1, Introduction Control valve is a very important part of automatic control system, like a human hand and foot. Control valve to regulate fluid flow, to overcome the interference to ensure that the controlled variable to achieve the given technical indicators. The valve part of the control valve consists of the inner part of the valve and the valve body. The inner part of the valve includes the valve plug, the valve stem, the stuffing box and the upper bonnet. The upper bonnet and stuffing box are used to seal the stem and guide the stem to prevent the process medium from leaking outward along the stem of the control valve, which is an integral part of the body. Conventional bonnet structure in general there are four types: ordinary type, heat sink type, long neck and bellows sealed type. Material generally cast iron, cast steel and stainless steel, stuffing box is generally PTFE or flexible graphite. The bonnet of a typical control valve is made of the same material as the valve body or an equivalent material. The bonnet withstands the same temperature and corrosive effects as the valve body, and stem seals wear out after hundreds of cycles of movement, and fluid pressure can also cause seal wear in engineering applications; the choice of filler is also a problem , Improper packing options, control valve friction increases cause dead zone control valve increases or it is easy to make the stem seal failure. Therefore, the choice of control valve, in addition to the valve body structure, material, actuator, caliber calculation, but also should be based on the control of fluid pressure, temperature, pressure, fluid properties, the choice of the valve cover structure and stuffing box to Prevent the fluid from leaking out along the stem of the control valve, which should fully consider the performance and service life of the stem seal. This is very important in engineering design. In the strong corrosive, volatile and toxic and hazardous process fluids, the control valve generally do not use ordinary type, heat sink type, long-necked bonnet and seal structure, as the structure of the sealing performance and life is extremely limited . The author has done experiments in the production site, the use of 304 stainless steel straight stroke control valve to adjust the concentrated nitric acid and concentrated sulfuric acid mixed acid flow, the use of PTFE packing seal structure, less than a month after about 600 times the entire trip control valve Rod up and down movement, PTFE packing seal that is completely destroyed. In strong corrosion, volatile and toxic and hazardous process conditions, once the stem seal is damaged, strong corrosion, volatile and toxic and hazardous process media leak from the stem of the control valve will bring the surrounding environment and personal safety serious consequence. The use of a bellows-sealed version is one way to solve the above problems. Bellows are generally made of stainless steel. This special bonnet structure protects the stuffing box of the control valve from fluid contact. Once the bellows is broken, the stuffing box structure above the bellows prevents the serious consequences of rupture of the bellows. In engineering practice, the choice of bellows seal should take full account of the bellows seal pressure rating will decrease with increasing temperature, the fluid can not have solid particles exist, and the bellows material longest cycle of action life . In stainless steel is not resistant to corrosion of some of the strong corrosion medium, such as the process medium is wet chlorine, chlorine contained in the wet chlorine will make stainless steel bellows quickly corroded, the control valve stem can not be sealed with a bellows form. In this case, the correct choice seems to be the use of diaphragm control valve. Diaphragm control valve with a corrosion-resistant lining and corrosion-resistant diaphragm, without filler, can completely avoid the leakage of corrosive media, but its flow characteristics are not very good, and by the diaphragm liner, pressure, temperature performance is poor , The most deadly is the corrosion-resistant diaphragm will be folded up and down due to fatigue damage, short life expectancy, once the corrosion-resistant diaphragm rupture, corrosive, volatile and toxic and hazardous process media will also leak out from the gap, The use of diaphragm control valves has been greatly limited. Therefore, looking for a reliable, economical and practical control valve bonnet and sealing structure under the conditions of strong corrosion, volatile and toxic and harmful process is the key to ensure the control valve work reliably and realize the automatic control of the production process. 2, a new type of sealing valve stem valve nitrification process nitrification nitration process that concentrated nitric acid and concentrated sulfuric acid mixed with cotton fiber reaction of nitrocellulose, control concentrated nitric acid and concentrated sulfuric acid mixed acid flow and cotton fiber ratio is to ensure the quality of nitrocellulose products The essential. In the process of reforming the nitrocellulose nitration process automatic control system, we designed a new type of stem sealing method and successfully solved the valve stem sealing problem of adjusting the mixed acid flow control valve of concentrated and concentrated sulfuric acid. Above the polytetrafluorethylene packing seal, we designed a cavity and introduced an inert gas (such as nitrogen or air) into the cavity and required the pressure of the inert gas to be slightly higher than the process in the pipeline The maximum pressure of the medium. In this way, the control valve in use, even if the control valve PTFE packing seal is damaged, due to the gas pressure is slightly higher than the highest pressure in the process medium in the pipeline, strong corrosion, volatile, toxic and harmful gases can not Leak out. In order to ensure the pressure of the gas in the cavity is stable and the gas is prevented from flowing up and down along the valve stem, the PTFE packing is divided into upper and lower parts in the lower part of the cavity, a stainless steel septum is added in the middle, Seals embedded stainless steel septa, septum up and down with PTFE packing to clamp it. Fluoroelastomer O-ring is also embedded in the bonnet in the upper part of the cavity, and the bonnet is pressed tightly with the bolt. In general, fluorine rubber O-ring service life is relatively long, up to millions of times, as long as the fluorine rubber O-ring seal is not damaged, the inert gas pressure in the cavity will always be higher than the pipeline in the process The maximum pressure of the medium, therefore, this control valve stem seal structure is very reliable, but also very economical. 3, concluding remark This kind of seal structure control valve was successfully used for the reforming of a company's nitration process automatic control system in 2001, realized the nitration process production process automation, the control valve operation so far, never occurred because of the control valve stem packing Sealing damage caused by concentrated nitric acid and concentrated sulfuric acid along the control valve stem leakage accident.
