How to seal better able to control valve repair stem? The valve repair factory Jiangyin Oulei Si that must control valve control valve repair stem sealing principle is a very important aspect of the automatic control system, like the hands and feet. Control valve regulate fluid flow to overcome interference to ensure the controlled variable reaches a given process indicators.
Control valve valve part of the inner parts of the valve and valve body, including the inner parts of the valve spool valve stem, stuffing box and bonnet. On the bonnet and stuffing for stem seal and stem-oriented control of pneumatic control valve stem to prevent the process medium along the moving parts to leak out, it is an integral part of the valve body.
Conventional bonnet structure are generally four types: ordinary, fin type, long-necked and bellows sealed type. Material is generally cast iron, cast steel and stainless steel, the stuffing is usually PTFE or flexible graphite.
Typical control valve bonnet from the valve body of the same material or an equivalent material. The valve cover is exposed to the same valve body temperature and corrosive effects stem seal after a cycle of action after a few hundred times, it will wear in engineering applications, the fluid pressure will cause the seal to wear; filler of choice is also a problem , the improper packing select control valve friction increases lead to increased control valve dead or very easy to make the stem seal failure.
Therefore, the control valve, the addition to the body structure, material, the executing agency, the method of calculation should be based on the pressure of the control fluid, temperature, pressure, fluid nature, a reasonable choice on the bonnet structure and stuffing to to prevent the leakage of fluid along the control valve stem, that should give full consideration to the performance and service life of the valve stem seals. This is very important in engineering design.
In highly corrosive, volatile and toxic and harmful to the process fluid, control valve generally not used in the ordinary type, fin type, flasks on the valve cover and the sealing structure, since the seal performance and life of such structures in the form of extremely limited . The author has done related experiments at the production site, 304 stainless steel straight trip control valve regulating concentrated nitric acid and concentrated sulfuric acid mixed acid flow, PTFE packing seal structure, about 600 times less than a month after the full stroke control valve valve the vertical movement of the rod, PTFE packing seal that completely destroyed. Strong corrosive, volatile and toxic and hazardous process conditions, once the stem seal is damaged, strong corrosive, volatile and toxic and hazardous process media from the control valve stem leak out, would bring surrounding environment and personal safety serious consequences.
The bellows seal-type form is one way to solve the above problems. The bellows is generally made of stainless steel. This particular valve cap structure to protect the control valve stuffing box to avoid contact and fluid, once broken bellows, bellows above the stuffing box structure would prevent the the broken bellows failure of serious consequences. In engineering practice, the choice of the form of bellows seal should take full account of the bellows seal pressure rating will be reduced with the increase of temperature, not the presence of solid particles in the fluid, and the longest cycle life moves bellows material . Stainless steel the strong corrosion workplace impatient with some of the process medium corrosion, such as process medium wet chlorine, the wet chlorine containing the trace hydrochloric make stainless steel bellows soon corrosion control valve stem bellows seal can not be used form.
In this case, the correct choice seems to be the use of the diaphragm control valves. Diaphragm control valve with a corrosion-resistant lining and corrosion-resistant diaphragm, no filler, can completely avoid outward leakage of corrosive media, but its flow characteristics is not very good, and is limited by the membrane lining, pressure, temperature performance is poor The most deadly corrosion resistant diaphragm will cause fatigue damage due to the upper and lower folding a very short life, resistance to corrosion ruptured diaphragm, strong corrosive, volatile and toxic and hazardous process media will also leak out from the gap at the The use of the diaphragm control valve to be greatly restricted.
Therefore, in highly corrosive, volatile, toxic and hazardous process conditions, looking for a reliable, economical and practical control valve on the valve cover and seal structure is to ensure that the production process in order to achieve the automatic control of key control and reliable operation of the valve.
Nitrocellulose nitration process, ie concentrated nitric acid and concentrated sulfuric acid mixture and cotton fibers reaction generating nitrocellulose control of concentrated nitric acid and concentrated sulfuric acid mixed acid flow and the ratio of the amount of cotton fibers is to ensure that the key to nitrocellulose product quality. In the transformation of nitrocellulose nitrification process automatic control system, we have designed a new type of stem seal, successfully resolved the issue of regulation of concentrated nitric acid and concentrated sulfuric acid mixed acid flow control valve stem seal.
PTFE packing seals above, we have designed a cavity, and pass into the inert gas (such as nitrogen, also may be air) to the cavity, and requires an inert gas pressure to be slightly higher than the process of the pipeline the maximum pressure of the medium. In this way, the control valve during use, even if the control valve PTFE packing seal is damaged, due to the pressure of the gas is slightly higher than the maximum pressure of the pipeline process medium, strong corrosion, volatile, toxic and harmful gases can not leak out. To ensure the stability of the cavity in the gas pressure to prevent gas string gas along the stem up and down in the lower part of the cavity, PTFE packing is divided into upper and lower parts, in the middle of a stainless steel septum, fluorine rubber O- seals embedded stainless steel within the septum, the septum with PTFE packing clamped. Viton O-ring is also embedded in the valve cover of the upper portion of the cavity, the bonnet pinch bolts. General, fluoro-rubber O-ring of life is relatively long, up to a million times, as long as the fluoro-rubber O-ring seal is not destroyed, the inert gas pressure in the cavity will always be higher than the process of the pipe the maximum pressure of the medium, so that this control valve stem seal structure is very reliable, but also very economic.
Conclusion
This seal structure control valve in 2001 for a company nitrification process automatic control system transformation, to achieve the nitrification process production process automation, control valves run so far and never had any control valve stem packing seal damage resulting in concentrated nitric acid and concentrated sulfuric to mixed acid along control valve stem leakage accident.
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