Metallic Ring-Joints are produced according to the standard established by the American Petroleum Institute, API 6A and API 17D and the American Society of Mechanical Engineers, ASME B16.20, for applications at elevated temperatures and/or pressures. A typical application of Ring-Joints is the “Christmas Tree” used in oil fileds. The seal is obtained in a line of contact by a wedge action with high seating pressures, thus forcing the gasket material to flow. The small sealing area with high contact pressure results in great reliability. However the contact surfaces of the gasket and the flange should be carefully finished. Some styles of Ring-Joints are pressure activated, that is, the greater the pressure the better the sealability.
Table below shows the standard materials recommended by the ASME B16.20 for Ring Joint Gaskets.
a) The code of each material is engraved on the gasket next to its size, according to Standard ASME B16.20.
Style 950, which is frequently referred to as the oval ring, was the gasket that was initially standardized. Later developments resulted in other styles. If the flange was designed using the older version of the gasket standard, for use with an oval ring, then it should be used only with style 950 gaskets.
Style 951 has an octagonal section as shown in the Figure on the right. Style 951 has better sealing performance than Style 950 and its use is recommended for new applications. For this style flanges are manufactured according to new issues of ASME and API standards and have grooves with a profile designed to work with both 950 and 951.
Style BX gasket has a square cross section with bevelled corners as shown in the Figure on the right. Designed for use only in flanges API 6BX. Style BX is recommended for pressures from 5000psi (34.5MPa) up to 20000 psi (138MPa). The average diameter of the gasket is slightly greater than that of the flange groove. This way when the gasket is seated it stays pre-compressed by the outside diameter creating a high seating stress.
Style RX is a pressure-activated gasket. Its shape is designed to use the fluid pressure to increase the sealability. The outside sealing surface of the gasket makes the initial contact with the flange seating the gasket. As the internal pressure of the piping or equipment is increased, the contact pressure between gasket and flange also increases due to the shape of the gasket. High seating pressure is created increasing the sealability. This design characteristic makes this gasket style more resistant to vibrations, pressure surges and shocks that occur during oil well drilling. Style RX is interchangeable with style 950 and 951, using the same flange grooving.
The maximum hardness for each gasket material is shown in Table 1. It is recommended that the hardness of the gasket be always less than that of the flange so as not to damage it. When the materials of the flange and the gasket are similar, it is recommended to heat-treat the gasket to have the minimum possible hardness.
For ASME Code calculation values for Ring Joint gaskets are shown in the Table below.
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