Part One of this series explained how the clamping force on a bolted flange connection (BFC) can play a major role in preventing gasket blowout. In fact, more often than not, it is the single most important factor. Part Two discusses the two most important strategies to use for applying the correct clamping force. End users control these strategies. One involves developing a bolt load solution that optimizes the inherent strength of the BFC components. The other is following an installation procedure that ensures that the specified bolt load is reached and evenly distributed around the face of the gasket.
Determination of Optimum Clamping Load
A BFC is composed of three primary components: the gasket, fasteners (bolts/studs, nuts and washers) and flanges. Each component has a role in developing and maintaining the clamping force. Together, they work as a system to produce a successful seal. The state of stress of one component depends on and affects the state of stress of the others. A sealing solution that only considers one or two will miss the opportunity to maximize the clamping force and, therefore, the gasket seal’s reliability. Likewise, developing a sealing solution that does not consider the interaction and strength of all components can cause blowout.
The gasket provides the properties that create the seal against the face of the flanges. For the gasket to seal under installed conditions, a minimum stress value is necessary. This value should include the relaxation that will occur during the gasket’s installed life. To prevent damage, it also has a maximum stress value that should not be exceeded, its crush strength, which can be obtained from the gasket manufacturer. When a gasket’s crush strength is exceeded, the properties that provide the seal break down. An optimized solution targets a stress level that is as high as possible above its minimum value but a comfortably safe margin below its maximum value and a value that may cause a problem to the bolts or flanges.
The bolts provide the mechanical leverage that allows a high and controlled value of force to be transmitted to the gasket. The force from tightening the bolts must be at least high enough to satisfy the gasket’s sealing requirements but not so high that it creates a problem with the gasket (exceeded crush strength) or the flange (exceeded strength limit or cause excessive rotation).
Additionally, the bolt has its own limit that must not be exceeded during tightening or under the internal pressure load. The targeted torque must not exceed this limit, and the specification of the bolts must be known. Two bolts of the same size and material can have significantly different limits. For instance, the yield strength at ambient temperature of an American Society for Testing and Materials A 193 Grade B8 Class 1 and Class 2 bolt is 30 ksi and 100 ksi, respectively, for diameters ¾ inch and less. Attempting to tighten a Class 1 bolt to a Class 2 bolt’s level can cause damage that results in a loss of clamping force.
As mentioned in Part One of this series, the amount of force required to overcome any incidental loads that would resist the torque load from compressing the gasket must be known. Table 1 in Part One provided a list these of items.
The primary function of the flanges is to transmit the force from the bolts to the gasket and allow for separating the connected pressure equipment for future maintenance. They must be strong enough to accept this force without damage and rigid enough to ensure that they do not excessively rotate across the face of the gasket. In many ways, the flanges are the most important component of a BFC and are often not evaluated in a clamping solution.
Figure 1. Flange rotation—conceptual examples
Specifying bolt loads without understanding the implications to the flanges can result in blowout or, at the very least, process leakage. Without considering the flanges, such problems are often inaccurately diagnosed as a gasket problem. To help understand the possible effect of excessive rotation, Figure 1 (A, B and C) shows the bending that can occur when the bolt load exceeds the capability of the flanges to resist the associated rotation. See the descriptions below:
A. Flange-gasket fit-up—No gasket compression occurs.
B. Non-excessive bolt load—The gasket deflects axially, mostly toward the outer diameter, because of flange rotation.
C. Excessive bolt load—The flange lifts off the inner portion of the gasket creating potentially damaging load and seal breakdown towards outer diameter of gasket.
As a flange rotates, the compressive gasket stress increases from the inner to the outer diameter. When excessive, the gasket stress’ value can become high enough to cause damage because the bolt load is transferred across a much smaller gasket area. Often, more leaks are caused from over-tightening than under-tightening, especially from specifying fastener loads that fail to account for a flange pair’s limitations.
Sequence to Determine Optimum Bolt Load
To ensure an optimum level of clamping these steps should be followed: