Engineering guide
This module calculates flat walls, stays and stiffening girders to TRD 305 (1996 edition with 2002 supplements), part of the German Technical Rules for Steam Boilers (TRD). Flat ends and plates — circular, oval or rectangular, with or without a flanged rim, with or without openings — are sized for the design pressure at design temperature. The required wall thickness follows from the design diameter, the design coefficient from Table 2 (degree of edge restraint and construction type) and the allowable stress of the material.
In steam boiler practice, this covers flat boiler end plates, reversal chamber walls, tube plates outside the tube field, and wall areas supported by stays or stiffening girders, among others. For flanged flat ends, the knuckle radius, the inside height and the wall thickness in the flanged region are additionally verified; the required wall thickness of the adjoining shell is checked as well.
The TRD rules were withdrawn at the end of 2012. For recalculations and recurring inspections of existing boilers, TRD 305 remains the governing reference; for new designs, EN 12952-3 and EN 12953-3 apply, which treat flat walls with comparable coefficient concepts.
Scope
- Flat walls, anchorages and stiffening girders
Typical workflow
Define the design quantities
The design pressure is derived from the maximum allowable working pressure; the design temperature results from the reference temperature plus the temperature margin for the heating conditions of the wall.
Determine the allowable stress
The material yields the strength value at design temperature; dividing by the safety factor gives the allowable stress.
Determine the design diameter and the coefficient
The design diameter describes the governing unsupported plate dimension; for oval and rectangular panels it is derived from the aspect ratios. The design coefficient from Table 2 captures the construction type and the edge restraint — for example welded-in plates, flanged flat ends, stress-relief grooves or panels supported by stays.
Calculate the required wall thickness
From the design diameter, the coefficient, the pressure and the allowable stress, the required wall thickness follows; the allowance for wall thickness undertolerance is added and the result is compared with the as-built wall thickness.
Verify the flanged rim and the adjoining shell
For flanged flat ends, the as-built knuckle radius, the inside height and the wall thickness in the transition are checked against the minimum requirements; in addition, the required wall thickness of the adjoining shell is checked. Finally, the module reports whether all conditions are satisfied.
Frequently asked questions
What does the design coefficient from Table 2 depend on?
The coefficient C captures the support and edge restraint conditions of the plate: a rigidly clamped, welded-in end receives a different value from a flanged flat end or a plate with a stress-relief groove. The weld detail also enters, as does the question of whether the plate is supported by stays, stay bolts or stiffening girders. Choosing the wrong coefficient is the most common source of error in flat wall calculations.
Why are flat walls so much thicker than dished ends of the same size?
A flat plate carries the pressure in bending, not in membrane tension like a dished shell. The bending stress grows with the square of the span-to-thickness ratio, so the required wall thickness scales with the square root of the pressure and is significantly larger than that of a dished shape. Large flat panels are therefore subdivided into smaller design panels by stays or stiffening girders.
What do stays and stiffening girders contribute in the calculation?
Stays and stiffening girders reduce the governing design diameter of the plate panel and take over a defined share of the pressure load. They must themselves be verified for the introduced tensile force or bending, as must their attachment welds. The 2002 supplements to TRD 305 refine these verifications.
Is TRD 305 still applicable?
Not for new designs — the TRD rules were withdrawn at the end of 2012; the successors are EN 12952-3 and EN 12953-3. For assessing existing boilers whose design is based on the TRD, TRD 305 remains the consistent calculation basis, especially when modifications or changed operating parameters have to be recalculated.
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