What Went Wrong Part 12 pps

Materials of Construction 303 During design the life expectancy, due to creep or other forms of corrosion, should be estimated and examination or replacement planned. Cheap fittings, such as studs, bolts. and nuts, should be replaced in good time. Not to do so is penny-pinching and expen- sive in the end. Here are two more examples of penny-pinching. The piston of a reciprocating engine was secured to the piston rod by a nut, which was locked in position by a tab washer. When the compressor was overhauled, the tightness of this nut was checked. To do this, the tab on the washer had to be knocked down and then knocked up again. This weakened the washer so that the tab snapped off in ser- vice, the nut worked loose, and the piston hit the end of the cylin- der, fracturing the piston rod. The load on a 30-ton hoist slipped, fortunately without injuring anyone. It was then found that a fulcrum pin in the brake mecha- nism had worked loose. as the split pin holding it in position had fractured and fallen. The bits of the pin were found on the floor. Split pins and tab washers should not be reused but replaced every time they are disturbed. Perhaps we cannot be bothered to go to the store for a fresh supply. Perhaps there is none in the store. 16.2 HYDROGEN PRODUCED BY CORROSION Hydrogen produced by corrosion can turn up in unexpected places, as shown by the following incidents: (a)An explosion occurred in a tank containing sulfuric acid. As the possibility of an explosion had not been foreseen, the roof/wall weld was stronger than usual, and the tank split at the base/wall weld. The tank rose 15 m into the air, went through the roof of the build- ing. and fell onto an empty piece of ground nearby, just missing other tanks. Fortunately no one was hurt. If the tank had fallen on the other side of the building it would have fallen into a busy street. Slight corrosion in the tank had produced some hydrogen. The tank was fitted with an overflow pipe leading down to the ground, but no vent. So the hydrogen could not escape, and it accumulated under the conical roof. The hydrogen was ignited by welders working nearby. (Presumably some found its way out of the over- flow.) [2]. 304 What Went Wrong? The tank should have been fitted with a vent at the highest point, as shown in Figure 16-1. Many suppliers of sulfuric acid recommend that it is stored in pressure vessels designed to withstand a gauge pressure of 30 psi (2 bar). The acid is usually discharged from tank trucks by com- pressed air. and if the vent is choked the vessel could be subjected to the full pressure of the compressed air. (b) Hydrogen produced by corrosion is formed as atomic hydrogen. It can diffuse through iron. This has caused hydrogen to turn up in unexpected places, such as the insides of hollow pistons. When holes have been drilled in the pistons. the hydrogen has come out and caught fire [3]. In another case, acidic water was used to clean the inside of the water jacket that surrounded a glass-lined vessel. Some hydrogen diffused through the wall of the vessel and developed sufficient pressure to crack the glass lining. Corrosion uses up oxygen, and this has caused tanks to collapse (see Section 5.3 d) and persons to be overcome when entering a vessel (see Section 11.1 d). (c) The sudden failure of six bonnet studs on an %in. valve caused a release of hydrogen fluoride, which killed two men and hospital- ized ten others. The failure was the result of hydrogen-assisted stress corrosion cracking. In this phenomenon, hydrogen, produced Overflow should end a little above ground level Figure 16-1. Acid tanks should be fitted with a high-point vent. as well as an overflow. so that hydrogen can escape. Materials of Construction 305 by corrosion, migrates to flaws in areas of high tensile stress. where it lowers the energy needed for cracks to grow. When the cracks reach a critical size, the equipment fails suddenly. The grade of steel used in this case was unsuitable; Reference 8 lis& the types that should be used. 16.3 OTHER EFFECTS OF CORROSION Corrosion usually results in a leak or failure of a support because a ves- sel or support gets too thin. It is then not strong enough to withstand the pressure or load. However, rust can cause failure in another way. It occu- pies about seven times the volume of the steel from which it was formed. When rust occurs between two plates that have been bolted or riveted together, a high pressure develops. This can force the plates apart or even break the bolts or rivets (see Section 9.1.2 g). Corrosion of the reinforce- ment bars in concrete can cause the concrete to crack and break away. 16.4 LOSS OF PROTECTIVE COATINGS Aluminum pump impellers are often used to pump fluorinated hydro- carbon refrigerants. If the impeller rubs against the casing, the protective film of aluminum oxide is removed. and combined with the local heating produced by the rubbing, which allows the aluminum to react with the refrigerant, the impeller may disappear. Contact between the impeller and the casing may be a result of worn bearings, which in turn are the result of compressor surges. so the reasons for any surging should be investigated [4]. A special type of high-pressure joint incorporated copper gaskets. A change was made to aluminum after laboratory tests showed no sign of reaction with the process material. The gaskets normally lasted for many years, but one failed after a few days. It was then found that the man who installed it. anxious to do a good job, had cleaned the gasket immediately before installing it. In doing so he removed the film of oxide. and the alu- minum now dissolved in the process liquid. It was usual to clean the gas- kets a few clays before they were installed. Though it was not realized at the time, this allowed a fresh oxide film to form. A change was made back to copper. It is more user friendly than alu- minum and will tolerate cleaning or scratching of the surface. 306 What Went Wrong? 16.5 SOME OTHER INCIDENTS CAUSED BY CORROSION (a)An oil company took a section of plant out of use and, due to an oversight, did not remove process materials from all the pipework. For 18 years a pipe was left with a mixture of hydrogen fluoride and benzene boxed up inside it. Finally, the walls became so thin that they burst, and ten men were taken to the hospital suffering from the effects of acid gas [9]. (b)A plant made an evaporator for liquid nitrogen by running hun- dreds of meters of copper piping through a steel tank filled with water. Although the steel was painted, it corroded right through in six months as the result of galvanic corrosion-that is, the steel and the copper formed an electrolytic cell. Paint never gives 100% cover, and if 1% of the steel was uncovered. all the current would have passed though this area, and its corrosion rate would have been increased 100 times. Painting the copper, which did not cor- rode, would be more effective than painting the steel! [lo]. This incident illustrates the hazards of do-it-yourself engineering by people who do not fully understand the properties of the materi- als they are using. (c) Minute amounts (up to 300 pg/m3) of mercury in natural gas have caused brittle failure of certain alloys. Valves have failed as a result. In addition, reaction of the mercury with ammonia can pro- duce explosive compounds [ 111. (d) Some catalyst tubes in a reactor failed as a result of chloride- induced stress corrosion cracking soon after startup. A materials expert, called in to investigate, found that all the failures had occurred in one corner of the reactor, that men had been working on the roof, day and night, for several weeks after the tubes had been fitted, that this area of the roof could not be seen from the rest of the plant, and that to reach the nearest restroom the men had to negotiate three ladders [ 121. Reference 13 describes some other corrosion problems. 16.6 FIRES We know that metals, especially aluminum (see Section lO.l), can be affected by fire, but we do not usually consider the possibility that they Materials of Construction 307 will burn. H[owever. some metals. including titanium, will burn when powdered or finely divided, and bulk titanium will also bum. Three titani- um heat exchangers were set alight and destroyed by burning operations. In one case. ignition was started by direct contact with the torch and in the other two cases by contact with hot slag [15]. Great care is needed if welding or buming is carried out anywhere near titanium equipment. 16.7 CHOOSING MATERIALS In choosing materials of construction. we have to compromise between various factors. Kirby [ 161 uses the acronym SHAMROCK to summarize and remember them. S = Safety: what are the consequences of failure? If they are serious, a more resistant material than usual may be justified. For example. on a plant where leaking water would react violently with process materials, the water lines were made from a grade of steel resistant to stress corro- sion cracking (from the chloride in the cooling water) as well as rust. H = History: if a plant has used material successfully for rrlany years. and the staff members know its strengths and limitations, how to weld it, etc hesitate before making a change. For example, a fiberglass-rein- forced plastic had given excellent service for many years; when another composite from the same company. with the same name but a different number, was used instead. it failed overnight. A = Availability: before a salesman sells you the latest wonder-working material. ask how easy it will be to get replacement supplies in a hurry. M = Maintenance. a plant engineer saved $10,000 per year by no longer neuzralizing the slightly acidic cooling water. In time, rust forma- tion in 30 jacketed reactors increased reaction times by 25%. I have known several engineers who gained a reputation for efficiency by simi- lar measures. including neglecting maintenance. and then left their suc- cessors to pick up the tab. R = Reparability: a plant bought some vessels with a new type of plas- tic lining instead of the one they had used for many years. The new mater- ial had better temperature resistance than the old, but when it did need repair. the patches would not stick. In time the problems were overcome, but reparability should have been considered before the change was made. 0 = Oxidizingh-educing nature of process fluids: in acidic solutions, this affects the choice of alloys. 308 What Went Wrong? C = Cost: an important consideration, but look at lifetime costs, including maintenance, not just at initial costs. Penny-pinching (Section 16.1 n) is rarely worthwhile. K = Kinetics of corrosion mechanisms: unless we understand these, we will not know which materials will be suitable and which will not. REFERENCES 1. G. C. Vincent and C. W. Gent, Armzonia Plant Safety, Vol. 20, 1978, 2. Chemical Safety Sumnzary No. 192, Chemical Industries Association, London, Oct./Dec. 1977. 3. Case Histories of Accidents in the Chemical Inclustry, No. 1807, Manufacturing Chemists Association, Washington, D.C Apr. 1975. 4. R. Stevens, Plant/Operations Progress, Vol. 4, No. 2, Apr. 1985, p. 68. 5. Loss Prevention Bulletin, No. 097, Feb. 1991, p. 9. 6. B. Eyre, Atom, No. 407, Oct. 1990, p. 11. 7. DOE Quality 4lert, Bulletin No. DOE/EH-0266. U.S. Dept. of Energy, Washington, D.C., Aug. 1992. 8. Loss Preet'ention Biilletirz, No. 089, Oct. 1989, p. 27. 9. Health and Safeh at Mbrk, Vol. 14, No. 6, June 1990, p. 4. p. 22. 10. M. Turner, The Chernical Engineer; No. 468, Jan. 1990, p. 28. 11. S. M. Williams, Plant/Opel-ations Progress, Vol. 10. No. 4, Oct. 12. M. Turner, The Cheinical Engineel; No. 492. Mar. 14, 1991, p. 40. 13. Corrosion Awal-erzess-A Three-Part Videotape Series, Gulf Publish- 14. S. J. Brown, Plant/Operations Progress, Vol. 6, No. 1, Jan. 1987, p. 20. 15. G. E. Mahnken and M. T. Rook, Process Safeg Progress, Vol. 16, 16. G. N. Kirby, Chemical Engineering Progress, Vol. 92, No. 6, June 1991,~. 189. ing Co., Houston, Texas. 199 1. No. 1, Spring 1997, p. 54. 1996, p. 38. Chapter 17 . ~ . people place their faith in systems either because they're new (so they simply must be good) or because they're old and have worked a long time. -Wendy Grossman, Daily Telegraph (London). July 29, 1997 This chapter describes some accidents that occurred because operating procedures were poor. It does not include accidents that occurred because of defects in procedures for preparing equipment for maintenance or ves- sels for entry. These are discussed in Chapters 1 and 11. 17.1 TRAPPED PRESSURE Trapped pressure is a familiar hazard in maintenance operations and is discussed in Section 1.3.6. Here we discuss accidents that have occurred as a result of process operation. Every day. in every plant, equipment that has been under pressure is opened up. This is normally done under a work permit. One man pre- pares the job, and another opens up the vessel. And it is normally done by slackeninlg bolts so that any pressure present will be detected before it can cause any damage-provided the joint is broken in the correct way, described in Section I S.1. Several fatal or serious accidents have occurred when one man has carried out the whole job-preparation and opening up-and has used a quick-release fastening instead of nuts and bolts. One incident. involving a tank truck, is described in Section 13.5. Here is another: 309 310 What Went Wrong? A suspended catalyst was removed from a process stream in a pressure filter. After filtration was complete, the remaining liquid was blown out of the filter with steam at a gauge pressure of 30 psi (2 bar). The pressure in the filter was blown off through a vent valve, and the fall in pressure was observed on a pressure gauge. The operator then opened the filter for cleaning. The filter door was held closed by eight radial bars, which fit- ted into U-bolts on the filter body. The bars were withdrawn from the U- bolts by turning a large wheel, fixed to the door. The door could then be withdrawn. One day an operator started to open the door before blowing off the pressure. As soon as he opened it a little, it blew open and he was crushed between the door and part of the structure and was killed instantly. In situations such as this, it is inevitable that sooner or later an opera- tor will forget that he has not blown off the pressure and will attempt to open up the equipment while it is still under pressure. On this particular occasion the operator was at the end of his last shift before starting his vacation. As with the accidents described in Section 3.2, it is too simple to say that the accident was due to the operator’s mistake. The accident was the result of a situation that made it almost inevitable. Whenever an operator has to open up equipment that has been under pressure: (a) The design of the door or cover should allow it to be opened about !4 in. (6 mm) while still capable of carrying the full pressure, and a separate operation should be required to release the cover fully. If the cover is released while the vessel is under pressure, then this is immediately apparent, and the pressure can blow off through the gap, or the cover can be resealed. (b) Interlocks should be provided so that the vessel cannot be opened up until the source of pressure is isolated and the vent valve is open. (c) The pressure gauge and vent valve should be visible to the operator when he or she is about to open the door or cover [ 11. Pressure can develop inside drums, and then when the lid is released, it may be forcibly expelled and injure the person releasing it. Most of the incidents reported have occurred in waste drums where chemicals have reacted together. For example, nitric acid has reacted with organic com- Operating Methods 31 f pounds. Acids may corrode drums and produce hydrogen. Rotting organ- ic material can produce methane. Materials used for absorbing oil spillages can expand to twice their original volume. Some absorbent was placed in drums with waste oil; the drums were allowed to stand for two days before the lids were fitted, and 10% free space was left, but never- theless pressure developed inside them. If drums are found to be bulged, lid-restraining devices should be fitted before they are opened or even moved [9]. 17.2 CLEARING CHOKED LINES (a) A man was rodding out a choked %-in. line leading to an instrument (Figure 17-la). When he had cleared the choke he found that the valve would not close and he could not stop the flow of flammable liquid. Part of the unit had to be shut down. Rodding out narrow bare lines is sometimes necessary. But before doing so, a ball valve or cock should be fitted on the end (Figure 17-lb). It is then possible to isolate the flow when the choke has been cleared, even if the original valve will not close. (b) Compressed air at a gauge pressure of 50 psi (3.4 bar) was used to clear a choke in a 2-in. line. The solid plug got pushed along with such force that when it reached a slip-plate (spade), the slip-plate was hocked out of shape, rather like the one shown in Figure 1-6. On another occasion. a 4-in diameter vertical U-tube, part of a large heat exchanger, was being cleaned mechanically when the cleaning tool, which weighed about 25 kg, stuck in the tube. A sup- ply of nitrogen at a gauge pressure of 3,000 psi (200 bar) was available, so it was decided to use it to try to clear the choke. The Rod M- Rob Figure 1’7-1. The wrong (a) and right (b) ways of clearing a choked line. 312 What Went Wrong? tool shot out of the end of the U-tube and came down through the roof of a building 100 m away. Gas pressure should never be used for clearing choked lines. (c) A 1-in. line, which had contained sulfuric acid, was choked. It was removed from the plant, and an attempt was made to clear it with water from a hose. A stream of acid spurted 5 m into the air, injur- ing one of the men working on the job. Those concerned either never knew or had forgotten that much heat is evolved when sulfu- ric acid and water are mixed. (d) When clearing chokes in drain lines, remember that there may be a head of liquid above the choke. The following incident illustrates the hazards: The drain (blowdown) line on a boiler appeared to be choked. It could not be cleared by rodding (the choke was probably due to scale settling in the base of the boiler), so the maintenance foreman pushed a water hose through the drain valve and turned on the water. The choke cleared immediately, and the head of water left in the boiler pushed the hose out of the drain line and showered the foreman with hot water. Although the boiler had been shut down for 15 hours, the water was still at 80°-90"C and scalded the foreman. Clearing the choke should not have been attempted until the temperature of the water was below 60°C, the foreman should have worn protective clothing, and if possible a second valve should have been fitted to the end of the drain line as described in (a) above. The accumulation of scale suggests that the water treatment was not adequate [3]. (e)An acid storage tank was emptied so that the exit valve could be changed. The tank was then filled with acid, but the new valve seemed to be choked. After the tank had been emptied again (quite a problem, as the normal exit line was not available), the staff found that the gasket in one of the flanged joints on the new valve had no hole in it! (f)An operator who tried to clear a choke in a pump with high-pres- sure steam was killed when the seal gave way and sprayed him with a mixture of steam and a corrosive chemical (2, 4-dichlorophenol). He was not wearing protective clothing. The seal was the wrong type, was badly fitted, and had cracked. When the company was prosecuted, its defense was that the operator should have notified [...]... to a chimney effect air entered the base of the column and displaced the lighter nitrogen [ 121 (b) A hydrogen line, about 12 in diameter, had to be repaired by welding The hydrogen supply was isolated by closing three valves in parallel (one of which was duplicated) (Figure 17-5) The line was 324 What Went Wrong? - !Gas flow Vessel Vent Welding Gas supply Heat exchanger - Heat exchanger purged with... incompatible chemicals were kept in the same store; if mixed they became, in effect, a firework easily ignited One of the chemicals 320 What Went Wrong? was stored in cardboard kegs on a shelf close to a hot condensate pipe As it was known to decompose at 50°C, the electrical department staff members were asked to disconnect the power supply to the steam boiler, but instead of doing so they merely turned... of Chemical Engineers Rugby, UK, 1991, Chapter 2 2 Accident at Mnrklzain Colliery, Der-byshire, Her Majesty’s Stationery Office, London, 1974 326 What Went Wrong? 3 Loss Prevention Bidletin, No 092, Apr 1990, p 9 4 Health and Safe5 at Work, Vol 14, No 12, Dec 1992, p 10 5 J S Arendt and D K Lorenzo “Investigation of a Filter Explosion,” Paper presented at AIChE Loss Prevention Symposium, San Diego... with the kick-back in use-and instead the operators controlled the level in the suction vessel by switching the pump on and off The control room operator watched the level and asked the outside 318 What Went Wrong? operators over a loudspeaker to start up and shut down the pump as required The two outside operators worked as a team; both could do every job, and they shared the work One day the control... an earlier transfer of another material) Some of the solution went into the filter When the operator realized what was Operating Methods 319 happening he closed the filter inlet valve but did not remove the solution that was in the filter; he did not know that it would decompose on standing There was no relief valve on the filter, and about 12 hours later the pressure broke the head bolts and blew the... The remotely operated valve was leaking, the air met the reactor contents in the feed line, and reaction took place there The heat developed caused the line to fail, and a major fire followed 314 What Went Wrong? / 1 Reactor - Choke Remotely operated valve closed The air line should have been provided with remotely operated double block and bleed valves, operated by a single button Other incidents in... often say what they think is wrong; for example, if a pump is not working correctly they ask the maintenance team to check or clean the suction strainer Sometimes the strainer is found to be clean, or the pump is no better after the strainer has been cleaned We then find that there is a low level in the suction tank, the suction temperature is too high, the impeller is corroded, or a valve is partially... the measurements were not correct Several possible explanations were considered Figure 17-4 A simple mechanical interlock: the lid could not be moved until the pin was withdrawn from the slot 322 What Went Wrong? 1 The pin might have been seized inside the solenoid Unfortunately the operator, believing this to be the case, had squirted a lubricant into the solenoid chamber before any investigation could... reduce its speed immediately He did so, but did not tell any of the operating team members straight away The cooling water rate fell, the process was upset, and a leak developed on a cooler 316 What Went Wrong? (b) A tank truck, which had contained liquefied petroleum gas, was being swept out before being sent for repair The laboratory staff was asked to analyze the atmosphere in the tanker to see... should give guidance on both these matters Operating Methods 317 id) Designers often recommend that equipment is “checked” or ‘-inspected” regularly But what do these words mean? Designers should state precisely what tests should be carried out and what they hLope to determine by the test In 1961 a brake component in a colliery elevator failed fortunately without serious consequences An instruction . clear the choke. The Rod M- Rob Figure 1’7-1. The wrong (a) and right (b) ways of clearing a choked line. 312 What Went Wrong? tool shot out of the end of the U-tube and came down. the chemicals 320 What Went Wrong? was stored in cardboard kegs on a shelf close to a hot condensate pipe. As it was known to decompose at 50°C, the electrical department staff members. nature of process fluids: in acidic solutions, this affects the choice of alloys. 308 What Went Wrong? C = Cost: an important consideration, but look at lifetime costs, including maintenance,