NAB - Pressure and tempreture Ratings

In order to assist standardisation of dimensions, flange and valve standards generally adopt a philosophy of specifying a standard set of dimensions for a pressure class and then identifying a pressure-temperature rating associated with these dimensions and a particular group of materials. The pressure-temperature rating then becomes the all important factor. The rating is based on the mechanical properties, but also takes account of the performance at varying temperatures. So the true comparison is between the ratings of the different materials.

Pressure ratings and tempreture ratings of nickel aluminium bronze and other selected alloys

The chart shows the cold working pressure (CWP) for the various materials. The comparison is not straightforward because the bronze and cast iron figures relate only to flat-face flanges and all the other dimensions relate to the ANSI B16.5 raised face dimensions. This fact will be discussed in more detail later as will the fact that the NAB and titanium ratings have been specially calculated.

A comparison between the pressure-ratings and the mechanical properties shows that the difference between the various materials has been flattened. The way the ratings are determined imposes an upper limit on the pressure, a ceiling is applied in order to limit deflection. The materials affected by this ceiling figure are 6Mo, the duplexes and Ni alloys

For comparison, the rating at 250oC is included. This illustrates the shortcomings of copper alloys at raised temperatures. Whilst there is no rating for bronze, NAB is less than most of the other materials, as are the titaniumís. The maximum temperature and pressure for each material is given in Appendix II. The difference between materials is further reduced by the use of ceiling values at higher pressures.

Mechanically, NAB can be seen to compare well enough to compete with its rivals. There is a limitation once high temperature performance is considered, however this is not generally an issue for seawater service.

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