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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10. Tubing. This can be a significant cost element when ordering large heat exchangers. The cost of the tube can vary appreciably depending on the fabrication requirements specified. It’s not our intent to steer you away from the highest quality tube but merely to point out subtleties that can noticeably affect price.

• Diameter. Tubing is specified based on OD. For quickest delivery, stick to commonly stocked sizes, typically ¾-in. and 1-in. tubes for the chemical industry. Specifying smaller tubes (e.g., ½ in.) will increase the exchanger’s tube count and cost; this will improve duty but will cause higher pressure drop and may make mechanical cleaning more difficult. Therefore, only consider tubes smaller than ¾ in. for cleaner services or when increasing the shell diameter/length isn’t an option. Larger tubes (>1 in.) have the opposite effect but may be necessary to satisfy process conditions. Another option to increase effective surface area without changing tube diameter is to specify finned tubes or twisted tubes — but those are limited to clean applications.

• Length. Tubes are stocked in 20-ft lengths. Seamless tubes are made from individual billets or hollows and so can vary in length by one to two feet. The length of welded tubes is more exact because they’re produced from a continuous strip coil. The most wasteful and costly option for stocked tubes is ordering units just over 10 ft in length because nearly 50% of the tube is discarded. As tube count increases, direct mill orders become economically attractive; in such cases, any length tube can be supplied, if your schedule allows. Mills have minimum orders (i.e., 2,000 lb.–2,500 lb.), though “mini-mills” will take orders at half these quantities.

• 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].

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