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Pipe spools

Piping system for process piping is prefabricated in the a sections called pipe spools. The figure below is snapshot taken from one of my drawings, shows typical piping spool.
1-pipe-spool-drawing
Please note that some pipe spools are supplied 10″ to 15″ longer to accommodate structural and equipment tolerances. Normally, engineering contractor will show location of field welds (FW) on isometric drawing and pipe manufacturing contractor will make pipe spool drawings based on the field welds location. Please see the figure below.
2-field-weld-spool-dwg
However, that might not be case for all projects. Engineering contractor might be required to make piping spool drawing, depending of a contract. Field welds location will determine pipe spools and there are two things needs to be considered. Firstly, pipe spool overall dimensions shall be fit into imaginary shipping box allowing normal transportation. For North America shipping box size is 40’x8’x8′. Larger size of shipping box will required special transportation permit. Please see the figure below.
3-pipe-spool
Secondly, pipe spools shall be design in such way that will allow proper installation. Most companies will not allow pulling a pipe in the case pipe misalignment during installation. Please see the figure below.
4-pipe-misalignment
The best way is to provide longer spools to accommodate construction and equipment tolerance in all three directions. Please see the figure below.
5-field-weld-location
Normally, last spool that is connect to equipment will be the last installed. Please note that equipment tolerances are usually small, depending on size, but generally they are not large than 0.25″. On the other hand, structural construction tolerances are much larger and in some cases might reach 1/2″. Maximum structural tolerances are given in structural code and standard. API RP 686, also, provides data for pipe misalignment. As per the code, if deviation of piping system and rotary equipment flange centre line is within +/-1.5 mm and rotation is within 0.0573 degree then alignment is accepted. Normally, this misalignment is neglected in stress analysis, but it can be taken into consideration by using cold spring (Caesar II) in your model. Proper field weld location will reduce field labor and have big impact on a price. Piping installation is about 25% to 30% of total project cost.