What Went Wrong Part 8 pdf

Pipe and Vessel Failures 183 in Section 7.3.2. When flange leaks are likely, or their consequences seri- ous, flanges should be left uninsulated [ 141. Dead-ends in domestic water systems can provide sites for the growth o-F the bacteria that cause Legionnaires' disease [ 151. Some vertical drain lines in a building were no longer needed. so they were disconnected and capped but left connected EO the horizontal main drain below. The caps were fixed with tape but were not made watertight as there was no way. it seemed, that water could get into them. Fifteen years later a choke developed in the main drain, water backed up into the disused legs and dripped into an electrical switch box. All power was lost. and some of the switch gear was damaged beyond repair [23]. 9 1.2 Poor Support Pipes have often failed because their support was insufficient and they were free to vibrate. On other occasions they failed because their support was too rigid and they were not free to expand. (a) Many small-diameter pipes have failed by fatigue because they were free to vibrate. Supports for these pipes are usually mn on-site, and it is not apparent until startup that the supports are inadequate. It is very easy for the startup team, busy with other matters. to ignore the vibrating pipes until the team has more time to attend to them. Then the team gets so used to them that it does not notice them. Vibration and failure are particularly liable to occur when a small-diaineter pipe carries a heavy overhung weight. Within 30 minutes of the start of a new compressor, a pressure gauge fell off for this reason [24.]. When equipment receives impulses at its own natural frequency of vibration, excessive vibration (resonance) occurs, and this can lead to rapid failure. A control valve was fitted with a new spindle with slightly different dimensions. This changed its natural fre- quency of vibration to that of the impulses of the liquid passing through it (the frequency of rotation of the pump times the number of passages in the impeller). The spindle failed after three months. Even al small change in the size of spindle is a modification [24]. (b) A near' failure of a pipe is illustrated in Figure 9-4. An expansion bend on a high-temperature line was provided with a temporary 184 What Went Wrong? Weld A Figure 9-4. A construction support on an expansion bend was left in position. support to make construction easier. The support was then left in position. Fortunately, while the plant was coming onstream, some- one noticed it and asked what it was for. (c) After a crack developed in a 22-in diameter steam main, operating at a gauge pressure of 250 psi (17 bar) and a temperature of 365°C the main was checked against the design drawings. Many of the sup- ports were faulty. Here's an example from four successive supports: 1. On No. 1 the spring was fully compressed. 2. No. 2 was not fitted. 3. No. 3 was in position but not attached to the pipe. 4. No. 4 was attached, but the nuts on the end of the support rod Piping with a 12-in. diameter and larger is usually tailored for the particular duty. There is a smaller factor of safety than with smaller sizes. With these large pipes, it is even more important than with smaller ones that the finished pipework is closely inspected, to confirm that the construction team has followed the designer's instructions. (d) A pipe was welded to a steel support, which was bolted to a con- crete pier. A second similar support was located 2 m away. The pipe survived normal operating conditions. But when it got excep- tionally hot, a segment of the pipe was torn out. The fracture extended almost completely around the weld. The bolts anchoring the support to the concrete pier were bent. This incident was reported in the safety bulletin of another com- pany. The staff members dismissed the incident. "Our design pro- cedures," they said, "would prevent it happening." A little later it did happen. A reflux line was fixed rigidly to brackets welded to the shell of a distillation column. At startup the differential expan- sion of the hot column and the cold line tore one of the brackets were slack. Pipe and Vessel Failwres 185 from the column. Flammable vapor leaked out but fortunately did not catch fire. (e) A 10-in. pipe cawing oil at 300°C was fitted with a %-in. branch on its underside. The branch was located 5 in. from a girder on which the pipe rested. When the pipe was brought into use, the expansion was sufficient to bring the branch into contact with the girder and knock it off. Calculations showed that the branch would move more than 6 in. Cs> On many occasions pipe hangers have failed in the early stages of a fire, and the collapse of the pipes they were supporting has added to the fire. Critical pipes should therefore be supported from below. (g) An extension was added to a 30-year-old pipebridge that carried pipes containing flammable liquids and gases. To avoid welding, the extension was joined to the old bridge by bolting. Rust was removed from the joining surfaces, and the extension was painted. Water penetrated the crack between the old and new paint and pro- duced rust. As rust is more voluminous than the steel from which it is formed. the rust forced the two parts of the pipebridge apart-a phenomenon known as rust-jacking (see Section 16.3). Some of the bolts failed, and a steam main fractured. Fortunately, the liquid and gas lines only sagged [ 161. (h) Eleven pipelines, 2-8 in. (50-200 mm) in diameter, containing hydrocarbon liquids and gases, were supported on brackets of the type shown in Figure 9-5 (a), 2.1 m tall and 6 m apart. The pipes were fixed to two of the brackets and rested on the others. The pipe run passed through a tank farm, and the wind flow through the gaps between the tanks caused the upright part of the supports to incline 2" from the vertical. This was noticed when the pipe run was inspected, but no one regarded it as serious. As the result of a power failure, the flow through many of the pipes suddenly stopped, and the surge caused the angle of inclination to increase to 6". The tops of the supports were now 5 in. (125 mm) out of line. The supports were now unstable. Eleven hours after the power failure and three hours after the flows had been restored, the pipe pun collapsed over a length of 23 m; 14 tons of gasoline were spilled. Three hours later a further length collapsed. The pipe sup- ports were replaced by the type shown in Figure 9-5 (b). 186 What Went Wrong? Figure 9-5 (a). The original pipe supports. Figure 9-5 (b). The supports used after the collapse. 9.1.3 Water Injection Water was injected into an oil stream using the simple arrangement shown in Figure 9-6. Corrosion occurred near the point shown, and the oil leak caught fire [5]. The rate of corrosion far exceeded the corrosion allowance of 0.05 in. per year. A better arrangement is shown in Figure 9-7. The dimensions are cho- sen so that the water injection pipe can be removed for inspection. However, this system is not foolproof. One system of this design was assembled with the injection pipe pointing upstream instead of down- stream. This increased corrosion. As discussed in Section 3.2.1, equipment should be designed so that it is difficult or impossible to assemble it incorrectly or so that the incoirect assembly is immediately apparent. 9.1.4 Bellows Bellows (expansion joints) are a good example of equipment that is intolerant of poor installation or departure from design conditions. They Pipe and Vessel Fail&fres 187 Figure 9-6. Water injection-a poor arrangement. Figure 9-7. Water injection-a better arrangement. should therefore be avoided on lines carrying hazardous materials. This can be done by building expansion loops into the pipelines. The most spectacular bellows failure of all time (Flixborough) was described in Section 2.4. Figure 9-8 illustrates a near-failure. A large distillation column was made in two halves, connected by a 42-in. vapor line containing a bellows. During a shutdown this line was Figure 9-8. A large bellows between the two halves of a distillation column. 188 What Went Wrong? steamed. Immediately afterward someone noticed that one end of the bel- lows was 7 in. higher than the other, although it was designed for a maxi- mum difference of 3 in. Someone then found that the design contractor had designed the line for normal operation. But the design contractor had not considered conditions that might be developed during abnormal pro- cedures, such as startup and shutdown. 9.1.5 Water Hammer Water hammer (also known as hydraulic shock) occurs in two distinct ways: when the flow of liquid in a pipeline is suddenly stopped, for exam- ple, by quickly closing a valve [13]. and when slugs of liquid in a gas line are set into motion by movement of gas or condensation of vapor. The lat- ter occurs when condensate is allowed to accumulate in a steam main, because the traps are too few or out of order or in the wrong place. High- pressure mains have been ruptured, as in the following incident. (a) A 10-in diameter steam main operating at a gauge pressure of 600 psi (40 bar) suddenly ruptured, injuring several workers. The incident occurred soon after the main had been brought back into use after a turnaround. It was up to pressure, but there was no flow along it. The steam trap was leaking and had been isolated. An attempt was made to get rid of condensate through the bypass valve. But steam entered the condensate header, and the line was isolated, as shown in Figure 9-9. Condensate then accumulated in the steam main. Faulty Steam Trap I Isolated Steam Main Valve Almost I Closed Valve Closed Condensate Recovery Header Figure 9-9. Arrangement of valves on steam main that was broken by hammer. water Pipe and Vessel Failures 189 When a flow was started along the steam main by opening a %- in. valve leading to a consuming unit, the movement of the conden- sate fractured the main 161. (b) Figure 9-10 shows how another steam main-this time one operating at a gauge pressure of 20 psi (1.4 bar)-was burst by water hammer. Two drain points were choked and one isolated. In addition. the change in diameter of the main provided an opportunity for con- densate to accumulate. The main should have been constructed so that the bottom was straight and so the change in diameter took place at the top. (c) An operator went down into a pit to open a steam valve that was rarely operated and had been closed for nine months. Attempts to open the valve with a reach rod, 8 m long, had been unsuccessful. The pit was recognized as a confined space, and so the atmosphere was tested, the operator wore a rescue harness, and a stand-by man was on duty outside. The steam main was up to pressure on both sides of the valve, and the gauge pressure was 120 psi (8.3 bar) on the upstream side, 115 psi (7.9 bar) on the downstream side. There was a steam trap on the downstream side of the valve but not on the upstream side, and as the valve was on the lowest part of the sys- tem, about 5 tons of cold condensate had accummulated on the upstream side. - Blank Valve that failed Flow e Condensate r Built Up Here Drain Point Isolated Drain points choked (Steam trap bypasses not shown) Figure 9-10. Arrangement of drains on steam main that was broken by water hammer. 190 What Went Wrong? The operator took about one to two minutes to open the valve halfway; very soon afterward, there was a loud bang as a 6-in. cast- iron valve on a branch (unused and blanked) failed as a result of water hammer. The operator was able to climb out of the pit. but later died from his burns, which covered 65% of his body [17]. Figure 9- 11 explains the mechanism. Water Hammer in Pit This frame illustrates the valve lineup prior to the accident. About 1,500 gal of 55°F condensate had collected upstream of valve MSS-25, which was located at the dead-end of an 800-ft pipe and was the lowest point in the system. XOt)ft stem line, containing jYF condensate 6-in. wIw As valve MSS 25 was opened, the water mixed with the steam on the downstream side of the valve As the water and steam interacted, the turbulence sealed off a pocket of steam, which quickly condensed, lowering the pressure in the pocket and creating a void. (Reprodirred by permission of the Office of Environment, Safen, and Health, US. Department of Eiieyp.) Figure 9-11. Condensate collected in a steam main. A valve was opened quick- ly. Sudden movement of the condensate fractured another valve. The figure explains how this occurred. Pipe and Vessel Failures I91 The accident would not have occurred (or would have been less serious) if 0 Cast iron had not been used. It is brittle and therefore not a suit- able material of construction for steam valves, which are always liable to be affected by water hammer. * There was a steam trap upstream of the valve. * The valve had been located in a more accessible place. *The operator had taken longer to open the valve. On previous occasions operators had taken several hours or even longer, but there were no written instructions, and the operator on duty had not been trained or instructed. * The operating team as a whole had been aware of the well-known hazards of water hammer in steam mains. For another failure due to water hammer. see Section 10.5.3. 9.1.6 Miscellaneous Pipe Failures (a) Many failures “nave occurred because old pipes were reused. For example, a hole 6 in. long and 2 in. wide appeared on a 3-in. pipe carrying flammable gas under pressure. The pipe had previously been used on a corrosive/erosive duty, and its condition was not checked before reuse. In another case. a 4%in diameter pipe carrying a mixture of hydrogen and hydrocarbons at a gauge pressure of 3,600 psi (250 bar) and a temperature of 350-400°C burst. producing a jet of flame longer than 30 m (Figure 9-12). Fortunately. the pipe was located high up, and no one was injured. The grade of steel used should have been satisfactory for the operating conditions. Investigation showed. however, that the pipe had previously been used on another plant for 12 years at 500°C. It had used up a lot of its creep life. Old pipes should never be reused unless their history is known in detail and tests show they are suitable (see Section 9.2.1 h). (b) Many failures have occurred because the wrong grade of steel was used for a pipeline. The correct grade is usually specified. but the wrong grade is delivered to the site or selected from the pipe store. 192 What Went Wrong? k a 1 ”‘ Figure Y-12. An old pipe was reused and failed by creep. The most spectacular failure of this sort occurred when the exit pipe from a high-pressure ammonia converter was constructed from carbon steel instead of 1!4% Cr, 0.5% Mo. Hydrogen attack occurred, and a hole appeared at a bend. The hydrogen leaked out, and the reaction forces pushed the converter over. Many companies now insist that if use of the wrong grade of steel can affect the integrity of the plant, all steel must be checked for composition before use. This applies to flanges, bolts, welding rods, etc., as well as the raw pipe. Steel can be analyzed easily with a spectrographic analyzer. Other failures caused by the use of the wrong construction material are described in Section 16.1. (c) Several pipe failures have occurred because reinforcement pads have been welded to pipe walls, to strengthen them near a support or branch, and the spaces between the pads and the walls were not vented. For example, a flare main collapsed, fortunately while it was being stress-relieved. [...]... of knowledgeable people and discussed the safety issue with them Consensus at the time supported our conclusion But after the explosion, these was some dispute over 196 What Went Wrong? exactly what was said and what was meant Knowing what we know now, there can be no other course in the future than to shut down operations in the event of a leak from a weep hole under similar circumstances." [SI (b)... The cylinder then became airborne, hit a platform 6 m above, and went through a sheet metal wall into a building It went through the roof of this building, 15 m above, and then fell back through the roof and landed 40 m from the point where it had started its journey Remarkably, no one was injured Four things were wrong: 202 What Went Wrong? The operating lever should have been removed before the cylinder... Prevention in the Chemical and Ol Processing Industries, Symposium i Series No 120, Institution of Chemical Engineers, Rugby, UK, 1990, p 589 204 What Went Wrong? 17 Occzipational Safety Observer; Vol 2, No 9, U.S Dept of Energy, Washington, D.C., Sept 1993, p 1 18 A B Smith, Loss Prevention Bulletin, No 102, Dec 1991, p 29 19 M L Griffin and F H Gurry, “Case Histories of Some Powerand Control-based... plates from which the vessels were made did not get the correct postwelding heat treatment Once a vessel has been constructed, it is not easy to check that it has had the correct heat treatment The 1 98 What Went Wrong? /qqQ\ Vent (Fill) \ \ \ Figure 9-14 Unusual arrangement of relief valve and pipework on tank truck used to transport liquid carbon dioxide The relief valve was cooled by the liquid and became... leak But this will not occur with a spiral-wound gasket 9.1 .8 Catastrophic Failures The fire and explosions in Mexico City in 1 984 , which killed more than 500 people (see Section 8. 1.4), started with a pipe failure The cause is not known, but the pipe may have been subjected to excessive pressure Earlier the same year, in February, at least 5 08 people, most of them children, were killed in Cubatao, Sao... Protection Manual for Hyrlrocarbori Processing Plants, Vol 1, 3rd edition, Gulf Publishing Co., Houston, Texas, 1 985 p 122 8 L B Patterson Ammonia Plum Safety, Vol 21, 1979, p 95 9 J E, Hare, Plant/Operatioizs Progress Vol 1, No 3, July 1 982 , p 166 10, Hcizardoiu Cargo Bulletin June 1 984 , p 34 11 J A Davenport, “Hazards and Protection of Pressure Storage of Liquefied Petroleum Gases,” Proceedings of... International Symposirirn on Loss Preipention and Safety Proinotion in the Process Industries, SociktC de Chimie Industrielle, Paris, 1 986 , p 22- 1 12 A C Barrell, Hazard Assessment Workshop, Atomic Energy Authority, Hanvell, UK, 1 984 13 D Clarke The Chemical Engineer; No 449, June 1 988 , p 44 14 B E Mellin, Loss Prererzriorz Bulletin, No 100, Aug 1991, p 13 15 Loss Prevention Bzillerirz, No 091, Feb 1990, p 23... of a reactor When the pipework was cold, any liquid in the branch leading to the rupture disc drained out; when it was hot, it remained in the branch, where it caused corrosion and cracking [ 181 194 What Went Wrong? 12mm differential vertical expansion Drain 7.5mm when cold Adverse fall 4.5mrn when hot Figure 9-13 The vent arrangements at the top of the reactor Liquid drained out when the pipework... ignition is unimportant In equipment containing moving parts, such its a centrifuge, sources of ignition can easily arise In another incident the nitrogen flow was too small The range of the rotameter in the nitrogen line was 0-60 Wmin (0-2 ft3/min) although 150 L/rnin ( 5 ft'imin) was needed to keep the oxygen content at a safe level 205 206 What Went Wrong? On all centrifuges that handle flammable solvent,... valves closed The pump disintegrated bits being scattered over a radius of 20 m If remote starting must be used then some form of interlock is needed to prevent similar incidents from occurring 2 08 What Went Wrong? Pumps can overheat if they run with the delivery valve almost closed In one incident a pump designed to deliver 10 tonshr was required to ! deliver only 4 todhr The delivery valve was gagged, . clusion. But after the explosion, these was some dispute over 196 What Went Wrong? exactly what was said and what was meant. Knowing what we know now, there can be no other course in the future. the wrong grade of steel was used for a pipeline. The correct grade is usually specified. but the wrong grade is delivered to the site or selected from the pipe store. 192 What Went Wrong? . was Figure 9 -8. A large bellows between the two halves of a distillation column. 188 What Went Wrong? steamed. Immediately afterward someone noticed that one end of the bel- lows was