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1, the classification of the ball valve
Ball valve is divided into two categories: floating, fixed (see Figure 1)
a, floating ball valve
b, fixed ball valve
Figure 1 Floating, fixed ball valve structure
1.1, floating ball valve
Ball valve ball is floating, under the media pressure, the ball can produce a certain displacement and squeeze the sealing surface of the outlet side, to ensure that the ball valve seal. Floating ball valve structure is simple, good sealing, but the ball to withstand the load of the working medium all to the export seal, so to consider whether the seal material can withstand the working load of the ball medium. This structure is widely used in low pressure ball valve.
1.2, fixed ball valve
Ball valve ball is fixed, no pressure to move. Fixed ball valve with a floating valve seat, subject to media pressure, the valve seat to move so that the ring is pressed on the ball in order to ensure the seal. Usually with the ball on the lower shaft equipped with bearings, operating torque is small, suitable for high pressure and large diameter of the valve. In order to reduce the operating torque of the ball valve and increase the reliability of the seal, in recent years there have been oil seal ball valves, both in the sealing surface injection of special lubricants to form a layer of oil film, which enhances the tightness and reduces operating torque , More suitable for high pressure large diameter ball valve.
This paper mainly on the floating large-diameter ball valve seal opening and closing moments for shallow discussion, hoping to help the application of the ball valve.
2, floating ball valve seal
Floating ball valve structure is simple, good sealing, but the ball to withstand the load of the working medium are all passed to the export seal, the seal material can withstand the ball of the media work load is the key. In general, seals are made of polytetrafluoroethylene and other elastic materials, metal and non-metallic materials to form a sealing pair, which is commonly known as the soft seal. Sealing the soft seal easy to ensure, but also on the sealing surface of the machining accuracy and surface roughness requirements are not high. Under the pressure of the medium, the ball displaces to the outlet end and compresses the valve seat. At this moment, the force Q MJ of the ball on the valve seat is:
Formula 1
Where: D MW - valve seat sealing surface diameter
D MN - Seat sealing surface inside diameter
P-media working pressure
In order to ensure low-pressure seal, the pre-tightening pressure of the ball valve and the valve seat should not be less than 1200N · M, the pre-tightening seal mainly depends on the elasticity of the seat itself, so the width of the sealing surface is limited by the formula :
Formula 2
Where: [q] - allowable stress (Teflon allowable stress 1530N · M)
For a valve seat with a resilient element, its operational reliability and service life depend very much on the pre-compression force of the correct choice of sealing seat.
Under low pressure, under the condition of insufficient pre-tightening force, the sealing of the ball valve can not be guaranteed. The transitional compression will cause an increase of the friction torque between the ball and the valve seat and may cause the plastic deformation of the valve seat material.
The gap δ1 between the valve seat and the valve body groove is the basic condition to ensure the normal operation of the ball valve. When selecting the gap δ1, the uniform elongation of the pre-compressed valve seat must be considered within the range of the total elastic deformation. For this purpose,
Formula 3
Where: D MP - average seat diameter; σ MJ - allowable stress on the cross-section of the valve seat in the elastic deformation range, for PTFE seat, σ MJ = 820 (NM); Elastic modulus, seat made of Teflon, E = 48000 ~ 81600 (NM). Seat sealing materials are generally used PTFE, in the existing plastics, chemical stability is the strongest polytetrafluoroethylene, its stability and chemical stability even more than the precious metal gold and platinum, glass, ceramics, Enamel, special steel and alloy. Chemically corrosive substances - concentrated acid, dilute acid, concentrated lye, the strongest oxidant can not play any role in the PTFE, a "king of plastic division," said. In addition to some amine halide or aromatic hydrocarbon polytetrafluoroethylene plastic swell a slight phenomenon, the ketones, acids, esters and other organic solvents have no effect on the PTFE, will not occur weight changes and swelling . It can only play a role in the molten alkali metal, chlorine trifluoride and elemental fluorine, but only under high temperature and high pressure was significant. In addition, PTFE is not affected by oxygen or ultraviolet light, non-absorbent, non-flammable.
It has a high heat resistance and cold resistance, the use of temperature range -180 ~ 250 ℃, 250 ℃ 240h treatment, no reduction in mechanical properties, 390 ℃ per hour 0.006% weight loss. Its excellent dielectric properties, and electrical properties are not affected by the frequency and temperature, long-term work at 250 ℃. It can be seen from the above, with PTFE as a ball valve seal is more ideal. Teflon elastic modulus value is very large, therefore, the material properties determine its elastic deformation is small, but the long-term use, the material's elasticity is not enough to make up for sealing surface wear, then the ball valve seal will be destroyed Therefore, considering how to structurally increase its elasticity when designing the valve seat, we chose a flexible structure (see Figure 2).
Figure 2 Seat ring
This valve seat has the following characteristics:
(1) sealing surface has an angle of 40 ° ± 1 ';
(2) seal the bottom of the valve seat has a semi-circular groove R2;
(3) with the valve body leaving a gap δ1 flexibility can be increased.
The principle of the structure is as follows: During the assembly without pretightening force, the ball is tangent to point A, the pre-tightening force is adopted to displace the ball and the displacement is generally between 0.2 and 0.4 (mm). When the ball is under pressure, Close to the sealing surface, then the ring outward expansion, so that the sealing surface and the ball will not be too tight and cause torque increases, after the pressure relief, sealing the valve seat is basically restored to the original installation position.
Since the tight seal, is a more special form of seal, I plant the production of large-diameter ball valve, stem seal all adopt the structure. Working principle is: the use of media pressure self-tight seal, and its sealing ring mounted inside the cone, with the media side of an angle, the media pressure transmitted to the cone, to pass to the ring, cone at a certain angle The surface produces two components, one with the valve body center line parallel to the outside, the other pressure to the valve body wall, behind the force is self-tightening force, the greater the media pressure, the greater the self-tightening, taking into account the low pressure Of the seal, in the self-tight seal above must increase a certain pre-tightening force, to ensure its sealing performance.
3, the ball valve torque
The size of the ball valve friction not only depends on the working pressure of the medium and the diameter of the valve, but also on the form of the ball valve, the structure of the sealing and supporting components and the nature of the sealing material. In addition, in each specific structure, Friction also depends on the ball valve rotation angle.
Torque at normal differential pressure: The maximum value occurs when the ball valve opens at 30 ° and is zero at full open (90 °). In fact, when the ball valve is turned to 80 °, the pressure drop is usually much less than the maximum value. When closing the ball valve when the maximum pressure drop, with its opening degree increases and gradually decreases, so under the conditions of constant pressure, the maximum torque generated when the rotation to 30 ° ~ 40 °. Ball valve work can be divided into two main stages.
(1) From the start of the ball to the beginning of the opening of the ball valve phase, this phase of the ball acts only on the frictional force generated by the preload in the sealing element and the static pressure of the working medium.
(2) From the beginning to open the valve to the hole and the valve channel completely coincide stage, this stage of static pressure differential friction, but also due to the liquid or gas medium force force. With the valve closed, the ball will block the through hole, so that the fluid on the ball will reduce the force and differential pressure increases.
4, the total friction torque calculation
The friction torque of the valve is calculated as follows:
M F = M QZ + M FT + M FC (NM) (4)
Where: MQZ-ball in the seat in the friction torque
M FT - Frictional torque of packing and stem
M FC - Frictional torque on the stem head
Analysis of the force generated by the ball valve shows that the ball valve under the action of the medium, the floating ball displacement to the outlet, all the force generated by the outlet valve seat ring to withstand the ball valve seat ring does not withstand the force, but also Does not affect the friction torque.
Formula 5
Where: f-coefficient of friction, for Teflon ring f = 0.5
Frictional torque of the valve stem in the packing depends on the form of the seal and the size of the working pressure. During the pressing, the upper and lower sealing rings are subjected to uniform pressure. The friction in the packing does not depend on the rotation angle of the ball. Therefore, Friction torque according to the following formula:
M FT = πd F h 1 Z 1 1.2P f (NM) (6)
Where: d F - stem diameter
h1-single ring packing and stem contact height
Z1-packing circle
Stem head of the friction torque can be the following formula:
Formula 7
Where: d F , d FT - stem diameter calculated from the above, the ball valve opening and closing torque is determined by many aspects, for diameter ≤ 100mm, the use of pressure within 1.6Mpa ball valve, the opening and closing torque Can not exceed 60N-M. The above analysis shows that the floating ball valve has its limitations, due to structural reasons, large-diameter high-pressure will appear torque multiplied. Therefore, we must choose a fixed structure of the ball valve, we will further explore its performance.
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