A new sealing method for the stem of the hottest c

Introduction control valve is a very important part of automatic control system, just like human hands and feet. The control valve regulates the fluid flow and overcomes the interference to ensure that the controlled variable reaches the given process index

the valve part of the control valve is composed of the valve internals and the valve body. The valve internals include the valve core, valve stem, stuffing box, upper bonnet, etc. The upper bonnet and stuffing box are used to seal and guide the valve stem to prevent the process medium from leaking outward along the movable part of the valve stem of the control valve. It is an integral part of the valve body

there are generally four types of conventional upper bonnet structure: ordinary type, radiator type, long neck type and bellows sealing type. The materials are generally cast iron, cast steel and stainless steel, and the stuffing box is generally polytetrafluoroethylene or flexible graphite

the bonnet of a typical control valve is made of the same material as the valve body or equivalent material. The bonnet bears the same temperature and corrosive effects as the valve body. The valve stem seal will wear after hundreds of cycles. In engineering applications, fluid pressure will also cause seal wear; The selection of packing is also a problem that the Brinell hardness value measured for 30s (seconds) is 120n/mm2 (MPA). If the packing is improperly selected, the friction of the control valve increases, resulting in the increase of the dead zone of the control valve or the failure of the valve stem seal

therefore, when selecting the control valve, in addition to the test method standard that the valve body structure, material, actuator and port meet: diameter calculation, the structural form of the upper bonnet and stuffing box should also be reasonably selected according to the pressure, temperature, differential pressure and nature of the control fluid, so as to prevent the fluid from leaking out along the control valve stem, that is, the performance and service life of the valve stem seal should be fully considered. This is very important in engineering design

in highly corrosive, volatile, toxic and harmful process fluids, the control valve generally does not adopt the common type, radiator type, long neck upper bonnet and sealing structure, because the sealing performance and service life of this calculation structure are extremely limited. The author has done relevant experiments on the production site. The 304 stainless steel straight stroke control valve is used to adjust the flow of concentrated nitric acid and concentrated sulfuric acid mixed acid, and the PTFE packing seal structure is used. After about 600 full stroke control valve stem up and down movements in less than a month, the PTFE packing seal is completely destroyed. Under the strong corrosive, volatile, toxic and harmful process conditions, once the valve stem seal is damaged, the strong corrosive, volatile, toxic and harmful process medium will leak out of the control valve stem, which will bring serious consequences to the surrounding environment and personal safety

adopting bellows sealing type is a 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 contacting with the fluid. Once the bellows breaks, the stuffing box structure on the bellows will prevent the serious consequences of the rupture failure of the bellows. In engineering practice, the selection of bellows seal form should fully consider that the pressure rating of bellows seal will decrease with the increase of temperature, there should be no solid particles in the fluid, and the longest cycle action life of bellows material. In places where stainless steel is not resistant to the corrosion of some process media, for example, when the process medium is wet chlorine, the trace hydrochloric acid contained in the wet chlorine will quickly corrode the stainless steel bellows, so the control valve stem cannot be sealed by bellows

in this case, the correct choice seems to be to use diaphragm control valve. The diaphragm control valve adopts a corrosion-resistant lining and a corrosion-resistant diaphragm. Without packing, the leakage of corrosive media can be completely avoided, but its flow characteristics are not very good, and limited by the diaphragm lining, its pressure resistance and temperature resistance are poor. The most fatal thing is that the corrosion-resistant diaphragm will cause fatigue damage due to folding up and down, and its service life is very short. Once the corrosion-resistant diaphragm breaks, it will be strongly corroded Volatile, toxic and harmful process media will also leak out from the crack, so the use of diaphragm control valves is greatly limited

therefore, under the strong corrosive, volatile, toxic and harmful process conditions, looking for a reliable, economic and practical upper bonnet and sealing structure of the control valve is the key to ensure the reliable operation of the control valve and realize the automatic control of the production process

2. New sealing method of control valve stem

nitrocellulose nitration process is that concentrated nitric acid and concentrated sulfuric acid mixed with acid react with cotton fiber to produce nitrocellulose. Controlling the ratio of concentrated nitric acid and concentrated sulfuric acid mixed with acid flow and cotton fiber quantity is the key to ensure the quality of nitrocellulose products. In the reconstruction of the automatic control system of nitrocellulose nitrification process, we designed a new valve stem sealing method, which successfully solved the valve stem sealing problem of regulating the flow control valve of concentrated nitric acid and concentrated sulfuric acid mixed acid

above the PTFE packing seal, we designed a cavity and filled it with inert gas (such as nitrogen, or air), and the pressure of the inert gas is required to be slightly higher than the maximum pressure of the process medium in the pipeline. In this way, even if the PTFE packing seal of the control valve is damaged during the use of the control valve, the strongly corrosive, volatile, toxic and harmful gases cannot leak out because the gas pressure is slightly higher than the maximum pressure of the process medium in the pipeline. In order to ensure the pressure stability of the gas in the cavity and prevent the gas from stringing up and down along the valve stem, the polytetrafluoroethylene packing is divided into upper and lower parts at the lower part of the cavity, a stainless steel spacer is added in the middle, the fluororubber O-ring is embedded in the stainless steel spacer, and the spacer is clamped up and down with polytetrafluoroethylene packing. Fluorine rubber O-ring is also embedded in the valve cover on the upper part of the cavity, and the valve cover is compressed with bolts. Generally speaking, the service life of fluororubber O-ring is relatively long, up to millions of times. As long as the seal of fluororubber O-ring is not damaged, the inert gas pressure in the cavity will always be higher than the maximum pressure of the process medium in the pipeline. Therefore, the cost of this new material automobile is several times that of the existing automobile. The sealing structure of valve stem is very reliable and economical

3. Conclusion

the control valve with this sealing structure was successfully used in the transformation of the automatic control system of the nitrification process of a company in 2001, realizing the automation of the production process of the nitrification process. So far, the control valve has been operated, and there has never been an accident that the mixed acid of concentrated nitric acid and concentrated sulfuric acid leaks along the control valve stem due to the damage of the packing of the control valve stem