Modeling Piping Connection to Storage Tank

Learn more about PASS/START-PROF pipe stress analysis software

Tank connections require special modeling considerations compared to pressure vessels. PASS/START-PROF automatically accounts for:

Analysis results appear in the Nozzle and Equipment Loads Table. See also Nozzle Load Reduction Methods in PASS/START-PROF

Tank nozzle objects at pipe ends automatically model settlement and thermal expansion. For intermediate nozzles, model these effects manually using rigid elements and cylindrical shells.

Tank object at end node:

Tank object at intermediate node. Model thermal expansion with rigid elements and settlement with anchor movement:

Vertical rigid element replaced by cylindrical shell:

Properties

 

Property

Description

Name

Element identifier. Sort and select elements in project tree

Material of Tank

Material from materials database

Manufacturing technology of Tank

For ASME B31.1, ASME B31.3, DL/T 5366-2014, seamless pipe uses Wl=1.0. Electric-welded pipe uses database Wl values. More...

For GOST 32388-2013, material properties vary by pipe type (seamless/welded).

Temperature of Tank

Design temperature in operating condition. More...

This property varies by operating condition. View values across conditions using button

Remove restraints for hanger selection

When enabled, removes specified restraints during spring hanger selection weight analysis. Restraints remain active during main analysis. This technique isolates weight loading from nozzle elements.

  • Remove vertical restraint - Releases vertical restraint during hanger selection

  • Remove custom restraints - Manually select restraints to release

  • Remove all restraints - Releases all directional restraints during hanger selection

Radius of Tank, R

Length from bottom to nozzle axis, L

Wall thickness of tank, t

Outer diameter of nozzle, 2a

Reinforcement On Neck, On Shell

Filling Height, H

Height from bottom to liquid surface. This property varies by operating condition. View values across conditions using button

Density of Product, G

Liquid density

Settlement of Tank, s

Tank settlement value. This property varies by operating condition. View values across conditions using button

Nozzle flexibility

Flexibility options:

  • Rigid - Zero flexibility, nozzle treated as rigid
  • Manually - Specify six flexibility values
  • By API 650 - Automatic calculation per API 650
  • By FEM - Automatic calculation via PASS/NOZZLE-FEM integration

FR - Axial flexibility along nozzle axis

FC - Transverse flexibility perpendicular to nozzle and vessel axes

FL - Transverse flexibility perpendicular to nozzle axis, parallel to vessel axis

MT - Torsional flexibility around nozzle axis

ML - Angular flexibility in nozzle-vessel plane

MC - Angular flexibility out of nozzle-vessel plane

Allowable loads

Allowable load options:

  • No check - No nozzle load verification
  • Manually - Specify six allowable loads
  • By API 650
  • By STO SA 93-002-2009
  • Stress check by FEM - Nozzle stress verification via PASS/NOZZLE-FEM integration
  • Allowable loads by FEM - Automatic allowable load calculation via PASS/NOZZLE-FEM integration

 

Tank Nozzle Movement from Thermal Expansion

Large tank diameters cause significant nozzle axial movement from thermal expansion. Calculated as . PASS/START-PROF models this movement plus vertical nozzle displacement from thermal expansion.

 

Tank Nozzle Movement from Settlement

Large tank diameters typically preclude foundations that distribute weight over large soil areas, resulting in settlement. Settlement magnitude depends on soil type, tank weight, and dimensions per geotechnical reports. Maximum settlement occurs at tank center, minimum at edges.

Piping connected to tank shell nozzles must account for settlement in stress analysis.

Reduce settlement effects by:

  1. Position first support well away from tank nozzle

  2. Use variable spring supports instead of rigid supports near nozzle

  3. Install lateral or gimbal expansion joints near nozzle

  4. Distinguish between short-term (post-filling/hydrotest) and long-term (continuous) settlement. Connect piping after short-term settlement completes. Analyze only long-term settlement.

First method works for minor settlement or large-diameter pipes with adequate self-support.

Second method may cause high nozzle loads since springs pull pipe upward and affect drainage. Manually select springs with travel range exceeding settlement and adequate capacity for pre/post-settlement reactions.

Third method increases cost and reduces service life due to expansion joints.

Model settlement in PASS/START-PROF as restraint downward movement using "From Other Influences" option.

Tank Nozzle Movement from Bulging Effect

Varying liquid levels create changing shell pressure, maximum at bottom. Shell expansion at bottom constrained by tank base causes radial nozzle movement and vertical plane rotation. Significant for tanks over 36 m diameter.

API 650 Appendix P provides shell radial growth from hydrostatic pressure:

Shell rotation from hydrostatic pressure:

G - Design liquid specific gravity

H - Maximum filling height (mm/in)

L - Vertical distance from nozzle centerline to tank bottom (mm/in)

R - Nominal tank radius (mm/in)

t - Shell thickness at opening (mm/in)

β - Characteristic parameter, 1.285/(R*t)^0.5 (1/mm)(1/in)

E - Modulus of elasticity (MPa/lbf/in²)

ΔT - Design temperature minus ambient temperature (°C/°F)

α - Shell material thermal expansion coefficient (mm/[mm-°C]/in/[in-°F])

 

Reduce nozzle rotation effects by turning pipe 90° close to tank nozzle.

Tank Nozzle Flexibility

PASS/START-PROF satisfies code flexibility requirements by creating rigid element length L, connecting tank nozzle to upper end, and applying flexible springs KR, KL, KC at lower end.

Tank Nozzle Allowable Loads

Two methods calculate allowable loads.

First method per API 650. Allowable moment (ML, MC) and axial force (FR) envelopes:

Second method per STO SA 93-002-2009. Allowable moment and axial force envelopes:

Applicable when D and DN values fall within specified envelope:

Menu and Toolbar Access

To insert tank nozzle, select the desired node and use: Insert > Equipment > Tank Nozzle