Ligaments in Pressure Vessels

This pertains to tube sheets in heat exchangers or boilers. The ligaments are the thin parts of a tube sheet between adjacent holes for the tubes. A common criterion is that their thickness is at least 0.25 x tube diameter.

You mean to say the distance b/w one edge of hole to other edge of hole should be 0.25 * tube dia.....?????????

The ligament is the thickness between one tube hole and the next, rather than the thickness of the whole tube sheet.

For example, a heat exchanger might use 1.500 inch O.D. tubes, with a tube sheet thickness of 2.000 inches. The center-to-center distance between adjacent tubes would be 1.25 x 1.500 = 1.875 inches; and thus the ligament between the tubes would be 0.25 x 1.500 = 0.375 inches.

Thank you, Tornado. Didn't know that the metal thickness between boiler tubes is called "ligament." A life is just an un-interrupted learning curve, isn't it?

Thank You very much Tornado ................ U have been very helpful............ Does Ligament has same meaning in Pressure Vessel what it has in boilers or exchangers................?

Yes, these are all the same idea.

To add a bit more to this picture, consider that if you expand tubes into a tube sheet, this process exerts significant pressure. If the ligaments between the tubes are too thin, they will distort as the tube is expanded, and then they will distort in the opposite direction as the adjacent tube is expanded. This may "break" the joint with the previous tube, causing leaks. Hence the need for sufficiently thick ligaments.

I haven't done the actual formulas for quite some time; what I have described is just the basic idea.

Both the ASME Boiler and Pressure Vessel Code and the TEMA (Tubular Exchanger Manufacturers' Association) standards have further information and formulas on this topic.

So far I have mentioned only tube sheets. In a D-tube (water-tube) boiler, the same idea applies. The upper and lower drums will have cylindrical shells penetrated by an array of holes to accommodate the tubes. Just as before, the portions of shell material between the tubes are called ligaments. Because of the different shapes, the formulas also differ somewhat, but the underlying concept is the same.

Similar, but it's simpler in an ordinary pressure vessel. It's the distance between 2 welds. So e.g. if you want to weld a pipe connection into a vessel straight side, the ligament is the distance between the straight side to dished end weld and the pipe connection weld.

Ligament would typically be specified as minimum 50mm.

Cheers........Codey

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