Application of regulating valve
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In the chemical industry, the control valve is the direct contact with the control mechanism of the process medium. This article discusses the principle of the selection of the control valve, and introduces the application of the control valve in the POM plant. Text / He Deming Process Profile Yunnan Yuntianhua Co., Ltd. POM unit is the first set of the same type of device, technology package imported from Poland ZAT, the use of copolymerization of formaldehyde technology. 30 Section formaldehyde concentration: divided into vacuum concentration of formaldehyde, methanol removal, formaldehyde concentration; raw materials of 40% formaldehyde, the product is 60% formaldehyde. 40 section of the synthesis of trioxane: trioxane synthesis in the liquid phase, the catalyst is sulfuric acid, the raw material is 60% formaldehyde solution, trioxane equilibrium concentration of 4%. The following section 42: removal of low-boiling substances such as methylal and methanol; Section 43 of paraformaldehyde concentrated technetium 60% trioxane; 44 section of paraformaldehyde crystallization; 46 section purification: alkali extraction; 47 section: distilled 99.99% Trioxymethylene. 50 Section Preparation dioxane: The raw material is 60% formaldehyde and ethylene glycol, dioxane obtained after the dehydration with NaOH to obtain 99% DOX; and then after distillation to 99.99% dioxane. As can be seen from the process flow, the purity of raw materials is very demanding, the concentration of industrial formaldehyde is generally not more than 55%, this process requires 60%; and the equilibrium concentration of the reaction is low, the need for multiple separation, which repeated distillation column operation, POM and dioxane products are required to achieve 99.99% concentration. The control valve is in direct contact with the control medium of the process medium and is exposed to fluid pressure, temperature, erosion, corrosion and wear. There are many bad condition jamming valve, vibration, leakage, flow characteristics and even the spool off the phenomenon. In the long-term mechanical fatigue, thermal shock and chemical corrosion under the control valve performance gradually declined. In fact, the life of the control valve is difficult to estimate, the most important factor is the need for designers to understand the process requirements and the use of conditions. Selection of control valve In the selection of the control valve, we must first consider whether the process can achieve the purpose, to meet the control requirements and long-term stable use. As we all know, adjust the parameters of the loop time-consuming and labor-intensive, the important reason is that the flow characteristics of the control valve is difficult to achieve the ideal curve. Controllers using computer technology have enabled PID algorithms to approach mathematical expectations, but the flow characteristics obtained by regulating the valve action always deviate from the ideal value and require multiple adjustments to stabilize. For a single loop, the control strength can be relaxed to get a better stability. In the secondary loop, the secondary loop needs good fast response and self-stability to follow the main loop. In the polyoxymethylene process, in order to ensure the separation efficiency of the rectification column, obtaining a high-purity product requires accurate and stable tray temperature control. Cascade adjustment is usually adopted: the main circuit is temperature regulation and the secondary circuit is steam flow regulation, as shown in Figure 2. However, the rapid response of the flow loop, easy to cause oscillation valve, the entire cascade loop fluctuations. Relative to the P-shaped spool, the use of V-shaped spool is conducive to improving control quality. It has asymmetric structure, fixed point radial force, small vibration, which is conducive to the stability of the spool, prolongs the service life, and is far away from the fluid scour zone, which is conducive to protecting the valve core / seat to avoid turbulence damage. In addition, the equal percentage flow characteristics are Strict equal percentage curve, in any caliber, does not change the curve characteristics, the working range of 20% to 90% (see Figure 3). In organic chemical plants, there are often a variety of toxic media. Typical paraformaldehyde mediators are formaldehyde and dioxolane. From the perspective of safety and environmental protection, low leakage valves should be used. Leakage of the control valve is mainly stem and packing gap, in order to ensure the stem action freely, must have a certain gap with the filler. For low boiling point media can easily leak from here. Corrugated pipe sealing can completely solve this problem. The traditional corrugated pipe material and processing technology constraints, the service life is not long. SAMSON valve with metal bellows patented sealing technology to prevent any toxic and explosive gases burst into the atmosphere, its design is 2 to 5 layers of structure, using a molding method, non-segmented welding, the use of soft-sealed valve Core / seat with zero leakage can be achieved, and the use of the number of (full-valve to fully open) in more than 1.6 million times, at the same time can be loaded bellows leak detection device to better ensure the safety of the valve to protect the personal and environmental safety (See Figure 4). As the main raw material for the synthesis of polyoxymethylene trioxane need to step-by-step to 99.99%. In the high concentration to maintain about 70 ℃ in order to maintain liquid. In the event of cold that is crystalline, accompanied by the formation of paraformaldehyde side effects, and irreversible reaction. Such pipes on the craft are designed as jacketed pipes with hot water at 75 ° C. The control valve must also be a jacketed valve. Many manufacturers to reduce manufacturing costs, take the jacket outsourcing approach, the jacket and valve body together, through the jacket wall heat transfer. Metal is difficult to make without gaps, resulting in heat transfer dead ends. SAMSON will jacket welded on the valve body, using the way in and out, independent circulation system enables the medium to be evenly distributed to all parts of the valve body; does not change the size of the original valve, easy to install, easy to remove; compact structure, Not affected by the pipeline, nor the other pipe fittings have an impact. This jacket must be hydrostatically tested and flawed, resulting in increased costs but no dead ends (see Figure 5). The above process from the perspective of the valve to discuss the special requirements of selection, to meet the needs of the process under the premise of the valve life is directly related to working conditions. In addition to the calculation of the opening book, noise, etc., should pay close attention to the value of the outlet flow rate, in principle, the liquid outlet flow rate should not exceed 3m / s, while the gas or steam should not exceed 3Mach, in the selection process If you ignore this value and simply look at the opening and noise, excessive flow rate will cause damage to the valve trim, so that the valve life can not be guaranteed. Valve manufacturing process, the valve body manufacturing process is very important, SAMSON valve body manufacturing (forging / casting) using computer stress analysis, testing to ensure standard temperature and pressure specifications; the same time using the robot hand turning body cavity To ensure the minimum drag coefficient; the casting valve body patented technology in the body cavity flow path surface to form an oxide film, greatly improving the corrosion resistance and life of the valve. Conclusion POM plant for the process requirements, after market research, the final selection of SAMSON control valve. After 4 years of operation, the operation is very stable. Disintegration inspection found no wear inside parts, so that the daily maintenance workload significantly reduced. In the 20% increase after the transformation, the existing control valve does not need to be replaced still be able to meet the requirements. In addition to the ruggedness of the valve itself, SAMSON also favors the conservative selection of style, Kv value and the implementing agencies have left a large margin, reducing the second investment.