There are five types of flange sealing surfaces. They are FF, RF, FM, M, T, G, and RTJ (RJ).
However, according to the working conditions, media, pressure, specifications, temperature, and other circumstances, the types we can choose are not the same.
Here we come together to get to know these flange sealing surface types. As they are an important factor for valve production and selection.
What’s the FF-Flat Face Full Face？
This type has an entirely flat sealing surface. Therefore, it is suitable for applications where the pressure is not high and the medium is non-toxic.
What’s the RF-Raised Face？
We can use the sealing surface in several most extensive types. First, it is the most commonly used one. In the international standard with the European system and the domestic standard, it is fixed height. But in the American Standard flange, we also need to pay attention to the high-pressure height that will increase the height of the sealing surface. And the use of gaskets also has a variety of types.
Many gaskets apply to the sealing surface of the flange. For example, there are various non-metallic flat gaskets, clad gaskets, metal-clad gaskets, wound gaskets (including with outer ring or with inner and outer ring), etc.
What’s the M/FM–Male Face and Female Face？
The two sealing face types, Male Face and Female Face, are a female and one male pair. Therefore, we must use them together. It facilitates alignment during installation and also prevents the gasket from being extruded. Consequently, they are suitable for applications with high pressures.
There are many sealing gaskets for which the male face and female face sealing surfaces are suitable. For example, there are various non-metallic flat gaskets, clad gaskets, metal-clad gaskets, basic or wound gaskets with inner rings, etc.
What’s the T/G–Tongue Face and Groove Face？
The Tongue Face and Groove Face are similar to the Concave and Convex Face. They are also a male and a female pair of sealing faces. They are also used in pairs. The gasket is located in an annular recess and is restricted by the metal wall on both sides. Due to pressure deformation, it prevents the gasket from being squeezed into the pipe. As a result, the gasket does not come into direct contact with the fluid medium in the line. It also suffers less from leaching or corrosion of the liquid medium. Therefore, we can use them in high pressure, flammable and explosive, toxic media, etc., where the sealing requirements are more stringent. This sealing surface gasket has good alignment performance. However, the sealing surface is challenging to process. We can easily damage the sealing surface when replacing the gasket. And we have to separate the flange in the axial direction during installation and disassembly. Therefore we have to consider the possibility of separating the flanges in the axial direction when designing the pipeline.
There are many gaskets available with tongue and groove sealing surfaces. Examples include metal and non-metal flat gaskets, metal-clad gaskets, and basic wound gaskets.
What’s the RTJ–Ring Joint Face？
Ring joint surface sealing flange also belongs to the narrow surface flange. A ring-shaped trapezoidal groove is cut into the flange’s protruding face as the flange sealing surface. It is the same as the tongue and groove face flange. You must separate these flanges axially during installation and dismantling. Therefore, the possibility of separating the flange in the axial direction is taken into account when designing the pipeline.
This sealing surface is specifically designed to fit solid metal gaskets machined from metal materials in an octagonal or oval shape. It enables to make a sealed connection. The sealing performance of this sealing surface is good because the metal ring gasket can be based on the inherent properties of the various metals. It also has less stringent installation requirements. It is suitable for high temperature and high-pressure service conditions. However, the sealing surface is machined to a high degree of precision.
Solutions for matching flange sealing surfaces to gaskets
If we want the gasket to seal successfully, we must control the level of defects on the surface of the flange with which the gasket is to be used. For example, we should avoid surface defects such as large scratches, grooves, or abrasions. It is because it is difficult to seal or fill these defects. The surface finish can be described by concepts such as “roughness, grain, waveform.”
Roughness is calculated as the average of protrusions and depressions from the center of the flange surface. Therefore, we can express it in parts per million inches (or meters). For serrated flange surfaces, the typical roughness is 125-500 microinches. And for non-serrated flange surfaces, the roughness is 125-250 microinches. With flanges that are too rough, the gasket often fails to seal. Therefore, we can only try to use softer gaskets.
On the other hand, flange surfaces with a very high finish are also tough to seal. We also have to avoid, for example, a polished surface. The sealing surface needs to be sufficiently “nibbled” to provide sufficient friction. It prevents the gasket from being blown out or extruded or from excessive creep.
The grain is the dominant direction of the flange surface roughness pattern. Most of these are as follows: multidirectional, record spiral, serrated, etc. The surface finish pattern should, as far as possible, coincide with the centerline of the gasket. A round flange, for example, should have its surface grain machined in a concentric circle or record spiral pattern. It will avoid having a line through the flange surface. If there is a straight grinding line through the sealing surface, it will be a direct leak path.
The waveform is the deviation relative to the overall flatness. Under normal conditions, we do not need to consider waviness. However, we need to pay special attention to the following two situations. Too many waveforms are also difficult to handle.
The first is in the case of glass-lined equipment. In this case, the natural flow of the molten glass creates a large waveform. Also, the glass-lined flange tends to deform at higher bolt torques. The solution is to use thicker and highly compressible gaskets.
The second situation is that of warped flanges. We can rework the flange if this warpage is due to thermal deformation or internal stress. We generally call this ” bowing ” if the warpage is caused by excessive bolt loading or short flange thickness, we generally call this “bowing.” Again, the solution is to redesign the flange with greater stiffness. Sometimes, we can reinforce the structure instead of replacing the flange. Another option is adding more bolts or changing from a small and large bolt solution to a large and small solution. It increases the elasticity of the bolts and allows the joint to bond better.
When you select a suitable valve, please consider the right seal surface for the valve flange.