Petrochemical piping

In petrochemical industry, process piping is mostly carbon steel pipes, up to 12″ size. Because of process requirement, piping is exposed to very large temperature range and piping routing required special consideration. Since almost all refinery plants has same type of equipment, we can at least have some guidelines, if not standards, for pipe routing concerning each type of equipment. Typical equipment piping (pumps, heaters, air collier, tower piping) can be found visiting websites: Piping Engineering and What is Piping.  In this article I will discuss requirements for pipe loop and I will briefly touch pipe rack modularization, equipment installation and plant costs.
Pipe loops
There is simple rule saying that pipe expansion between two anchor points should not exceed 10″. Let’s see how this rule is applied to pipe loop.
Suppose that point “C” has a thermal expansion of 16″. This is larger than 10″ and therefore a pipe loop is required. New pipe configuration with the pipe loop is shown on the figure bellow. Note that the pipe rack is not shown on the figure.
Suppose that point “A” and “B” has a thermal expansion of 4″ and 8″ respectively. Total expansion between anchors is 8″+4″=12″. This will exceed value of 10″ and, obviously, we have to redesign our piping adding one pipe loop more. Our final design is shown on the figure below.
Thermal expansion for the first loop is 4+3=7″ and for the second loop is 3+6=9″. Both values are less than 10″ and therefore our design is satisfactory. For calculating thermal expansion, I suggest you to use excel spreadsheet. Figure below is snapshot of my spreadsheet for thermal calculation.
Please note that if thermal expansion between two anchor points is larger than 14″, most likely you will need expansion loop. If thermal expansion is in the range 10″ to 14″ you might need thermal expansion, depending of piping configuration. I have seen the pipe on the pipe rack having 12″ thermal expansion and stress was within allowable. If thermal expansion is less than 10″ most likely pipe loop is not required. If comprehensive analysis of pipe loop is not required, preference would be given to guided cantilever method over empirical equation given by B31.3 (Dy/[(L-U)(L-U)<K ).
Modularization and plant costs
In plant cost structure, plant installation is 25% to 30% of total plant costs, 25% to 30% is engineering and the rest is material and fabrication. Since installation has big impact on overall cost and field labor is very expensive, it makes a sense to consider modularization. Cost of field labor is usually three times higher than shop labor, but that can vary depending of geographical location. Most of pipes must be installed in field especially those pipes that connecting equipment and pipes around pumps, but pipes on the pipe rack can be installed and shipped as modules, thereby reducing field labor and contributing to overall plant costs. Typical pipe rack module is shown on the figure below.
Columns will be installed fist then a pipe rack module. If required, bracing and connecting beams will be installed in the field. Module must be sized in such way that allows module transportation. Please see my blog Pipe spool for shipping box. Some pipe rack module due to size will require special transportation permit. Please see the figure below.
Tower structure or smaller platforms can also be shipped in modules and assembled in the field. Please see the figure below.
Of course, structural connections and installation procedure will be responsibility of structural department. Lifting brackets must be so positioned that after module is lifted, it remains in approximately horizontal position (Centre of gravity analysis). Analysis of module weight must be also conducted to ensure that crane will has capacity to lift module. (Crane capacity will vary with crane boom lifting angle). Interesting article about pipe rack modularization can be found on Structuremag site. In the case that mechanical equipment cannot be shipped as one piece due to size (e.g. towers), it will be shipped in parts, assembled in the field (welded or bolted) then installed by crane. Mechanical department is responsible for installation procedure. Typical site installation is shown on the figure below.
It is also evident that engineering cost is great portion of overall costs and carefully planned project execution will contribute to overall cost saving. From a design point of view, milestones are set at 30%, 60% and 90% model review. In a 30% model review following should be established and resolved: nozzle orientation, equipment elevation and location, structure and mechanical equipment should be outlined. In a 60% model review all major process lines should be routed and stress analysis should be conducted. In a 90% model review all technical questions should be resolved and all pipe lines should be shown on a 3D model. Finally, after having touch-up, model is 100% done and closed. This is just rough outline of design stages. For check points at each design stage, always refer to your company standards.