Forensics

Thermal Oil Heater Failures

Thermal Heaters

Common design of a thermal heater, which can be horizontal or vertical.

This type of heater is used in industrial applications to supply heat. The basic design consists of a chamber in which fuel oil is burnt, around which thermal-oil-circulating steel tubes convey the heat to the application. These heaters can be vertical or horizontal, and the larger ones have the heat passing between banks of tubes in serpentine fashion to remove as much heat as possible before the flue gas is exhausted to a stack. The thermal oil returns and is reheated.

Oil heaters are normally operated at a temperature just below the boiling point of the thermal oil. As such, they are not 'boilers' or even pressure vessels. The internal pressure comes from the pumps. Once these are switched off, the pressure drops to atmospheric. Therefore they are not usually subject to the close scrutiny that government departments apply to pressure boilers. The tubing can be made of thinner walled pipe and alterations to the tube configuration can be made by the factory without the need for qualified welders and NDT.

The inside of a heater after a fire. The tubes were heavily oxidised and the oil had gelled and would not flow.

Perhaps in part because of the above, failures of thermal heaters are common. Explosions are rare, but leaks can result in serious fires which at best will cause severe damage to the heater. This is because the thermal oil is itself combustible, with flash point and auto-ignition temperatures typically being >170ºC and >550ºC respectively. While the oil pumps may be switched off quickly in the event of a problem, there is a lot of oil in the tubes and this can flow by gravity and feed the fire. The oil in the tubes can be 'cooked' by the heat such that circulation cannot be restored after the fire. At worst the fire can escape from the heater and destroy surrounding equipment or even the whole factory.

More Serious Accidents

More serious accidents occasionally occur with thermal heaters. As with any combustion chamber where oil is injected, it is possible for fuel to be injected for a time without burning, followed by the sudden combustion of the accumulated fuel such that an explosion occurs. A variation of this has involved a fire from a tube leak, with the pressure relief valve being unable to cope with the rise in temperature from the uncontrolled fire, resulting in an explosion.

Reasons for Failure

The most common problem is the development of thermal fatigue cracks in the tubes. Typically these are circumferential and occur on the side facing the burner, generally at the burner end. It is the leaking of oil from these cracks that cause most of the fires.

Fatigue cracks sometimes occur where plates are welded between tubes to prevent short circuiting of the exhaust gas. Deterioration of the tubes can also occur from the burning of high sulphur fuel oil, resulting in acid attack.

Stalactites of resolidified steel/slag following a fire in a thermal heater. The temperature exceeded the melting point of the carbon steel tubes.

Fatigue cracks almost always initiate on the outside diameter of the tube and are therefore capable of being detected before a leak occurs. However the detection by eye, magnetic particle or dye penetrant requires the tube surface to be descaled*. In the absence of regular scrutiny a fire might be the first indicator of the existence of cracking. Another feature we have observed is the repair by welding of the crack that leaked, with others remaining undetected or ignored. We have been called to fires in thermal oil heaters where it has been apparent there is a history of leak, repair, leak, repair without there ever having been a thorough inspection and proper repair.

Cracks can also occur out of sight at the webs previously described. These can be very difficult to repair, requiring access by cutting the shell with extensive refractory refurbishment thereafter.

Failure Analysis

FSM has been asked to investigate a large number of thermal oil heater failures. Our findings in this field of failure are usually unequivocal, with the only difficulty being when a fire is so severe that the tubes in the area of leaking are melted away. Methodology involves inspecting the tubes insitu, selection of samples, followed by metallurgical examination of such (see our pamphlet Metallurgical Failure Analysis for a description of our laboratory facilities).

*We have an eddy current technique that can detect cracks through scale.