With the centerline ID being much smaller than the tube ID, you will not get the tubes clean by pigging the heater. The other options are on-line spalling or off-line steam-air decoke. On-line spalling consists of removing oil from one of the passes while the unit is in service and replacing the oil with steam. The tubes expand due to higher wall temperatures and break the coke off of the inside surface. The steam carries the coke out of the heater and into the transfer line where it travels with the oil from the other passes and into a coke drum. Care should be taken to manage the velocities in the heater to prevent erosion of the return bends.
Steam-air decoking was used for years to decoke heaters. It consists of an off-line spall using steam similar to the on=line spall that removes the bulk of the coke out of the heater and then an air burn to remove the rest of the coke. A steam-air decoke is normally slower than a pig, but can usually be completed in 4 days total, depending on the number of passes.