Engineering guide
This module calculates dished furnace-tube end plates (furnace end heads) to TRD 304, part of the German Technical Rules for Steam Boilers (TRD). In shell boilers and fire-tube boilers, these dished ends connect the furnace tube to the cylindrical shell; the spherical cap is loaded by the operating pressure from the water side and additionally by the forces introduced by the attached furnace tube. From the maximum operating pressure, the design temperature, the material strength value and the safety factor, the required wall thickness of the end including its allowances is determined.
Geometrically, the inside diameter of the spherical cap and the outside diameter of the connected cylinder enter the calculation. The allowances for wall thickness undertolerance and for corrosion and wear are reported separately, and the wall thickness including allowances is compared with the as-built end plate thickness.
TRD 304 dates from 1964 and, like the entire TRD code, was withdrawn at the end of 2012. For recalculating and assessing existing fire-tube boilers, it remains the governing historical design document; new shell boilers are designed to EN 12953-3.
Scope
- Dished tubesheets
Typical workflow
Define the operating conditions
The maximum operating pressure and the design temperature are taken from the boiler data. For flue-gas-wetted dished ends, the thermal loading due to heating must be reflected in the design temperature.
Determine the material and the allowable stress
The selected material yields the strength value at design temperature; together with the safety factor, this gives the allowable stress of the end plate.
Record the geometry
The governing dimensions are the inside diameter of the spherical cap (crown radius region) and the outside diameter of the connected cylinder. The dished geometry determines how the pressure load is carried into the end as membrane and bending stresses.
Calculate the required wall thickness
The theoretical wall thickness of the end is determined from pressure, geometry and allowable stress. The allowances for wall thickness undertolerance and for corrosion and wear are added.
Compare with the as-built wall thickness
The wall thickness including allowances is compared with the as-built end plate thickness. The verification is satisfied if the as-built thickness equals or exceeds the required value.
Frequently asked questions
How does a furnace-tube end plate differ from an ordinary dished end?
In addition to the pressure from the water space, a furnace-tube end plate also carries the forces from the attached furnace tube, which expands thermally and is supported by or anchored to the end. The end is also frequently flue-gas-wetted and therefore runs hotter. TRD 304 addresses exactly this combination, whereas dished ends without a furnace tube connection are calculated to TRD 303.
Does TRD 304 still apply to new boilers?
No. The TRD rules were withdrawn on 31 Dec 2012; new shell boilers are designed to EN 12953-3 in conjunction with the Pressure Equipment Directive. However, TRD 304 is still needed to trace the original design of existing fire-tube boilers, to assess repairs and to support remaining-life evaluations.
Which allowances are particularly important for flue-gas-wetted ends?
Besides the allowance for the plate's wall thickness undertolerance, the corrosion and wear allowance is decisive, because flue-gas-side corrosion and water-side deposits reduce the load-bearing wall thickness over the service life. When recalculating existing plants, the actually measured remaining wall thickness should be used instead of the nominal wall thickness.
Technical content source: LV-Solved-Problems-Content 628d313247e7