Over the years, the FSA Gasket Division has explored many specific sealing and gasketing issues in Sealing Sense. Regardless of all the insights provided, chances are that users of gaskets will be confronted at some point with a bolted flange connection that leaks. When this happens, those involved will (most ...
Over the years, the FSA Gasket Division has explored many specific sealing and gasketing issues in Sealing Sense. Regardless of all the insights provided, chances are that users of gaskets will be confronted at some point with a bolted flange connection that leaks. When this happens, those involved will (most of the time) still consider the gasket as the probable cause of leakage.
Our purpose this month is to share with our readers much of the experience of the gasket material manufacturers and to help the reader understand how other factors related to the gasket, but not the gasket itself, may contribute to the sealing difficulties they face.
The experience of most gasket material manufacturers suggests that a very high percentage of bolted flange connections that leak (perhaps 75 to 85 percent) do so as a result of non-gasket related factors. These factors usually relate to installation and assembly problems and limitations.
We will discuss some of the most typical circumstances gasket material manufacturers find that relate to installation and assembly issues resulting in leakage. But before moving forward, please accept and understand two fundamentals relating to flanged connections.
First, a bolted flange connection is a complex combination of many factors. All these various elements are interrelated and depend upon one another to achieve a successful result. Figure 1 shows the complexity of these relationships.
Second, the user must understand that one of the most important factors for success is correctly following the gasket installation procedure. Quoting from John H. Bickford's book, An Introduction to the Design and Behavior of Bolted Joints:
"That all important clamping force which holds the joint together - and without which there would be no joint -is not created by a good joint designer, nor by high quality parts. It is created by the mechanic on the job site, using the tools, procedures, and working conditions we have provided him with . . ." And further: The final, essential creator of the force is the mechanic, and the time of creation is during assembly. So it's very important for us to understand this process."
The industry has recognized the critical nature of installation and assembly for several years. Work began in 2001 to develop formal guidelines for flange assembler training and certification. In September 1998 the original draft document, "Guidelines for Pressure Boundary Bolted Flange Joint Assembly," was created. Later known as PCC-1-2000, it was first available for purchase in April of 2001.
Today, a special working group of the ASME is undertaking an update of this document with focus on developing standard qualifications and requirements for the training and certification of flange assemblers. This process will be similar to that followed to train and certify welders, and is intended to ultimately be a revised Appendix A for PCC-1.
Actual Installation- and Assembly-Related Issues
Some of the most frequent issues dealing with the installation of a gasket and the assembly of a flange include:
Underloading of the Gasket
This may comprise a vast majority of the reasons a bolted flange leaks. There are many reasons for this to occur, including these most common ones:
- Pressure and Temperature create loss of initial gasket load. The higher the pressure and temperature, the more loss of the initial gasket load that occurs. Solution: Before assembly, properly calculate the amount of torque required considering these two critical application conditions.
- Misaligned flanges (whether axial or radial) make the bolt "work" to correct the misalignment instead of loading the gasket. The applied torque is reduced significantly, resulting in insufficient achieved bolt load or stress on the gasket. Solution: Try to bring the flanges into an acceptable alignment per ASME B31.3 (ref. PCC-1 Appendix E). Special Tip: Bolts should pass freely through bolt holes!
- Lack of lubrication may reduce the applied load you are targeting by as much as 50 percent. Un-lubricated bolts develop lower force every time they are used, due to increased thread friction as the nut/bolt thread surfaces are "worked."
Solution: Once the proper lubricant is determined (being sure to check compatibility with process fluid, temperature, and materials of construction), apply lubrication to threads and "working" surfaces, i.e. all internal and external thread surfaces and nut and washer surfaces.
- Do not apply lubrication to those already coated with lubricant.
- Do not apply lubrication to the gasket.
- Reused bolts may no longer be functional. There are many related issues in this category: