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Failure analysis and solution of hard seal ball valve

2024-10-12
Hard sealed ball valve in the use of conditions than the soft seal ball valve is much more stringent, high temperature, strong corrosion and dust, particles, mud and other media on the continued use of hard sealed ball valve caused considerable obstacles. During the use of hard sealed ball valves, valve failures are often forced to be repaired due to working conditions or human reasons. Understanding the causes of valve failures and finding ways to prevent them is necessary and effective for valve industry workers and product users.
This paper focuses on the analysis of hard sealed ball valve. And from the problems found in the process of customer use, expand the problems that should be paid attention to in the design of hard sealed ball valve products and ancillary equipment, and put forward the direction of improvement.
From the perspective of product use classification, hard seal ball valve failures can be roughly divided into the following two categories: non-technical failures and technical failures.


First, non-technical failures

This kind of failure is not related to technology, that is, there is no problem with the internal structure of the valve or the operation of the internal parts. Therefore, the cause of the failure is generally caused by external conditions.
1. Valve selection error
Some customers in the selection of valve model, due to the lack of understanding of the product working conditions, or in cost considerations, the selected valve can not meet the working conditions of the working temperature, working pressure, medium corrosion, erosion requirements, resulting in the valve once put into use, the rapid failure.
Therefore, in the selection process, the customer or the design institute must put the reliability of the use of the product in the first place, because the cost of choosing too low-end products, failure caused damage, but not worth the loss. This is particularly important in the selection of hard sealed ball valves under harsh conditions.
The material selection of ball seat sealing surface spraying carbide surface is wrong
The key to the performance of hard sealed ball valve is the surface hardening treatment of steel ball and seat. In order to ensure the metal to metal seal, the sealing pressure between the hard seal ball seat is much greater than that of the ordinary ball valve, which makes the metal material prone to bite or strain, both of which will directly lead to the normal use of the ball valve. At present, the most widely used, the best effect of ball seat surface hardening method is spraying carbide technology (mainly nickel base alloy and chromium base alloy), hard sealed ball valve ball and seat coating to a large extent determines the performance and life of the ball seat sealing surface, so in the actual complex working conditions, to choose the carbide layer according to the actual situation.
The choice of cemented carbide layer has two directions: first, the pursuit of its hardness, high hardness alloy layer wear resistance, erosion resistance is strong, often used in the presence of small, hard solid particle medium conditions (such as coal chemical industry, polysilicon industry); The second is to pursue its corrosion resistance, some highly corrosive media, corrosion resistant alloy layer can effectively protect the internal substrate of the ball seat, thereby extending the life, often used in highly corrosive media conditions (such as petrochemical, paper industry). In the selection of hard sealed ball valve materials, the material of spraying carbide layer on the ball seat sealing surface is the most important thing, and the hard alloy that does not match the working condition and the use of medium conditions must not be selected.
② The selection of some inner parts of the valve is inconsistent with the working temperature
In the hard sealed ball valve, the selection of some internal parts is closely related to the temperature of its working condition or the temperature of the medium, such as packing, sealing gaskets, valve stem jacket and so on. The choice of hard sealed ball valve generally has two reasons, one is the working temperature is high, the soft seal seat can not withstand, the other is the medium abrasion ability, erosion ability is strong, soft seal seat is not hard enough. It can be seen that the hard sealed ball valve is not equal to the high temperature ball valve, in some conditions, its use temperature is not necessarily very high.
When the medium temperature is lower than 200 ° C, the packing, sealing gasket (including the body cover sealing gasket, lower valve stem sealing gasket, middle flange sealing gasket, etc.), valve stem sheath, etc., can be selected to use synthetic plastics. This configuration is similar to the configuration of related accessories in soft seal ball valves.
When the medium temperature is higher than 200 ° C, the filler needs to use graphite (graphite temperature resistance is strong, the performance is basically unchanged at high temperature), the sealing gasket is generally used to wrap the metal wire around the graphite ring (sealed with graphite, wire shape), and the valve stem sheath should also be replaced with metal material.
Therefore, in the selection process of hard sealed ball valves, the operating temperature or medium temperature must be used as one of the necessary factors to determine the selection direction. Wrong selection of internals, light will cause cost waste, heavy will lead to valve failure, can not be used.
2. The selection failure of accessories
The accessory parts of the valve include a driving device, a supporting device of the driving device, and a bracket connecting the driving device, a connecting shaft and other accessories. The drive device of the hard sealed ball valve can be divided into: lever handle, steel pipe handle, turbine actuator, handwheel clutch reducer (used to switch manual operation or automatic operation), automatic actuator (including pneumatic actuator, electric actuator, electro-hydraulic actuator, etc.) according to its power source.
For lever handles, steel pipe handles, gear mechanisms and other manually operated drive devices, calculate the torque according to the working torque value of the product, and then choose the appropriate length or specifications, of course, the torsional tensile strength of the material also needs to be considered.
The selection of automatic actuators is relatively complicated. Take the pneumatic actuator as an example, the pneumatic actuator is divided into single acting type and double acting type. Double-acting pneumatic actuator switches have spring force action, and the torque is consistent. Single-acting pneumatic actuator is opened to overcome the spring action, closed by the spring force, so that the output torque is a range of torque value, increasing the difficulty of selection, general installation of pneumatic actuators, need to work on the valve torque value based on a safety factor, Prevent the occurrence of insufficient operating force due to the variable range of actuator output torque.
Generally, when leaving the factory, the actual working torque value can not be accurately measured before the actual working condition, and the commonly used standard is to estimate the working torque value according to the no-load torque value multiplied by a medium coefficient. For hard sealed ball valves, the medium coefficient should be selected according to the different working conditions, if it is a particle, dust, mud medium, the coefficient should be obtained as large as possible, generally in the range of 1.5 ~ 2 choice.
In addition to the selection of the actuator itself needs attention, its accessories should also be selected according to the actual situation, such as the relevant solenoid valve, pressure reducing valve, valve position switch, etc., such as a large intake air actuator, but equipped with a small air flow solenoid valve, so that the air inlet of the pneumatic actuator will be slow, the valve is closed slowly, and the sealing surface will be corroded longer. Not conducive to valve life. Moreover, the significant advantage of the valve of the automatic device is that the opening and closing time is fast, and many customers require the opening and closing action to be completed within N seconds, which puts higher requirements on the reasonable configuration of relevant accessories.
3. Other non-technical faults
In addition to the non-technical faults caused by the above common selection errors, in the actual use of hard sealed ball valves, due to the installation direction, steps, etc., are not in accordance with the specifications, may also produce use failures, so in the use of the valve, should follow the instructions, the connection position is connected to ensure that there is no mistake.

Second, technical failures

The non-technical faults mentioned above are caused by human factors, from subjective reasons, if it is prudent enough or experienced enough, it is completely avoidable, and the cause of the failure has nothing to do with the internal structure of the valve. And technical failures. As the name suggests, it is due to the current hard ball valve technology is not perfect, many times we can only try to reduce the impact of it, and it is difficult to completely eliminate. Of course, if a hard-sealed ball valve with a service life of 3 months can extend its life to half a year after troubleshooting or prevention, even if the fault cannot be completely eliminated, it can already be considered a success.
1. Leakage caused by damage of sealing surface
For ball valves, and even for valves, the sealing surface is the most important part that affects the work of the entire valve. From the working principle of the ball valve, the ball valve is a opening and closing part, and its operation process is the process of opening or closing the steel ball on the valve seat. If the damage of the sealing surface is caused in the process of opening or closing, the sealing surface may leak, resulting in the internal leakage of the valve, and the valve is invalid.
① Reasonable selection of valve seat spring
The biggest difference between hard sealed ball valves and soft sealed ball valves is that the steel ball and the seat are metal materials, different from the soft sealed steel ball is metal, and the seat is non-metal. Therefore, the sealing mechanism is also different, soft seal ball valve sealing principle is: The sealing force between the steel ball and the seat causes the non-metal seat to deform, transforming the uneven line contact between the seat and the steel ball into contact. At the same time, the soft seal seat is squeezed and deformed, and further fits with the surface of the steel ball to compensate the uneven surface of the steel ball. If the extrusion deformation of the steel ball and the seat is larger or smaller, the sealing performance is not affected. The sealing principle of the hard sealed ball valve is that the sealing force between the steel ball and the seat is difficult to make the metal seat deform, so it is necessary to use a larger sealing force (compared to the soft sealed ball valve) to make the sealing surface of the seat deform, so as to achieve the fit of the steel ball and the seat, so as to achieve the seal, but this deformation is only subtle, if the extrusion pressure of the steel ball seat changes slightly, It may not be sealed. It is not difficult to draw two conclusions from the difference between the two mechanisms: first, the extrusion pressure between the steel ball and the seat of the hard sealed ball valve is larger; Second, the adjustment function of the hard seal ball valve is weaker than that of the soft seal ball valve.
Because the extrusion pressure between the hard sealed ball valve and the seat is larger, the sliding friction between the metal ball and the metal seat occurs when the ball valve is switched on and off. According to the mechanics of materials, the sliding friction between the metals with similar hardness is likely to strain the sealing surface (produce scratches on the sealing surface). In hard-sealed ball valves, spring (disc spring or column spring) is generally added after the seat to compare the pressure for fine tuning. In this way, the sealing force can be achieved within a certain range to a certain extent.
The adjustment function of the valve seat pressure ring
The adjustment function of the spring is analyzed above. The elasticity and compression of the spring are linear or conic functions, and the range of effective values is very short, so it is only adjusted in a very small range. When the valve leaks and is much higher than or below the standard valve position, it needs to be adjusted by the thickness of the seat pressure ring installed behind the seat.
③ Sealing surface leakage caused by working conditions
In fact, for the role of hard sealed ball valves, the biggest problem is not a technical problem, even if the design is precise enough, in the actual use of the process, there will still be some problems, this problem generally appears in the coal chemical industry, polysilicon industry, the common feature of these conditions is that the medium is not composed of a single gas or liquid, which has a large number of solid particles, These solid particles cause strong erosion to the runner of the steel ball and the valve seat, and even enter the sealing surface through the runner entrance, which can cause fatal damage to the sealing surface.
The valve repair caused by this situation accounts for more than 20% of the maintenance. The steel ball and seat sealing surface of the ball valve are very demanding, once the scratch exceeds 0.1mm, there is basically no possibility of grinding again, and once this situation occurs, it is often scrapped at the same time. Therefore, we should try to reduce this situation to a minimum.
Two steps on both sides of the sealing surface of the valve seat are repaired, that is, the "double scraper" structure. The steps on both sides are like two scrapers. In the process of opening and closing the steel ball, the solid particles adsorbed on the steel ball can be effectively scraped to avoid them entering the sealing surface. Of course, this valve seat is generally only used in the case of more solid particles, for water and steam media, the scraper structure reduces the strength of the valve seat, and increases the processing cost, it is not necessary.
2. Excessive torque value
According to statistics, the sealing surface leakage accounts for 20% of the total failure, and the problem of excessive torque accounts for at least 40% of the probability of failure. Hard tight ball valve due to its special sealing mechanism, the torque is 3-4 times larger than the same specification soft seal ball valve, in the mud, dust and other adsorption capacity of the medium working conditions, the solid liquid mixture adsorbed on the outside of the steel ball and seat sealing surface, resulting in the steel ball opening and closing torque is great, and even "stuck" phenomenon, if forced to open and close, even cause the valve stem twisted.
There is no perfect solution. In most cases, it is disassembled and repaired, scraping away the mud. In terms of prevention, we can start from the following aspects:
① "double scraper" structure, scrape the mud away from the sealing surface.
② The shape of the runner mouth should be streamlined as far as possible, and the corners are easier to accumulate sediment.
③ The use of automatic drive, one is to speed up the opening and closing speed, reduce mud erosion, adhesion sealing surface time. The second is the safety effect, because the torque of the drive is capped, when the steel ball is stuck, it will not be forced to break the valve stem or cut the sealing surface.

Third, the conclusion

To sum up, the hard sealed ball valve production precision requirements are high, the working condition is harsh, in order to make the hard sealed ball valve can be less failure, maintain a long life, need to grasp the following aspects:
1. The design and selection should be combined with the actual working conditions, and the appropriate material and structure should be selected.
2. The production process should strictly grasp the accuracy, and the factory should do every step of testing.
3. Strictly follow the production instructions during use.
4. Check and maintain frequently. If there is a problem, repair or replace it in time to avoid safety accidents.
Hard sealed ball valve in the current petroleum, chemical and other industries more and more widely used, but also gradually towards the direction of large specifications, high pressure, in the actual production of use, will encounter a variety of problems to be solved, I believe that every valve industry workers will feel full of challenges and opportunities.
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