What are the main factors affecting flange sealing performance?
Bolt preload
An important factor affecting sealing. The preload force must make the gasket compression to realize the initial sealing.
Appropriately increasing the bolt preload can increase the gasket's sealing ability because increasing the preload can make the gasket in normal working conditions retain a larger contact surface pressure.
However, the preload should not be too large, otherwise, it will make the gasket overall yielding and loss of resilience, or even the gasket will be extruded or crushed.
In addition, the preload should be as uniform as possible on the gasket. Usually taken to reduce the bolt diameter, increase the number of bolts, take an appropriate pre-tightening method, and other measures to improve the sealing performance.
Gasket performance
A Gasket is an important element of the composition of the seal, the role of the gasket is to seal the gap between the two flange sealing surfaces, to prevent fluid leakage. Types of gaskets are non-metallic gaskets, non-metallic and metal combinations of gaskets, and metal gaskets.
Suitable gasket materials require gaskets in the appropriate preload that can produce the necessary elastic deformation, but not be crushed or extrusion;
When the distance between the flange sealing surface is stretched, the gasket material should have enough resilience, so that the gasket surface and the flange surface are in close contact, to continue to maintain good sealing performance; selection of gasket material should also consider the working medium and working temperature.
The width of the gasket is also an important factor affecting the sealing, the wider the gasket, the greater the required preload, and thus the larger the bolt and flange size requirements.
(1) rubber, asbestos rubber, PTFE, and other non-metallic gaskets are commonly used on medium and low-pressure equipment and piping flanges, which have better corrosion resistance and flexibility, but poorer strength and temperature resistance.
They are usually cut from the whole gasket sheet, the whole gasket shape is a circle, and the cross-section is rectangular.
(2) To improve the strength and heat resistance of the gasket, a thin steel strip and asbestos tape (or PTFE tape or flexible graphite tape) together with the winding made of twisted gaskets or in asbestos or other non-metallic materials outside the package made of metal sheet metal package gaskets.
It has a multi-channel sealing effect and good resilience, is utilized in higher temperature and pressure ranges, and can maintain good sealing under pressure and temperature fluctuation conditions, thus it is widely used.
Winding gaskets are made by winding a steel strip with a filler strip of asbestos PTFE or flexible graphite. To prevent loosening, the beginning and end of the metal strip are welded.
To increase the elasticity and resilience of the gasket, metal and non-metallic tape are rolled into a wave.
There are two kinds of wave shapes: V-shape and W-shape. The one shown in the figure below is V-shaped, and there are 4 structural types.
Type B - also known as the basic type, without reinforcing ring, for tongue and groove sealing surface.
Type B - with internal reinforcement ring, used for concave and convex sealing surfaces.
Type C - with the external reinforcing ring, used for flat sealing surfaces.
Type D - with internal and external reinforcing rings, used for flat sealing surfaces.
(3) Metal clad gasket is made of asbestos rubber sheet as the inner core, the outer thickness of 0.2 ~ 0.5mm thick thin metal sheet composition.
The material of the metal plate can be aluminum, steel, and its alloys, and can also use stainless steel or high-quality carbon steel. Metal-clad gasket is also only used for B-type flat welding and long neck welding on two types of flanges.
(4) On the flanges of high-pressure equipment and pipelines, metal gaskets are commonly used, and the materials are soft aluminum, copper, soft steel, and stainless steel. In addition to the rectangular cross-section of the metal gasket, there are also cross-sectional shapes of the oval or octagonal and other special shapes of the metal ring gasket.
When the operating pressure is very high or the leakage rate requirements are very strict, plus the temperature is very high or extremely corrosive, metal gaskets can be used.
Metal gasket specific pressure value is very large, to reduce the bolt force, the compression surface must be very narrow, relying on a very narrow compression surface to maintain good sealing, and must have a small surface roughness, Ra ≤ 2.5μm to Ra ≤ 0.63μm.
Sealing surface type
the flange gasket and press play a role in sealing the contact surface known as the flange sealing surface or compression surface, the choice of sealing surface type and operating conditions, the consequences of leakage and the nature of the gasket, the most common structural types are as follows.
1. Flat sealing surface:
Its structure is shown in the following chart (a), the sealing surface is not a smooth plane, in the plane is often open 2 ~ 4 concentric distribution of triangular cross-section grooves (i.e., flange waterline).
The Plane sealing surface structure is simple, easy to manufacture, and easy to carry out anti-corrosion lining. Secondly, the width of the sealing surface of this structure is larger, so the use of a non-metallic or metal soft gasket is often used.
However, after the bolt is tightened, the gasket material is easy to stretch to both sides. It is used for occasions where the required compression force is not high and the medium is non-toxic.
2. Concave sealing surface:
The structure of the sealing surface is shown in Fig. (b) below, which is equivalent to a pair of flat sealing surfaces of the flange, one of which is made with a raised platform of the compression surface, and this flange is called convex flange;
Another corresponding made into a concave surface is called a concave flange, and the concave surface of the same size as the gasket embedded in it, the gasket facilitates the centering. The height of the raised plane is slightly greater than the depth of the concave surface, with the bolt compression sealing effect.
This structure can limit the radial deformation of the gasket, which can prevent the gasket from being extruded, and will improve the sealing performance to a certain extent. Suitable for higher-pressure occasions.
3. Tongue and groove sealing surface:
In the middle of the width direction of a pair of flat sealing surfaces, one of which is made into a cross-section such as a tongue and a groove, another cross-section such as groove compression surface paired use, as shown in the following figure (c), the former is called tongue and groove flange, the latter is called groove flange.
Groove-type compression surfaces can limit the radial deformation embedded in the gasket, and sealing performance is good, at the same time, the gasket can be less subject to medium scouring and corrosion. However, the tongue and groove part is easy to destroy. Commonly used in flammable, explosive, toxic media and higher pressure occasions.
There are also trapezoidal groove sealing surfaces and conical sealing surfaces. The former with oval section ring metal gasket, and the latter with lens a type of ring metal gasket.
The Flange sealing surface of the form and surface properties of the sealing effect play a vital role. Flange sealing surface straightness, sealing surface, and the perpendicularity of the flange centerline directly affect the gasket force uniformity and gasket and flange good contact.
Flange stiffness
the flange stiffness is not enough to produce excessive warping deformation (as follows), which is often caused by the actual production of bolted flange connection sealing failure of one of the main reasons.
The flange with large stiffness has small deformation, which can transfer the bolt preload to the gasket uniformly, thus improving the sealing performance of the flange.
The roughness of the sealing surface of the flange should match the requirements of the gasket. The surface is not allowed to have radial knife marks or scratches, not to mention surface cracks.
Flange stiffness is related to many factors, among which increasing the thickness of the flange ring appropriately, reducing the diameter of the center circle of the bolt, and increasing the outer diameter of the flange ring can improve the flange stiffness.
The use of a necked flange or an increase in the size of the cone neck part can significantly improve the bending capacity of the flange. However, unprincipled improvement of flange stiffness will make the flange bulky and heavy, and increase the cost.
Operating conditions
Pressure, temperature, and the physical and chemical properties of the medium on the sealing performance of the impact are very complex, simple pressure and medium on the sealing effect is not significant.
However, the combined effect of temperature, especially the fluctuation of high temperature, will seriously affect the sealing performance, and even make the seal due to fatigue and complete failure.
Because at high temperatures, the viscosity of the medium is small, permeability is large, easy to leak; medium on the gasket and flange corrosion effect intensifies, increasing the possibility of leakage;
Flange, the sealing performance of the flange, bolts, and gaskets will produce a large high-temperature creep and stress relaxation so that the seal fails; some non-metallic gaskets will also accelerate aging, deterioration, or even burn.
