Corrosion allowance in a local model: inside, outside, pads and clips

A shell element sits on the mid-surface of the wall, so the allowance changes two things at once: the thickness and where the wall is. The corroded thickness is the nominal one less the inside and the outside allowance. The inside diameter grows by the inside allowance, so the mid-surface sits at the inside radius, plus the inside allowance, plus half the corroded thickness. A thickness with nothing left is refused.

The decay length sets the mesh near the attachment and the size of the zone where local stress is read, so both follow the corroded wall. A thinner wall also raises the bending part of the local stress more than the membrane part, because bending stress goes with the square of the thickness.

A nozzle takes its own inside and outside allowance. Its wall loses both and its bore stays at the drawn size. The same holds for a heavy barrel or a variable neck. An inside projection keeps the inside allowance only.

Lengths along the nozzle are measured from the uncorroded outside surface of the vessel: the projection, the barrel, the neck points and the gusset height. So the loaded end of the nozzle does not move when the shell corrodes. The nozzle loads from the load table act where the drawing puts the flange face, in both conditions.

• A reinforcing pad or a clip pad loses its allowance off its outer face, the face exposed to the weather.
• A clip, lug or leg plate loses its allowance off each face. Each plate is then twice the allowance thinner. This is the conservative reading, chosen because both faces of a standing plate are exposed.
• The shell under a pad keeps its own outside allowance as well. The pad is welded over it, but the model does not credit that face as protected.
• Fillet welds are not in the shell geometry. Their sizes are taken as entered.

A clip plate 12 mm thick with a 1 mm allowance is checked at 10 mm. On a slender bracket that can move the stress more than the shell's own allowance does, because the plate's bending stiffness goes with the cube of its thickness and sets how much moment the plate passes into the shell.

Corroded is usually the worse case for stress, but not always. A thicker, stiffer new wall draws more of a displacement-controlled load, such as thermal growth of the attached line, so secondary stress can be higher new than corroded. The hydrostatic test is normally made on the new vessel. And a fatigue check on the new wall sees different weld-toe stresses.

With the new condition ticked, the elastic checks run a second time on the same model with every allowance set to zero: inside and outside, nozzle, pad and member. The worse condition decides the verdict, and the results line and the report table show both. Plots, solid details, limit load and buckling stay on the corroded model.

The WRC 537 calculator takes one allowance. The shell is checked at its thickness less the allowance and a pad at its thickness less the same allowance. There is no outside allowance and no second run. When the two tools are compared on one nozzle, check that the same allowances went into both.