Q&A – For column overhead line, why is the first support always a Rest support?
The simple answer is “That’s all that is needed”. You have to understand the full picture of the top of the vessel along with the configuration of the “Overhead Line”.
a) If the “Overhead Line” exits the vessel from a side nozzle and turns down with an elbow then you want a solid (Rest) support as close to the elbow as possible. When the solid support is a minimum distance from the nozzle then the growth in the vessel shell from the support to the center line of the nozzle is equalized by the growth in the pipe. There is absolutely no need for anything fancy or expensive such as springs. The vessel will “Grow” up due to the thermal expansion created by the process temperature. The pipe will grow down thus equalizing some of the vessels vertical growth. The support attachment connected to the vessel needs to be designed to support the weight of the pipe, the weight of hydro-test water, any insulation, any added load concentrations such as a large valve, the normal wind/snow/rain/etc. loading and a safety factor.
b) If the “Overhead line” exits the vessel from a nozzle on top of the head then the line will go up, turn flat and run horizontal to near the outside edge of the vessel then turn down and pass the tangent line of the top vessel head. The solid (rest) support for this type of configuration should be placed a minimum distance below the vessel tangent line. The vertical expansion of the vessel shell, the vessel head, the nozzle and the first vertical piping elements will be balanced by the vertical growth in the pipe from the support up to the top horizontal segment. Any slight difference in the exact amount of growth between the two vertical legs will be compensated for by the horizontal “leg”. All other issues with this configuration below the solid support will be the same as with method “a”.
This text is an extract from questions gathered from numerous queries of anonymous people in the Piping Engineering and Design profession. The answers or discussion of the topics were provided by James O. Pennock. Images obtained from www.coastalflange.com
