Heavy oils are widely used as fuel oils, solvents, lubricants, and heat transfer oils, as well as process materials. This term is used to describe oils which have a flash point above ambient temperature. They will therefore not burn or explode at ambient temperature but will do so when hot. Unfortunately many people do not realize this and treat heavy oils with a disrespect that they would never apply to gasoline, as shown by the incidents described below.
TRACES OF HEAVY OIL IN EMPTY TANKS
Repairs had to be carried out to the roof of a storage tank which had contained heavy oil. The tank was cleaned out as far as possible and two welders started work. They saw smoke coming out of the vent and flames coming out of the hole they had cut. They started to leave, but before they could do so the tank's roof lifted and a flame 25 m long came out. One of the men was killed and the other was badly burned. The residue in the tank continued to burn for 10-15 minutes.
Though the tank had been cleaned, traces of heavy oil were stuck to the sides or behind rust or trapped between plates. These traces of oil were vaporized by the welding and ignited. Some old tanks are welded along the outside edge of the lap only, thus making a trap from which it is hard to remove liquids. Even light oils can be trapped in this way.
A similar incident is described in an official report. A tank with a gummy deposit on the walls and roof had to be demolished. The deposit was unaffected by steaming but gave off vapor when a burner's torch was applied to the outside. The vapor exploded, killing six firemen who were on the roof at the time.
It is almost impossible to completely clean a tank (or other equipment) which has contained heavy oils, residues or polymers or material that is solid at ambient temperature, particularly if the tank is corroded. Tanks which have contained heavy oils are more dangerous than tanks which have contained lighter oils such as gasoline. Gasoline can be completely removed.
Note also that while light oils such as gasoline can be detected with a combustible gas detector, heavy oils cannot be detected. Even if a heavy oil is heated above its flash point the vapor will cool down in the detector before it reaches the sensitive element.
Before welding is allowed on tanks which have contained heavy oils, the tanks should be filled with inert gas or with fire-fighting foam generated with inert gas, not with fire-fighting foam generated with air. Filling the tank with water can reduce the volume to be inerted.
TRACES OF HEAVY OIL IN PIPELINES
Some old pipelines had to be demolished. They were cleaned as far as possible and then tested with a combustible gas detector. No gas or vapor was detected, so a burner was given permission to cut them up. While doing so, sitting on the pipes 4 m above the ground, a tarry substance seeped out of one of the pipes and caught fire. The fire spread to the burner's clothing and he ended up in the hospital with burns to his face and legs. The deposit did not give off enough vapor when cold for it to have been detected by the combustible gas detector.
It is almost impossible to completely clean pipes which have contained heavy oils or polymers. When demolishing old pipelines there should be as many open ends as possible so that pressure cannot build up. And good access should be provided so that the burner or welder can escape readily if he needs to do so.
POOLS OF HEAVY OIL
An ore-extracting process was carried out in a building with wooden floors. But this was considered safe because the solvent used had a flash point of 42'C and it was used cold. Leaks of solvent drained into a pit inside the building. While welding was taking place, a burning piece of rag fell into the pit and in a few seconds the solvent film which covered the water in the pit was on fire. The rag acted as a wick and set fire to the solvent, although a spark or a match would not have done so. The fire spread to the wooden floor, some glass pipes burst and these added more fuel to the fire. In a few minutes the building was ablaze and two-thirds of the contents were destroyed.
SPILLAGES OF HEAVY OIL, INCLUDING SPILLAGES ON INSULATION
The heat transfer section of a plant was filled with oil after maintenance by opening a vent at the highest point and pumping oil into the system until it overflowed out of the vent. The overflow should have been collected in a bucket but sometimes a bucket was not used or the bucket was overfilled. Nobody worried about small spillages because the flash point of the oil was above ambient temperature and its boiling point and auto-ignition temperature were both above 300'C.
A month after such a spillage the oil caught fire. Some of it might have soaked into insulation and, if so, this would have caused it to degrade, lowering its auto-ignition temperature so that it ignited at the temperature of the hot pipework.
The oil fire caused a leak of process gas which exploded causing further localized damage and an oil fire. All spillages, particularly those of high boiling liquids, should be cleaned up promptly. Light oils will evaporate but heavy oils will not. Besides the fire hazard, spillages produce a risk of slipping.
Insulation which has been impregnated with heavy oil-or any other organic liquid-should be removed as soon as possible before it ignites. If oil is left in contact with insulation materials, the auto-ignition temperature is lowered by 100-200'C.
HEAVY OIL FIREBALLS
There have been instances when heavy oils, at temperatures above 100'C, came into contact with water. The water vaporized with explosive violence and a mixture of steam and oil was blown out of the vessel, after rupturing it.
In another incident of the same nature, the oil caught fire. A furnace supplied heat transfer oil to four reboilers. One was isolated for repair and then pressure tested. The water was drained out of the shell, but the drain valve was eight inches above the bottom tube plate and so a layer of water was left in the reboiler.
When the reboiler was brought back on line, the water was swept into the heat transfer oil lines and immediately vaporized. This set up a liquid hammer which burst the surge tank. It was estimated that this required a gauge pressure of 450 psi (3100 kpa). The top of the vessel was blown off in one piece and the rest of the vessel was split into 20 pieces. The hot oil formed a cloud of fine mist which ignited immediately forming a fireball 35 m in diameter.
Recommendations which followed from this incident are:
1 . Adequate facilities must be provided for draining water from heat transfer and other hot oil systems.
2. Oil rather than water should be used for pressure testing.
3. Surge vessels should operate about half full, not 90% full as in this case.
4. In new plants, water should be considered as a heat transfer medium instead of oil.
A decision to use water has to be made early in the design because the operating pressure will be higher. Although this will add to the cost, there will be a saving in lower fire protection costs. In some plants the heat transfer oil is a bigger fire hazard than the process materials.