the stack, so close that valve.
Let the heater cool down naturally, it'll take several hours to happen. When the temperature allows it, have someone get into the heater and take new hardness measurements.
I wouldn't be surprised if the BHN has lowered to acceptable levels.
Sounds an unusual procedure? Marmaduke Surfaceblow used to solve the most complicated engineering problems by means of unusual procedures. And Marmaduke Surfaceblow was the greatest engineer human kind has ever known (after Leonardo da Vinci, of course)
In any case, before scrapping the tubes, as 3.2 recommends, wouldn't it be worthwhile to test this suggestion?
Giovanni S. Crisi
Sao Paulo - Brazil