1. Introduction Guangzhou Ethylene Cracking Unit According to the design of Italy's TCM Company, the ethylene plant adopts the switch ball valves produced by 77 Italian ALFA companies. These ball valves are used in the cleavage device part of the interlock, cut off the seal demanding, generally more than five. Such as for the six cracking furnace interlocking fuel gas shut-off valve, leaks can cause major accidents. However, these switch ball valves have more problems. During the test run (purge, airtight), most of the valves were found to have large leakage and could not hold or close properly. Leakage problems overhaul can meet the requirements, but often bring the problem is not in place. These valves are good and bad, very unreliable. Ethylene has always been a major safety hazard. 2, ALFA switch ball valve is not in place The reason for the analysis of these products is Italy's ALFA's early products, the structure is irrational. The valve actuators are single cylinder type, but no spring pressure adjustment device. Ball valve and seat seal, sealed by PTFE gasket, and adjust the thickness of the gasket to adjust, to ensure that the seal, the seal surface is often too much resistance, it is difficult to adjust to the appropriate thickness. Furthermore, due to the influence of the stress and thermal stress of the process pipe, even if the valve is repaired during the overhaul, the tested valve is often not closed after being put into use. 3 ALFA on-off valve of the transformation program According to the ALFA valve structure defects, we consider the following transformation programs: Option One: modification with new products, each valve 3 to 40,000 yuan, 77 sets of valves are expected to invest 200 to 3 million yuan. Due to installation size restrictions, domestic and foreign brands such as KTM, NESS valve size is not suitable, the transformation is very difficult; and the investment is too large, should not be used. Scenario 2: A spring washer is installed in the seat groove to adjust the seal automatically. This is the same as the new structure of the ALFA switch ball valve, as shown in Figure 1 below. Figure 1 before and after the transformation of the sealing surface structure but the following problems: 1 in the valve seat groove, destroy the original structure; 2 high technical requirements, spring washer valve manufacturing engineers to design calculations, we currently do not have the technical strength; 3 new Increased spring washer, the domestic do not know whether there is any suitable product, the original imported parts price is too expensive. In the case of little grasp, should not use this program. Option three: After repeated tests and found ALFA valve there is more than the lack of the reason is due to insufficient force when the valve is closed, it is difficult to overcome the ball valve sealing surface resistance, the best solution is to install a pilot valve on the cylinder spring side, according to Pneumatic signal side of the control spring side of the gas off, the use of instrument air pressure to increase the pressure on the side of the spring, so that the closing force increases, to overcome the resistance of the sealing surface to ensure that both sealed, but also close in place. The structure of the transformation shown in Figure 2 below. Figure 2 Schematic diagram of the modified structure Description: The thin line for the original part (solenoid valve for the redundant structure), bold line for the transformation to increase some of the expected results: due to normal operation, the spring side of the cylinder connected with the atmosphere, the other side and Wind instrument connected. Under the action of the instrument wind, the valve is fully open. Under the condition of the pilot valve, the valve is controlled by the pilot valve to introduce the instrument wind into the spring side cylinder. The spring force is increased and the other side is communicated with the atmosphere. The larger pressure difference Can overcome the pressure on the seat sealing surface, which can ensure the valve is fully closed, and has a good sealing effect. The main materials of this program are shown in the following table. (Each valve) is expected to invest about 2,000 yuan per valve renovation, this program does not undermine the original valve, the investment is relatively small, the entire project (transformation of 77 ALFA switch ball valve) a total investment of about 150,000 yuan. 4 Conclusion After the implementation of ethylene cracker 77 ALFA switch ball valve all the transformation and put into use successfully. After more than two years of actual production and testing, the project has achieved good results and eliminated a serious long-term hidden danger for the safety of ethylene plant and created considerable economic benefits. [Author] Liu Lizhong, (1969-), male, Guangzhou, Guangdong Province, bachelor degree, bachelor's degree, engineer. 92 years graduated from Shanghai China Textile University Production Process Automation (now Donghua University), now China Petrochemical Corporation Guangzhou Branch Instrument Control Center, mainly engaged in DCS, PLC systems and field instrumentation applications, maintenance and design management . Tel: (020) 82128350 (O) 82399837 (H) 13042061415 Address: Sinopec Guangzhou Branch Instrument Control Center Workshop III Zip Code: 510725 EMAIL: liulz2001969@21cn.com
