BEU U-Tube
Heat Exchangers

You cut it, which is why tube-side nozzles on a BEU should be flanged with a short removable spool on each. Hard-piped bonnets are the most common reason a removable bundle never gets removed. Both tube nozzles are on the bonnet, so both spools come out before it moves. Leave room to swing the head clear and set it down, and leave the bundle-pull length open beyond that.

Yes; the two design pressures are set independently and the tubesheet is designed for the worst differential. The check that gets missed is the tube wall under external pressure, and on a U-tube it is worst at the bend, where the outer radius thins during forming. TEMA requires the wall before bending to be the design wall multiplied by one plus the tube OD over four times the bend radius. At the TEMA minimum centerline radius of 1.5 times OD that is 17 percent extra wall on the innermost rows. Specify the shell-side design pressure honestly, including any relief or vacuum case, so that allowance is in the tube order.

No, and mixed builds are routine: stainless or 90/10 CuNi tubes in a carbon steel tubesheet, or carbon steel tubes in a clad tubesheet. A rolled joint works between dissimilar metals because it relies on the tube expanding past yield into the hole. A strength-welded joint needs a compatible filler and often a weld overlay on the tubesheet face. The galvanic question lives on the tube-side wetted face, where a large area of noble tube metal meets a small area of carbon steel; in aggressive water, specify that face clad.

Yes. The bundle is free to grow, but the tubesheet is not: steam on one face while the other is at ambient sets up a gradient through the plate that tries to dish it, and the tube joints at the hot face take the strain. Rolled joints loosen under repeated shock, and the symptom is weeping at the tubesheet that appears months after a unit was commissioned with a snap-open steam valve. Establish tube-side flow first, then bring steam on over several minutes through a bypass or a slowly opened valve. The U-bend handles the differential growth; the warm-up protects the joints.

No; absorbing that difference is the job the U-tube does. Stainless grows about 9.6 millionths of an inch per inch per degree F and carbon steel about 6.5, so a 120 inch stainless bundle running 200 F above ambient grows about 0.23 inch while a carbon steel shell at 100 F above ambient grows about 0.08 inch. A fixed tubesheet unit forces that 0.15 inch into the tube joints or into a bellows. In a BEU the bend simply flexes.