Nozzle Loads on Pipe Fittings and Equipment: WRC 107, WRC 297, and Allowable Load Assessment
Every pipe fitting connected to a pressure vessel, heat exchanger, pump, or compressor nozzle imposes loads on that nozzle from the piping system — forces and moments arising from thermal expansion, pipe weight, pressure thrust, wind, and seismic loads. Excessive nozzle loads can crack the vessel shell, distort the nozzle flange face, overload equipment casings, or fracture the fitting at the nozzle connection weld. Assessing nozzle loads requires the pipe stress engineer to check both fitting stress (per ASME B31.3 or B31.1) and vessel/equipment nozzle capacity (per WRC 107, WRC 297, or FEA).
WRC Bulletin 107: Local Stresses in Cylindrical Shells
WRC Bulletin 107 provides dimensionless stress coefficients for calculating local membrane and bending stresses in a cylindrical or spherical shell at a nozzle attachment, as functions of nozzle-to-shell diameter ratio (d/D), shell mean radius-to-thickness ratio (R_m/T), and attachment type. Six load components — three forces (F_x, F_y, F_z) and three moments (M_x, M_y, M_z) — are applied independently and stresses are superimposed. WRC 107 is applicable when d/D < 0.3 approximately and R_m/T is in the range 10–100. Outside these bounds, FEA is required. Local stresses are compared to allowable limits — typically 3S_m (allowable design stress intensity from ASME Section VIII or B31.3) for primary plus secondary stress. The fitting at the nozzle connection carries the piping loads directly into the nozzle interface; SIF factors from B31.3 Appendix D apply at the fitting, meaning local fitting stress can be roughly twice the nominal pipe stress.
WRC Bulletin 297 and FEA for Large Nozzles
WRC Bulletin 297 extends the method to d/D up to approximately 0.5 and provides coefficients for loads applied to the nozzle itself — more representative of how piping loads are actually transmitted. Preferred for large process nozzles on heat exchangers and columns. For d/D > 0.5, neither bulletin is reliable — FEA of the nozzle-shell junction is required, per ASME Section VIII Division 2 Part 5.
Rotating Machinery: API 610 Nozzle Load Limits
For rotating equipment (pumps per API 610, compressors per API 617), nozzle load limits are far more restrictive than shell stress limits for static vessels. Exceeding API 610 limits causes casing distortion (mechanical seal failure), shaft deflection (increased bearing loads), and in severe cases rotor-to-casing contact. The pipe stress engineer must design pump suction and discharge piping — including all elbows, tees, and reducers within the first 5–10 diameters of the pump nozzle — to keep forces and moments within API 610 allowables. This typically requires careful routing, flexible pipe loops, or spring supports close to the nozzle. Cold spring at the pump nozzle connection is used for large pumps with tight allowables.
Sustained vs Occasional Load Combinations
Nozzle load assessment must consider sustained loads (weight, pressure, thermal expansion — acting continuously) and occasional loads (wind, seismic, pressure surge — acting for a fraction of plant life). ASME B31.3 allows 1.33× overstress for occasional loads. The sustained + occasional combination is assessed against 1.33 × S_h (hot allowable stress) for each load case. A tee adjacent to a nozzle has in-plane SIF ≈ 2.0 and out-of-plane SIF ≈ 1.8 per B31.3 Appendix D — the fitting governs the piping flexibility design even when nominal pipe stress is within allowable.