Entry to Vessels 243 7. OccLipaiional Safety and Health, Supplement, Oct. 1975. 8. Peti-oleunz Reiiiew: May 1977. p. 49. 9. Chemical Safety Sumnmr-j: Vol. 55, No. 2 19, Chemical Industries Association, London, 1984. p. 66. 119. R. C. Melchior. Anrerican InclLrstrial Hygiene Association Journal. Vol. 48. No. 7. 1987, p. 608. 11. The Sentinel, Vol. XLIV, No. 2, Industrial Risk Insurers, Hartford, Conn 2nd quarter 1987, p. 8. 12. Health and SLfety at Work, Vol. 6, No. 8. Apr. 1984. p. 13. 13. Cl~ernical SafeQ Suminmy Vol. 55, No. 219. Chemical Industries 11. P. A. Carson and C. J. Mumford, Loss Preverztion Bidletin. No. 091, 15. Loss Prevention Bidletin, No. 110, Apr. 1993, p. 8. 16. Health arzdS~rfety at Work, Vol. 15, No. 5, May 1993, p. 9. 17. Safeh lMranagement (South Africa), Vol. 16, No. 9, Sept. 1990, p. 79. 18. Sqfe~ Managerneizt (South Africa). Vol. 16, No. 4, Apr. 1990, p. 23. 19. Loss Prelwtiori Bidletin, No. 112. Aug. 1993, p. 20. 20. Loss ?miention Bidletin, No. 122, Apr. 1995, p. 7. 2 I. Occupational Safen, atid Environmental Netvs (South Africa), Vol. 2, 22. Safety Managenzeiit (South Africa), Apr. 1997, p. 36. 23. Occupational Safety and Health Obsenvi; Vol. 3, No. 10, U.S. Dept. of Energy, Washington, D.C., Oct. 1994, p. 2. 24. An OSHA report quoted in Operating Experience Weekly Siiiimm?. No. 97-40, Office of Nuclear and Safety Facility. U.S. Dept. of Ener- gy. Washlington, D.C., 1997. p. 7. 25. Operating Experience Weekly Surnmary, No. 97-16, Office of Nuclear and Safety Facility, U.S. Dept. of Energy, Washington, D.C Association, London. 1984, p. 63. Feb. 1990, p. 7. No. 1, 1996, p.1. 1997, p. 5. Chapter 12 Hazards of Common This chapter is not concerned with the hazards of obviously dangerous materials, such as highly flammable liquids and gases, or toxic materials. Rather, the focus is on accidents involving those common but dangerous substances: air, water, nitrogen, and heavy oils. 12.1 COMPRESSED AIR Many operators find it hard to grasp the power of compressed air. Sec- tion 2.2 (a) describes how the end was blown off a pressure vessel, killing two men, because the vent was choked. Compressed air was being blown into the vessel. to prove that the inlet line was clear. It was esti- mated that the gauge pressure reached 20 psi (1.3 bar) when the burst occurred. The operators found it hard to believe that a pressure of “only twenty pounds” could do so much damage. Explosion experts had to be brought in to convince them that a chemical explosion had not occurred. Unfortunately, operators often confuse a force (such as 20 lbs) with a pressure (such as 20 psi) and forget to multiply the 20 lbs by the number of square inches in the end of the vessel. Section 13.5 describes a similar accident, while Section 5.2.2 describes other incidents in which equipment was damaged by compressed air. Because employees do not always appreciate the power of compressed air, it has sometimes been used to remove dust from workbenches or clothing. Consequently, dust and metal splinters have been blown into people’s eyes or into cuts in the skin. Worse still, compressed air has been 244 Hazards of Common Materials 245 used for horseplay. A man was killed when a compressed air hose was pushed up his rectum [ 11. Fires have often occurred when air is compressed. Above 140°C lubri- cating oil oxidizes and forms a carbonaceous deposit on the walls of air conipressor delivery lines. If the deposit is thin. it is kept cool by conduc- tion through the pipework. But when deposits get too thick, they can catch fire. Sometimes the delivery pipe has gotten so hot that it has burst o-r the aftercooler has been damaged. In one case the fire vaporized some of the water in the aftercooler and set up a shock wave, which caused serious damage to the cooling-water lines. To prevent fires or explosions in air compressors: I. Keep the delivery temperature below 140°C. It is easier to do this if the inlet filters are kept clean and the suction line is not throttled. On some rotary air compressors, a large oil surface is exposed to the air. deposits readily form and ignite, and the temperatures should be kept lower. 2. Install a high-temperature alarm or trip on the delivery line. 3. Avoid long periods of operation at low rate. as this can increase oil 4. Avoid traps in the delivery pipework in which oil can collect. 5. Clean the pipework regularly so that deposits do not get more than W in. (3 mm) thick. One fire occurred in a compressor on which it was impossible to clean the pulsation dampers. deposition. 6. Use special lubricants that reduce the formation of deposits. 7. Use nonlubricated compressors. However, oil is still needed for bearings and gear boxes and may leak into the compressors unless special attention is paid to their design and maintenance [ 191. After passing through the aftercooler, the compressed air is usually tu0 cool for deposits to form or catch fire but not always. On one plant an instrument air drier became contaminated with oil and caught fire during the drying cycle. One company experienced 25 fires or explosions in air-compressor discharge pipework within 35 years. In one of the worst, the fire heated the air going forward into an air receiver, which was lined with bitumen to prevent corrosion. On heating, the bitumen gave off flammable vapors. which exploded, toppling the receiver and demolishing part of a building. 246 What Went Wrong? Thin films of oil in pipework can explode without a previous fire if subjected to sudden shock, for example, by rapid opening of a valve [20]. Unexpected concentration of oxygen can occur when compressed air is dried or purified by passing it over certain types of molecular sieves. Nitrogen is absorbed preferentially after regeneration, and the air first produced may be rich in oxygen. This can widen flammability limits and lower auto-ignition temperatures. At least one explosion has occurred as a result. If possible, use Type 3A molecular sieves [21]. Another hazard of compressed air is that it contains dust (organic and inorganic). water, and traces of hydrocarbons, which if they are not removed can cause excessive wear of tools or contamination of products. Morris writes, "Those who use air for pneumatic tools or even paint spray seem to have an inbuilt resistance to any idea that the quality of their com- pressed air is of any serious consequence. The fact that it transmits concen- trated quantities of abrasive particles and water into the finely machined orifices and cylinders of their tools seems to pass them by" [ 121. At one time it was believed that hydrocarbon vapor and air in the form of a foam could not explode, and it was even suggested that tanks con- taining flammable vapor could be made safe for welding or other hot work by filling them with fire-fighting foam. It is now known that this is incorrect and that such foams can explode. In fact, a method proposed for exploding antipersonnel mines laid during the Falkland Islands War is to cover the ground with foam. with a hydrocarbon-air mixture in the bub- bles, and then ignite it [13]. (Tanks can, of course, be made safe for welding by filling them with foam made from nitrogen instead of air. This method is often used if the tank contains openings through which nitrogen gas would rapidly disperse.) Other hazards of compressed air are described in Reference 2. 12.2 WATER The hazards of water hammer are described in Section 9.1.5 and the hazards of ice formation in Section 9.1.1. This section describes some accidents that have occurred as the result of the sudden vaporization of water, incidents known as boilovers, slopovers, foamovers. frothovers, or puking. Boilover is used if the tank is on fire and hot residues from the burning travel down to the water layer. Slopover is often used if water from fire hoses vaporizes as it enters a burning tank. Sections 9.1.1 and 12.4.5 describe incidents in which vessels burst because water that had Hazards of Common Materials 247 collected in a trap was suddenly vaporized. But most slopovers have occurred when a water layer in a tank was suddenly vaporized, as in the following incidents : (a) Hot oil, the residue from a batch distillation. was being moved into a heavy residue storage tank. There was a layer of water in the tank-the result of steaming the oil transfer line after previous movements-and this vaporized with explosive violence. The roof of the tank was lifted, and structures taller than 20 m were covered with black oil. A man who saw the incident said the tank exploded, though the sudden release of energy had a physical rather than a chemical cause. To prevent similar incidents from happening, if heavy oil is being transferred into a tank, incoming oil should be kept beloits 100°C. and a high-temperature alarm should be installed on the oil line. Alternatively. water should be drained from the tank, the tank kept above 100°C. and the tank contents circulated before the movement of oil into the tank starts. In addition, the movement of oil into the tank should start at a low rate. (b) In other cases a water layer has vaporized suddenly when it was heated by conduction from a hotter oil layer above. For example. to clean a tank that had contained heavy oil, some lighter oil was put into it and heated by the steam coil. There was a layer of water below the oil. The operators were told to keep the temperature of the oil below 100°C. But they did not realize that the height of the thermocouple (1.5 In) was above that of the top of the oil (1.2 m). Although the thermocouple was reading 77"C, the oil was above 100"C, the water vaporized, and the roof was blown off the tank, As the water started to boil and lift up the oil, the hydrostatic pressure on the water was reduced. and this caused the water to boil with greater vigor, (c) Some paraffin that had been used for cleaning was left in a bucket. There was some water under the paraffin. Some hot equipment set fire to some cleaning rags. and the fire spread to the paraffin in the bucket. To put out the fire, a man threw 3 shovelful of wet scale into the bucket. The water became mixed with the oil, turned to steam. and blew the oil over the man, who was standing 1-2 m away. He died from his burns. 248 What Went Wrong? 1. Never mix water and hot oil. 2. Do not use flammable solvents for cleaning. 3. Do not carry flammable liquids in buckets. Use a closed can (see Section 7.1.3). Water can be trapped behind heat exchanger baffles and then suddenly vaporized by circulation of hot oil. It can also be trapped in dead-ends and U-bends in pipework (see Section 9.1.1). Such U-bends can form when one end of a horizontal pipe is raised by thermal expansion. The trays in a distillation column were damaged during startup when hot gas met water, from previous steaming, dripping down the column [3]. Sec- tion 17.12 describes an incident somewhat similar to a foamover. Accidents have occurred because hot water was not treated with respect. Five men were killed when a plastic hot-water tank split along a seam [14]. On another plant, a man, about to make some tea, caught his sleeve on the tap of an electric water heater. The heater fell over, 2 gal of hot water fell on him, and he died in the hospital five days later [ 151. The heater should have been fixed to the wall. If it had contained a hazardous chemical, it would have been secured, but no one thought hot water was hazardous. Chemicals are not the only hazards on a plant. Other hazards of water are described in Reference 3. 12.3 NITROGEN [4] Nitrogen is widely used to prevent the formation of flammable mix- tures of gas or vapor and air. Flammable gases or vapors are removed with nitrogen before air is admitted to a plant, and air is removed with nitrogen before flammable gases or vapors are admitted. There is no doubt that without nitrogen (or other inert gas) many more people would be killed by fire or explosion. Nevertheless we have paid a heavy price for the benefits of nitrogen. Many people have been asphyxi- ated by it. In one group of companies in the period 1960-1978, 13 employees were killed by fire or explosion, 13 by toxic or corrosive chemicals. and 7 by nitrogen. It is our most dangerous gas. This section describes some accidents in which people were killed or overcome by nitrogen. Some of the accidents occurred because nitrogen was used instead of air. In others people were unaware of the dangers of nitrogen or were not aware that it was present. Hazards of Common Materials 249 The name irzer? gas, often used to describe nitrogen, is misleading. It suggests a harmless gas. Nitrogen is not harmless. If people enter an atmosphere of nitrogen, they can lose consciousness, without any warn- ing symptoms or distress, in as little as 20 seconds. Death can follow in three or four minutes. A person falls, as if struck down by a blow on the head. In German, nitrogen is known as stickstoff ("suffocating gas"). Perhaps we would have fewer incidents if we called it choking gas ins,tead of inert gas. 123.1 Nitrogen Confused With Air Many accidents have occurred because nitrogen was used instead of compressed air. For example, on one occasion a control room operator noticed a peculiar smell. On investigation it was found that a hose, con- nected to a nitrogen line, had been attached to the ventilation intake. This had been done to improve the ventilation of the control room, which was rather hot. On other occasions nitrogen has been used by mistake to freshen the atmosphere in vessels in which employees were working. And in another incident, nitrogen was used by mistake to power an air- driven light. used during entry to a vessel. In this case the error was dis- covered in time. More serious are incidents in which nitrogen has been connected to air masks. To prevent these errors. many companies use different fittings for compressed air and nitrogen. Nevertheless, confusion can still occur, as the following story shows: An operator donned a fresh-air hood to avoid breathing harmful fumes. Almost at once he felt ill and fell down. Instinctively he pulled off the hood and quickly recovered. It was then found that the hood had been connected by mistake to a supply of nitrogen instead of compressed air. Different connections were used for nitrogen and compressed air. so it was difficult at first to see how a mistake had been made. However, the place where the man was working was a long way from the nearest coin- pressed air connection, so several lengths of hose had to be joined togeth- er. This was done by cutting off the special couplings and using simple nipples and clamps. Finally, the hoses were joined to one projecting through an opening in the wall of a warehouse. The operator then went into the warehouse, selected what he thought was the other end of the pro- jecting hose and connected it to the air line. Unfortunately. there were sev- 250 What Went Wrong? era1 hoses on the floor of the warehouse, and the one to which he had joined the air line outside was already connected to a nitrogen line. To prevent incidents similar to those described, we should: 1. Use cylinder air for breathing apparatus. 2. Label all service points. 3. Use different connections for air and nitrogen and publicize the dif- ference so that everyone knows. Another incident occurred on a plant where the pressure in the instru- ment air system was maintained with nitrogen when the instrument air compressor failed. Two operators who were required to wear air masks attached them to the instrument air system. Unknown to them, the com- pressor had broken down, and the system was full of nitrogen. They both died [ 161. A third incident occurred at a U.S. government facility. An employee connected his air mask onto a nitrogen line and immediately blacked out, fell, and hit his head. Fortunately, a stand-by man came to his assistance, and he recovered without serious injury. The compressed air and nitrogen lines used the same couplings, and the nitrogen lines, which should have been a distinctive color, had not been painted [22]. When possible, air from cylinders or a dedicated system should be used instead of general-purpose compressed air. If the latter has to be used, it should be tested at the point of use immediately before use, every time. 12.3.2 Ignorance of the Dangers (a) A member of a cleaning crew decided to recover a rope, which was half inside a vessel and was caught up on something inside. While kneeling down, trying to disentangle the rope, he was overcome by nitrogen. Afterward he admitted that if necessary he would have entered the vessel. (b) On several occasions people who were working on or near leaky joints on nitrogen lines have been affected. Although they knew nitrogen is harmful, they did not consider that the amount coming out of a leaky joint would harm them (Figure 12-1). Two maintenance workers had just removed the cover from a manhole near the top of a distillation column, which had been Hazards of Common Materials 251 TAKE CARE \y Don't go too near nitrogen leaks! Figure 12-1. swept out with nitrogen, when one of them collapsed. The other pulled him free, and he soon recovered. The bottom manhole had already been removed, and it seems that a chimney effect caused nitrogen to come out of the upper manhole [23]. (c) Two men without masks were killed because they entered a vessel containing nitrogen. Possibly they had removed their masks on other occasions, when the atmosphere was not harmful to breathe, for a moment or two and did not appreciate that in a 100% nitrogen atmosphere they would be overcome in seconds. It is believed that one man entered the vessel, removed his mask, and was overcome and that the second man then entered, without a mask, to rescue him. Entry should not normally be allowed to vessels containing irres- pirable atmospheres. Special precautions are necessary if entry is permitted (see Section 11 3. (d)You do not have to get right inside a confined space to be over- come. Your head is enough. When a plant was being leak-tested with nitrogen after a shut- down, a leak was found on a manhole joint on the side of a vessel. The pressure was blown off, and a fitter was asked to remake the 252 What Went Wrong? joint. While he was doing so, the joint ring fell into the vessel. Without thinking, the fitter squeezed the upper part of his body through the manhole so that he could reach down and pick up the joint. His companion saw his movements cease, realized he was unconscious, and pulled him out into the open air, where he soon recovered. (e) In another incident the cover of a large converter was removed, but nitrogen was kept flowing through it to protect the catalyst. An inspector did not ask for an entry permit, as he intended only to “peep in^'* Fortunately someone noticed that he had not moved for a while. and he was rescued in time. (f) A contract welder was asked to repair some cracks near the man- hole on top of a vessel that had been swept out with nitrogen. To gain access. he removed the plastic sheet that covered the open manhole and placed a ladder inside the vessel. protruding through the manhole. He then stood on the ladder. in a position similar to that shown in Figure 11-3. He dropped the tip of his torch into the vessel, went part way down the ladder to see if he could see it, and collapsed. By the time he u7as rescued. he was dead [24]. As stated in Section 11.4, if a manhole has been removed but entry has not been authorized, the manhole should be covered by a fixed barrier, not just a plastic sheet. A ladder inside a manhole that is protected by only a loose cover is almost an invitation to enter. 12.3.3 Nitrogen Not Known to Be Present Some of the incidents described in Section 12.3.2 may fall into this category. Most of the incidents of this type. however. have occurred dur- ing construction when one group of workers has, unknown to others, connected up the nitrogen supply to a vessel. The following is an account of a particularly tragic accident of this type. Instrument personnel were working inside a series of new tanks. installing and adjusting the instruments. About eight weeks earlier, a nitrogen manifold to the tanks had been installed and pressure-tested; the pressure was then blown off and the nitrogen isolated by a valve at the plant boundary. The day before the accident, the nitrogen line was put back up to pressure because the nitrogen was required on some of the other tanks. [...]... occurred.” 13.2 BURST HOSES Hoses h.ave failed while tank trucks or cars were being filled or emptied for all the reasons listed in Section 7.1.6, in particular because damaged hoses or hoses made from the wrong material were used However, 264 What Went Wrong? the most common cause of hose failure is the tanker driving away before the hose is disconnected The following incidents are typical: (a) A tank... with flames 10 m high but no explosion The flames went out as soon as the mist had been burned Many thousands of tank trucks had been splash-filled with gas oil at this installation before conditions were exactly right for a fire to occur ’When handling flammable gases or liquids we should never say, “It’s OK We’ve been doing it this way for 20 years and have never had a 266 What Went Wrong? fire."... the trailer connected to the truck’s unit during loadinghnloading If it is not connected, the front compartments should be filled last and emptied first or a support put under the front of the trailer Figure 13-1 A tank trailer may tip up if the rear compartments are emptied first 268 What Went Wrong? Some tank trailers are fitted with folding legs Some designs are difficult to lubricate adequately... clean a tank (or other equipment) that has contained heavy oils, residues or polymers, or material that is solid at ambient temperature, particularly if the tank is corroded Tanks that have contained heavy oils are more dangerous than tanks that have 256 What Went Wrong? contained lighter oils, such as gasoline Gasoline can be completely removed by steaming or sweeping with nitrogen Note also that while... If oil is left in contact with insulation materials, the auto-ignition temperature is lowered by 100 -200°C [8] (see Section 7.3.2) 12.4.5 Heavy Oil Fireballs Sections 9.1.1 and 12.2 describe incidents that occurred when heavy oils, at temperatures above lOO"C, came into contact with water The 258 What Went Wrong? water vaporized with explosive violence, and a mixture of steam and oil was blown out of... valves are removed for routine examination 260 What Went Wrong? Heavy ends can further degrade into carbon deposits on the insides of furnace tubes and lead to tube failure Sometimes the tube blocks completely and prevents a serious spillage, but at other times spillages have produced costly and spectacular fires, as in the incident descried in Section 10. 7.2 (though that one was not due to accumulation... color and unusual fittings, and he thought it strange that only one was delivered Usually several cylinders 270 What Went Wrong? were delivered at a time Nevertheless he accepted the cylinder He did not notice that the invoice said Oxygen The invoice was sent as usual to the purchasing department for payment The young clerk who dealt with it realized that oxygen had been delivered to a unit that had... Baker, Measiireiiierzt and Corztrol, Vol 18, Apr 1985, p 104 9 Petroleunz Reviett: June 1985, p 36 10 T H Pratt, Process Safety Progress, Vol 15, No 3, Fall 1996, p 177 11 Operating Experience Weekly Sunznzaq No 97-06, Office of Nuclear and Safety Facility, US Dept Of Energy, Washington D.C., 1997, p 3 12 The Chemical Engineel; No 620, Oct 10 1996 p 10; and No 621, Oct 24 1996 p 7 13 The Iiidiutrial En?ergencyJourml,... competent, experienced person who will certify that it is isolated and free from danger While a tank is being built, when the walls reach a certain height (say, greater than the diameter) the 254 What Went Wrong? tank should be deemed to be a confined space, and the entry procedure should apply 4 All managers and supervisors should be aware of the procedure for handover and entry to vessels 12.3.4 Liquid... carried epichlorohydrin The chlorite truck went to the customer who was expecting epichlorohydrin and was off-loaded into a tank that already contained some epichlorohydrin The result was an explosion and a serious fire; fumes and smoke led to the closure of the bridges over the Severn Estuary UK [ 12, 131 Suppliers’ papers tell us what they intend to deliver, not what is in the tank truck or car We can . operator then went into the warehouse, selected what he thought was the other end of the pro- jecting hose and connected it to the air line. Unfortunately. there were sev- 250 What Went Wrong? era1. off flammable vapors. which exploded, toppling the receiver and demolishing part of a building. 246 What Went Wrong? Thin films of oil in pipework can explode without a previous fire if subjected. asked to remake the 252 What Went Wrong? joint. While he was doing so, the joint ring fell into the vessel. Without thinking, the fitter squeezed the upper part of his body through the