Engineering guide
The NOLO module is a building block for the verification of nozzles under local loads: it determines the nozzle parameters that enter the calculation of the allowable local stresses at the opening. These include the nominal design stress of the nozzle material derived from the material strength value and safety factor, the analysis wall thicknesses reduced by the wall thinning and corrosion allowances, and the equivalent wall thickness of shell and reinforcement (e.g. a reinforcing pad).
Such parameters are needed whenever a nozzle must carry not only internal pressure but also external forces and moments from connected piping – a standard case in pressure vessel engineering, since piping reactions from thermal expansion and dead weight act directly on the nozzles. The dimensionless ratios and the nozzle coefficient that the module evaluates, together with their conditions of validity, follow the systematics of the code procedures for local loads on nozzles (such as EN 13445-3, Clause 16).
Typical workflow
Determine the nozzle material parameters
From the selected nozzle material, its strength value at the design temperature and the safety factor, the nominal design stress of the nozzle is formed.
Determine the effective wall thicknesses
Nominal wall thicknesses are reduced by the wall thinning allowance (minus tolerance) and the corrosion allowance; for the load-bearing wall at the opening, the analysis thickness of shell and reinforcement is combined into an equivalent wall thickness.
Check the geometry ratios
The dimensionless ratios (such as diameter and wall thickness ratios) are formed and checked against the conditions of validity of the procedure – the stored coefficient curves are applicable only within these limits.
Evaluate the nozzle coefficient
From the geometry ratios follows the nozzle coefficient together with its associated condition; it subsequently enters the determination of the allowable local loads and stresses at the nozzle attachment.
Frequently asked questions
What does the equivalent wall thickness of shell and reinforcement mean?
If the opening is reinforced by a reinforcing pad or a locally thicker shell, the additional wall within the load-bearing length may be credited. The code procedures combine this into an equivalent wall thickness with which the coefficients and allowable loads are calculated – it is always less than or equal to the sum of the individual thicknesses, since the reinforcement contributes only partially.
Why are the corrosion and wall thinning allowances deducted?
The verification must hold for the most unfavourable condition over the service life: the minus tolerance of the wall thickness may be missing from the start, and the corrosion allowance is consumed in operation. The calculation is therefore performed with the analysis wall thickness (nominal wall thickness minus both allowances), not with the as-ordered wall thickness.
What happens if a condition of validity of the ratios is violated?
The coefficient curves of the codes are validated only for certain ranges of diameter and wall thickness ratios. Outside these, the procedure does not deliver reliable values; a different verification method (e.g. stress analysis by stress categories or FEA) or a modified design is then required.
Technical content source: LV-Solved-Problems-Content 628d313247e7