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Piping Flange Fundamentals
Basic Flange Math
To understand how flanges work, we must first understand the concept of pressure. Pressure is defined as:
Pressure = Force / Area
P = F / A
Flanges seal because pressure is applied to the mating sealing surfaces; this pressure is known as the ‘gasket compression’
or ‘sealing pressure’. The applied pressure causes the two faces to either:
•
Crush a gasket between the two mating faces.
•
Press the two mating faces against each other.
In the gasket example, the gasket is deformed due to the pressure applied; this deformation causes the gasket to ‘flow’
into any surface imperfections that may be present on either sealing face. Because the surface imperfections have been
filled by the gasket material, leakage is no longer possible.
The second example assumes no gasket is present and that two flange faces are pressed together. It is hard to create a
leak tight seal using this method, although it is possible if the surfaces are well machined and very clean. The sealing
pressure applied will often need to be significant, as the flange surface may be manufactured from metal, which does not
easily deform under pressure (material and flange class dependent). Metal to metal flange face sealing is expensive and
thus not common.
To create the necessary sealing pressure, the variables of force and area can be adjusted.
•
Force refers to the tightening torque (bolting load) applied to the mating flange faces when the nuts on a flange
assembly are tightened. Force (F) depends upon the torque (T) applied, torque friction (K) and nominal bolt
diameter (D). The force described is classed as ‘bolt pretension’ or ‘bolt preload’, or ‘bolt prestress’, and is
represented by the equation F = T/(KD)
•
Area refers to the size of the sealing face area.
The amount of pressure on the flange sealing faces corresponds to the amount of force applied when tightening the flange
assembly. Thus it is possible to regulate the pressure by adjusting the amount of effort that is exerted when tightening the
bolts during flange assembly.
The sealing area of a flange cannot be as easily adjusted as the force used during assembly. A larger sealing face requires
more force to obtain a certain amount of pressure, compared to when using a smaller sealing face. The below example
highlights this point, but without the use of units.
Example
A given flange assembly requires a pressure of 10 to seal. This can be achieved by applying a lot of force onto a small sealing
face:
Pressure = Force / Area
10 = 40 / 4
Or, it is possible to decrease the size of the sealing face (area) and thus reduce the amount of force required to create the
same amount of pressure3:
3 Standards such as ASME B16.5 and B16.47 dictate the size of the sealing face required.