by Greg Hollinger and George Antaki
One of the qualification requirements for a piping system is to keep the loads imparted by the piping on equipment nozzles within certain allowable limits. These loads consist of sets of three forces and three moments, for the various load combinations. There are basically two types of nozzle load limits: (1) nozzle loads applied to active equipment, and (2) nozzle loads applied to passive equipment.
Active Equipment Nozzle Loads
Active mechanical equipment consist of equipment with moving parts, such as pumps, compressors, and fans. The pipe nozzle load limits are developed by the equipment manufacturer and are intended to prevent malfunction, such as shaft misalignment, or distortion of the casing that could impede the movement of impellers. These limits are typically based on actual testing of the equipment, and not on analysis.
Some standards have published standard pump nozzle loads, but these are only valid for the particular pumps for which they are published. This is the case for the American Petroleum Institute’s API-610 and the hydraulic institute’s HI 184.108.40.206 standard.
A second consideration is the translation of the pipe nozzle loads to the equipment baseplate and anchorage.
Passive Equipment Nozzle Loads
Passive mechanical equipment consist of tanks and pressure vessels. In this case, the pipe load limits are based on stress or strain limits on the vessel or tank nozzle, its shell, and the translation of the pipe nozzle loads to the equipment supports, baseplate and anchorage.
The stress or strain limits are obtained either from the tanks and vessels codes (such as ASME VIII or API), or local stress using WRC Bulletins 537/107 and 297, or by finite element analysis to the rules of codes such as ASME VIII Div. 2 or ASME III NB-3200. The photograph illustrates that excessive nozzle loads can cause the nozzle-to-shell transition to rupture.
Nozzle Load Limits Using WRC Bulletins 537/107 and 297
WRC Bulletin 537 “Precision Equations and Enhanced Diagrams for Local Stresses in Spherical and Cylindrical Shells Due to External Loadings for Implementation of WRC Bulletin 107” is an enhanced replacement of WRC Bulletin 107 (WRC Bulletin 107 is no longer available) in which the WRC 107 Bulletin parametric curves have been re-drawn and polynomial curves provided for each parametric curve. WRC Bulletin 537/107 provides methodology for calculation of stress intensities in the shell-side juncture of a shell and a nozzle or solid attachment caused by “P, V, M and T” loads on the nozzle or solid attachment. When the shell is spherical or elliptical WRC-537/107 parameters U, U and r are used, and the nozzle/attachment may be solid (rigid) or hollow. When the shell is cylindrical, the nozzle or attachment is solid (rigid) and the parameters b and g are used. These parameters are valid in specific ranges for each parametric curve, and use of the parametric curves outside the applicable range must be avoided or at least carefully considered. Extrapolation significantly beyond the ranges of application should not be used. For the example of a nozzle represented by a rigid round attachment on a cylindrical shell, some of the curves, such as Figure 3A, for b approaching 0, the curve value, “Y” steeply approaches the minimum value of 0.001. Although it is possible to read this type of curve as b approaches 0, the use of the polynomial representation of these curves become inaccurate, and some of the commercial applications of WRC-537/107 generate warning messages even before b reaches the end of the curves near b=0. Other curves, e.q. Figure 1b are less problematic as b approaches 0. Interpolation between the g curves is permissible, but extrapolation of the g curves is not valid. Whatever the case, remaining within the range of parameters is important.
WRC Bulletin 297, “Local Stresses in Cylindrical Sells Due to External Loadings on Nozzles – Supplement to WRC Bulletin No. 107” provides parametric curves that were generated analytically with shell theory analysis software by C. R. Steele. The parametric curves in WRC Bulletin 537/107 were generated by a combination of testing analytical methods synthesized by P. P. Bijlaard of Cornell University. WRC Bulletin 297 applies only to nozzles on cylindrical shells loaded by “P, V, M and T” loads and where the nozzle is circular and hollow. Stress intensities on the shell-side of the juncture are provided as in WRC Bulletin 537/107. However, unlike WRC Bulletin 537/107, stress intensities are also provided on the nozzle side of the juncture. WRC-297 often computes higher stress intensities than WRC Bulletin 537/107, and sometimes the juncture stresses are extraordinarily high as reported in Appendix A of WRC Bulletin 297.
These two WRC Bulletins provide easy-to-model and use methods for evaluating nozzle/shell junctures, but they should be used within the range of parameters indicated in the Bulletins, and within other limitations discussed in the Bulletins. They provide primary local membrane stress intensities and primary-plus-secondary membrane-plus-bending stress intensities at four equally spaced locations circumferentially around the nozzle/shell juncture, and at the inside and outside surfaces. The Bulletins explain that it is possible that under some conditions of loading and geometry, stresses may be higher at other locations around the circumference, i.e., between points A, B, C and D and at locations away from the juncture.
Neither Bulletin includes primary local stress intensities due to internal pressure, although primary local stress intensities due to internal pressure can be superimposed, considering that the local primary membrane stress intensities due to pressure are magnified by a membrane stress concentrating factor on the general primary membrane stress intensity in the shell or in the nozzle. Such an approach is approximate compared to performing a more rigorous evaluation, such as using a finite element model.
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