In most of the organizations there is no defined criteria for designing and placing an expansion loop in a pipe rack. So most of the time the expansion loop is designed and located based on user experience.
The important parameters which governs the design of expansion loop are listed below:
- Design/Maximum operating temperature of line
- Allowed Displacement or movement (Normally allowed thermal displacement is 250-300mm inside a loop, and 75-100mm in outside turns).
- Allowed Expansion stress (normally within 80% of code allowable).
- Line size (Bigger sizes require more leg to absorb expansion)
- Loop Supporting Requirements (locations at which the loop will be supported)
- Fluid type (Normally Flare and condensate lines require 2D loop)
- Line sagging criteria from Project specification (Sometimes Steam, Condensate, Two Phase flow lines and Flare lines require sagging limited within 3-5 mm for others it can go upto 15 mm)
- Rack length and width
After having the above mentioned parameters ready one can proceed to locate the loops over the rack. Follow the below mentioned steps for a preliminary guideline:
- Select an elevation of pipe rack and check what are the lines running over that rack.
- The growth of such utility headers should be determined by multiplying the coefficient of expansion by the length of the line.
- The coefficient of expansion is based on a particular material operating at a specific temperature.
- Upset temperatures take precedence over operating temperatures.
- Select the line with maximum temperature first. Check the allowed maximum movement outside loop (say 75mm) and place the first anchor at a distance which will be nearer to the allowed thermal movement (75mm) as mentioned above.
- Assuming that an anchor is located in the center of the header, the designer should calculate the growth of various branches to determine whether they have enough flexibility to absorb the header growth.
- Now as one anchor is fixed one can easily calculate the thermal displacement at design temperature towards other end/turn. If the displacement is within allowed displacement (75mm) then an expansion loop is not required. But if the calculated displacement is more (>75mm) then expansion loop is required. From this displacement you can decide how many expansion loops are required for the straight run allowing a maximum of 250-300mm displacement inside the loop. (Care should be taken for expansion leg requirement as sometimes allowing 300 mm displacement may cause expansion failure or huge anchor load. In that case increase no of expansion loops.)
- It is better to place lines with high temperature at outside of the rack so that longer loop length can be achieved on the other side.
- Placing the headers along one side of the pipe rack allows the expansion loops to sit with a slight overhang along the adjacent side of the pipe rack.
- It is better to nest the loops in a single location (same structures can be utilised for supporting)
- Don’t mix lines which required 2D loops with lines which required 3D loop in same elevation.
- It is better to place anchors in similar locations for deciding anchor bay.
- After deciding the loops check the loop length requirements from Pipe-Data-Pro, Caesar modelling (most optimized approach), Nomograph, Manual calculation etc.
Although conducting the final stress analysis is the responsibility of the mechanical or stress engineer, the pipe rack designer makes preliminary calculations using relevant books and nomograms to ensure that the design will not require major rework during the formal stress check.
Following drawing highlights the steps involved in making a preliminary flexibility check, which are discussed in the following sections.
As a result of imposing stop loads on a particular bent, bracing may be required to grade, prohibiting the location of any equipment in that particular bay. A means of removing condensate build-up must be provided on either side of the expansion loop.
The most common way to accomplish this is to add drip legs and traps, as shown in following figure.
Header growth causes another problem that is often not as obvious. The line spacing chart may have been used to set distances between lines, or lines may have been set close to a column.
Following drawing reveals that the movement of a line must not be restricted by an adjacent line or column, because it will act as a line stop and could cause a problem.
Enough space must be provided for the line to move its maximum distance and still have an ample clearance of 3 in (75 mm).