CAESAR II Pipe Stress Workflow: Sustained, Thermal Expansion, and Occasional Load Cases

Accurate pipe stress analysis begins with precision modeling of isometric geometry and realistic restraint boundary conditions. In CAESAR II, engineers must never treat equipment connections as infinitely rigid anchors without verifying vessel flexibility or skid deflection. Key initialization inputs required before solving equations: 1. **Material Properties per ASME II-D:** Modulus of elasticity ($E_h$), Poisson's ratio ($ u = 0.3$), thermal expansion coefficients ($alpha$), and yield strength ($S_y$) across operating temperature extremes. 2. **Corrosion Allowance & Mill Tolerance:** Deduct corrosion allowance ($CA$) and 12.5% mill tolerance from nominal wall thickness for pressure hoop stress verification, while evaluating flexibility on uncorroded nominal thickness. 3. **Flange & Valve Modeling:** Model heavy gate and control valves with concentrated weight points and rigid length segments to accurately capture eccentric center-of-gravity moments.

CAESAR II solves matrix stiffness equations based on user-configured load combinations. A robust ASME B31.3 stress dossier mandates at minimum the following load case architecture:

// Standard Static Load Case Matrix in CAESAR II
L1: W + P1 + T1            (OPE) -> Operating Case 1: Operating Restraint Loads
L2: W + P1                 (SUS) -> Sustained Case: Gravity + Design Pressure
L3: L1 - L2                (EXP) -> Expansion Range 1: Thermal Range Stress
L4: W + P1 + T1 + WIN1     (OPE) -> Operating + Wind Case: Max Restraint Reactions
L5: L4 - L1                (OCC) -> Pure Occasional Component
L6: L2 + L5                (OCC) -> Sustained + Occasional: Evaluated vs 1.33*Sh

While pipe stress percentages may show a comfortable 65% utilization against ASME B31.3 allowable limits, the reactions exerted on equipment nozzles often exceed vendor tolerances by 200–300%. For centrifugal pumps adhering to **API 610 Table 5**: - Individual nozzle forces ($F_x, F_y, F_z$) and moments ($M_x, M_y, M_z$) must not exceed allowable values. - Combined resultant force ($F_R$) and resultant moment ($M_R$) must meet equation requirements. - Combined pump casing qualification (sum of suction and discharge nozzles transferred to pump center) must satisfy the API 610 interaction formula. When reactions exceed allowables, our stress engineers iteratively introduce expansion loops, directional guide supports, or variable spring hangers rather than oversized piping anchors that transfer destructive loads to foundation pedestals.

When vertical thermal growth ($Delta Y$) exceeds 0.25" (6 mm) at a support location, rigid supports are non-viable because the pipe either lifts off the support (transferring entire weight to adjacent nozzles) or binds downward (causing support failure). CAESAR II features automated hanger selection algorithms (Anvil, Lisega, Carpenter & Paterson, Grinnell). The software calculates: 1. **Operating Load (Hot Load):** Vertical reaction at operating temperature. 2. **Installation Load (Cold Load):** Restraint force when the system is cold. 3. **Variability Index:** Defined as $( ext{Hot Load} - ext{Cold Load}) / ext{Hot Load} imes 100%$. Under MSS SP-58, variability must remain under 25%. If variability exceeds 25%, constant effort hangers must be deployed.