Avoid Costly Design Mistakes

Common errors keep plants from getting the most reliable and suitable vessels

By Chip Eskridge, Jacobs, Mike James, DuPont, and Steve Zoller, Enerfab

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• Gauge. Tubes come in different wall thicknesses (or gauge). Industry standards [7] detail the appropriate wall thickness based on material type and service. Table 3 provides guidance for a ¾-in. tube where no prior service history is available.

• Corrosion allowance. This typically isn’t added because tubes are considered a replaceable feature of the exchanger. If designing for a corrosive service, specifying the next-heavier-gauge wall thickness or choosing a higher alloyed tube material is more appropriate.

• Seamless versus welded tube. There’s a perception that seamless tubes are more reliable than welded tubes. This is currently less valid as some manufacturers have developed specialized techniques for making welded tubing that give products that show no preferential weld corrosion and have properties equal to those of seamless tubing [8,9]. Seamless tubing will cost more and usually has longer delivery. Welded tubing requires a greater amount of non-destructive examination (NDE), but this typically only adds pennies per foot of tubing if done at the mill [8,9].

Figure 3 -- Welded tubing (left) is
inherently more concentric than seamless
tubing (right). Eccentricity exaggerated
for illustration purposes; “A” is the
governing dimension, minimum wall thickness.
Click on illustration for a larger image.

Eccentricity is inherent in producing seamless tubes [10]. They typically are made by piercing, extrusion or pilgering, generally followed by sizing to obtain final dimensions. The inner mandrel/die can’t stay perfectly centered during the tube forming process. Welded tubes on the other hand begin with strip material which is very consistent in wall thickness. So, welded tubes tend to be more concentric (Figure 3). Seamless tube standards permit larger wall-thickness tolerances than those allowed by welded tube standards [11].

• Minimum versus average wall thickness. Minimum wall tubes cost a bit more than average wall tubing. When it’s unnecessary to use minimum wall tubing, such as for high pressure or corrosive service where metal loss is anticipated, it may be more economical to permit the use of average wall welded tubing and specify additional NDE or corrosion evaluation of the tube seam.

• Tube pattern. Shell and tube heat exchangers are the workhorses of the chemical industry. These units typically are fabricated with one of four types of tube patterns — 30°, 60°, 45° and 90° (Figure 4). Duty, pressure drop, cleanability, cost and vibration all depend on which pattern is chosen. Consider process needs, not cost, when making the selection.

A 30° or 60° pattern is laid out in a triangle configuration. The main benefit is that approximately 10% more tubes can fit in the same area as a 45° or 90° pattern. There’s very little difference between the 30° and 60° patterns. Often a thermal designer will run analyses of both patterns and select the one that provides the best pressure drop and vibration results. The disadvantage of a 30° or 60° pattern is that it’s difficult to mechanically clean on the shell side. Therefore, such a pattern is chosen for cleaner services; frequently the bundle isn’t removable.

Figure 4 – More tubes can fit with 30°

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