【 Weak current College 】 control valve stem of the new seal---Power By 【 China power house network 】

Control valve adjustment fluid flow, overcome interference to guarantee an accused the variable reaches the given process indicators. Control valve is automatic control system is very important aspect, like the hands and feet.
Control valve of body parts from the valve internals and valve body, valve internals including valve, valve stem and the stuffing and the valve cover, etc. On the bonnet and the stuffing box is used to seal the valve stem and valve stem is oriented to prevent process media along control valve stem of the moving parts to leak, it is an integral part of the body.

General structure of the valve cover form generally has four: ordinary, fin-type, long-necked type and bellows seal type. Materials generally have cast iron, cast steel and stainless steel, the stuffing is PTFE or graphite.

Typical control valve and the valve cover the same material or equivalent material. Valve and the valve cover and bear the same temperature and corrosive effects, stem seal after hundreds of times the loop action after wear, in engineering, fluid pressure can also cause seal wear; packing of choice is a choice, stuffing, control valve friction increases the resulting control valve dead increase or easily make the valve stem seal failure.

Therefore, select the control valve, in addition to the body structure, material quality, implementing agencies, calculated, under the control of fluid pressure, temperature, pressure, fluid properties, select the structure on the bonnet and the stuffing box form in order to prevent fluid control valve stem leakage out along, that is, should take full account of the valve stem seal performance and service life. This design is very important.

In strong corrosive, volatile and toxic process fluid, control valve normally not use ordinary, fin-type, long-necked type valves are covered and sealed structure, because this kind of structure in the form of the sealing performance and service life are extremely limited. The author has done in the production site, related experiments 304 straight-stroke valve for regulation of concentrated nitric acid and concentrated sulphuric acid mixed acid flow, using PTFE packing seal structure, less than a month after 600 meters full stroke of the valve stem of the up and down movement, PTFE packing seal that is completely destroyed. In strong corrosive, volatile and toxic and harmful under the conditions of the process, once the stem seal is compromised, strong corrosive, volatile and toxic process media from control valve stem leakage out of the surrounding environment and personal safety have serious consequences.


Using bellows seal type is address these issues in a way. Made from stainless steel bellows General. This special valve cover structure protection control valves, stuffing avoid and fluid contacts, once the bellows, bellows above stuffing box structure prevents the bellows failure resulting in serious consequences. In project real, bellows seal forms of choice should give full consideration to the bellows seal pressure temperature rating will increase and decrease, fluid can not have solid particles exist, and bellows material maximum cycle action life, etc. In stainless steel is not resistant to corrosion by some process of strong corrosive medium, as technology media for wet wet chlorine, chlorine containing micro hydrochloric acid causes corrosion of stainless steel bellows quickly, the valve stem will not be able to use bellows seal form.

In this case, the proper selection seems to be using diaphragm valve. Diaphragm valve with corrosion-resistant and corrosion-resistant lining of diaphragm, no filler, you can completely avoid corrosive medium leakage to the outside, but its flow is not very good, and diaphragm liner, pressure resistance, heat resistance, the most deadly is corrosion-resistant diaphragm will collapse because the up-and-down cause fatigue damage, life is very short, once the corrosion-resistant diaphragm rupture, strong corrosive, volatile and toxic process media will leak from a crack at it, therefore, the diaphragm valve using a lot of restrictions.

Therefore, in a highly corrosive, flammable, toxic or harmful under the conditions of the process, looking for a reliable, economical and practical control valve, the valve is covered and sealed structure to ensure the reliable operation of control valve to achieve automatic control of the production process.

New control valve stem seal method

Nitrocellulose nitrification process i.e. concentrated nitric acid and sulphuric acid and cotton fibers mixed reaction of nitrocellulose, control of concentrated nitric acid and concentrated sulphuric acid mixed acid flow and the amount of cotton fiber is key to quality nitrocellulose. In the automatic control system for NC nitrification process of transformation, we designed a new type of stem seal way, successfully resolved the regulation of concentrated nitric acid and concentrated sulphuric acid mixed acid flow control valve stem seals.

At the top of the PTFE packing seal, we designed a cavity and in the cavity leads to inert gas (e.g. nitrogen or air), and inert gas pressure to slightly higher than the pipeline processes the maximum pressure of the media. In this way, the control valve in the course of using, even if valve with PTFE packing seal is broken, because the gas pressure to slightly higher than the pipeline technology media of the maximum pressure, strong corrosive, volatile and toxic gases cannot leak out. In order to guarantee the cavity pressure in gas stabilized to prevent gases from top to bottom along the stem, series gas in the lower part of the cavity, the PTFE packing is divided into upper and lower section, in the middle with a stainless steel diaphragm cushion, Viton o-ring embedded within the stainless steel diaphragm cushion pad from top to bottom, with PTFE packing be clamped. Void at the top of the bonnet is also embedded in the Viton o-rings, bolted cover pressing. In General, Viton o-ring service life is longer, up to millions of times, as long as fluoro rubber o-ring seal not broken, the cavity pressure of inert gas in always higher than the pipeline technology media of the maximum pressure, therefore, the control valve stem seal structure is very reliable and economic.

The sealed structure of control valves in 2001 successfully used for a certain company nitrification process automatic control system, has realized the nitrification process, production process automation, control valves run so far, never happens as controls the valve stem packing seal damage resulting in concentrated nitric acid and concentrated sulphuric acid mixed acid leakage along control valve stem.