Classification and technical indexes of the hottes

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Classification and technical index of large diameter ball valve

ball valve is one of the widely used valves. It appeared very early and was popularized and used late. Its working principle is the same as that of the plug valve. The valve is opened and closed by rotating the valve core, but it is lighter than the plug valve. It is relatively small in size and can be made into a large-diameter valve. The ball valve has reliable sealing, simple structure and convenient maintenance. The sealing surface and the ball are often closed, which is not easy to be eroded by the medium. At present, the caliber of foreign ball valves has reached 3050cm, which are widely used in petroleum, chemical industry, metallurgy, light textile, food, atomic energy, aviation and other departments

I. classification of ball valves:

ball valves are divided into two categories: floating and fixed (see Figure 1)

A, floating ball valve

b, fixed ball valve

Figure 1 floating and fixed ball valve structure

1. The ball of floating ball valve is floating. Under the action of medium pressure, the ball can produce a certain displacement and press tightly on the outlet end. If a loose sealing surface is found, the sealing of the ball valve can be guaranteed. The floating ball valve has simple structure and good sealing performance, but the load of the working medium borne by the ball is transmitted to the outlet sealing ring. Therefore, it is necessary to consider whether the sealing ring material can withstand the working load of the ball medium. This structure is widely used in medium and low pressure ball valves. 2. Fixed ball valve the ball of the ball valve is fixed and does not move after being pressed. Fixed ball valves are equipped with floating valve seats. After being pressed by the medium, the valve seats move, so that the sealing ring is tightly pressed on the ball to ensure sealing. Usually, bearings are installed on the upper and lower shafts of the ball, and the operating torque is small, which is suitable for high-pressure and large-diameter valves. In order to reduce the operating torque of ball valve and increase the reliability of sealing, oil sealed ball valve has appeared in recent years. It not only injects special lubricating oil between sealing surfaces to form an oil film, which not only enhances the sealing performance, but also reduces the operating torque, which is more suitable for high-pressure large-diameter ball valve. This paper mainly discusses the sealing and opening and closing torque of floating large diameter ball valve, hoping to be helpful to the application of ball valve. 2、 Technical indicators of large-diameter ball valve: 1. Sealing of floating ball valve. The floating ball valve has simple structure and good sealing performance, but the load of the working medium borne by the ball is transmitted to the outlet sealing ring. Therefore, whether the sealing ring material can withstand the working load of the ball medium is the key. (1) Generally speaking, sealing rings are made of elastic materials such as polytetrafluoroethylene, and metal and non-metallic materials form a sealing pair, which is usually called soft seal. The sealing performance of the soft seal is easy to ensure, and the requirements for the processing accuracy and surface roughness of the sealing surface are not very high. Under the pressure of the medium, the ball moves towards the outlet end and compresses the valve seat. At this time, the force QMJ of the ball pressing on the valve seat is:

Formula 1

where: DMW - outer diameter of the valve seat sealing surface

DMN - inner diameter of the valve seat sealing surface p - working pressure of the medium. In order to ensure the sealing at low pressure, the pre tightening specific pressure necessary for the ball valve and valve seat should not be less than 1200n. Because dust, like moisture, is a major killer of computers and other electronic products m, The pre tightening seal mainly depends on the elasticity of the valve seat itself, so the width of the sealing surface is limited to a certain extent. The calculation formula is:

formula 2

in the formula: [q] - allowable stress (the allowable stress of polytetrafluoroethylene is 1530n · m). For the valve seat with elastic elements, its working reliability and service life largely depend on the correct selection of the pre compression force of the sealing valve seat. At low pressure, under the condition of insufficient preload, the sealing of the ball valve will not be guaranteed. The excessive compression will increase the friction torque between the ball and the valve seat, and may cause the plastic deformation of the valve seat material. Clearance between valve seat and valve body groove δ 1. It is the basic condition to ensure the normal operation of the ball valve δ 1, it must be considered that the uniform extension of the pre compressed valve seat should be within the short service life of the total elastic deformation range. For this, the following:

formula 3

formula: DMP - average diameter of the valve seat; σ MJ - allowable stress of valve seat cross section within the range of elastic deformation, for PTFE valve seat, σ MJ=820(N-M); E - elastic modulus of valve seat material, for PTFE valve seat, e=48000 ~ 81600 (n-m). The sealing material of the valve seat is generally polytetrafluoroethylene. Among the existing plastics, polytetrafluoroethylene has the strongest chemical stability, and its stability to chemical action is even more stable than precious metal gold and platinum, glass, ceramics, enamel, special steel and alloy. Substances with strong chemical corrosivity - concentrated acid, dilute acid, concentrated lye, and the strongest oxidant can't play any role on polytetrafluoroethylene, which is known as the "king of plastics". Except that some halogenated amines or aromatic hydrocarbons cause slight swelling of polytetrafluoroethylene plastics, ketones, acids, esters and other organic solvents have no effect on polytetrafluoroethylene, and weight changes and swelling will not occur. It can only be affected by molten alkali metals, chlorine trifluoride and elemental fluorine, but it is only significant under high temperature and high pressure. In addition, polytetrafluoroethylene is not affected by oxygen or ultraviolet rays, and it does not absorb water and is non combustible

it has high heat resistance and cold resistance. The service temperature range is -180 ~ 250 ℃. After treatment at 250 ℃ for 240h, the mechanical properties do not decrease, and the weight loss per hour at 390 ℃ is 0.006%. Its dielectric properties are excellent, and its electrical properties are not affected by frequency and temperature. It can work for a long time at 250 ℃. It can be seen from the above that it is ideal to use polytetrafluoroethylene as the seal of ball valve. The elastic modulus value of polytetrafluoroethylene is very large. Therefore, the performance of the material determines that its elastic deformation is very small. Once it is used for a long time, the elasticity of the material is not enough to make up for the wear of the sealing surface. At this time, the sealing of the ball valve will be destroyed. Therefore, when designing the valve seat, we consider how to increase its elasticity from the structure, and we choose the elastic structure (see Figure 2)

Figure 2 valve seat sealing ring

this valve seat has the following characteristics: (1) there is an angle of 40 ° ± 1 'in the sealing surface; (2) There is a semicircular groove of R2 at the bottom of the sealing valve seat; (3) There is clearance left in the fit with the valve body δ 1 can increase elasticity. The structural principle is as follows: when assembling without preload, the sphere is tangent to point a, and the assembly preload causes the displacement of the ball shaft, which is generally between 0.2 and 0.4 (mm). When the sphere is under pressure, the sphere clings to the sealing surface with the pressure direction. At this time, the sealing ring expands outward, so that the sealing surface and the sphere will not be too tight, resulting in an increase in torque. After pressure relief, the sealing seat basically returns to the original installation position

self tightening seal is a relatively special sealing form. This structure is used for all valve stem seals of large-diameter ball valves produced by our factory. The working principle is: the medium pressure is used for self tightening sealing. Its sealing ring is installed outside the inner cone at a certain angle with the opposite side of the medium. The medium pressure is transmitted to the inner cone to transmit to the sealing ring. Two components are generated on the cone at a certain angle, one is parallel to the centerline of the valve body, the other is pressed to the inner wall of the valve body. The latter force is the self tightening force. The greater the medium pressure is, the greater the self tightening force is, Considering the sealing at low pressure, a certain preload must be added to the self tightening seal to ensure its sealing performance

2. Torque of ball valve. The magnitude of friction force of ball valve depends not only on the working pressure of medium and the diameter of valve, but also on the form of ball valve, the structure of sealing and supporting parts and the properties of sealing materials. In addition, in each specific structure, the friction force on the sealing seat also depends on the rotation angle of ball valve. Torque acting under normal differential pressure: when the ball valve is turned and opened to 30 °, the maximum value appears, and when it is fully opened (90 °), it is zero. In fact, when the ball valve is turned to 80 °, the differential pressure is usually much smaller than the maximum value. When the ball valve is closed, the pressure difference is the largest, and gradually decreases with the increase of its opening degree. Therefore, under the condition of constant pressure, the maximum force and torque will rotate to 30 ° ~ 4. The energy difference before and after the pendulum impacts the sample will be used to determine the resistance and brittleness of the sample at 0 °. The work of ball valve can be divided into two main stages. (1) From the start of the ball to the start of opening the ball valve, in this stage, the ball only acts on the friction generated by the preload in the sealing element and the static pressure of the working medium. (2) From the beginning of opening the valve to the stage when the ball hole completely coincides with the valve channel, the friction caused by the static pressure difference at this stage should be added with the force caused by the liquid or gas medium force. With the valve closed, the ball will block the through hole, so that the fluid action force on the ball will decrease and the differential pressure will increase. 3. Calculation of total friction torque the friction torque of the valve is calculated according to the following formula: mf=mqz+mft+mfc (n-m) (4) in the formula: MQZ - friction torque of the ball in the valve seat MFT - friction torque of the packing and valve stem MFC - friction torque of the valve stem head analysis of the force generated by the ball valve shows that under the action of the medium, the floating ball moves to the outlet end, and the force generated is borne by the valve seat sealing ring at the outlet, The sealing ring of the valve seat in front of the ball does not bear the force and does not affect the friction torque

formula 5

in the formula: F - friction coefficient. For PTFE sealing ring, f=0.5

the friction torque of the valve rod in the packing depends on the sealing form and the working pressure. When pressing, the upper and lower sealing rings bear uniform pressure, and the friction in the packing does not depend on the rotation angle of the ball, so, The friction torque of the packing is as follows: mft= π dfh1z11.2pf (n-m) (6), in which: DF - the diameter of the valve stem H1 - the contact height between a single ring of packing and the valve stem Z1 - the number of rings of packing. The friction torque of the valve stem head can be as follows:

formula 7

in which: DF, DFT - the calculated diameter of the valve stem head. From the above, it can be seen that the opening and closing torque of the ball valve is determined by many aspects. For the ball valve with a diameter ≤ 100mm and a pressure within 1.6Mpa, The opening and closing torque shall not exceed 60n-m. The above analysis shows that the floating ball valve has its limitations. Due to structural reasons, the torque will increase exponentially under large diameter and high pressure. Therefore, the ball valve with fixed structure must be selected, and we will further discuss its applicable performance

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