Non-technical failure analysis of hard seal ball valve - Wuzhong instrument

From the perspective of product application, hard seal ball valve defects can be roughly divided into the following two categories: non-technical faults and skill defects. The following text mainly introduces the analysis and solution of non-technical problems of hard seal ball valves;

Non-technical failure analysis of hard seal ball valve - Wuzhong instrument

Non-technical failure analysis of hard seal ball valve - Wuzhong instrument

1. Non-skill questions

Non-skill problems are literally caused by non-skill problems, that is, it is not a problem with the internal structure of the valve or a problem with the operation of internal parts. Therefore, the cause of the valve problem is generally caused by external conditions.

1.1 Fault in valve selection

Many customers in the purchase of valves because of the lack of understanding of the product operating conditions, perhaps in the cost of consideration, the selection of valves can not meet the working temperature, operating pressure, medium corrosiveness, erosion requirements under the working conditions, resulting in the valve once put into use immediately presented problems.

Therefore, in the selection process, the customer or the design institute must put the reliability of the product in the first place, and the harm caused by the failure of the selection of cheap valves outweighs the losses. This is especially important in the selection of hard-seated ball valves under severe operating conditions.

1.1.1 The selection of some internal parts of the valve does not match the working temperature

In the hard seal ball valve, the selection of some internal parts is closely related to the temperature of its application condition or medium temperature, such as packing, sealing gasket, valve stem sheath, etc. There are generally two reasons for choosing a hard seal ball valve, one is that the working temperature is high, and the soft seal seat is unacceptable, and the other is that the medium can be abrasive and erosion can be strong, and the soft seal valve seat is not strong enough. It can be seen that the hard seal ball valve is not equal to the high temperature ball valve, and in some working conditions, its operating temperature does not have to be very high.

When the medium temperature is higher than 200 °C, the packing needs to be graphite (graphite has strong temperature resistance, and the function is basically unchanged at high temperature), the sealing washer is generally wrapped in the periphery of the graphite ring (sealed with graphite, and the metal wire is shaped), and the stem sheath should also be replaced with metal materials.

When the medium temperature is lower than 200 °C, packing materials, sealing gaskets (including body cover sealing gaskets, lower stem sealing gaskets, middle flange sealing gaskets, etc.), valve stem sheaths, etc. can be selected as constituent plastics. This equipment is similar to that of the relevant fittings in the soft-seated ball valves.

Therefore, in the selection process of hard seal ball valve, the working condition temperature or medium temperature must be one of the necessary elements to determine the selection direction. The wrong selection of internal parts will lead to cost waste, and at worst it will lead to valve failure and cannot be used.

1.1.2 The sealing surface of the ball seat is sprayed with cemented carbide surface and the material selection is wrong

The key to the function of the hard-seated ball valve is the case hardening of the steel ball and the valve seat. In order to ensure the metal-to-metal sealing, the sealing specific pressure between the hard seal ball seats is much greater than that of ordinary ball valves, which makes it easy to bite or strain between metal materials, both of which will directly lead to the failure of the ball valve to operate normally. At present, the most widely used, the best effect of the ball seat surface hardening method is spraying cemented carbide skills (first of all, nickel-based alloy and chromium-based alloy), the steel ball and valve seat spraying coating of the hard seal ball valve largely determines the use of the ball seat sealing surface function and life, so in the practice of messy working conditions, to select the cemented carbide layer according to the practice.

There are two directions for the selection of cemented carbide layer: one is to pursue its hardness, and the high-hardness alloy layer is wear-resistant and corrosion-resistant, and is often used in the presence of fine and solid solid particle media conditions (such as coal chemical industry, polysilicon occupation); The second is the pursuit of its corrosion-resistant function, under some strong corrosive media, the corrosion-resistant alloy layer can effectively maintain the internal substrate of the ball seat, thereby extending the lifespan, and is often used in strong corrosive media working conditions (such as petrochemical, papermaking occupations). In the material selection of hard seal ball valve, the material of the cemented carbide layer sprayed on the sealing surface of the ball seat is the top priority, and the cemented carbide that does not match the working conditions and application medium conditions must not be selected.

1.2 Selection of subordination

The subordinate parts of the valve include the driving equipment, the supporting equipment of the driving equipment, and the brackets, connecting shafts and other accessories of the driving equipment. The driving equipment of the hard seal ball valve can be divided into lever handle, steel pipe handle, turbine actuator, handwheel dispersion reducer (used to switch manual operation or active operation), active actuator (including pneumatic actuator, electric actuator, electro-hydraulic actuator, etc.).

Regarding lever handles, steel pipe handles, gear structures and other drive equipment that rely on manual operation, calculate the torque according to the operating torque value of the product, and then select the appropriate length or specification, of course, the torsional tensile strength of the material is also considered.

The selection of active actuators is a bit more messy. Take pneumatic actuators as an example, pneumatic actuators are divided into single-effect type and double-effect type. Dual-effect pneumatic actuator switches have a spring force effect and consistent torque. When the pneumatic actuator of the single effect type is opened, it is necessary to overcome the spring elastic effect, and when it is closed, it is closed by the spring force, so that its output torque is a range torque value, which increases the difficulty of selection, and the pneumatic actuator is generally installed, and the operation torque value of the valve is multiplied by a safety factor on the basis of the operating torque value of the valve, so as to avoid the situation that the output torque of the actuator presents insufficient operating force because of the range change.

Generally, when leaving the factory, the practical working torque value cannot be accurately measured before the practical working conditions, and the commonly used standard is to estimate the operating torque value according to the no-load torque value multiplied by a medium coefficient. Regarding the hard seal ball valve, the medium coefficient should be selected according to the different working conditions, if it is particle, dust, mud medium, this coefficient should be as large as possible, generally in the range of 1.5-2.

In addition to the actuator's own selection needs, its subordinate accessories should also be selected according to the practical situation, such as related solenoid valves, pressure reducing valves, valve position switches, etc., such as a pneumatic actuator with a large air intake, but equipped with a solenoid valve with a small air flow, so that the air intake of the pneumatic actuator will be very slow, the valve closure will be slow, and the longer the time when the sealing surface is eroded, which is not conducive to the service life of the valve. Moreover, the significant advantage of the valve of the active equipment 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 forward higher requirements for the reasonable equipment of related accessories.

1.3 Other non-skill-based questions

In addition to the above common selection mistakes caused by non-technical faults, in the practical application of hard seal ball valves, because the installation direction and process are not in accordance with the standards, there may also be application problems, so in the process of using the valve, the instructions should be followed, and the direction of the connection should be well connected to ensure that nothing goes wrong.

Non-technical failure analysis of hard seal ball valve - Wuzhong instrument

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