Common problems and failure analysis of the hottes

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Common problems and fault analysis in the use of valves

common problems in the use of valves

first, the double seat valve is easy to oscillate when working with a small opening

for a single core, when the medium is flow open, the stability of the valve is good; When the medium is flow closed, the stability of the valve is poor. The double seat valve has two valve cores, the lower valve core is in flow closed and the upper valve core is in flow open. In this way, when working with a small opening, the flow closed valve core is easy to cause the vibration of the valve, which is the reason why the double seat valve cannot be used for working with a small opening

second, the double seal valve cannot be used as a shut-off valve

the advantage of the double seat valve spool is the force balance structure, allowing a large differential pressure, while its prominent disadvantage is that the two sealing surfaces cannot be in good contact at the same time, resulting in large leakage. If it is artificially and forcibly used in cutting off occasions, the effect is obviously not good. Even if many improvements have been made for it (such as double seal sleeve valve), it is not desirable

III. the stem of the straight stroke regulating valve is thin

it involves a simple mechanical principle: large sliding friction and small rolling friction. When the valve rod of the straight stroke valve moves up and down, and the packing is slightly compressed, it will wrap the valve rod tightly, resulting in a large return difference. For this reason, the valve stem is designed to be very small, and PTFE packing with small friction coefficient is often used for packing, so as to reduce the return difference, but the problem is that the valve stem is thin, it is easy to bend, and the service life of packing is also short. The best way to solve this problem is to use the travel valve stem, that is, the angle stroke regulating valve. Its stem is 2 ~ 3 times thicker than the straight stroke stem, and graphite packing with long service life is selected. The stem stiffness is good, the packing life is long, and its friction torque is small and the return difference is small

IV. what is the poor anti blocking performance of the straight stroke regulating valve? The anti blocking performance of the angular stroke valve is good.

the valve core of the straight stroke valve is vertical throttling, and the medium flows in and out horizontally. The flow channel in the valve cavity must turn upside down, making the flow path of the valve quite complex (such as the inverted "s" shape). This 1. In some experimental machine parts design manuals and experimental machine manufacturing monographs, there are many dead zones, which provide space for the precipitation of the medium. In the long run, it will cause blockage. The throttling direction of the angle travel valve is the horizontal direction. The medium flows in and out horizontally, which is easy to take away the dirty medium. At the same time, the flow path is simple, and there is little space for medium sedimentation, so the angle travel valve has good anti blocking performance

v. the cut-off pressure difference of angle stroke valve is large

the cut-off pressure difference of angle stroke valve is large because the resultant force produced by the medium on the valve core or valve plate has a very small torque on the rotating shaft, so it can withstand a large pressure difference

VI. the service life of rubber lined butterfly valve and fluorine lined diaphragm valve used in desalted water medium is short

the desalted water medium contains low concentration of acid or alkali, which is corrosive to rubber. The corrosion of rubber is characterized by expansion, aging and low strength. The use effect of rubber lined butterfly valves and diaphragm valves is poor. The essence is that rubber is not corrosion resistant. The rear rubber lined diaphragm valve is improved to a fluorine lined diaphragm valve with good corrosion resistance, but the diaphragm of the fluorine lined diaphragm valve cannot be folded up and down and is broken, causing mechanical damage and shortening the service life of the valve. Now the best way is to use special ball valve for water treatment, which can be used for 5 ~ 8 years

VII. The shut-off valve should be hard sealed as far as possible

the shut-off valve requires that when analyzing the current situation, the leakage must be further strengthened. The lower the confidence, the better. The leakage of the soft seal valve is the lowest. Of course, the shut-off effect is good, but it is not wear-resistant and has poor reliability. Judging from the double standards of small leakage and reliable sealing, the soft seal cut-off is not as good as the hard seal cut-off. For example, the full-function ultra light regulating valve is sealed and stacked with wear-resistant alloy protection, with high reliability and a leakage rate of 10 ~ 7, which can meet the requirements of the shut-off valve

VIII. Sleeve valve replaced single and double seat valve, but it did not succeed.

sleeve valve, which came out in the 1960s, was widely used at home and abroad in the 1970s. Sleeve valve accounted for a large proportion of petrochemical devices introduced in the 1980s. At that time, many people believed that sleeve valve could replace single and double seat valve and become the second generation product. Up to now, this is not the case. Single seat valve, double seat valve and sleeve valve are all used equally. 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 the same as those of the single and double seat valve. How can it replace the single and double seat valve? Therefore, it can only be used together

IX. model selection is more important than calculation

compared with model selection, model selection is much more important and complex. Because calculation is only a simple formula calculation, it does not lie in the accuracy of the formula itself, but in the accuracy of the given process parameters. The initial clamping force involved in the selection is too small. A little carelessness will lead to improper selection, which will not only cause a waste of human, material and financial resources, but also the use effect is not ideal, resulting in a number of use problems, such as reliability, service life, operation quality, etc

X. piston actuators will be used more and more in pneumatic valves.

for pneumatic valves, piston actuators can make full use of the air source pressure, making the size of the actuator smaller and the thrust larger than the membrane type. The O-ring in the piston is also more reliable than the membrane, so it will be used more and more

what are the common faults of valves

1. Stuffing box leakage

this is the main aspect of running, emitting, dripping and leakage, which is often seen in factories

the causes of stuffing box leakage are as follows:

① the stuffing is incompatible with the corrosivity, temperature and pressure of the working medium; ② The filling method is wrong, especially when the whole filler is put in spirally, it is most likely to leak; ③ The processing accuracy or surface finish of the valve rod is not enough, or there is ovality or scoring; ④ The valve rod has pitted or rusted due to lack of protection in the open air; ⑤ The valve rod is bent; ⑥ The filler has been used for too long and aged; ⑦ The operation is too fierce

the methods to eliminate packing leakage are: ① correct selection of packing; ② Fill according to the correct; ③ If the processing of the valve rod is unqualified, it should be repaired or replaced. The minimum surface finish should reach ▽ 5, and more importantly, it should reach more than ▽ 8 without other defects; ④ Take protective measures to prevent corrosion, and replace those that have been corroded; ⑤ The bent valve rod shall be straightened or renewed; ⑥ The filler should be replaced after a certain period of time; ⑦ Pay attention to stable operation, slow opening and slow closing, to prevent sudden temperature change or medium impact

2. Leakage of closing parts

usually, the leakage of stuffing box is called external leakage, and the leakage of closing parts is called internal leakage. The leakage of the closing part is not easy to find in the valve

the leakage of closing parts can be divided into two categories: one is the leakage of sealing surface, and the other is the leakage of sealing ring root

the causes of leakage are: ① the sealing surface is not well ground; ② The sealing ring is not tightly matched with the valve seat and disc; ③ The valve disc is not firmly connected with the valve rod *; ④ The valve rod is bent and twisted, so that the upper and lower closures are misaligned; ⑤ Close too fast, the sealing surface is not in good contact or has been damaged; ⑥ The material is improperly selected and cannot withstand the corrosion of the medium; ⑦ The stop valve and gate valve are used as regulating valves. The sealing surface cannot withstand the erosion of high-speed flowing medium; ⑧ Some media will gradually cool down after the valve is closed, causing fine cracks on the sealing surface and erosion; ⑨ Some sealing surfaces are connected with valve seats and valve clacks by threads, which is easy to produce oxygen concentration cells and cause corrosion and looseness; ⑩ Because welding slag, rust, dust and other impurities are embedded, or mechanical parts in the production system fall off and block the valve core, the valve cannot be closed tightly

preventive measures are:

① pressure and leakage test must be carried out carefully before use. If leakage is found on the sealing surface or at the root of the sealing ring, it should be handled before use; ② Check whether all parts of the valve are in good condition in advance, and do not use the valve whose valve stem is bent or twisted or the valve disc cannot be connected with the valve stem; ③ When the valve is closed tightly, it should be firm, not strong. If it is found that the contact between the sealing surfaces is poor or blocked, it should be opened immediately for a while to let the debris flow out, and then close it carefully; ④ When selecting valves, we should consider not only the corrosion resistance of the valve body, but also the corrosion resistance of the closing parts; ⑤ According to the structural characteristics of the valve, it should be used correctly, and the components that need to adjust the flow should use the regulating valve; ⑥ If the medium is cooled and the temperature difference is large after closing the valve, close the valve again after cooling; ⑦ When the valve seat, disc and sealing ring are connected by threads, polytetrafluoroethylene tape can be used as the filler between threads to make it free of gaps; ⑧ For valves that may fall into impurities, a filter should be added in front of the valve

3. Valve rod lifting failure

the reasons for valve rod lifting failure are: ① excessive operation causes thread damage; ② Lack of lubrication or lubricant failure; ③ The valve rod is bent and twisted; ④ Insufficient surface finish; ⑤ The fitting tolerance is inaccurate and the bite is too tight; ⑥ The valve rod nut is inclined; ⑦ Improper material selection, for example, the valve rod and valve rod nut are of the same material, which is easy to bite; ⑧ The thread is corroded by the medium (refers to the Non rising stem valve or the valve with the stem nut at the lower part); ⑨ The open-air valve is lack of protection, and the valve rod thread is covered with dust, or corroded by rain, dew, frost and snow

prevention methods:

① operate carefully. Don't use force when closing, and don't reach the top dead center when opening. After opening enough, turn the handwheel back oneortwo turns to make the upper side of the thread close, so as to avoid the upward impact of the valve stem driven by the medium; ② Regularly check the lubrication condition and maintain the normal lubrication state; ③ Do not use a long lever to open and close the valve. Workers who are used to using a short lever should strictly control the degree of force to prevent twisting the valve stem (referring to the valve with the handwheel directly connected to the valve stem); ④ Improve the processing or repair quality to meet the specification requirements; ⑤ The material should be corrosion-resistant and adapt to the working temperature and other working conditions; ⑥ The valve rod nut shall not be made of the same material as the valve rod; ⑦ When plastic is used as the valve stem nut, the strength should be checked, not only considering good corrosion resistance and small friction coefficient, but also considering the strength. If the strength is not enough, do not use it; ⑧ Open air valves shall be equipped with valve stem protective sleeve; ⑨ When the valve is normally open, the handwheel should be turned regularly to prevent the valve stem from rusting

4. Other

gasket leakage: the main reason is that it is not corrosion-resistant and does not adapt to the working temperature and pressure; There are also temperature changes of high-temperature valves. Prevention method: adopt gaskets suitable for working conditions. For new valves, check whether the gasket material is suitable. If not, replace it. For high-temperature valves, tighten the bolts again during use

valve body cracking: generally caused by freezing. When it is cold, the valve should be provided with thermal insulation and heat tracing measures, otherwise the water in the valve and connecting pipeline should be drained after shutdown (if there is a plug at the bottom of the valve, the plug can be opened for drainage)

hand wheel damage: caused by impact or heavy operation of long lever. As long as the operators and other relevant personnel pay attention, it can be avoided

the packing gland is broken: the force is uneven when pressing the packing, or the gland (usually cast iron) is defective. When pressing the packing, rotate the screws symmetrically without deflection. During manufacturing, attention should be paid not only to large and key parts, but also to secondary elements such as glands, otherwise the use will be affected

the connection between the valve rod and the valve plate fails: the gate valve adopts many forms of connection between the rectangular head of the valve rod and the T-shaped groove of the ram, and sometimes the T-shaped groove is not processed, so the rectangular head of the valve rod is worn faster. Mainly from the aspect of manufacturing. But the use order

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