1, Introduction
Control valves, automatic control system is a very important, like people's hands and feet. Regulation of fluid flow control valve to overcome the interference to ensure the accused of variables to achieve a given process indicators.
Control valves by the valve inside the valve body parts and the composition of the inner valve spool pieces, including, stem, packing on the valve cover letter and so on. On the valve cover and fill for the letter of the stem seals and valve stem to guide, to prevent the process medium along the valve stem to control the outward leakage of movable parts, it is an integral part of valve body.
Conventional structure of the valve cover there are four general forms: ordinary type, heat sink type, long-necked and bellows-type seal. Materials are generally cast iron, cast steel and stainless steel, PTFE packing or letter is generally flexible graphite.
A typical control valve of the valve cover and valve body from the same material or equivalent material. Valve cover and valve body to bear the same temperature and corrosive effects of stem seal after a few hundred times after the cycle of action, they will wear, in engineering applications, the fluid pressure can lead to seal wear and tear; fill the choice is also a problem filling materials selection, control valves, increasing the friction caused by dead-zone control valves easily increase or stem seal failure.
Therefore, the choice of control valve, in addition to body structure, material, implementing agencies, the calculation caliber but also under the control of fluid pressure, temperature, pressure, fluid nature, a reasonable choice of the structure of the valve cover and fill the form of letters to to prevent fluid leakage along the control valve stem out, that should give full consideration to stem seal performance and service life. This is in engineering design it is very important.
Corrosion in strong, volatile and toxic and hazardous fluids in the process, control valves are generally not the ordinary type, heat sink type, long-necked type seal on the valve cover and structure, because such a structure of sealing performance and life of great limited. The author has done the scene in the production of related experiments, the use of 304 stainless steel control valves regulate straight trip concentrated nitric acid and concentrated sulfuric acid mixed acid flow, the use of PTFE packing seal structure, less than a month after about 600 of the control of the entire trip valve up and down movement of the rod, PTFE packing seal that is completely undermined. Corrosion in strong, volatile and toxic and hazardous process conditions, once the stem seal is damaged, strong corrosive, volatile and toxic and hazardous process media control valve stem from leaking out on the surrounding environment and personal security serious consequences.
Adopted the form of bellows-type seal of the problem is a way. Bellows made of stainless steel in general. This special structure to protect the valve cover of the fill control valves and fluid contact with you to avoid, once broken bellows, the bellows structure of the above letter will fill the bellows to prevent the breakdown of failure have serious consequences. In engineering practice, the form of bellows seal selection should take full account of the pressure of bellows sealed the rating would be higher with lower temperature, fluid particles can not have a solid presence, and corrugated pipe materials, such as the longest life cycle of action . Intolerance of certain processes in the stainless steel corrosion medium places strong corrosion, such as wet chlorine process medium, the wetlands of trace chlorine in hydrochloric acid containing stainless steel corrugated pipe will soon be corrosive, it can not be used to control valve stem bellows sealed form.
In this case, the right to choose seems to be the use of diaphragm control valve. Diaphragm control valves used in corrosion-resistant and corrosion-resistant lining membrane, do not fill, you can completely avoid the leakage of corrosive media field, but its not very good flow characteristics, and by the membrane lining of the restrictions, pressure, low temperature performance The most deadly is corrosion-resistant film will divide up and down folding as a result of damage caused by fatigue, a very short life span, once piece corrosion-resistant diaphragm rupture, strong corrosive, volatile and toxic and hazardous process media will be the same Department from leaking out of cracks, so diaphragm control valve by the use of a lot of constraints.
Therefore, in strong corrosion, volatile, toxic and hazardous process conditions, to find a reliable, economical and practical control valves on the valve cover and seal structure is to ensure reliable control valves work in order to achieve automatic control of the key production processes. 2, a new type of control valve stem sealing method
Nitrocellulose nitrification process that concentrated nitric acid and concentrated sulfuric acid and cotton fiber mixed reaction Nitrocellulose, control of concentrated nitric acid and concentrated sulfuric acid mixed acid flow and the ratio of the volume of cotton fiber is to ensure that the key to Nitrocellulose product quality. Nitrocellulose nitrification processes in the transformation of self-control system, we designed a new type of stem seal and successfully resolved the regulation of concentrated nitric acid and concentrated sulfuric acid mixed acid flow control valve stem seal problem.
PTFE packing seal in the top, we designed a cavity and the cavity to pass into the inert gas (such as nitrogen, can also be air), and requested the pressure inert gas pipeline to be slightly higher than the process the maximum pressure medium. In this way, control valves in the use of the process, even if the control valve of PTFE packing seal is damaged, due to the pressure of the gas pipeline to be slightly higher than the maximum pressure on media technology, strong corrosive, volatile, toxic and harmful gas can not leak out. Cavity in order to ensure the stability of gas pressure to prevent the gas valve stem up and down along the string gas in the cavity of the lower part of the PTFE packing divided into two parts from top to bottom, in the middle of a stainless steel pad traps will Viton O - stainless steel ring embedded in every other pad, the pad separated using PTFE packing its clamping. The upper cavity of the valve cover is also embedded in fluorinated rubber O-ring, and valve cover bolts will be pressed. In general, the fluorine rubber O-ring of life is relatively long, up to a million times, as long as the fluorine rubber O-ring seal is not damaged, cavity pressure inert gas pipeline technology has always been higher than the medium the highest pressure, the control valve stem sealing structure is very reliable and very economic.
3, Conclusion
That the control valve sealing structure in 2001, the company successfully used a modified nitrification process automatic control system, the realization of the production process of the nitrification process automation, control valve operation so far and there have never been as a result of the control valve stem packing seal damage resulted in concentrated nitric acid and concentrated sulfuric acid mixed acid along the valve stem to control the leakage of the accident.
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