What Went Wrong Part 9 pptx

Other Equipment 21 3 Figure 10-4. This relief-valve tailpipe was not adequately supported. 10.4.5 Relief-Valve Faults Here are a few examples of faults in relief valves themselves. These are not the results of errors in design but of poor maintenance practice. The following have all been seen: 1. Identification numbers stamped on springs, thus weakening them 2. The sides of springs ground down so that they fit. 3. Corroded springs. 4. A small spring put inside a corroded spring to maintain its strength. Sometimes the second spring was wound the same way as the first spring so that the two interlocked (Figure 10-6). (Figure 10-5). 5. Use of washers to maintain spring strength. 6. Welding of springs to end caps (Figure 10-7). 7. Deliberate bending of the spindle to gag the valve (Figure 10-8). 8. Too many coils allowing little, if any, lift at set pressure (Figure 10-9). 214 What Went Wrong? Figure 10-5. Identification marks on body coils could lead to spring failure Figure 10-6. Use of additional inner springs of unknown quality in an atten to obtain set pressure. Figure 10-7. End caps welded to spring. Failure occurred at weld. Figure 10-8. Deliberate bending of the spindle to gag the valve. Other Equipment 215 Figure 10-9. Example of too many coils. Do not assume that such things could not happen in your company (unless you have spent some time in the relief-valve workshop). All relief valves should be tested and inspected regularly. Reference 3 describes model equipment and procedures. When a large petroleum company introduced a test program, it was shocked by the results: out of 187 valves sent for testing, 23 could not be tested because they were leaking or because the springs were broken, and 74 failed to open within 10% of the set pressure-that is, more than half of them could not operate as required [4]. Testing, of course, must be thorough. The following incident is described in the form of a conversation between an inspector investigat- ing a boiler explosion and the maintenance foreman [ 121. Inspector: “When was the relief valve last checked?’ Foreman: “After the last overhaul.” Inspector: “How was it checked?’ Foreman: “I set it myself, using the boiler’s own pressure gauge.” Inspector: “Why didn’t you use a master gauge?’ Foreman: “I didn’t need to. The gauge had been checked and found Inspector: “Who checked it?, Foreman: “Mr. X, one of my fitters. He has often done so in the past.” accurate only two weeks before.” The inspector then spoke to Mr. X. Inspector: “I understand you checked the pressure gauge two weeks Mr. X: “Yes, the foreman told me to do so.” before the explosion.” 216 What Went Wrong? Inspector-: “When was your master gauge last calibrated?” MK X: “I didn’t use one.” Inspector: “You didn’t use a master gauge? Then how did you check it?’ MI: X: ”I checked it against the relief valve. I knew it was correct because the foreman told me he had adjusted it himself.” This incident occurred in the 19th century when boiler explosions were much more frequent than they are today. But are you sure some- thing similar could not occur today? Read Sections 10.7.2 (b) and (c) before you decide. Similar incidents have occurred in technical reports. A writes some- thing in a book or paper. B copies it without acknowledgment. A then repeats it in another report, citing B as the source and thus giving it an authenticity it lacked in its first publication. 10.4.6 Disposal of Relief Discharges Material discharged from relief valves and rupture discs should not be discharged to atmosphere unless: 9 It will have no harmful effects, for example, steam, compressed air, or nitrogen. It is a gas at a pressure high enough to disperse by jet mixing. It is necessary to use a pilot-operated relief valve that is either open or shut and is not a type that will hover. Although it is safe to discharge gases such as ethylene and propylene in this way, there may be objections on environmental grounds. The amount released is negligible, for example, the relief valves that protect a pipeline that has been isolated. . A system of trips or interlocks makes the probability that the relief valve will lift very low, say, less than once in 1,000 years for flam- mable liquids and less than once in 100 years for flammable gases. The relief valve will lift only after prolonged exposure of the equip- ment to fire and will discharge within the fire area so that the dis- charge will ignite. Here are some examples of the results of letting relief valves discharge to atmosphere: Other Equipment 21 7 0 A 6-in. (150-mm) relief valve on a petrochemical plant discharged ben- zene vapor to atmosphere. It was ignited by a furnace and exploded, rupturing piping, which released more than 100 tons of various flam- mable liquids. One man was killed, and damage was extensive [5]. 1 At Seveso in Italy in 1976, a runaway reaction led to the discharge of the reactor contents, including dioxin, a toxic chemical, through a rupture disc direct to the atmosphere. Although no one was killed, many people developed chloracne, an unpleasant skin disease, and an area of about 17 km2 was made uninhabitable. A catchpot after the relief device would have prevented the reactor contents from reach- ing the atmosphere. No catchpot was installed as the designers did not foresee that a runaway might occur, although similar runaways had occurred on other plants (see Section 21.2.5) [6]. e Naphtha vapor from a relief valve on a town gas plant in the UK was ignited by a flare stack. The flame impinged on the naphtha line, which burst, starting a secondary fire [7]. A relief valve sprayed liquid into the face of a passing operator with such force that it ?docked his goggles off. 0 A reaction involving concentrated sulfuric acid was carried out at atmospheric pressure in a vessel with an opening to the atmosphere at the top. When a runaway occurred, acid was ejected over the sur- rounding area [ 131. The rupture discs on some water compressors were allowed to dis- charge inside a building as the water was clean. However, by the time it had drained down through several floors to the basement of the building, it had dissolved some solid material that had been spilt on one of the intervening floors and became hazardous. Discs had failed on several occasions, for unknown reasons. Possible causes were vibration, hammer pressure, and low-cycle fatigue. If2 despite my advice, you let relief devices discharge to atmosphere, make sure that if the discharge ignites, the flame will not impinge on other equipment and that no one will be in the line of fire. 10.4.7 Vacuum Relied Valves Some large equipment, though designed to withstand pressure, cannot withstand vacuum and has to be fitted with vacuum relief valves. These 218 What Went Wrong? usually admit air from the atmosphere. If the equipment contains a flam- mable gas or vapor, then an explosion could occur with results more seri- ous than collapse of the vessel. Experience shows that a source of igni- tion may be present even though we have tried to remove all possible sources (see Section 5.4). It is therefore better to protect equipment that cannot withstand vacuum by means of a pressure control valve that admits nitrogen or, if nitrogen is not available, another gas such as fuel gas. Very large amounts may be necessary. For example, if the heat input to a large refinery distillation column stopped but condensation contin- ued, 8,000 m3/hr of gas, the entire consumption of the refinery, would be required. Instead, a much smaller amount was supplied to the inlet of the condenser, thus blanketing it and stopping heat transfer [ 141. The sim- plest solution, of course, is to design equipment to withstand vacuum. Protection of storage tanks against vacuum is discussed in Sections 5.3 and 5.4. 10.5 HEAT EXCHANGERS 10.5.1 Leaks into Steam and Water Lines Hydrocarbons can leak through heat exchangers into steam or conden- sate systems and appear in unexpected places. Some hydrocarbon gas leaked into a steam line that supplied a heater in the basement of a con- trol building. The gas came out of a steam trap and exploded, killing two men. The operators in the control building had smelled gas but thought it had entered via the ventilation system, so they had switched off the fan. The control gear was ordinary industrial equipment, not suitable for use in a flammable atmosphere, and the sparking ignited the gas. It was for- tunate that more people were not killed, as the building housed adminis- trative staff as well as operators. A leak in another heat exchanger allowed flammable gas to enter a cooling-water return line. The gas was ignited by welding, which was being carried out on the cooling tower. The atmosphere had been tested before work started, five hours earlier (see Section 1.3.2). 10.5.2 Leaks Due to Evaporative Cooling If the pressure on a liquefied gas is reduced, some of the liquid evapo- rates, and the rest gets colder. All refrigeration plants, domestic and Other Eqoipmenf 21 9 industrial, make use of this principle. This cooling can affect equipment in two ways: it can make it so cold that the metal becomes brittle and cracks. as discussed in Sections 8.3 and 10.5.2 (g) and it can cause water, or even steam. on the other side of a heat exchanger to freeze and rupture a tube or tubes. The leak that caused the explosion in a control building (see last section) started this way. Figure 10-10 illustrates another incident. When a plant was shutting down, the flow of cooling water to the tubes of a heat exchanger was iso- lated. The propylene on the shell side got colder as its pressure fell. The water in the tubes froze, breaking seven bolts. The operators saw ice forming on the outside of the cooler but did not realize that this was haz- ardous and took no action. When the plant stai-ted up again, propylene entered the cooling-water system, and the pressure blew out a section of the 16-in. (400-mm) line. The gas was ignited by a furnace 40 m away. and the fire caused serious damage. Reduction of pressure here will cause liquefied gas to evaporate and cool and may freeze the water in the tubes. L Waterto tubes Figure 10-10. Evaporative cooling. The cooling water should have been kept flowing while the plant was depressured This would have prevented the water from freezing, provid- ed that depressuring took more than ten minutes. 10.5.3 Damage By Water Hammer Water hammer (hydraulic shock) in pipelines is discussed in Section 9.1.5. It can also damage heat exchangers, and Figure 10- 11 illustrates such an incident. 220 What Went Wrong? A Impingement plate ‘7- Calandria f Figure 10-11. Condensate in the steam-the result of too few steam traps- knocked off the impingement plate and damaged the calandria tubes. The steam supplied to the shell of a distillation column reboiler was very wet, as there was only one steam trap on the supply line although at least three were needed. In addition. condensate in the reboiler drained away only slowly because the level in the drum into which it drained was only 1.4 m below the level in the reboiler. An impingement plate was fitted to the reboiler to protect the tubes, but it fell off, probably as a result of repeated blows by slugs of conden- sate. The condensate then impinged on the tubes and squashed or broke 30 of them. The impingement plate had fallen off several times before and was merely put back with stronger attachments. When something comes apart, we should ask why, not just make it stronger (see Section 1.5.5). Buildup of condensate in a heat exchanger can cause operating prob- lems as well as water hammer. If the steam supply is controlled by a motor valve and the valve is not fully open, the steam pressure may be too low to expel the condensate, and its level will rise. This will reduce heat transfer, and ultimately the steam supply valve will open fully and expel the condensate. The cycle will then start again. This temperature cycling is bad for the heat exchanger and the plant and may be accompa- Other Equipment 226 nied by water hammer and corrosion. Proprietary devices are available for overcoming the problem [SI. 10.5.4 An Accident During Maintenance The tube bundle was being withdrawn from a horizontal shell and tube heat exchanger. It was pulled out a few inches and then became stuck. The mechanics decided that the cause was sludge, and to soften it they reconnected the steam supply to the shell. The tube bundle was blown out with some force, causing serious injuries [9]. 10.6 COOLING TOWERS These are involved in a surprisingly large number of incidents; one is described in Section 10.5.1. Wooden packing. after it dries out, is veiy easily ignited. and many cooling towers have caught fire while they were shut down. For example, the support of a force draft fan had to be repaired by welding. An iron sheet was put underneath to catch the sparks, but it was not big enough, and some of the sparks fell into the tower and set the packing on fire. Corrosion of metal reinforcement bars has caused concrete to fall off the corners of cooling towers. A large natural-draft cooling tower collapsed in a 70-mph (1 lo-kmihr) wind, probably due to imperfections in the shape of the tower. which led to stresses greater than those it was designed to take and caused bending collapse [ lo]. An explosion in a pyrolysis gas plant in Rumania demolished a cooling tower. It fell on the administration block, killing 162 people. Many people who would not build offices close to an operating plant would consider it s.afe to build them close to a cooling tower. It is doubtful if this is wise. 10.7 FURNACES 10.7.1 Explosions While Lighting a Furnace Many explosions have occurred while furnaces were being lit. The two incidents described below occurred some years ago on furnaces with simple manual ignition systems, but they illustrate the principles to be followed when lighting a furnace, whether this is carried out manually or automatically. 222 What Went Wrong? (a) A foreman tested the atmosphere inside a furnace (Figure 10-12) with a combustible gas detector. No gas was detected, so the slip- plate was removed, and two minutes later, a lighted poker was inserted. An explosion occurred. The foreman and another man were hit by flying bricks, and the brickwork was badly damaged. The inlet valve was leaking, and during the two minutes that elapsed after the slip-plate was removed, enough fuel gas for an explosion leaked into the furnace. (Suppose the leak was equiva- lent to a 1.6-mm [X/;h-in.] diameter hole, and the gauge pressure of the fuel gas was 0.34 bar [5 psi]. The calculation shows that 80 L [3 ft’] of gas entered the furnace in two minutes. If this burned in 0.01 second, the power output of the explosion was 100 MW.) The correct way to light a furnace (hot or cold) that bums gas or burns light oil is to start with a positive isolation, such as a slip- plate, in the fuel line. Other positive isolations are disconnected hoses, lutes filled with water (if the fuel is gas at low pressure), and double block and bleed valves; closed valves without a bleed are not sufficient. Then: 1. Test the atmosphere inside the furnace. 2. If no gas is detected, light and then insert the poker (or switch on the electric igniter). 3. Remove the slip-plate (or connect the hose, drain the lute, or change over the double block and bleed valves). If the isolation valve is leaking, the leaking fuel will be ignited by the poker or igniter before it forms an explosive mixture. (The solenoid valve shown in Figure 10-12 should open automatically when the poker is inserted or the igniter is switched on. If it does not, it should be held open until the main burner is lit.) 4. Open the fuel-gas isolation valve. The furnace had been lit in an incorrect way for many years before the isolation valve started to leak and an explosion occurred. Never say, “It must be safe because we have been doing it this way for years and have never had an accident.” On furnaces with more than one burner, it may be possible to light a burner from another one if the two are close to each other. If they are not, the full procedure just described should be followed. Explosions have occurred on multiburner furnaces because opera- tors assumed that one burner could always be lit from the next one. [...]... London, 199 3 5 C H Vervalin Hydrocarbm Processing, Vol 51, No 12, Dec 197 2 p 49 6 T A Kletz Learning from Accidents, 2nd edition, ButterworthHeinemann, Oxford, UK, 199 4, Chapter 9 7 FPAJournal, No 84, Oct 196 9, p 375 8 Loss Preiventioiz Bulletin, No 103, Feb 199 2, p 29 9 C Butcher, The Clzenzical Engineel; No 5 49, Sept 16, 199 3 p 24 10 Report on the Collapse o the Arrleer Cooling %fer Tower; 27 Sept f 197 3,... failure 228 What Went Wrong? REFERENCES 1 User 5 Guide for the Safe Operation of Centrifciges with Parriciilar Reference to Hazardous Atmosphere, 2nd edition, Institution of Chemical Engineers, Rugby, UK 198 7 2 T A Kletz, Loss Prevention, Vol 6, 197 2, p 134 3 R E Sanders and J H Wood, Chemical Engineering, Feb 199 3 p 110 4 A B Smith, Safety Relief Valves on Pressure Systenzs, Reprint No C454/001 /93 , Institution... London, 197 4 11 Furnace Fires arid Explosions Hazard Workshop Module No 005, Institution of Chemical Engineers, Rugby, UK, undated 12 R Weaver, Northern Arrow (newsletter of the Festiniog Railway Society Lancashire and Cheshire Branch), No 147, Oct 199 4 13 S J Brown and T J Brown Process Safely Progress, Vol 14, No 4, Oct 199 5, p 244 14 I M Duguid, Loss Prevention Bulletin, No 134, Apr 199 7, p 10 15... Prevention Bulletin, No 134, Apr 199 7, p 10 15 R E Sanders, Process Safely Progress, Vol 15, No 4 Winter 199 6, p 1 89 16 R E Sanders, Management of Change in Chemical Plants-Learning from Case Histories, Buttenvorth-Heinemann, Oxford, UK, 199 3 17 S Mannan, Process Safety Progress, Vol 15, No 4, Winter 199 6, p 258 Chapter 11 Entry to Vessels Many people have been killed or overcome because they entered vessels... deposit in the right-hand half caught fire The welder got out without serious injury but bruised himself in his haste If a part of the vessel cannot be inspected and be seen to be safe, then we should assume the vessel contains hazardous materials 2 29 230 What Went Wrong? Figure 11-1 If part of a vessel cannot be seen, assume it is dirty If the previous contents were flammable, we should assume there is... York, 199 4, Chapter 12 2 F P Lees, Loss Prevention in the Process Irzdustries, Butterworths, London, 198 0, Chapter 21 3 Health and Safe0 at Work, July 198 2, p 38 4 W W Cloe, Selected Occupatioizal Fatalities Related to Fire and/or Explosion in Confined Work Spaces as Found in Reports of OSHA Fataliiy/Catastrophe lnvestigatiorzs, Report No OSHA/RP-82/002, U.S Dept of Labor, Washington D.C Apr 198 2, p... dirty [20] 11.2 HAZARDOUS MATERIALS INTRODUCED Sometimes, after a vessel has been freed from hazardous materials, they are then deliberately reintroduced, as in the following incidents 232 What Went Wrong? (a) Two men went into a reactor to carry out a dye-penetrant test on a new weld using trichlorethylene Because the weld was 8 m long, the solvent was soon used up, and the man who was on duty at the... workers have connected up nitrogen supplies (see Section 12.3.3) 242 What Went Wrong? Section 11.2 (d) describes an incident in which no tests were carried out and the vessel was not recognized as a confined space because ventilation was good When the vessel was moved indoors, ventilation was no longer good, and a man was overcome 11 .9 EVERY POSSIBLE ERROR Earlier sections have described how people were... poker The fuel oil supply was still positively isolated, but nevertheless an explosion occurred and the foreman was injured, fortunately not seriously 224 What Went Wrong? Fuel oil Figure 10-13 Lighting a furnace heated by heavy fuel oil When the burner went out, the solenoid valve took a few seconds to close, and during this time some oil entered the furnace In addition, the line between the last valve... than one occasion complete reliance seems to have been placed on the presence and reliability of a boiler fitter and his pair of keys On this occasion, unfortunately, his memory let him down.“ 236 What Went Wrong? The blowdown valves on the boilers were operated by a special key which had a lug on it so that it could not be removed when the valve was open It was therefore impossible in theory, for two . FPAJournal, No. 84, Oct. 196 9, p. 375. 8. Loss Preiventioiz Bulletin, No. 103, Feb. 199 2, p. 29. 9. C. Butcher, The Clzenzical Engineel; No. 5 49, Sept. 16, 199 3. p. 24. 10. Report. Buttenvorth-Heinemann, Oxford, UK, 199 3. 17. S. Mannan, Process Safety Progress, Vol. 15, No. 4, Winter 199 6, p. 258. p. 110. p. 49. Heinemann, Oxford, UK, 199 4, Chapter 9. 197 3, Imperial Chemical. If a part of the vessel cannot be inspected and be seen to be safe, then we should assume the vessel contains hazardous materials. 2 29 230 What Went Wrong? Figure 11-1. If part of