Heater tubes may require periodic cleaning to remove internal fouling and coking deposits. These deposits can be detrimental to heater performance and reliability. Tubes and fittings usually require cleaning when deposits cause an increase in coil pressure drop, an increase in firing rate to maintain the desired coil outlet temperature, a decrease in coil outlet temperature, or tube hot spots.
Internal cleaning of heater tubes may be accomplished by several methods such as gas oil circulation, chemical cleaning, steam-air decoking, thermal spalling, mechanical pigging, hydroblasting, and abrasive grit. These methods are typically performed off-line, although some heater arrangements can allow on-line thermal or steam spalling. The effectiveness of each method to remove deposits varies with the deposit type. For instance, circulating gas oil prior to steaming and water wash can be an effective cleaner for soft deposits dissolved by gas oil. However, it will not be effective in removing heavy coke deposits. Therefore, when selecting a cleaning method the nature of the deposit, in addition to safety, potential risk of damage, allotted time, and cost should be considered. In addition to potential damage from the particular technique, cleaning can cause leaks in the tube rolls or header plugs of removable headers from thermal forces or the removal of coke. Chemical cleaning consists of circulating an inhibited acid or other proprietary chemical cleaner through the coil until all deposits have been softened and removed. Water washing to flush all deposits from the coil usually follows this method. When the tubes are made of austenitic stainless steel, the chloride content of the water used for flushing should be maintained at less than 50 ppm. Care must be used in chemical cleaning to avoid corrosion damage to the tubes.
High-pressure water jet blasting is another option for cleaning tubing with plug-type fittings. Other cleaning options for welded coils include mechanical decoking pigs, and abrasive blasting (shot blasting or sand jet blasting) with metal shot or an abrasive medium.
Steam-air decoking consists of the use of steam, air, and heat to burn the coke out of the tube. Only trained, experienced personnel should use this cleaning method, because improper procedures or control could result in overheating the tubes and supports causing serious, costly damage to the heater. Steam-air decoking will not always remove the coke from a heater fitting. If this is the case, it may be necessary to use mechanical cutters on the U-bends and remove them for cleaning. This is an expensive and destructive method of cleaning.
Thermal spalling is a technique that uses alternating heating and cooling to spall coke off the tube wall. Steam is often used as the process medium to control heating and cooling. Care should be exercised with this technique for the coke particles removed from the wall have caused localized erosion damage of return bends.
Abrasive pigs can be used to clean tubes mechanically. The technique involves propelling a pig equipped with metal appendices through the tubes with water. The pig is sent back and forth through the tubes and deposits are removed much like using a wire brush to clean a surface. This technique often involves some modifications to heater piping to create a location to launch and receive the pigs. The potential advantages of abrasive pigs are that they can potentially reduce damaging the tubes as with other techniques like steam-air decoke and acid cleaning.