A fixed-tubesheet exchanger ran about 18 months before a minor leak appeared at the shell outlet. Inspection found no corrosion in the tubesheet and intact tubes, but the tubesheet centre had visibly bulged and several expanded joints near it had loosened. Material certificates and welding records were in order. The problem was not manufacturing. It was the load combination: differential pressure, differential thermal expansion and bolt load had aligned at the same location, and exceeded the assumption the tubesheet was checked against — pressure alone.
This article does not list failure modes. It answers the three questions engineers ask on site, and each answer traces back to a combination of the five loads acting on every tubesheet: tube-side and shell-side differential pressure, axial and hoop stress from pressure, thermal expansion mismatch, thermal stress from the tubesheet-to-shell temperature difference, and flange bolt load.
The symptom is familiar: a bulge near the centre, a leak at the tube-to-tubesheet joint, a slow loss of bolt preload. Three loads combine to produce it.
Differential pressure creates a transverse bending load, resisted by tubesheet bending stiffness and by the tubes as an elastic foundation. The tubesheet-to-shell temperature difference adds an edge bending moment at that junction, and flange bolts add a third through gasket compression. Acting in one direction, these three push bending stress past the allowable; where the tubesheet is too thin for the combined load, deflection opens a leakage path at the tube joint. This is the most common in-service failure, a thickness and load-combination issue, not a material one.
Here the tubesheet is intact and the tubes are the visible damage: bowed, bulged in the middle of the bundle, sometimes collapsed against each other. The cause is the one load a fixed-tubesheet design cannot release.
When shell and tubes run at materially different temperatures they expand differently. A fixed-tubesheet design cannot absorb that differential by relative movement, so it converts into axial stress in tubes and shell; beyond the critical buckling value the tubes bow. Floating-head or U-tube designs relieve it by allowing the bundle to move. The check that catches it is an explicit tube axial stress and critical buckling calculation — tubesheet bending alone does not.
The tube slides out of the tubesheet hole, the joint leaks, the tube end sits below the tubesheet face. The cause is axial force on the tube exceeding the holding capacity of the expanded joint.
That force sums the pressure-driven and thermal-expansion axial loads. When the sum exceeds joint capacity the tube pulls out. Pull-out is a joint design issue as much as a load issue: expansion length, expansion degree, groove geometry and tube wall thickness set the holding force, and it is not predicted by tubesheet bending stress — it needs a separate joint capacity check.
For procurement and project engineers the review reduces to three questions on the drawing: which load combination governs tubesheet thickness, has the fixed-tubesheet buckling case been checked, and what axial load does the joint capacity correspond to? When those answers exist, the strength design is verifiable, not just plausible.
Differential pressure, junction thermal stress and bolt load can act in one direction; too thin a tubesheet then deflects and opens a leakage path at the tube joint.
Shell-to-tube temperature difference creates axial compressive stress that fixed tubesheets cannot release; beyond the critical buckling value the tubes bow. Floating-head and U-tube designs relieve it.
Axial force from pressure plus differential expansion exceeds joint capacity, set by expansion length, expansion degree, groove geometry and tube wall thickness.
Differential pressure interacting with differential expansion: each may be acceptable alone, but together they can exceed allowable bending stress or joint capacity.
