Engineering guide
The TEM module verifies the longitudinal (axial) stresses in the shell and tubes of shell-and-tube heat exchangers with two fixed tubesheets to TEMA (1999 edition). In this configuration, shell and tube bundle are rigidly coupled through the tubesheets: operating pressures and the differential thermal expansion of shell and tubes generate axial tensile and compressive forces, which are captured via reference pressures per RCB-7.161 through RCB-7.164 and converted into stresses. On the compression side, stability (buckling of the tubes, buckling of the shell) must additionally be checked via factor A per ASME UG-23.
The TEMA 1999 method has been withdrawn or superseded in this form: for new designs to ASME Section VIII-1, the tubesheet and longitudinal stress calculation per Part UHX is now mandatory, which models the elastic interaction of the tubesheet–tube field–shell system more rigorously. The module nevertheless remains relevant for the reassessment of existing units designed and documented to TEMA 1999 — for repairs, re-ratings or changed operating conditions.
A typical application is answering the question of whether a fixed tubesheet unit can do without an expansion joint, or whether the axial restraint loading of the tubes and the tube-to-tubesheet joint requires an expansion joint in the shell.
Scope
- Longitudinal stresses
Typical workflow
Define operating conditions
The operating pressure and temperature of the shell side and tube side are recorded, together with the geometry (shell outside diameter, shell wall thickness, tube dimensions). Besides normal operation, start-up and shutdown conditions must be considered, in which only one side is under pressure or at temperature.
Form the equivalent expansion pressure
The differential thermal expansion between shell and tubes is converted into the equivalent expansion pressure per RCB-7.161; an expansion joint enters via its spring rate and reduces this component.
Calculate reference pressures of the load cases
Per RCB-7.163 and RCB-7.164, the reference pressures are formed that combine the shell-side and tube-side pressures with the expansion pressure — including the difference quantity P1 = Pt − Pt'.
Determine axial stresses
From the reference pressures, the longitudinal stresses in the shell and in the tubes follow for each load case — as tension or compression, depending on the sign of the resulting axial force.
Compare against allowable values
Tensile stresses are checked against the allowable tensile stress at design temperature; compressive stresses against the allowable compressive stress, which follows via factor A per ASME UG-23 from buckling stability. In addition, the load capacity of the tube-to-tubesheet joint (pull-out load of the expanded joint or weld) must be verified.
Frequently asked questions
Why can the tubes fail axially in compression even though the unit is under internal pressure?
The direction of the axial force in the tubes follows from the interplay of both pressures and the differential expansion. If the shell expands more than the tubes (hot shell, cold tubes), the tubes are pulled into tension; in the opposite case they come under axial compression. Since heat exchanger tubes are slender, it is then not the yield strength but buckling between the support points that governs — hence the limitation via the stability values per UG-23 with the free buckling length between the baffles.
What does 'Replaced by UHX modules' mean in concrete terms?
ASME has superseded the TEMA longitudinal stress and tubesheet calculation with Part UHX in Section VIII-1, which is based on the elasticity-theory solution of the perforated plate on an elastic tube-field foundation. For new code equipment, UHX is mandatory; the TEMA 1999 calculation of this module serves the reassessment of existing units within their original design code. Results of the two methods can differ noticeably, particularly for thin tubesheets and stiff shells.
Which load case typically governs for fixed tubesheets?
Frequently not steady full-load operation, but a transient condition: for instance start-up with hot fluid on one side only, shutdown, or the hydrostatic test of one side with the unit cold. In these conditions, the partial compensation of the components is absent, and the equivalent expansion pressure or the difference P1 = Pt − Pt' reaches its maximum. TEMA therefore requires the systematic investigation of all combinations.
When does an expansion joint help — and what is the price?
A shell expansion joint, through its low axial spring stiffness, often lowers the restraint forces from differential expansion by an order of magnitude and relieves the tubes, tubesheets and the tube-to-tubesheet joint. Disadvantages: an additional fatigue-loaded component with a limited number of load cycles, pressure limitations (thin-walled bellows) and additional inspection effort. Alternatively, U-tube or floating-head designs solve the problem by design, but make tube-side cleaning more difficult or make the unit more expensive.
Technical content source: LV-Solved-Problems-Content 628d313247e7