Engineering task and calculation objective
The TEB1 module calculates the required thickness of tubesheets to TEMA Section 5, paragraph RCB-2.21 (1978 edition). The tubesheet — the perforated plate into which the tubes of the bundle are expanded or welded — is treated as a plate in bending under the governing design pressure; the boundary conditions of the gasketed joint enter via the mean gasket diameter and a configuration-dependent factor.
In practice, this calculation is needed for the design and reassessment of shell-and-tube heat exchangers to TEMA standards, particularly when older units are documented to the 1978 edition and have to be retraced for repair, modification or periodic inspection. The input quantities are the design pressures of both sides, the design temperature, the mean gasket diameter, the material and allowable stress of the tubesheet, and the corrosion allowance; the results are the required and the final tubesheet thickness.
The TEMA tubesheet formula is deliberately semi-empirical: the supporting effect of the tube field and the edge restraint are captured by lump-sum factors, which makes the calculation fast and robust — in contrast to the elasticity-theory methods of more recent codes.
Standard and calculation basis: TEMA Section 5 - RCB-2.21: 1978
Calculation workflow
- Define load cases and design pressures: The governing case is the most unfavourable of the load cases examined: shell-side pressure, tube-side pressure or both together — in each case observing whether the pressures may add up at the tubesheet or partially cancel.
- Determine edge restraint conditions and factor: From the configuration of the tubesheet edge connection (gasketed, integral, supported), the TEMA factor F follows, which describes the boundary condition of the plate. For gasketed tubesheets, the mean gasket diameter G serves as the effective diameter of the pressure-loaded plate.
- Determine allowable stress: For the tubesheet material, the allowable stress at design temperature is determined from the material tables.
- Calculate required tubesheet thickness: With the factor F, diameter G, governing pressure and allowable stress, the TEMA bending formula yields the required thickness; in addition, the shear check at the perimeter of the tube field must be verified, which can govern at small diameters and high pressures.
- Allowances and final thickness: The corrosion allowance (on both sides, depending on the fluid) and, where applicable, groove depths for gaskets and pass partitions are added to the calculated thickness; the final tubesheet thickness must cover the sum.
Input quantities
| Quantity | Symbol | Unit |
|---|---|---|
| Design pressure | p0 | bar |
| Design pressure | P0 | N/mm² |
| Design temperature | T0 | °C |
| Mean diameter of gasket | G | mm |
| Factor | F | – |
| Final head thickness | Te | mm |
| Allowance (corrosion) | c2 | mm |
| Material | Werkstoff | – |
| Allowable stress | S | N/mm² |
| Required head thickness | T = | mm |
Frequently asked questions
Why may the tubesheet be thinner than an unperforated plate of the same size?
Because the tube field shares the load: the expanded or welded-in tubes support the plate like elastic springs and take over part of the shear force. The TEMA formula captures this in a lump-sum manner through its factors. Conversely, the holes weaken the load-bearing cross-section — both effects together are calibrated into the semi-empirical formula. This works reliably within the geometries typical of TEMA, but cannot be transferred to arbitrary special designs.
Which load case governs the tubesheet thickness?
Not automatically the highest individual pressure. For integral configurations with two tubesheets, thermal restraint forces and flange moments can generate additional equivalent pressures, and a one-sided pressure (for instance tube-side only during start-up) can be more unfavourable than the operating condition with both pressures. TEMA therefore requires the investigation of all combinations that can realistically occur; the largest resulting differential pressure determines the thickness.
Is the 1978 edition of the TEMA standards still applicable?
For new designs, the current TEMA standards apply, or — for ASME code equipment — the mandatory tubesheet calculation to ASME VIII-1 Part UHX. However, RCB-2.21:1978 remains important for the reassessment of existing units that were designed and documented to this edition: repairs and evaluations are sensibly carried out first within the original code before switching to a newer method.
How does the gasket enter the calculation?
For gasketed tubesheets, the mean gasket diameter G limits the pressure-loaded area and the lever arm of the plate bending — it is the effective span of the calculation, not the bolt circle or outside diameter. In addition, the gasket type influences the boundary condition: a gasketed seat acts more like a simple support, an integral connection with shell or channel more like a fixed edge, which TEMA captures via different F values.