The regulating valve is a very important part of the automatic control system, just like human hands and feet.Control valveAdjust the fluid flow to overcome interference to ensure that the controlled variable reaches the given process index.
The valve part of the regulating valve is composed of the inner parts of the valve and the valve body. The inner parts of the valve include the valve core, the valve stem, the stuffing box and the upper valve cover.The upper valve cover and stuffing box are used to seal the valve stem and guide the valve stem to prevent the process medium from leaking out along the movable part of the valve stem of the regulating valve. It is an integral part of the valve body.
There are generally four types of conventional upper valve cover structures: common type, fin type, long neck type and bellows sealed type.The materials are generally cast iron, cast steel and stainless steel, and the stuffing box is generally PTFE or flexible graphite.
The valve cover of a typical regulating valve is made of the same material or equivalent material as the valve body.The bonnet bears the same temperature and corrosive influence as the valve body. The stem seal will wear after hundreds of cycles. In engineering applications, fluid pressure will also cause seal wear; the choice of packing is also a problem , The packing is not properly selected, the friction of the regulating valve increases, which causes the dead zone of the regulating valve to increase or it is easy to make the valve stem seal failure.
So chooseControl valveIn addition to the calculation of the valve body structure, material, actuator, and caliber, the structure of the upper valve cover and the stuffing box should be reasonably selected according to the pressure, temperature, pressure difference, and nature of the control fluid to prevent the fluid from being adjusted along If the valve stem leaks out, the performance and service life of the valve stem seal should be fully considered.This is very important in engineering design.
In strong corrosive, volatile, toxic and harmful process fluids, the regulating valve generally does not use ordinary, fin type, long-necked upper valve cover and sealing structure, because the sealing performance and service life of this structure are extremely limited. .The author has done relevant experiments on the production site, using 304 stainless steel straight-stroke control valve to adjust the flow of concentrated nitric acid and concentrated sulfuric acid mixed acid, using PTFE packing seal structure, less than one month, about 600 times of full stroke control valve When the rod moves up and down, the PTFE packing seal is completely destroyed.Under strong corrosive, volatile and toxic and harmful process conditions, once the valve stem seal is damaged, strong corrosive, volatile, toxic and harmful process media leaks from the valve stem of the regulating valve, which will bring about the surrounding environment and personal safety. serious consequence.
The use of a bellows seal type is a way to solve the above-mentioned problems.Bellows are generally made of stainless steel.This special bonnet structure protects the stuffing box of the regulating valve from contact with fluid. Once the bellows is broken, the stuffing box structure above the bellows will prevent serious consequences when the bellows breaks and fails.In engineering practice, the selection of the bellows seal should fully consider that the pressure rating of the bellows seal will decrease with the increase in temperature, there should be no solid particles in the fluid, and the longest cycle life of the bellows material, etc. .In places where stainless steel is not resistant to corrosion by certain process media, such as wet chlorine gas, the small amount of hydrochloric acid contained in the wet chlorine gas will quickly corrode the stainless steel bellows, so the control valve stem cannot be sealed with bellows. form.
In this case, the correct choice seems to be the use of a diaphragm regulating valve.Diaphragm regulating valve adopts corrosion-resistant lining and corrosion-resistant diaphragm. Without packing, it can completely avoid the corrosive medium from leaking out, but its flow characteristics are not very good, and limited by the diaphragm lining, it has poor pressure and temperature resistance. The most fatal thing is that the corrosion-resistant diaphragm will cause fatigue damage due to folding up and down, and its life is very short. Once the corrosion-resistant diaphragm ruptures, the highly corrosive, volatile, and toxic and harmful process media will also leak from the crack. Therefore, The use of diaphragm control valves is greatly restricted.
Therefore, under the process conditions of strong corrosion, volatile, toxic and harmful, finding a reliable, economical and practical valve cover and sealing structure of the regulating valve is the key to ensuring the reliable operation of the regulating valve and realizing the automatic control of the production process. 2. New sealing method for regulating valve stem
The nitrification process of nitrocellulose is the reaction of concentrated nitric acid and concentrated sulfuric acid mixed acid with cotton fiber to produce nitrocellulose. Controlling the ratio of the flow of concentrated nitric acid and concentrated sulfuric acid mixed acid to the amount of cotton fiber is the key to ensuring the quality of nitrocellulose products.In the transformation of the nitrocellulose nitrification process automatic control system, we designed a new type of valve stem sealing method, which successfully solved the valve stem sealing problem of adjusting the flow control valve of concentrated nitric acid and concentrated sulfuric acid.
Above the PTFE packing seal, we designed a cavity, and inert gas (such as nitrogen, or air) is introduced into this cavity, and the pressure of the inert gas is slightly higher than the process in the pipeline The highest pressure of the medium.such,Control valveDuring use, even if the PTFE packing seal of the regulating valve is damaged, the gas pressure is slightly higher than the highest pressure of the process medium in the pipeline, and strong corrosive, volatile, toxic and harmful gases cannot leak out.In order to ensure the stability of the gas pressure in the cavity and prevent the gas from flowing up and down along the valve stem, in the lower part of the cavity, divide the PTFE packing into two parts, add a stainless steel septum in the middle, and make the fluorine rubber O-shaped The sealing ring is embedded in the stainless steel septum, and the septum is clamped up and down with PTFE packing.A fluorine rubber O-ring seal is also embedded in the valve cover at the upper part of the cavity, and the valve cover is pressed tightly with bolts.Generally speaking, the service life of fluorine rubber O-rings is relatively long, up to millions of times. As long as the sealing of fluorine rubber O-rings is not damaged, the pressure of inert gas in the cavity will always be higher than that in the pipeline. The highest pressure of the medium, therefore, the sealing structure of this regulating valve stem is very reliable and economical.
3. Conclusion
The regulating valve of this kind of sealing structure was successfully used in the transformation of the automatic control system of the nitrification process of a certain company in 2001, and realized the automation of the nitrification process production process. The regulating valve has never been in operation due to the damage of the regulating valve stem packing seal. This resulted in an accident of the mixed acid of concentrated nitric acid and concentrated sulfuric acid leaking along the valve stem of the regulating valve.