Control valve is a very important part of automatic control system, like a man's hands and feet. Control valve regulating the flow of fluid, to overcome the interference to ensure the controlled variable to a given process indicators.
Valve control valve is composed of valve parts and valve body, internal parts, including stem, valve core and valve cover, stuffing box. On the valve cover and stuffing box used for valve stem seal and the orientation of the stem, prevent the process media control valve stem along the moving parts out of this leakage, which is part of the body can not be separated.
Regular structure on the valve cover, in general there are four types: ordinary type, heat sink type, long-necked type and bellows seal type. Material generally have cast iron, cast steel and stainless steel, stuffing usually PTFE or graphite.
A typical control valve of the valve cover and the valve body is made from the same material or equivalent material. The valve cover bearing temperature and corrosive effects with the same body, valve stem seal after hundreds of cycles of action, will wear, in engineering applications, the fluid pressure can lead to seal wear; the choice of filler is also a problem, the improper selection of filler, increase the friction control valve control valve deadband caused increases or it is easy to make the valve stem seal failure.
Therefore, the choice of control valve, in addition to body structure, material, actuator, the method of calculation, but also under the control of fluid pressure, temperature, pressure, fluid properties, a reasonable choice of the structure on the valve cover and stuffing box, to prevent fluid leakage along the valve stem out, which should give full consideration to the performance and the service life of valve stem seals. This is very important in engineering design.
In highly corrosive, volatile and toxic and hazardous process fluids, control valve is generally not the ordinary type, heat sink type, long-necked type valve cover and seal structure, because the sealing performance and the service life of the structure form of 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 structure, less than a month after the full stroke control valve stem 600 or so up and down movement, is completely destroyed the PTFE packing seal. In highly corrosive, volatile and toxic and hazardous conditions, once the stem seal is damaged, strong corrosive, volatile and toxic and hazardous process media control valve stem from leaking out, bring about serious consequences to the surrounding environment and personal safety.
With bellows seal form is a way to solve the above problems. Generally made of stainless steel bellows. The stuffing of the special structure of the valve cover to protect the control valve to avoid contact with the fluid, once broken bellows, bellows above the stuffing box structure can prevent serious consequences of corrugated pipe rupture failure. In engineering practice, the bellows seal in the form of selection should take full account of the bellows seal pressure ratings will increase and decrease with temperature, not fluid solid particles, and the corrugated pipe materials such as the longest life cycle actions. In the strong corrosion resistant stainless steel is not some process of corrosion media sites, such as the process medium to wet chlorine gas, wet traces of hydrochloric acid containing chlorine will cause corrosion of stainless steel bellows quickly, the control valve stem seal bellows can not be used in the form of. In this case, the correct choice seems to be the use of diaphragm control valve. Diaphragm control valve with corrosion resistant lining and corrosion resistance diaphragm, no filler, can completely avoid the corrosive medium leakage, but its flow characteristics is not very good, and the membrane lining limit, pressure resistance, temperature resistance performance is poor, is the most lethal corrosion-resistant diaphragm due to the upper and the lower folding caused by fatigue damage, life is short, once corrosion resistance diaphragm rupture, strong corrosive, volatile and toxic and hazardous process medium will also from the cracks at the leak out, therefore, the use of diaphragm control valve limits the.
Therefore, in highly corrosive, volatile, toxic and harmful conditions, to find a reliable, economical and practical control valve on the valve cover and seal structure is to ensure reliable operation of control valves in order to achieve automatic control of the key production process.
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