DEU High-Pressure
U-Tube Exchangers

Three forms are common: a threaded breech-lock plug, a shear-key ring that locks a cover inside the channel barrel, and a bolted cover with a thin welded diaphragm as the pressure seal. All three move the pressure load off a bolted flange and into the forged barrel, which is why bolt count and gasket area stop growing with pressure. Each opens differently, and the diaphragm type has a seal weld that must be cut and remade at every opening. Agree on the closure type before the drawing is approved, because it sets the tools, the lifting gear and the outage time for the life of the unit.

No. The TEMA tubesheet formula makes thickness proportional to diameter times the square root of pressure over allowable stress, so doubling pressure adds about 41 percent while doubling diameter doubles it. Diameter, not pressure, is therefore the lever to pull when a tubesheet forging is on the critical path: a narrower, longer bundle at the same surface takes a thinner and more readily available forging. A thinner tubesheet also shortens the tube hole, which makes the expanded or welded joint easier to make and inspect.

Often, yes. Division 2 permits higher allowable stresses than Division 1 in exchange for design-by-analysis, tighter material control and fuller examination, and the thickness saving on a forged closure and a thick tubesheet can pay for the extra engineering. The gain is largest where sections are thickest, which is exactly where a D head lives. In small diameters Division 1 stays simpler and cheaper. HeatX can run the comparison on the actual duty before the forging is ordered.

Because a tube carries pressure in a wall sized by its own small diameter, while the shell has to carry the same pressure across the full shell diameter. Hoop stress is pressure times diameter over twice the wall, so a tube with one-twentieth the diameter of the shell needs one-twentieth the wall for the same stress. Put the high-pressure stream in the tubes and the D closure and tubesheet handle it while the shell stays an ordinary vessel. Reverse it and the shell becomes the forging, the shell flange at the tubesheet becomes the problem joint, and the removable-bundle advantage is spent on a closure nobody wanted to pay for.