Controlling the load placed on a gasket is perhaps the most important factor for ensuring a successful seal. Applying too much or too little load via the flange bolts is a major cause of leaks and failure of gasket and seals.
It is important to note that the force applied to flange bolts can be both measured and controlled. “Torque” is the turning force applied to tighten a nut on a bolt, measured in foot-pounds (ft-lb) or inch-pounds (in-lb). Torque can be measured during flange assembly with a properly calibrated torque wrench. In a bolted flange, the applied torque generates the axial load in the bolt.
The bolt acts like a spring and tightening the nut stretches the bolt, which increases the load on the gasket. Bolts need to be loaded sufficiently to maintain the load on the gasket as the system is put into operation. It is usually recommended that a bolt be loaded to approximately 50 to 60 percent of its yield strength to ensure the bolt is stretched enough. However, overloading bolts can cause them to stretch beyond their yield strength and actually result in lower loads exerted on the gasket. Continued tightening of the bolts will not necessarily increase gasket load or stop a leak, and if severe enough, the overloading may even lead to bolt failure.
Over-tightening of flange bolts is a major cause of gasket failure. Often, when a sealed joint begins to leak, maintenance personnel typically respond by re-tightening the flange bolts. The over-tightened bolts then exert a force in excess of the recommended load for the gasket, causing the gasket to be crushed on the flange face. The net result – full gasket failure and increased leakage.
The loading force needed to compress the gasket for an effective seal will vary with the type and style of gasket, the degree of flange tightness, media being sealed, as well as the operating temperature and pressure. In all cases, load force recommendations provided from the gasket manufacturer should be followed to avoid injury or gasket failure.
As important as applying the correct force is the need to ensure that the force is evenly distributed around the flange face. Usually this means going from side to side around the joint in a star crossing pattern. See our previous blog The Correct Procedure for Gasket Installation for detailed instructions on how to ensure that loading forces are evenly distributed around the flange face.
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