Document 9344oeox4MMQwZodZB7LBQ0Lq

htlo^v'i/w-X report on an incident at the edc-cracking unit / VCSA-HEETING IN BOSTON ON SEPTEMBER 28, 1989 I. Incident On December 11, 1988, at 9.28 a.m., a pipe burst in the eastern coil of the double-piped cracking furnace. The damage was noticed by a plant operator, who, because of a change in temperature, went into the plant, saw the smoke coming out of the flue gas stack and immediately pressed the "emergency stop". The alarm of the fire department was given at approximately the same time, at 9.31 a.m. Due to a pipe burst, the gas mixture (HC1/VC/EDC) escaped into the combustion chamber of the cracking furnace, which is heated with natural, gas. There the mixture burnt and hydrochloro and soot was set free. With the emergency stop the whole VC-plant was shut down, the plant units were separated by remoted valves and the security position was taken. Due to the pressure rise in the combustion chamber the excess pressure release flap- valve opened consistently, so that the escaping gases were within reach of the water guns. During the cooling-down phase the remaining unburnt gases detonated when air entered the furnace. This lead to an even bigger damage on the ceramic lining of the furnace, than the damage on the pipe. II. Measures taken to prevent greater damage The fire department, arriving at the scene of accident after 3 minutes, ascertained that no person was injured and started at once to build up water walls to fight the HCl-gas. In my opinion they succeeded quite good. Apart from the first escape of about 1 ton HCl-gas from the flue gas stack, no further grave escape of gas happened. Altogether 2000 m3 water were sprayed during the time of action from 9.35 until 11.51 a.m. Most of the water was showered directly in front of the flap valve of the slowly degassing furnace and the connected plant units. BOR 000619 Observation drives to the periphery of the plant also indicated no ill effects. III. Consequences The aforementioned damages lead to a breakdown of the VCMproduction for several days, from December 11 until December 15. The effects on the environment kept within reasonable limits, so that it was not necessary to report an "occurrence of failure" to the communities. The first assumption on the immediate HC1emission amounted to 1 ton, after an exact analysation, the maximum possible emission was about 12.5 tons HC1 in about 2 hours. However, most of this slowly escaping gas was kept down by the water walls. 2 days after the incident 2 cases of affected health (irritation of the respiratory tract) became known. The symptoms ceased after a few days, and, according to the medical report, will leave no injuries to health. Within the company no more injuries are known and solely the persons working at the residual waste incinerator, situated at the lee-side of the EDC-plant, needed to wear gas-masks for a short period of time. There was no danger of polluting the rivers, because the water used to build up the water walls was led directly to the waste water facilities. IV. Analysis on the causes According to our department for material testing, the reason for this incident is undoubtably a high-temperature-corrosion due to a local overheating of the pipes (made of the material Inconel 600). As the investigations for the final analysis have not been completed, I am not able to tell you more on this subject. However, at the next VCSA-meeting the results concerning this incident will be reported. BOR 000620 BOR 0 0 0 6 2 1 2 EXPLOSION IN VCM PLANT IN 1988 1. BRIEF DESCRIPTION OF THE PLANT Norsk Hydro's VCM plant Is located at Rafnes, some 120 km south west of Oslo. The plant is based on 8. F. Goodrich Technology. Design capacity was 300 000 metric tons VCM per year in 8000 operating hours. This corresponds to 900 tons VCM per day. The plant was started up in 1978. As a result of several debottlenecking activities, the capacity has been increased. At the time of the accident, the capacity was 1200 tons a day. Figure 1 shows the general lay-out of the plant. 2. MODIFICATIONS IN THE CRACKING AREA The plant had two crackers with accompanying quench towers. (A cracker No. 3 has been Installed during summer 1989). Two important modifications in the cracking quench system are shown in figure 2. Originally the quench tower had a bottom circulation system. EDC/VCM/HC1 at a ratio of about 87/9/4 and a temperature around 175 C was circu lated by a pump. The liquid contained some coke and there were constant troubles with the pump's mechanical seal. He also had indications of stress corrosion cracking in pipes and valves. In 1981 we therefor replaced this system with a submerged inlet into the quench tower. The second important modification was carried out in 1984. To conserve energy, a steam boiler and a feed preheater were installed in the quench overhead. 890905V.05/HH 3. BRIEF DESCRIPTION OF THE INCIDENT 3 A pipe ruptured in the reflux line. The gas cloud that formed was ignited by one of the gas burners in the cracker3. The distance from the leakage point to the burner was about 35 meters. The amount of EDC/VCM in the vapour cloud i3 estimated to 2-3 tons. In addition some 5 tons of liquid leaked down to the ground and was ignited. The fire already started had serious secondary effects: The outlet line from a liquid hold up tank ruptured and poured some 35 tons of EDC/VCM into the fire. The cracker feed line also ruptured, and a similar amount of EDC/VCM was released. This line also was bent out of position and directed a torch onto a cable rack. The line feeding chlorine to the DC-reactors also ruptured, and son 5 tons of chlorine were released. No persons were injured. No main equipment, e.g. heat exchangers, tanks, towers, pumps etc, was destroyed. The main areas of destructions were: pipes; several hundred meters of piping had to be replaced cables; both Instrument (pneumatic) and electric - instrumentation a concrete structure in three levels was severely exposed during the fire. It was not damaged beyond repair. A special technique was applied in order to extract chloride from the concrete. 890905V.05/HH t* 4. 4 DETAILS ABOUT THE PIPE RUPTURE A detailed drawing of the pipe fitting where the rupture accured is shown in figure 3. The design of this fitting was unfavourable in several respects; The liquid flow was turned ISO degrees. - The T-plece had a much greater wall thickness than the rest of the construction. Soae Internal welding seaas had a protrusion of about 5 u>. Thera are indications that the bend that ruptured. Initially had a wall thickness of only 3 mm Instead of 3,6 mm. In appendix 4 are given some data for the reflux stream. The most important differences between present (1988) and initial figures are: - Higher velocity * pressure * temperature The coke concentration given is very uncertain. It represents just one single sample. However, It seems likely that higher throughput and submerged inlet Instead of bottom spray nozzles will have a tendency to Increase carry over of coke. The higher reflux flow on the other side should have the opposite effect. * 5. CAUSES OP THE RUPTURE The most likely causes for the thickness reduction In the bend that ruptured, ara erosion and corrosion. These two factors will be discussed In more detail. 090905V.05/HH t 5.1 Corrosion The piping was made in carbon steel. This is considered an adequate material as long as this process fluid is dry. The water content has normally been below 10 ppm. At this water level corrosion is very slow. In order to monitor the condition of the piping system, a program for ultrasonic thickness measurements was established at plant start up. The program included 51 measuring points at the reflux system. At the time the program was worked out, erosion was not considered as a problem. Accordingly, all measuring points were on straight parts of the pipes. The reason for this was that ultrasonic measurements with the available equipment were less reliable on bends. In the period from plant start up In 1978 to th fire in 1908, som leakages occured, both in b nds and straight pipe. These leakages were caused by external corrosion under insulation. Further inspections revealed several cases of external corrosion, and also som cases of internal pittings. The general lmpr salon was that there was no significant thickness reduction due to internal corrosion. 5.2 Erosion According to what is said in the preceding section, erosion must have been the main cause for the rupture of the bend. Erosion severeness will be dominated by the following three conditions*. Fluid velocity, coke partic les and geometry of the actual fitting. 5.2.1 Fluid velocity Recommended values for maximum velocities of liquids flowing in pipes are hard to find. In a textbook being used at the Norwegian Technical University it is stated: "Liquid velocities over 4,5 to 6 m/s (14,8 to 19,7 'n-:i should be avoided to reduce erosion". CM ^ to O O O In a paper in Chemical Engineering by Charles R. Kent (September 25., 1970> is given a formula for maximum velocities for clean fluids. In our case this formula gives 3,8 m/s (12,5 fp3). Compared with the two previous figures, the actual velocity, 3,27 m/s (10,7 fps), does not seem excessively high. This is true as long as the reflux stream can be considered as a clean fluid. 5.2.2 Coke particles Under normal operating conditions of the quench tower the carry over of coke is considered to be small. During ~fopshort upsets, l.e. low liquid level and loss of beMan spray, the carry over will increase. Some weeks before the incident a defect (leakage) was discovered in the spray system of one of the quench towers. This might have contributed to an increased coke carry over. A sample of the reflux fluid was analysed for coke. The result was 300 mg/liter (0,03 t). The value seems low, but it is hard to predict the influence upon the erosive prop rties of the liquid. 5.2.3 Geometry of fitting (return bend) The flow characteristics of the return bend was investi gated using a fluid dynamis program (FLUENT). The investi gation showed that high local velocities occured, especial ly in the inner curve of the bend. This is Just where the rupture took place. 6. CONCLUSION 7 The fire in Norsk Hydro's VCM plant at Rafne3, Norway, was caused by a rupture in a return bend. Three factors are considered as the main reasons for the rupture: High liquid velocities Coke paricles in the liquid Return bend geometry with high local velocities. The relative weight of these three factors are uncertain. In rebuilding the plant all three have been taken care of: Bigger pipe diameter Pipes with greater wall thickness Zig-zag baffles in the top of the quench tower to hold back coke particles and liquid droplets. Redesign of return bend - Thickness measurement program to include also bends. When operating with three crackers (see section 2), the velocity in the reflux line to each quencher will be 1,13 m/s (3,7 fps). This Is by all standards a low velocity. This in combination with greater wall thickness, 1 ss probability of coke in the fluid, better design of return bend and a more extensive program for thickness measure ment make us confident that the situation is under control In this specific part of the plant. Rafnes, 06.09.89 H. Hovden BOR 0 0 0 6 2 3 090905V.05/HH 890905V.05/HH TO JETTY [ VCM- PLANT. RAFNES O W.WT. O C 080 O O ADMINISTRATION BUILDING CONTH ROOM jrn~T: DIRECT CHLORINATION ~i " QXY ..CHLORINATinN 7a oO o i KWfSTSlSBfSlfc o o o N-S- PIPE-RACK =L ooo (CX. CONDENSERS |I and RX.TANKS U--------- aa4 gP VCM TANKS O oO EDC- TANKS OOO NORSK HYDRO Fij.'t BOR REFLUX FLOW CONDITIONS APPENDIX 4 Fluid flow Fluid velocity EDC/VCM/HCl Pressure Temperature Coke m3/h gpm m/s fps bar psig C F wt * mg/ltr Initial desiqn Present (1988) situation 66 290 106 466 2,04 6,as 3,27 10,7 91/^5/0,5 88/10/2 24 348 30 433 57 135 146,5 296 _ 0,03 - 300 Bor 000625 f=`3 3. 09O9O5V.05/HH