Two Rules-of-Thumb Regarding Piping Systems
Have you noticed, looking at the famous ASME B31.1 support spacing Table 121.5, that the spacing between pipe supports is roughly “size + 10”, in other words, the spacing between supports on an 1 inch instrument tubing would be “1 + 10” = 11 ft, while the spacing on a 6 inch line for example would be “6 + 10” = 16 ft. There are of course exceptions, for example, to mention one, supports should be added near concentrated weights such as valves or vertical risers.
A simple rule-of-thumb that was pointed-out to me years ago concerns the Class rating of flanges. The working pressure for a B16.5 flange at ambient temperature is approximately 2.4 times the flange Class. For example, the working pressure for a Class 400 flange at 100 F is approximately 2.4 x 400 = 960 psi. Referring to B16.5 for Group 1.1 (carbon steel) we read 990 psi … not bad. As most rules-of-thumb, there are exceptions and the rule does not work well for the lowest pressure Class, i.e. Class 150.
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