Floating Ball Valve NPT Ends
There are many kinds of floating ball valve npt endss, but the basic function is the same, that is, to connect or cut off the medium flow. Therefore, the sealing problem of the floating ball valve npt ends is very prominent.
To ensure that the floating ball valve npt ends can cut off the medium flow well and no leakage occurs, it is necessary to ensure that the sealing of the floating ball valve npt ends is intact. There are many reasons for floating ball valve npt ends leakage, such as unreasonable structure design, defective sealing contact surface, loosening of fastening parts, poor coordination between floating ball valve npt ends body and floating ball valve npt ends cover, and so on. All these problems may lead to poor floating ball valve npt ends sealing and leakage problems. Therefore, the floating ball valve npt ends sealing technology is an important technology related to the performance and quality of the floating ball valve npt ends, which needs systematic and in-depth study.
Floating ball valve npt ends sealing technology has also undergone great development since its emergence. So far, floating ball valve npt ends sealing technology is mainly embodied in two aspects, namely static seal and dynamic seal.
The so-called static seal usually refers to the seal between two static surfaces. The main sealing method of static seal is using gaskets.
The so-called dynamic seal mainly refers to the seal of the floating ball valve npt ends stem, that is, the medium in the floating ball valve npt ends does not leak with the movement of the floating ball valve npt ends stem. The main sealing method of the dynamic seal is to use the packing letter.
1. static seal
Static seal refers to the formation of a seal between two stationary sections. The main sealing method is to use gaskets. There are many kinds of gaskets. The gaskets often used include flat gaskets, O-rings, wrapped gaskets, special-shaped gaskets, wavy gaskets and winding gaskets. Under each type, they can be further divided according to the different materials used.
(1) Flat washer. Flat washer is a kind of washer which is flat and sticks between two static sections. Generally, according to the materials used, it can be divided into plastic flat washer, rubber flat washer, metal flat washer and composite flat washer. Each kind of material has its applicable scope.
(2) O-ring. O-ring refers to gasket with O-shaped cross-section. Because of its O-shaped cross-section, it has a certain self-tightening effect, so the sealing effect is better than that of flat gasket.
(3) Pack gaskets. Packing gasket refers to the gasket which wraps one material on another material. Such gasket generally has good elasticity and can enhance the sealing effect.
(4) Special-shaped gaskets. Special-shaped gaskets refer to those gaskets with irregular shapes, including elliptical gaskets, diamond gaskets, gear gaskets, swallow-tail gaskets, etc. These gaskets generally have self-tightening effect, mostly used in high and medium pressure floating ball valve npt endss.
Wave washer. Wave washer is only a washer with wave shape. This kind of washer is usually composed of metal and non-metal materials. It generally has the characteristics of small compression force and good sealing effect.
_Wrap gasket. Winding gasket refers to the gasket formed by tightly wrapping thin metal belt and non-metal belt. This kind of gasket has good elasticity and sealability.
The gasket manufacturing materials mainly include three categories: metal materials, non-metal materials and composite materials. Generally speaking, metal materials have high strength and high temperature resistance. Commonly used metal materials are copper, aluminium, steel and so on. There are many kinds of non-metallic materials, including plastic products, rubber products, asbestos products, hemp products and so on. These non-metallic materials are widely used and selected according to specific needs. There are many kinds of composite materials, including laminates, composite plates and so on. They are also selected according to specific needs. Generally, waveform washers and winding washers are used more.
2. dynamic seal
Dynamic seal refers to the seal that does not let the medium flow in the floating ball valve npt ends leak with the movement of the floating ball valve npt ends stem. This is a sealing problem in the relative movement process. The main sealing method is to use packing box. There are two basic forms of stuffing box, i. e. gland type and nut type. Cover type is the most commonly used form at present. Generally speaking, it can be divided into two types: combination type and integral type. Although each form is different, it basically contains bolts for pressing. Compression nut type is generally used for smaller floating ball valve npt endss. Compression force is limited because of its small size.
In the stuffing box, because the stuffing is directly in contact with the floating ball valve npt ends stem, all the requirements of the stuffing have good sealing, small friction coefficient, can adapt to the pressure and temperature of the medium, and corrosion resistance. At present, the commonly used fillers include rubber O-ring, PTFE braided packing, asbestos packing and plastic forming packing. Each filler has its own suitable conditions and scope, and can be selected according to specific needs.
Sealing is to prevent leakage, so the principle of floating ball valve npt ends sealing is also studied from the prevention of leakage. There are two main factors causing leakage, one is the most important factor affecting the sealing performance, that is, there is a gap between the sealing pairs, and the other is the pressure difference between the two sides of the sealing pairs. Floating ball valve npt ends sealing principle is also analyzed from four aspects: liquid sealing, gas sealing, leakage channel sealing principle and floating ball valve npt ends sealing pairs.
The sealing property of liquids is achieved by the viscosity and surface tension of liquids. When the capillary leaking from the floating ball valve npt ends is filled with gas, the surface tension may repel the liquid or introduce the liquid into the capillary. So the tangent angle is formed. When the tangent angle is less than 90 degrees, the liquid will be injected into the capillary tube, which will lead to leakage. Leakage occurs because of the different properties of the medium. Tests with different media will yield different results under the same conditions.
You can use water, air or kerosene. Leakage also occurs when the tangent angle is greater than 90 degrees. Because it is related to grease or waxy film on metal surface. Once the film on these surfaces is dissolved, the characteristics of the metal surface change. The previously excluded liquid will penetrate the wet surface and leak. In view of the above situation, according to Poisson's formula, the purpose of preventing leakage or reducing leakage can be achieved by reducing capillary diameter and medium viscosity.
According to Poisson's formula, gas sealing is related to gas molecule and gas viscosity. Leakage is inversely proportional to capillary length and gas viscosity, and to capillary diameter and driving force. When the diameter of the capillary is the same as the average degree of freedom of the gas molecule, the gas molecule will flow into the capillary with free thermal motion. Therefore, when we do the floating ball valve npt ends sealing test, the medium must use water to play the role of sealing, using air or gas can not play the role of sealing.
Even if we reduce the diameter of the capillary to below the gas molecule by plastic deformation, the flow of the gas can not be stopped. The reason is that gases can still diffuse through metal walls. So when we do the gas test, we must be more rigorous than the liquid test.
The floating ball valve npt ends seal consists of two parts: the irregularity scattered on the waveform surface and the waviness of the distance between peaks. Under the condition that the elastic strain force of most metal materials in our country is low, if we want to achieve the sealed state, we need to put forward higher requirements for the compressive force of metal materials, that is, the compressive force of materials should exceed its elasticity. Therefore, when designing the floating ball valve npt ends, the sealing pair is matched with a certain hardness difference. Under the action of pressure, it will produce a certain degree of plastic deformation sealing effect.
If all the sealing surfaces are metal materials, the uneven protrusions will appear earliest, and plastic deformation of these uneven protrusions can be produced initially with only a small load. When the contact surface increases, the surface roughness will become plastic-elastic deformation. At this time, the roughness of both sides in the concave will exist. It is necessary to apply a load which can cause serious plastic deformation of the underlying material and make the two surfaces contact closely. Only along the continuous line and circumferential direction can these remaining paths be closed.
Floating ball valve npt ends seals are the part of seats and closures that are closed when they are in contact with each other. The metal sealing surface is vulnerable to the damage of clamping medium, medium corrosion, abrasion particles, cavitation and erosion during its use. For example, wear particles. If the unevenness of wear particles is smaller than that of the surface, the surface accuracy of the wear particles will be improved without deterioration when the sealing surface runs in. On the contrary, the surface accuracy will be deteriorated. Therefore, when selecting wear particles, it is necessary to take into account the material, working conditions, lubricity and corrosion of the sealing surface.
Like wear particles, when we choose seals, we should consider all kinds of factors affecting their performance in order to play a leak-proof function. Therefore, it is necessary to select materials that are corrosion resistant, scratch resistant and erosion resistant. Otherwise, without any requirement, the sealing performance will be greatly reduced.
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