Gasket stress is a term commonly used to describe the unit load on its surface. It is one of the most important parameters of a bolted joint because it directly impacts the ability of the gasket to seal. Since the conditions under which a bolted joint operates during its life can be complex, compressive stress definitions have been established to describe conditions throughout this life cycle. Gasket types respond differently to a given stress range, so employing the guidance provided by the gasket manufacturers regarding how their materials react is important. A soft and conformable gasket may seal at a relatively low gasket stress while a hard metal gasket may require much higher stress.
Four Aspects of Gasket Stress
The compressive stress required on a gasket can be viewed in four ways.
Conform to the Flange Surfaces
A minimum amount of compression is needed to seat the gasket on the flange surfaces. The gasket must conform to the flange's irregularities to function effectively. If the flanges were perfectly flat and smooth, a gasket might not be needed. With greater imperfections, more compression is needed to form the gasket into that shape.
Block the Gasket Material's Permeability
Once the gasket has conformed to the flange surface, additional compression may be needed to block any permeability in the gasket body. Permeability through gaskets varies greatly for different types of material, but in almost all cases, leak rates decrease as the compressive load increases. This relationship is the basis of the gasket constants as determined by the room temperature tightness (ROTT) test. These constants were created specifically to provide more than one specific compressive stress that makes a particular gasket type seal.
The state of the fluid, including molecular size, determines the stress required. Required stresses, especially in gaseous services, will increase depending on how tight the seal needs to be. These stresses are higher than the minimum stresses that are necessary to make the gasket conform to the flanges.
Withstand Internal Pressure
When using nonmetallic gaskets, the ability of a bolted joint to hold internal pressure depends on friction, which is related to the compressive load on the gasket. The minimum compressive stress will need to be high enough to maintain the friction needed to keep the gasket from blowing out from the internal pressure.
The fourth consideration for determining an installation stress is temperature. Elevated temperature will create gasket relaxation and subsequent relaxation in the bolt load. Some load losses can be as high as 50 percent of the initial gasket stress. The initial installation stresses need to be high enough to compensate for this effect. This is the reason that some gasket manufacturers recommend a retorque after the first heat cycle depending on the gasket type (of course, observing the appropriate lock-out and tag-out safety procedures).
Characterization of Stresses
The minimum seating stress, ideal operating stress, minimum operating stress (considering internal pressure of the system) and maximum operating stress specific to a given gasket material need to be understood and taken into consideration. While many references to values for these stresses have been published, the most updated reference is found in an appendix to recently published ASME PCC-1-2010 Guidelines for Pressure Boundary Bolted Flange Joint Assembly. This valuable post construction document also offers insight and recommended guidance on diverse sealing challenges, such as surface finish acceptance for used equipment and misalignment limits for piping systems.
References to gasket stress in this document are shown, but further explanations are needed. Below are the terms and references used in the text and some suggested guidance agreed upon by gasket material manufacturers.
Minimum gasket seating stress (SgminS) can be defined as the Y value in ASME Code calculations. This is basically the absolute minimum stress needed to conform to the flanges, assuming that there is little or no internal pressure. Most gasket manufacturers can provide these values on their gasket materials. Often, these values are determined with low-pressure leakage tests on each gasket material. This minimum stress value will normally be used only in flange design calculations.
Minimum gasket operating stress (Sgmin-O) will normally depend on the design pressure of the assembly. It will be higher than the seating stress, or Y value, of the gasket. Most gasket suppliers can provide the minimum operating stress with consideration of the pressure. It is not uncommon for these values to increase with increasing gasket thickness. Gasket manufacturers will recommend that installation stress be higher than the minimum seating stress.
Maximum assembly gasket stress (Sgmax) is the stress that could damage the integrity of the gasket and detrimentally affect its ability to maintain a seal. Many gasket manufacturers will perform laboratory tests to determine the maximum stress on a gasket. Many variables are involved when considering the maximum stress or crush strength of a material, including surface finish, gasket width and thickness, material type and temperature. Most manufacturers will test with smooth surfaces as well as standard ASME serrated flange finishes. Thicker gaskets are usually less resistant to over compression and crushing. Also, serrated flanges tend to allow for higher compressive loads because the rougher surface will grab or hold the gasket better. Smooth surfaces allow the gasket to slip sideways and split at lower stresses.