Questions about the applicable scope of flexible graphite gaskets
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Among many gaskets, the applicable nominal pressure range often only has an upper limit, while flexible graphite gaskets (and high-temperature mica gaskets) also have a lower limit, as shown in the figure below (Figure 1 HG/T 20606 Table 3.3； Figure 2 NB/T47024 Table 1). What is the basis for determining this lower limit? Is it considered that in situations where the pressure is less than this, the flange and fasteners cannot be sealed due to the low grade? So can this lower limit be ignored after the flange calculation passes? Everyone is welcome to discuss.
The calculation should pass. It’s fine. PN6 is rarely used. The lower limit of PN10 is low enough.
Flexible graphite is a composite plate and requires a large pre-tightening force. Low-grade flanges often do not have necks. Excessive pre-tightening force will cause excessive flange deformation and cannot guarantee the sealing effect.
Therefore, in situations where the gasket coefficient is large and the specific pressure is high, necked flanges and studs are generally used. The studs can provide sufficient pre-tightening force, and the necked flange can ensure sufficient stiffness to achieve sealing.……
For flange sealing issues, wrapping gaskets can be used under low pressure conditions. If there is no problem, it will be fine. If there is a problem and there is no standard support, it will be easy to take the blame. It is not safe to just talk about the non-standard calculation of flanges without considering the empirical reference in the standard. The simplest example is to select high-strength bolts for low pressure. The calculation can be passed with a very small number of bolts, but the bolt spacing will be too large, and sealing may not be achieved.
Bolt spacing standards give ranges. The J coefficient can be calculated for the integral flange and the flange calculated as an integral flange for stiffness check. However, the stiffness of loose flange is difficult to calculate. Although it is not commonly used, when heat exchangers and other equipment use special materials, such as titanium, and the pressure is not high, loose flanges will be considered to save costs. As you said, it's more about standard support, so it's best to use suitable spacers.