Hard sealed ball valves are much more stringent than soft sealed ball valves in service conditions. High temperature, strong corrosion, dust, particles, mud and other media have caused considerable obstacles to the continuous use of hard sealed ball valves. In the process of using hard seal ball valve, the valve is often forced to be repaired due to failure caused by working conditions or human factors.

It is necessary and effective for valve industry workers and product users to understand the causes of valve failures and find ways to prevent them.

The technical researchers of our company specially analyze the problems of hard seal ball valves that commonly appear in the use of ball valves by market users. And expand the problems that should be paid attention to in the product design and accessory equipment of hard seal ball valve, and put forward the direction of improvement.

From the perspective of product use, the faults of hard sealed ball valves can be roughly divided into the following two categories: non-technical faults and technical faults. Then this paper will introduce the analysis and solutions of technical faults of hard seal ball valve;

1 technical fault

The non-technical faults described in the previous article are caused by human factors and come from subjective reasons. If they are careful enough or experienced enough, they can be completely avoided. The cause of the fault has nothing to do with the internal structure of the valve.;

The so-called technical fault. As the name suggests, it is caused by the current imperfect technology of hard tight ball valve. Many times, we can only try to reduce the impact it brings, but it is difficult to completely eliminate it. Of course, if a hard sealed ball valve with a service life of 3 months can extend its service life to half a year after troubleshooting or prevention, it can be considered successful even if the failure cannot be completely eliminated.

1.1 leakage caused by damage of sealing surface

For ball valves, even for valves, the sealing surface is the most important part that affects the work of the whole valve. From the working principle of the ball valve, the ball valve is a hoist, and its operation process is the opening or closing process of the steel ball on the valve seat. If the sealing surface is damaged during the opening or closing process, the sealing surface may leak, causing internal leakage of the valve, and the valve is invalid.

1.1.1 reasonable selection of valve seat spring

The biggest difference between hard seal ball valve and soft seal ball valve is that both the steel ball and the valve seat are made of metal, which is different from that the soft seal steel ball is made of metal and the valve seat is made of non-metal. Therefore, the sealing mechanism is also different. The sealing principle of the soft seal ball valve is that the sealing force between the steel ball and the valve seat causes the deformation of the non-metallic valve seat, transforming the uneven line contact between the valve seat and the steel ball into contact. At the same time, after the soft seal valve seat is extruded and deformed, it further fits with the surface of the steel ball to compensate for the bumps on the surface of the steel ball. If the extrusion deformation of the steel ball and the valve seat is larger or smaller, It has little effect on sealing performance; The sealing principle of the hard sealed ball valve is that the sealing force between the steel ball and the valve seat is difficult to deform the metal valve seat. Therefore, it is necessary to use a larger sealing force (compared with the soft sealed ball valve) to deform the sealing surface of the valve seat, so as to achieve the fit between the steel ball and the valve seat, so as to achieve sealing, but this deformation is only subtle. If the extrusion force of the steel ball valve seat changes slightly, it may not be sealed. It is not difficult to draw two conclusions from the difference between the two mechanisms: stunt valve

1、 The squeeze pressure between the steel ball and the valve seat of the hard sealed ball valve is greater;

2、 The regulating function of hard seal ball valve for sealing is weaker than that of soft seal ball valve.

Due to the greater squeeze pressure between the steel ball and the valve seat of the hard sealed ball valve, the sliding friction between the metal steel ball and the metal valve seat occurs when the ball valve is opened and closed. According to the mechanics of materials, it can be seen that the sliding friction between metals with similar hardness is likely to damage the sealing surface (scratches on the sealing surface). In hard sealed ball valves, spring (disc spring or column spring) is usually added behind the valve seat to compare the pressure for fine adjustment. In this way, the sealing force can be sealed within a certain range to a certain extent.

1.1.2 sealing surface leakage caused by working conditions

In fact, the biggest trouble caused by the function of hard sealed ball valve is not a technical problem. Even if the design is precise enough, there will still be some problems in the actual use process. This problem usually occurs in the coal chemical industry and polysilicon industry. The common feature of these working conditions is that the medium is not composed of a single gas or liquid, in which there are a large number of solid particles, These solid particles cause strong scouring to the port of the steel ball and valve seat, and even enter the sealing surface through the port of the port, which can cause fatal damage to the sealing surface.

The valve repair caused by this situation accounts for more than 20% of the maintenance volume. The steel ball and valve seat sealing surface of British sealed ball valve have high requirements. Once the scratch exceeds 0.1mm, there is basically no possibility of grinding and utilization. Once this happens, the ball seat is often scrapped at the same time. Therefore, this situation should be minimized.

Two ladders are built on both sides of the sealing surface of the valve seat, namely the "double scraper" structure. The ladders on both sides are like two scrapers. In the process of opening and closing the steel ball, it can effectively scrape off the solid particles adsorbed on the steel ball and prevent them from entering the sealing surface. Of course, this kind of valve seat is generally only used when there are many solid particles under working conditions. For water and steam media, the scraper structure reduces the strength of the valve seat and increases the processing cost, which is unnecessary.

1.1.3 adjustment function of valve seat pressure ring

The adjustment effect of spring was analyzed earlier. The relationship between spring elasticity and compression is linear or quadratic function, and the effective value range is very short, so it is only adjusted in a very small range. When the valve leaks and the pressure is higher than the standard or higher than the standard valve position, it needs to be adjusted by the thickness of the valve seat pressure ring installed behind the valve seat.

1.2 excessive torque value

According to statistics, the leakage of the sealing surface accounts for 20% of the total failure, and the problem of excessive torque accounts for at least 40% of the failure. Due to its special sealing mechanism, the torque of the hard sealed ball valve is 3-4 times larger than that of the soft sealed ball valve of the same specification. Under the working conditions of muddy, dust and other media with super strong adsorption capacity, the solid-liquid mixture is adsorbed on the outside of the sealing surface of the steel ball and valve seat, resulting in great torque when the steel ball is opened and closed, There is even a "stuck" phenomenon. If it is forced to open and close, it will even cause the valve rod to be twisted.

At present, there is no perfect solution. Most of the cases are disassembly and maintenance, scraping 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 channel mouth shall be streamlined as much as possible, and sediment is easier to accumulate at the edges and corners.

③ Automatic drive is adopted. First, speed up the opening and closing speed and reduce the time of mud erosion and adhesion of sealing surface. The second is the safety function, because the torque of the driver has an upper limit. When the steel ball is stuck, it will not forcibly twist the valve rod or cut the sealing surface.

2 Conclusion

To sum up, hard sealed ball valves require high production accuracy and poor working conditions. In order to reduce failures and maintain a long service life of hard sealed ball valves, we need to grasp the following aspects:

① The design and type selection should be combined with the actual working conditions, and appropriate materials and structures should be selected.

② The production process should strictly grasp the accuracy, and do a good job in every step of testing before leaving the factory.

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.

At present, hard sealed ball valves are more and more widely used in petroleum, chemical industry and other industries, and are gradually moving towards the direction of large specification and high pressure. In the actual production and use, there will be various problems to be solved. I believe that every worker in the valve industry will feel full of challenges and opportunities.