Flange-bolted joints enable connections of singular segments of pipes into more complex sections, as well as joining measuring and process devices, such as flowmeters, pumps, fans and pressure vessels. These joints are potential sources of leakage, which can introduce safety and environmental hazards, as well as process inefficiencies. This one-page reference reviews concepts associated with preventing leaks in flange-bolted joints.
Design considerations
The design of flange-bolted joints depends on the selection of a gasket, as well as the proper assembly. Required design data include the nominal diameter of the pipeline, the temperature and pressure of the process fluid and the chemical and physical properties of the transported medium. Additional data are external loads on the joint (and their variations in time), required tightness and durability of the joint. Over the past several decades, computational algorithms have been developed to help achieve the desired bolt tightness. In most cases, the algorithms apply to flange-bolted joints with circular gaskets, and the results of the calculations provide the torque at which the nuts should be tightened to ensure sufficient tightness [1].
Forces on joints
Three main forces act upon a bolted flange joint assembly (BFJA; Figure 1). The flange/bolt load acts to compress the gasket enough to fill any serrations or imperfections on the sealing surface. This helps prevent potential leak paths. The hydrostatic end load, caused by the internal pressure of the fluid in the system, acts to push the two flanges apart. The internal blowout pressure acts upon the gasket and tries to push it out through the gap between the flanges.
FIGURE 1. The three main forces at play in a bolted flange joint assembly (shown here) act to prevent leaks and gasket blowouts
A main concern surrounding the design and installation of the BFJA is determining the gasket stress or load that will be applied to the gasket (the flange/bolt load minus the hydrostatic end load). This remaining gasket load must then be greater than the internal blowout pressure to ensure the integrity of the seal. If it is not, a leak or gasket blowout can occur [2].
Leakage rate
The leakage rate refers to the quantity of fluid that passes through the body and over the faces of a gasket per unit periphery of the gasket over a specified time [3]. The leakage rate is highly influenced by process media, pressure, temperature, surface pressure and other factors. It is usually measured under a specific gasket load and at a specific fluid pressure. For given set of conditions, the lower the leakage rate, the better the fluid is retained by the gasket material.
As a general guideline, increasing internal pressure leads to higher leakage rates. Increasing surface pressure results in lower leakage rates. Increasing molecular size generally means lower leakage rates. Increasing temperature gives lower leakage rates for some gasket materials, although toward the end of service life, the leakage rate is likely to increase [3].
Installation tips
The following are tips to consider when installing BFJAs [2].
- Make sure the flange sealing surfaces are clean and free of dings, marks or indentations
- Flanges should be aligned to maximize sealing contact and to provide a uniform gasket load
- Working surfaces of bolts, nuts and washers should be lubricated to ensure uniform friction
- Verify the material, grade and condition of the bolts. Nuts should spin freely onto the bolt thread without binding
- Number and tighten bolts using a proven tightening sequence or assembly pattern, and use a calibrated torque-control device to ensure proper torque values are applied
- Keep assembly records to verify that proper procedures were followed
Gasket materials
Common gasket materials include elastomers, polytetrafluoroethylene (PTFE), flexible graphite, natural fibers or mineral-based materials. Gasket manufacturers provide pressure-versus-temperature charts to help determine whether or not the material is safe to use for a particular application.
Flange compressibility
Flange compressibility is considered to be the percentage reduction in thickness of a flange under compressive pressure (applied at a constant rate, at ambient temperature). This value provides a means to measure the deformation properties of a gasket, and is an indication of the ability of the material to conform to the flange surface irregularities [3]. Gasket materials with higher compressibilities will more effectively fill surface roughness.
References
1. Jaszak, P. and Adamek, K., Design and analysis of the flange-bolted join with respect to required tightness and strength, Open Engineering, 2019. https://doi.org/10.1515/eng-2019-0031
2. Norton, C., Back to Basics: Soft Gaskets, Fluid Sealing Association (FSA), Sealing Sense, 2016.
3. European Sealing Association (ESA) and the Fluid Sealing Association (FSA). Flange Gaskets: Glossary of Terms, ESA/FSA, Publication 018/09. 2009.