Engineering task and calculation objective
The KAP12 module calculates unpierced tubes and tube plates to Chapter 12 of DIN EN 12953-3:2016, the design and calculation standard for the pressure parts of shell boilers. It thus covers two closely related components of the smoke-tube boiler: the plain smoke tubes and stay tubes, which simultaneously penetrate and stay the end plate, and the tube plates perforated by these tubes with their ligament fields.
In practice, this verification is needed for every design or re-rating of a three-pass or smoke-tube boiler: for the tubes, the required wall thickness under pressure loading must be determined, with smoke tubes being externally pressurized from the water side and additionally carrying tensile loads from the flat end plates when acting as stays. For the tube plate, it must be verified that the ligaments between the tube holes, weakened by the perforated field, can safely carry the pressure and stay loads.
The module evaluates the corresponding equations of DIN EN 12953-3 and delivers required wall thicknesses, allowable loads and the utilization of the individual checks, so that tubes and tube plates of a shell boiler can be calculated and documented in accordance with the code.
Calculation workflow
- Define tube geometry and loading type: For the tubes, the outside diameter, wall thickness and direction of loading are recorded: boiler tubes and smoke tubes are under external pressure from the water side, while superheater tubes, for example, may be under internal pressure. It is also specified whether the tube acts as a stay tube carrying tensile loads from the end plates.
- Enter design pressure, temperature and material: From the design pressure and design temperature together with the tube and plate material, the module determines the allowable design stresses according to the strength rules of the standard, including the allowances for wall thickness tolerance and corrosion.
- Determine the required tube wall thickness: Using the equations of Chapter 12, the required wall thickness of the unpierced tubes is calculated or the existing wall thickness verified; for stay tubes, the additional axial tensile stress from staying the flat end plates is taken into account.
- Verify the tube plate in the perforated field: For the tube plate, the load-bearing capacity of the ligaments between the tube holes is assessed: pitch and hole diameter yield the weakening of the plate, while pressure and stay loads determine the stresses in the ligaments in the governing directions.
- Check the tube-to-tube plate connection and limits: Finally, the requirements for the connection between tube and tube plate (welded or expanded) and the geometric limits of application of the chapter are checked, so that the verification remains within the scope of the standard.
Input quantities
| Quantity | Symbol | Unit |
|---|---|---|
| load case | case | – |
| Type of componenet | componenet | – |
| Smoke tube | tube | – |
| Type of Boiler | Boiler | – |
| Subjected to heat transfer | transfer | – |
| Gas entery temperature | tG | – |
| Nominal wall thickness | e | mm |
| Heating by | by | – |
| Type of pressure | pressure | – |
| Calculation internal pressure | pc | MPa(p) |
| Calculation external pressure | pca | MPa(p) |
| Saturation temperature | ts | °C |
| Additional temperature | tz | °C |
| Calculation temperature | tc | °C |
| Bending Radius | R | mm |
| Outside diameter | do | mm |
| Geometric condition for do | do | – |
| Condition of R/do | R/do | – |
| Design factor intrados | Ci | - |
| Design factor extrados | Co | - |
| Ratio R/do | R/do | - |
| Nominal wall thickness | et | mm |
| Minimum thickness according to table 7 | eL | mm |
| Condition acc.to.table 7 | 7 | – |
Calculation options
load case
operation · test
Type of componenet
Tube · Tube bends
Smoke tube
No · Yes
Type of Boiler
Low Pressure Boiler · High Pressure Boiler
Subjected to heat transfer
Yes · No
Gas entery temperature
≤ 400 · 400-800 · > 800
Heating by
radiation · convection
Type of pressure
internal · external · both
Frequently asked questions
Why are smoke tubes calculated for external pressure even though the boiler is under internal pressure?
The boiler pressure acts in the water space, i.e. on the outside of the smoke tubes; the flue gas inside the tube is practically at ambient pressure. For the individual smoke tube, the loading is therefore external pressure. For thin-walled tubes of small diameter this is usually uncritical, but it must still be verified, because under external pressure stability plays a role in addition to stress.
What is the difference between a smoke tube and a stay tube?
Both carry flue gas, but stay tubes additionally take on a structural function: they stay the flat boiler end plates against each other and carry the tensile forces resulting from the pressure on the end plates. Stay tubes are therefore usually made with a thicker wall and are verified with the superimposed axial stress; their arrangement in the tube field also defines the plate panels for the end plate verification.
How does the tube pitch enter the tube plate verification?
The closer the tube holes lie together, the narrower the load-bearing ligaments between the holes and the greater the weakening of the plate. The verification is based on the ratio of remaining ligament width to pitch in the governing directions of the perforated field; the smallest ligament width determines the required plate thickness.
Does the chapter also apply to tubes with holes or branch connections?
No. Chapter 12 explicitly deals with unpierced tubes. As soon as a tube is weakened by holes, branches or nozzles, the opening rules of the standard apply, in the cylindrical shell for example Chapter 8 on openings and branches.