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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Equipment and piping in continuous processes also can experience fatigue due to the relentless mechanical loading/unloading of reciprocating compressors, piston pumps, bin vibrators or from vibration, etc., from any type of mis-aligned rotating equipment.

Fatigue failures in welded equipment most commonly occur in fillet welds where there’s an abrupt change in equipment geometry. Division 2 of the ASME Code designs around fatigue cracking in nozzles by limiting the use of fillet welds. However, fillet welds and sharp corners are ubiquitous in Div. 1 designs and can’t be avoided without cost.

Figure 2 -- Blend grinding of fillet
weld can prevent fatigue but the throat
of the weld (the distance from face
to root) must meet code requirements
after grinding.
Click on illustration for a larger image.

Crack initiation usually begins at the surface due to small microcracks. Therefore, surface smoothness is a good defense. Polished surfaces have four times the fatigue resistance [6] but polishing generally can’t be justified for fatigue life alone. Shot peening imparts compressive stresses into the metal surface that impede crack initiation but, again, only high-end applications can economically justify peening. For mid- to small-size process vessels, good weld quality often is the most economical defense against fatigue; so, state requirements in the equipment specifications. Because fatigue cracks often initiate at the toe or root of fillet welds, grinding the face to gently blend the weld into the base metal with a generous radius remarkably reduces stress risers (Figure 2). Another method to reduce stress risers is to TIG (tungsten-inert-gas) wash a weld toe to improve smoothness and remove microcracks. Initially target welds where cyclic loading is occurring. Experience has shown that most fatigue problems occur due to inadequately supported attachments or where saddles/supports lacked wear pads or rounded corners.

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.

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