The proper selection of gasket is critical to the success of achieving long-term leak tightness of flanged joints. Due to their widespread usage, gaskets are often taken for granted. Industry demands for reduced flange leakage in environments of increasing process temperatures and pressures have led gasket manufacturers to develop a wide variety of gasket types and materials, with new gaskets being introduced on an ongoing basis. This rapidly changing environment makes, and will continue to make, gasket selection difficult.
It is highly recommended that the gasket manufacturer be consulted on the proper selection of gaskets for each application. Gasket manufacturers are familiar with the industry codes and standards and conduct extensive testing of their products to ascertain performance under a variety of operating conditions.
Flange design details, service environment, and operating performance guide the gasket selection process. Start with the flange design. Identify the appropriate flange standard, outlining size, type, facing, pressure rating, and materials (i.e., ASME B16.5, NPS 4, Class 1500, RF, carbon steel). Identify the service environment of temperature, pressure, and process fluid. It is useful to highlight gasket-operating performance.
Gasket Operating Performance
New flange and gasket designs are incorporating tightness factors in their calculations to reduce leak rates. Traditional ASME Section VIII code utilizes m and y gasket factors in the design calculations of flanges. These factors are useful to establish the flange design required to help ensure the overall pressure integrity of the system; however, they are not useful parameters to predict flange leak rates.
All flanges leak to a certain degree. Industry requirements are demanding reduction in leak rates along with predictable performance. This has lead to a more rigorous approach to establishing gasket factors and the associated methods for gasketed flanged-joint design.
Significant progress has been made in the last six years in Europe by CEN and in North America by ASME’s Pressure Vessel Research Council (PVRC) to establish gasket test procedures and the development of design constants that greatly improve the gasketed flanged-joint design. Maximum allowable leak rates have been established for various classes of equipment. EPA Fugitive Emissions basic limits are shown below.
PVRC has established a new set of gasket factors, Gb, a, and Gs and a related tightness parameter, Tp, which can be used in place of traditional m and y factors in determining required bolt load.
Gb and a (Part A testing) represent the initial gasket-compression characteristics. Gb is the gasket stress at a tightness parameter (Tp) of 1; a is the slope of the line of gasket stress versus tightness parameter plotted on a log-log curve. This line shows that the tightness parameter (or leak tightness) increases with increasing gasket stress. That is, the higher the gasket stress, the lower the expected leakage. Gs is the unloading (Part B) gasket stress at a Tp = 1.
A low value of Gb indicates that the gasket requires low levels of gasket stress
for initial seating. Low values of Gs indicate that the gasket requires lower stresses to maintain tightness during operation and can tolerate higher levels of unloading, which maintain sealability. An idealized tightness curve showing the basis for gasket constants Gb, a, and Gs is shown in Fig. A7.17.
The data for many gasket styles and materials have been published in various
PVRC-sponsored publications. Typical PVRC gasket factors for a variety of gasket types are shown in Table A7.15.
PVRC Convenient Method. The PVRC Convenient Method provides an easy conservative method for determining bolt load (Wmo) used in flange and gasket design as an alternate to using m and y values.
The total bolt load to achieve the same leak tightness of T3 is 1,119,255 lb.
These examples would indicate that higher leak tightness can be achieved using the camprofile gasket versus the double-jacketed gasket under the design conditions outlined.
Types of Gaskets
As discussed earlier, gaskets can be defined into three main categories: nonmetallic, semimetallic, and metallic. The general applications for each gasket type are shown in Table A7.16.
High Temperature Selection
In high temperature applications, above 650°F (343°C), gasket selection becomes even more critical. Many gaskets may perform well at low temperatures but fail to meet leak-tightness requirements at elevated temperatures.
Many gaskets lose their resiliency at elevated temperatures, with changes in their elastic behavior. The gasket’s inherent stiffness will also tend to diminish, resulting in the gasket continuing to deform under the applied flange loads. This deformation (or creep) will result in loss of gasket stress, bolt load, and leak tightness. In elevated temperature applications, search out materials that retain their resiliency and gasket designs that will not change in thickness (retain its stiffness).
Considerable technical information on gasket selection is available from gasket manufacturers and from other technical sources such as the Pressure Vessel Research Council and industry trade associations such as the Fluid Sealing Association (FSA). #Little_PEng