Common problems in valve use:
1. Double seat valve is easy to oscillate when working at small opening. For single core, when the medium is flow open type, the valve stability is good; when the medium is flow closed type, the stability of the valve is poor. The double seat valve has two spools, the lower spool is closed, and the upper spool is open. Thus, when working at a small opening, the flow-closed spool is liable to cause vibration of the valve. This is a double seat valve. Can not be used for small opening work.
Second, the double seal valve can not be used as a shut-off valve The advantage of the double-seat valve spool is the force balance structure, which allows the pressure difference to be large, and its outstanding disadvantage is that the two seal faces cannot be in good contact at the same time, resulting in large leakage. If it is artificially and forcibly used to cut off the occasion, it is obviously not effective, even if it has made many improvements (such as double-sealed sleeve valves), it is not advisable.
Third, the straight stroke control valve stem is thinner It involves a simple mechanical principle: large sliding friction, small rolling friction. The valve stem of the straight stroke valve moves up and down, and the packing is slightly pressed tightly, which will tightly seal the valve stem and produce a large backlash. For this reason, the valve stem is designed to be very small, and the filler is also commonly used with a small friction coefficient of PTFE filler to reduce the backlash, but the problem is that the valve stem is thin, the bend is easy, and the filler life is short. The best way to solve this problem is to use the travel valve stem, that is, the angular stroke type regulating valve. Its valve stem is 2 to 3 times thicker than the straight stroke valve stem, and the graphite packing with long life is selected. Well, the filler has a long service life, and the friction torque is small and the backlash is small.
Fourth, what is the straight-stroke regulating valve anti-blocking performance, the anti-blocking performance of the angular stroke valve is good. The straight-stroke valve spool is vertical throttling, and the medium is horizontally flowing in and out, the flow passage in the valve cavity must turn and turn, causing the valve The flow path becomes quite complicated (the shape is like the inverted "S" type). In this way, there are many dead zones that provide space for the precipitation of the medium, which in the long run causes blockage. The direction of the angular stroke valve throttling is the horizontal direction. The medium flows horizontally and horizontally, which makes it easy to take away the dirty medium. At the same time, the flow path is simple, and the space for the medium to precipitate is also small, so the anti-blocking performance of the angular stroke valve is good.
5. The cut-off pressure difference of the angular stroke type valve is large. The cut-off pressure difference of the angular stroke type valve is large because the resultant force of the medium on the valve core or the valve plate produces a very small torque to the rotating shaft, so it can withstand the comparison. Large pressure difference.
Sixth, the desalinated medium uses a rubber-lined butterfly valve, and the fluorine-lined diaphragm valve has a short service life. The desalted medium contains a low concentration of acid or alkali, which is highly corrosive to rubber. The corrosion of rubber is characterized by expansion, aging, and low strength. The use of rubber-lined butterfly valves and diaphragm valves is inferior in that the rubber is not resistant to corrosion. The back-lined rubber diaphragm valve is improved to a fluorine-lined diaphragm valve with good corrosion resistance, but the diaphragm of the fluorine-lined diaphragm valve can be broken by being folded up and down, causing mechanical damage and shortening the life of the valve. The best way to do this now is to use a special ball valve for water treatment, which can be used for 5 to 8 years.
7. The shut-off valve should use hard seal as much as possible. The shut-off valve requires the lower the leak, the better, the leakage of the soft seal valve is the lowest, the cutting effect is of course good, but it is not wear-resistant and the reliability is poor. From the double standard of small leakage, sealing and reliability, the soft seal cut is not as good as the hard seal cut. Such as full-featured ultra-light regulating valve, sealed and protected by wear-resistant alloy, high reliability, leakage rate of 10 to 7, has been able to meet the requirements of the shut-off valve.
Eight, the sleeve valve replaced the single and double seat valve but did not have the wish to pay for the sleeve valve that came out in the 1960s. It was widely used at home and abroad in the 1970s. In the petrochemical plant introduced in the 1980s, the ratio of the sleeve valve was large. Many people believe that the sleeve valve can replace the single and double seat valve and become the second generation product. This is not the case today, single-seat valves, double-seat valves, and sleeve valves are equally used. This is because the sleeve valve only improves the throttling form, stability and maintenance better than the single seat valve, but its weight, anti-blocking and leakage indicators are consistent with single and double seat valves. How can it replace single and double seat valves? What? Therefore, they can only be used together.
Nine, the selection is more important than the calculation. Compared with the selection, the selection is much more important and more complicated. Because the calculation is just a simple formula calculation, it is not in itself the accuracy of the formula, but rather whether the given process parameters are accurate. There are many contents involved in the selection. If you are not careful, it will lead to improper selection, which not only causes waste of manpower, material resources and financial resources, but also has unsatisfactory effect, which brings about some use problems such as reliability, longevity and operation. Quality and so on.
10. In the pneumatic valve, the piston actuator will be used more and more. For the pneumatic valve, the piston actuator can make full use of the air source pressure, so that the size of the actuator is smaller than the diaphragm type, the thrust is larger, and the piston is O. The ring is also more reliable than the film, so it will be used more and more.
What are the common faults in the valve?
1. Stuffing box leakage This is the main aspect of running, running, dripping and leaking, which is often seen in factories.
The reasons for the stuffing box leakage are as follows:
1 The filler is not compatible with the corrosiveness, temperature and pressure of the working medium; 2 the filling method is wrong, especially when the whole packing is hovered, which is the most likely to cause leakage; 3 the valve stem has insufficient processing precision or surface finish, or has ovality. Or nicks; 4 stem has been pitting, or rust due to lack of protection in the open air; 5 stem bending; 6 filler used for too long, has been aging; 7 operation is too fierce.
The method for eliminating the leakage of the packing is: 1 correct packing is selected; 2 is filled correctly; 3 if the valve stem is unqualified, it must be repaired or replaced, and the surface finish should be at least â–½5, and more importantly, â–½8 or more. And no other defects; 4 take protective measures to prevent corrosion, have to be replaced with rust; 5 stem bending should be straightened or updated; 6 filler after a certain period of time, to be replaced; 7 operation should pay attention to smooth, slow opening and closing, Prevent temperature drastic changes or media impact.
2, closure parts leakage Usually the stuffing box leakage is called leakage, the closing piece is called internal leakage. The closing piece leaks inside the valve and is not easy to find.
Closed parts leaks can be divided into two categories: one is the leakage of the sealing surface, and the other is the leakage of the root of the sealing ring.
The causes of leakage are: 1 the sealing surface is not well grounded; 2 the sealing ring is not tightly fitted with the valve seat and the valve disc; 3 the valve disc is not firmly connected with the valve stem; 4 the valve stem is twisted and twisted, so that the upper and lower closing members are not centered. ; 5 close too fast, the sealing surface is not well contacted or damaged early; 6 material selection is improper, can not withstand the corrosion of the medium; 7 use the shut-off valve, gate valve as a regulating valve. The sealing surface can not withstand the erosion of high-speed flowing medium; 8 some media, after the valve is closed, gradually cool, so that the sealing surface appears slit, it will also produce erosion; 9 some sealing surface and valve seat, valve Thread connection is easy to produce oxygen concentration battery, corrosion is loose; 10 due to impurities such as welding slag, rust, dust, etc., or mechanical parts in the production system fall off and block the valve core, so that the valve can not be closed.
The preventive measures are:
1Before using, you must carefully test the leak and find that the sealing surface leaks or the root of the sealing ring leaks. It should be used after being processed. 2 Check that the valve parts are in good condition, you can not use the valve stem to bend or the valve flap to connect with the valve stem. 3 valves should be closed; 3 valves should be tight to ensure stability, do not make violent, if you find that the contact between the sealing surfaces is not good or has obstacles, you should immediately open a little, let the impurities flow out, and then carefully close; 4 when using the valve Not only the corrosion resistance of the valve body should be considered, but also the corrosion resistance of the closing member should be considered. 5 According to the structural characteristics of the valve, the correct use should be made. The parts that need to adjust the flow rate should adopt the regulating valve; 6 the medium is cooled after the valve is closed. In the case of large temperature difference, the valve should be closed after cooling; when the valve seat, valve disc and sealing ring are screwed, the Teflon tape can be used as the filler between the threads to make it have no gap; Valves that may fall into the impurity should be fitted with a filter in front of the valve.
3, valve stem lifting failure valve stem lifting failure reasons are: 1 excessive operation caused by thread damage; 2 lack of lubrication or lubricant failure; 3 stem bending; 4 surface finish is not enough; 5 tolerance tolerance is not allowed, bite too Tight; 6 stem nut tilt; 7 material selection is not appropriate, for example, the stem and stem nut are the same material, easy to bite; 8 thread is corroded by the medium (refer to the dark rod valve or stem nut in the lower valve); The valve lacks protection, the stem threads are dusty, or rusted by rain, frost and snow.
Prevention method:
1 Careful operation, do not make it strong when closing, do not go to the top dead point when opening, turn the handwheel one or two times after opening enough, make the upper side of the thread close, so as to avoid the medium pushing the valve stem upward impact; 2 often check the lubrication situation, Maintain normal lubrication; 3 Do not use long lever to open and close the valve. Workers who are accustomed to using short levers should strictly control the force to prevent the valve stem from being twisted (refer to the valve directly connected to the hand wheel and the valve stem); Repair quality, meet specifications; 5 materials should be corrosion resistant, adapt to working temperature and other working conditions; 6 stem nut should not use the same material as the valve stem; 7 when using plastic as the stem nut, to check the strength, can not only consider Good corrosion resistance and low friction coefficient. Also consider the strength problem. Do not use if the strength is not enough. 8 Open the valve to add the valve stem cover; 9 Normally open the valve, turn the hand wheel regularly to avoid the valve stem rust.
4. Other gasket leakage: The main reason is not corrosion resistance, not suitable for working temperature and working pressure; and temperature change of high temperature valve. Precautions: Use gaskets that are compatible with the working conditions. Check the gasket material for new valves. If it is not suitable, replace it. For high temperature valves, tighten the bolts again during use.
Valve body cracking: caused by general freezing. When the weather is cold, the valve should have heat preservation and heat tracing measures. Otherwise, the water in the valve and the connecting pipe should be drained after the production is stopped (if there is a plug at the bottom of the valve, the wire plug can be opened).
Handwheel damage: caused by impact or long lever violent operation. As long as the operator and other relevant personnel pay attention, it can be avoided.
Fracture of the packing gland: uneven force when compacting the packing, or defective gland (usually cast iron). To compress the packing, rotate the screws symmetrically without being biased. When manufacturing, not only should pay attention to large and key parts, but also pay attention to secondary parts such as gland, otherwise it will affect the use.
The connection between the valve stem and the valve plate is unsatisfactory: the gate valve adopts a large number of square stems connected to the T-shaped groove of the gate, and the T-shaped groove sometimes does not process, so that the long square head of the valve stem wears faster. Mainly from the manufacturing side to solve. However, the unit can also be used to fill the T-shaped groove to give it a certain degree of finish.
The gate of the double gate valve cannot be pressed tightly. The tension of the double gate is generated by the top wedge. Some gate valves have poor top wedge material (low grade cast iron), which will wear or break soon after use. The top wedge is a small piece. The material used is not much. The unit can be made of carbon steel and replaced with the original cast iron.
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