Heaters that have plugged headers (mule ears) have been successfully on-line spalled. There is no need to weld the plugs into the headers. That would defeat the reason for the plugs in the first place. You are right that plugged headers can leak and the most common time for them to leak is during thermal cycles. Plugged headers are placed in a header box that is separate from the radiant section of the heater to minimize the impact of any variation in radiant heat flux. These header boxes will have snuffing steam piped to them. If any of the plugs begin to leak, injecting steam into the header box will prevent them from igniting and will allow the leak to coke up.
If the concern for leakage is great, replace the plugged headers with welded return bends rather than welding the plugs in the header. This will reduce the pressure drop through the furnace as the pressure drop through a plugged header is about four times higher than a welded return bend.
Plugged headers are much more tolerant of high velocities during on-line spalling as coke particles collect near the top plug and provide a layer of coke that erodes rather than eroding the return bend. Managing the velocity during on-line spalling is imperative to protect against erosion of welded return bends.
