Engineering task and calculation objective
Half-pipe coils are the robust alternative to a jacketed vessel in equipment engineering: half pipes welded onto the vessel wall, routed longitudinally or helically, form heating or cooling channels for steam, thermal oil or coolant brine. Since the half pipes are themselves under pressure, not only the half pipe but also the vessel wall must be designed for this loading — the pressure in the half-pipe channel bends the shell wall between the welds like a plate.
The HLB module determines the required wall thicknesses for vessels with welded-on half pipes on the basis of the German AD 2000 code, Merkblätter B1 and B5. The half pipe is sized as a pressure-loaded shell element to B1; the vessel wall beneath the half-pipe channel is treated as a long rectangular plate to B5, whose span corresponds to the clear width of the half pipe. The orientation of the half pipes — longitudinal or circumferential — is taken into account in the superposition with the membrane stresses from the internal vessel pressure.
Typical applications are stirred vessels, reactors and storage tanks with temperature control, where half-pipe coils are preferred over jackets because of their pressure stiffness — especially at higher heating medium pressures, where a jacket would become uneconomically thick.
Standard and calculation basis: AD 2000 B1 & B5
Calculation workflow
- Define the orientation and geometry of the half pipes: First, the orientation of the half pipes is selected — circumferential (helical) or longitudinal along the vessel. Added to this are the half-pipe diameter, the pitch of the coils, the vessel diameter and the pressures in the vessel and in the half-pipe space, with the associated temperatures.
- Size the half pipe to AD 2000 B1: The half pipe is verified as a pressure-loaded shell to Merkblatt B1, using its diameter, the heating medium pressure and the design stress K/S of the half-pipe material; allowances for wall thickness undertolerance and corrosion are added.
- Calculate the vessel wall as a long rectangular plate: The shell wall beneath the half-pipe channel acts as a long rectangular plate to Merkblatt B5, supported by the two fillet welds of the half pipe. From the half-pipe pressure, the span and the applicable plate coefficient follows the required wall thickness of the vessel wall for this bending load.
- Check the superposition with the internal vessel pressure: The vessel wall simultaneously carries the membrane stresses from the vessel pressure to B1. Depending on the orientation of the half pipes, the plate bending is superimposed with the circumferential or the longitudinal stress; the most unfavorable combination of load cases governs, including the case where only one of the two spaces is under pressure.
- Define the welds and the final design: Finally, the attachment welds of the half pipes, which transfer the channel forces into the vessel wall, are evaluated and the as-built wall thicknesses of shell and half pipe are confirmed against the required values.
Input quantities
| Quantity | Symbol | Unit |
|---|---|---|
| Outside diameter of the half pipes | d | mm |
| Outside diameter of the shell | M | mm |
| Allowance c1 (half pipe) | c1R | mm |
| Allowance c2 (half pipe) | c2R | mm |
| Allowance c1 (shell) | c1M | mm |
| Allowance c2 (shell) | c2M | mm |
| Design temperature | δ | °C |
| Weld factor | v | – |
| Nominal design strength (half pipe) | KR | N/mm² |
| Nominal design strength (shell) | KM | N/mm² |
| Safety factor according to B0 table 2 | S1 | – |
| Safety factor according to B6 table 1 | S2 | – |
| Excess pressure in the half pipes | p1 | bar |
| Excess pressure in the shell vacuum = -1 | p2 | bar |
| Final wall thickness of the half pipes | s1 | mm |
| Final wall thickness of the shell | s2 | mm |
| Required shell wall thickness circumferential stress | s2' | mm |
| Required shell wall thickness longitudinal stress | s2'' | mm |
| Ruling wall thickness longitudinal/circumferential stress | s21 | mm |
| Required wall thickness according to AD design sheets | s22 | mm |
| Required wall thickness of the shell | s2 | mm |
| Design pressure (shell region) | p3 | bar |
| Required wall thickness of the half pipes | s1 | mm |
| Material (half pipe) | Rohr | – |
Calculation options
Select Half Pipe Orientation:
Half Pipes in Circumferential Direction · Half Pipes in Longitudinal Direction
Frequently asked questions
Why is the vessel wall calculated as a long rectangular plate?
The half-pipe channel clamps the shell wall in strips between its two welds. Since the channels are long compared with their width, the loaded wall strip behaves like a long rectangular plate under uniform pressure — the bending across the width dominates. Merkblatt B5 provides the appropriate plate formula with the corresponding coefficient for this case.
Half-pipe coil or jacket — when is which worthwhile?
Half-pipe coils stiffen the shell and carry the heating medium pressure through small cross-sections, so that even high pressures (e.g. steam from 6 bar upward or thermal oil systems) can be handled without thick walls; in addition, the coil enforces a defined flow with good heat transfer. The jacket offers the larger heat transfer surface, but at higher jacket pressures it quickly leads to thick walls or an external pressure problem for the inner vessel.
Must the vessel also be verified without internal pressure, when only the half pipes are under pressure?
Yes. The load case "half pipes under pressure, vessel unpressurized" frequently governs the plate bending of the wall, because the supporting membrane tension from the vessel pressure is missing. Conversely, the case of vessel pressure without heating medium pressure must also be checked. Both limiting load cases, alongside the combined operating condition, are part of the complete design.
What influence does the orientation of the half pipes have?
With half pipes routed helically or in the circumferential direction, the loaded wall strip runs circumferentially and the plate bending is superimposed with the longitudinal stress of the vessel; with longitudinally routed half pipes, it is superimposed with the hoop stress, which is twice as large. The orientation therefore influences the stress superposition in the vessel wall and is provided as a selection in the module.