Loop expansion

Multielevation piperacks are usually needed to handle all the required services for piping, electrical, utilities, and instmmentation. The two-level rack is one of the most common but three-level ones are also used. The utility lines are usually mn in the upper level and the process lines in the lower levels. The larger-diameter lines are located to the outside of the rack to be closest to the column supports. Access platforms are required at the battery limit to provide operators access to the block valves and blinds. If long mns of hot pipe are required, a portion of the pipe rack needs to be dedicated to an expansion loop. A horizontal space in the piperack is provided for a set of lines to be flat-turned into a set of expansion loops with the large pipes located on the outside. AH of the pipe turns are in the same horizontal plane, which is an exception to normal piping practice. A flat turn takes up and blocks space for other pipes. Flat turns are generally only made from the outside of the rack to minimize this blockage. 

Tolerance Whenever possible, equipment should tolerate poor installation or operation without failure. Expansion loops in pipework are more tolerant of poor installation than bellows are. Fixed pipes, or articulated arms, if rlexibihty is necessary, are fiiendher than noses. For most apphcations, metal is friendlier than glass or plastic. 

Thermal Expansion in Flare Header - Sliding-type expansion joints may be used in flare headers as an alternative to piping expansion loops, if required to achieve a reduction in pressure drop or where expansion bends may result in liquid surging, subject to the following conditions ... 

Using equipment that can tolerate a degree of misuse. Thus, fixed pipework is safer than hoses (see Section 7.1.6), and fixed pipework with expansion loops is safer than expansion joints (bellows). 

Flexibility is incorporated into piping systems to absorb the thermal expansion. A piping system will have a certain amount of flexibility due to the bends and loops required by the layout. If necessary, expansion loops, bellows and other special expansion devices can be used to take up expansion. 

The simplest and cheapest type of shell and tube exchanger is the fixed tube sheet design shown in Figure 12.3. The main disadvantages of this type are that the tube bundle cannot be removed for cleaning and there is no provision for differential expansion of the shell and tubes. As the shell and tubes will be at different temperatures, and may be of different materials, the differential expansion can be considerable and the use of this type is limited to temperature differences up to about 80°C. Some provision for expansion can be made by including an expansion loop in the shell (shown dotted on Figure 12.3) but their use is limited to low shell pressure up to about 8 bar. In the other types, only one end of the tubes is fixed and the bundle can expand freely. 

Equipment should tolerate maloperation, poor installation or maintenance without failure. E.g. expansion loops in pipework are more tolerant to poor installation than bellows. The construction materials should be resistant to corrosion and physical conditions. For most applications metal is safer than glass or plastic. 

Thermal expansion and the resultant pipe stresses must be considered in any piping system design. For example, if the temperature changes from 50 to 600°F, the increase in length would be 4.9 in. per 100 ft for steel pipe and 7.3 in. per 100 ft for brass pipe. This amount of thermal expansion could easily cause a pipe or wall to buckle if the pipe were fastened firmly at each end with no allowances for expansion. The necessary flexibility for the piping system can be provided by the use of expansion loops, changes in direction, bellows joints, slip joints, and other devices. 

The outlet collectors, of Incoloy 8(X) or Manaurite 900, must be followed by expansion loops (pigtails) to offset the mechanical forces due to rapid temperature variations. They lead to waste beat hollas that are normally of two types ... 

DPPC is prominent in the lipid bilayer making up the cell membrane and is also a major constituent of lung surfactant ( pulmonary surfactant). The lung membrane resembles a mixed surfactant monolayer at the air/water interface. Since the temperature in a lung is below the critical temperature for DPPC monolayers, the LE-LC transition may be of significance in the continuous compression and expansion loops that this membrane undergoes during respiration. We will say more about this in sec. 3.9. 

Sample Problem 1. For a simple U-type expansion loop as shown in Figure 7-14, check maximum stress in the loop, and if the calculated stress is much less than the allowable, suggest a loop which will produce a maximum stress equal to or near Sa. Space does not permit a change in J7, but G can be changed. 

Figure 7-14. Simple U-type expansion loop for example 1.

Design a 17-type symmetrical expansion loop to yield the maximum moment and stress equal to the allowable limits. 

Figure 7-35. Pipe expansion loop program. Note Subroutines not included in the program listing. Solve step-by-step using the manual method described, using the functions duplicated as comment statements in the mainline program listing. The comment statements reveal all the basic thought and method.

J7-bend expansion loops similar to above, (use the term legs instead of tangents)... 

The use of the square comer technique with its conservative answers (sometimes by a factor of two or more) tends to indicate problems where there are none and may result in wasting investment and operating capital for unnecessary expansion loops. 

Figure 7-41 b. The expansion loop can t be taken off the long side. 

As shown in Figure 7-41b, it is doubtful that the piping configuration can be extended enough to allow for proper flexibility without exceeding b if we take the expansion loop off the long side. Therefore, the short side is more desirable. 

Change the configuration by adding 2A/ to the length of the expansion loop and check to see that b is not exceeded by this change. 

Change the configuration by subtracting 2Al from the length of the expansion loop. In this case, b does not require a further check. 

Figure 7-54. Important points in this typical line position arrangement of yard piping are heavy lines over columns, utility lines near the center, and horizontal expansion loops with hot lines on the outside of the loops.

Ellison, G.L. Find Best Expansion Loop Quickly. Hydrocarbon Processing Petroleum Refiner, January 1962, p. 151. 

Hilker, R.W. Which to Use Ball Joints or Expansion Loops. Hydrocarbon Processing Petroleum Refiner, February 1962, p. 185. 

Shull, W.W. Bogel, G.N. A Simplified Computer Program for Pipe Expansion Loop Design. Hydrocarbon Processing, September 1967, p. 183. 

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