Tank coils or heating coils are placed in tanks and are designed to transfer heat from steam vapor or condensate to the tank. It is used when a LACT unit requires a certain temperature crude prior to shipment or in installations where heat loss will significantly viscosity of the tank fluid.
The photos below show a tank coil that is designed to be inserted into a 10,000 bbl tank. This particular unit consists of heavy walled pipe welded to form a rudimentary coil. It is also skid mounted so that it can be removed from service and replaced quickly.
It is important to note for these applications when sizing this pipe to pay special attention to the corrosion both inside and outside the pipe. A ruptured steam line discharging into an atmospheric tank will almost certainly damage the structure of the tank and could lead to catastrophic failure if the steam has no place to vent.