Pressure Vessels: 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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• 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°
and 60° configurations but mechanical
cleaning may be harder.
Click on illustration for a larger image.

The 45° or 90° pattern is selected if shell-side mechanical cleaning is required. Such a pattern also requires a removable bundle. The 45° is more common than the 90° because it provides more shell-side flow disturbance, which improves heat transfer. A 90° pattern is used to reduce pressure drop at the expense of duty and often is employed in boiling service to enable better vapor disengagement.

Make the right choices
In today’s chemical industry, too many engineers given the task of specifying welded equipment such as vessels, heat exchangers and tanks aren’t well versed in what’s necessary to develop an economic design that provides suitable safety and performance. Myriad choices must be made — and each will incrementally add to the final cost and schedule. When looking for savings, cutting the wrong corners may turn out to be very costly over the equipment’s service life.

Chip Eskridge is a principal engineer with Jacobs, Louisville, Ky. Mike James is a senior consultant, materials engineering, for DuPont, Houston. Steve Zoller is director of fabricated equipment for Enerfab, Cincinnati. Reach them via e-mail at, and

1. “Vessel Design Manual,” ITT Industries, White Plains, N.Y. (2000).
2. “Loadings,” Paragraph UG-22, ASME Code, Sect. VIII, Div. 1, ASME, New York City (2007).
3. Young, W.C. and R. Budynas, “Roark’s Formulas for Stress and Strain,” 7th ed., McGraw-Hill, New York City (2002).
4. Brooks, C.R. and A. Choudhury, “Metallurgical Failure Analysis,” McGraw-Hill, New York City (1993).
5. “Fatigue Screening Criteria,” Table 5.9, ASME Code, Sect. VIII, Div. 2, ASME, New York City (2007).
6. Deutschman, A.D., W.J. Michels and C.E. Wilson, “Machine Design: Theory and Practice,” Macmillian, New York City (1975).

7. “Shell and Tube Heat Exchanger Specification,” PIP VESST001, Process Industry Practices, Austin, Texas (2007).
8. James, M.M. and D. O’Donnell, “When to Specify Welded, Welded and Drawn, or Seamless Tubing,” Welding Journal (June 2008). Available online at
9. James, M.M., “Development of High Quality Welded Heat Exchanger Tubing in Lieu of Seamless,” presented at ATI Corrosion Solutions Conference, Sunriver, Ore. (2007).
10. Kearns, W.H., “Minimizing Wall Thickness Variation in Seamless Tubing,” The Fabricator, (Aug. 28, 2003),
11. “Specifications for General Requirements for Carbon, Ferrite Alloy, and Austenitic Alloy Steel Tubes,” ASME/ASTM SA/A-450, ASME, New York City (2004).


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