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Becht Engineering Blog

In this section of the site contributing authors submit interesting articles relating to the various services, industries and research & development efforts of Becht Engineering.

Discussions on events, projects and activities with a nuclear flavor.

The Meaning of the Class 1 Nuclear Piping Equations in NB-3650

piping_systems
Below we review the meaning of the six Class 1 piping stress equations in ASME III NB-3650, Class 1. The nomenclature is found in NB-3650. Two things must be kept in mind when applying these Class 1 piping equations: All the stresses are elastically calculated, i.e. the calculation model considers the material to be such that s = E e, even when the stress exceeds yield. All the stress terms are stress intensities, i.e. twice the maximum shear stress. Using the Tresca criterion for elastic behavior, the stress intensities can be compared directly to yield. EQUATION (9) B 1 (P D o )/(2 t) + B 2   D o /(2 I)  M i  ≤ k S m     Eq.(9) Equation (9) is a primary stress intensity amplitude equation. It is meant to prevent failure by structural instability of the joint or fitting (elbow, tee, reducer, etc.). For example,...
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Evolution of Pipe Stress Equations in the ASME Nuclear Codes (and where we are today)

piping_systems
The Pre-1955 Era Until 1955, the ASME Boiler and Pressure Vessel Codes, and the ASME B31 Pressure Piping code provided rules of good design practice with quantitative criteria for pressure design.  For example, quoting from ASME VIII Rules for the Construction of Unfired Pressure Vessels Section VIII A. S. M. E. Boiler Construction Code, 1927: U-20. For Internal Pressure. The maximum allowable working pressure on the shell of a pressure vessel shall be determined by the strength of the weakest course, computed from the thickness of the plate, the efficiency of the longitudinal joint, the inside diameter of the course, and the maximum allowable unit working stress.  (S×t×E)/R = maximum allowable working pressure Where S = maximum allowable unit working stress in lb. per sq. in. = 11,000 lb. per sq. in. for steel plate stamped 55,000 lb. per sq. in., 10,000 lb. per sq. in. for steel plate stamped...
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Capacity Reduction of a Compression Member with an Initial Deformation

piping_support_3D
A recent review of equipment installed in a power plant found a piping strut that was visibly bent. Would the bend in the strut reduce its load bearing capacity as a compression member? The answer is YES!  The load bearing capacity of a bent strut against buckling is reduced. The reduction in buckling capacity is a function of the strut’s section properties and its initial deformation. We will see here how this can be quantified analytically (closed-form solution) and numerically (FEA solution). The equation for the maximum axial load capacity of a column, before it buckles, was first formulated by Leonhard Euler in 1757. F cr  = (π 2  E I)/(K L) 2 Where E = Young’s modulus of column material; I = cross-sectional moment of inertia of the column; L = unbraced length of column; and K is a factor that depends on the degree of fixity in the...
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Evaluation of Corroded Pipe in Accordance with ASME B&PV Code Section XI - A Comparison of the Three Code Cases

Evaluation of Corroded Pipe in Accordance with ASME B&PV Code Section XI -
A Comparison of the Three Code Cases
The evaluation of wall thinning corrosion in steel pipes is addressed in three ASME XI code cases: N-513, N-597, and N-806. I have no ambition here other than to summarize in a table the differences between these three code cases. A brief commentary follows the table. The brief commentary... Line A – While these are Section XI Code Cases applicable to ASME III Class 2 and 3, technically, nothing would prevent from applying these Code Cases for B31.1 piping. Line B – There is no technical basis for limiting N-513 to moderate energy lines, i.e. pressure at or below 275 psi and (“or”, depending on the plant vintage) temperature at or below 200oF, other than the understandable reluctance to operate with hot water flashing to steam through a pinhole leak. Line D – It can be confusing that N-513 includes fracture mechanics consideration because it attempts to address cracks (so-called...
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