Hard seal ball valves in the use of working conditions than soft seal ball valves to be a lot more demanding, high temperature, strong corrosion and dust, particles, slurry and other media on the hard seal ball valves continue to use a considerable obstacle. In the use of hard seal ball valves, often because of working conditions or man-made reasons lead to valve failure and forced to repair. Understand the causes of valve failure, and find ways to prevent failure, for the valve industry workers and product users, are necessary and effective.

This paper is targeted to analyze the hard seal ball valve. And from the customer use process found in the problem, expand the hard seal ball valve product design and ancillary equipment should pay attention to the problem, and put forward the direction of improvement.

Classified from the point of view of the use of the product, hard seal ball valve failure can be roughly divided into the following two categories: non-technical failures and technical failures.

NO1, non-tec7  hnical failures

    This type of failure is not related to technology, that is, not the internal structure of the valve problems or internal parts operation failure. Therefore, the cause of the failure is generally caused by external conditions.

1.1Valve Selection Errors

Some customers in the selection of valve models, due to a lack of understanding of the product use conditions, or in the cost considerations, the choice of the valve can not meet the conditions of the operating temperature, operating pressure, media corrosivity, erosion requirements, resulting in the valve once put into use, the rapid failure.

Therefore, in the selection process, the customer side or the design institute must put the reliability of the product in the first place, because the cost reasons for choosing too low-end products, resulting in damage caused by failure, but the loss is not worth it. This is particularly important in the selection of hard seal ball valves in harsh working conditions.

1.1.1Ball seat sealing surface sprayed carbide surface selection error

The key to the performance of hard seal ball valves lies in the surface hardening treatment of the steel ball and seat. In order to ensure metal-to-metal sealing, the sealing ratio pressure between the hard seal ball seat is much greater than ordinary ball valves, which makes it easy to bite or strain between the metal materials, both of which will directly lead to the ball valve can not be used normally. Currently the most widely used, the best effect of the ball seat surface hardening method is spraying carbide technology (mainly nickel-based alloy and chromium-based alloy), hard seal ball valve steel ball and seat spraying layer largely determines the use of the performance and life of the ball seat sealing surface, so in the actual complex conditions, according to the actual situation to choose the carbide layer.

The choice of hard alloy layer has two directions: one is the pursuit of its hardness, high hardness alloy layer wear-resistant, erosion-resistant performance is strong, commonly used in the presence of fine, hard solid particles of the media conditions (such as coal chemical industry, polysilicon industry); the second is the pursuit of its corrosion-resistant performance, some of the strong corrosive media, corrosion-resistant alloy layer can effectively protect the internal base material of the ball seat to extend the life of the ball seat, commonly used in the strong corrosive medium Conditions (such as petrochemical, paper industry). Hard seal ball valve material selection, its ball seat sealing surface sprayed carbide layer of material is the most important, must not choose and working conditions, the use of media conditions do not match the carbide.

 1.1.2 Valve some internal parts of the material selection and operating temperature does not match the

Hard seal ball valve, there are some internal parts of the material selection is closely related to its use of temperature or medium temperature, such as packing, sealing gasket, stem sheath, etc.. Selection of hard seal ball valves generally have two reasons, one is the working temperature is high, soft seal valve seat can not withstand, the second is the medium abrasion ability, erosion ability is strong, soft seal valve seat is not hard enough. Visible hard seal ball valve is not equal to high temperature ball valves, in some cases, it is not necessarily very high temperature.

When the medium temperature is lower than 200℃, packing, sealing gaskets (including body cap sealing gasket, lower stem sealing gasket, center flange sealing gasket, etc.), stem sheath, etc. can be selected with synthetic plastic. This configuration is similar to the configuration of related accessories in soft seal ball valves.

When the medium temperature is higher than 200 ℃, packing need to choose graphite (graphite temperature resistance, high temperature performance is basically no change), sealing gasket is generally used metal wire wrapped in graphite ring periphery (sealed with graphite, metal wire shaping), the valve stem sheath should also be replaced with metal materials.

Therefore, in the selection process of hard seal ball valves, the working condition temperature or medium temperature is a must as one of the necessary factors to determine the direction of selection. Wrong selection of internal parts, light cause cost waste, heavy lead to valve failure, can not be used.

1.2Attachment Selection Failure

Valve accessories include drive device, drive device supporting devices and connect the drive device bracket, coupling shaft and other accessories. Hard seal ball valve drive device according to its power source can be divided into: lever handle, steel handle, turbine actuator, handwheel clutch reducer (used to switch manual operation or automatic operation), automatic actuator (including pneumatic actuators, electric actuators, electro-hydraulic actuators, etc.).

For lever handle, steel pipe handle, gear mechanism and other manually operated drive device, according to the working torque value of the product to calculate the torque, and then select the appropriate length or then specifications can be, of course, the torsional and tensile strength of the material also needs to be considered.

The selection of automatic actuators is relatively complex. Take the pneumatic actuator as an example, pneumatic actuators are divided into single-acting type and double-acting type. Double-acting pneumatic actuator switch have spring force, torque consistent. Single-acting type pneumatic actuator open to overcome the spring elasticity effect, closed by the spring force to close, so that its output torque is a range of torque values, increasing the difficulty of selection, general installation of pneumatic actuators, the need to work in the valve based on the torque value, multiplied by a safety factor, to prevent due to the range of changes in the output torque of the actuator to appear the occurrence of insufficient operating force.

Generally in the factory, not to the actual working conditions, its actual working torque value can not be accurately measured, the common standard is based on the unloaded torque value multiplied by a medium coefficient to estimate its working torque value. For hard seal ball valves, according to the different working conditions to choose the media coefficient, if it is particles, dust, slurry media, this coefficient should be made as large as possible, generally in the range of 1.5 ~ 2 this choice.

In addition to the actuator itself need to pay attention to the selection, its ancillary accessories should also be selected according to the actual situation, such as the relevant solenoid valves, pressure reducing valves, valve position switches, etc., such as a large air intake of pneumatic actuators, but equipped with a very small air flow solenoid valves, so that the air intake of the pneumatic actuator will be very slow, the valve closure slows down the sealing surface by the erosion of the longer the service life of the valve is not conducive. Moreover, the significant advantage of the valve of the automatic device lies in the fast opening and closing time, many customers require that the opening and closing action be completed within N seconds, which puts forward higher requirements for the reasonable configuration of the relevant accessories.

1.3Other non-technical faults

In addition to the above common selection errors caused by non-technical failure, in the actual use of hard seal ball valves, due to the direction of installation, steps, etc. are not in accordance with the norms, may also produce the use of faults, so in the use of the valve, should follow the instructions, the position of the connection to connect, to ensure that everything is safe.

NO2Technical faults

The previously mentioned non-technical failures are caused by human factors, originating from subjective reasons, if it is careful enough or experienced enough, it is completely avoidable, and the cause of its failure and the internal structure of the valve does not have a relationship. And technical failures. As the name suggests, it is due to the current hardened ball valve technology is not perfect enough caused by many times we can only try to minimize the impact it brings, but it is difficult to completely eliminate. Of course, if a hard seal ball valve with a service life of three months, after troubleshooting or prevention, can extend the service life to half a year, then even if the failure can not be completely eliminated, it can already be regarded as a success.

2.1Leakage due to sealing surface damage

For ball valves, and even for valves, the sealing surface is the most important part of the valve affecting the work of the whole valve. From the point of view of the working principle of the ball valve, the ball valve is the opening and closing parts, the operation process is the steel ball on the valve seat to open or close the process, if in the process of opening or closing the sealing surface caused by the destruction of the sealing surface, then the sealing surface at the leakage may be caused, resulting in the internal leakage of the valve, the valve that is, the valve is failing.

2.1.1Reasonable selection of valve seat spring

The biggest difference between hard seal ball valve and soft seal ball valve is that the steel ball and seat are metal materials, unlike soft seal steel ball is metal, while the seat is non-metallic. Therefore, the sealing mechanism is also different, soft seal ball valve sealing principle is: the sealing force between the steel ball and the valve seat leads to deformation of the non-metallic valve seat, the valve seat and the steel ball between the uneven line contact into contact, while the soft seal valve seat is squeezed deformation, and the surface of the steel ball to further fit, to compensate for the surface of the steel ball at the unevenness of the steel ball, if the steel ball and the valve seat of the squeeze deformation of the larger or smaller, on the sealing The hard seal ball valve sealing principle 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 to the soft seal ball valve) to make the seat sealing surface deformation, so as to achieve the fit of the steel ball and the valve seat, so as to achieve sealing, but the deformation is only subtle, if the extrusion pressure of the steel ball seat is a little bit of a change, then may be Can not be sealed. From the difference between the two mechanisms it is not difficult to draw two conclusions: First, the hard seal ball valve ball and seat squeeze pressure is greater; Second, the hard seal ball valve for the sealing of the regulatory function is weaker than the soft seal ball valve.

Due to the hard seal ball valve ball and seat extrusion pressure is greater, in the ball valve for opening and closing conversion, the metal ball and metal valve seat sliding friction, according to the mechanics of materials can be learned, similar hardness of the metal sliding friction is likely to strain the sealing surface (in the sealing surface to produce scratches). In the hard seal ball valve, generally used in the valve seat after the spring (disc spring or column spring) to compare the pressure for fine-tuning. In this way, a certain degree of sealing force within a certain range, are able to achieve sealing.

2.1.2Adjustment of the seat pressure ring

Previously analyzed the role of the spring regulation, the spring's elasticity and compression is a linear or quadratic function of the relationship, and the effective value of the range is very short, so only in a very small range of adjustment. When the valve leakage and than the pressure is higher than the standard or lower than the standard valve position is large, need to be installed in the back of the valve seat seat pressure ring thickness to adjust.

2.1.3Sealing surface leakage due to operating conditions

In fact, for the role of hard seal ball valves, causing the most trouble is not a technical problem, even if the design is sophisticated enough, in the actual use of the process, there will still be some problems, this problem is generally found in the coal chemical industry, polysilicon industry, the common characteristics of these working conditions is that the medium is not made up of a single gas or liquid, in which there are a large number of solid particles, these solid particles on the steel ball and valve seat These solid particles on the ball and the valve seat of the runner port caused by strong scouring, and even through the runner port into the sealing surface, can cause fatal damage to the sealing surface.

    For this situation leads to the valve repair situation accounts for more than 20% of the repair volume. Sealed ball valve steel ball and seat sealing surface requirements are very high, once the scratch more than 0.1mm, basically there is no re-grinding the use of the possibility of using once this situation occurs, often is the ball and seat at the same time scrapped. So should try to minimize this situation.

    Seat sealing surface on both sides of the repair out of two steps, that is, "double scraper" structure, both sides of the steps like two scrapers, in the process of opening and closing the ball, can effectively scrape off the adsorption of solid particles on the ball to avoid its entry into the sealing surface. Of course, this valve seat is generally only used for working conditions with more solid particles, for water, steam media, scraper structure reduces the strength of the valve seat, and increase the cost of processing, it is not necessary.

2.2Excessive torque value

Statistically, the sealing surface leakage in the total number of failures accounted for 20%, while the torque is too large problem, in the failure of at least 40% of the chance. Hard seal ball valve due to its special sealing mechanism, the torque would have been 3-4 times larger than the same specification soft seal ball valve, in the mud, dust and other adsorption capacity of the media conditions, solid-liquid mixture adsorbed in the ball and seat sealing surface of the external, resulting in the ball opening and closing of the torque is very large, and even appear "stuck! "phenomenon, if forced to open and close, and even cause the valve stem twisted.

    Currently did not find a perfect solution. Most of the cases are disassembled and repaired, scrape off the mud. Prevention, you can start from the following aspects:

    ① "double scraper" structure, scraping mud, away from the sealing surface.

    ② The shape of the runner port is streamlined as much as possible, the corners are more likely to accumulate sediment.

    ③ automatic drive drive, one is to speed up the opening and closing speed, reduce the mud erosion, adhesion sealing surface time. The second is the role of safety, because the driver's torque is an upper limit, when the ball jammed, not forcibly twisted stem or cut the sealing surface.

NO3 concluding remarks

In summary, the hard seal ball valve production precision requirements, harsh working conditions, to make the hard seal ball valve can be less failure, to maintain a long life, need to grasp the following aspects:

    ① design, selection should be combined with the actual working conditions, choose the right material, structure.

    ② The production process should strictly grasp the accuracy, factory do every step of the test.

    ③The use of the process should be in strict accordance with the production instructions.

    ④ often do inspection and maintenance, if there is a problem timely repair or replacement, to avoid safety accidents.

    Hard seal ball valves in the petroleum, chemical and other industries are becoming more and more widely used, but also gradually towards the direction of large specifications, high pressure, in the actual production of the use of a variety of problems that need to be solved, I believe that every worker in the valve industry will feel full of challenges and opportunities.