HDPE Piping Stress Analysis (and other Thermoplastic Piping PP, PVC, PVDF, PE-RT, etc.)

 

General Information

The function described in this topic should be used only for non-reinforced, stiff plastic piping.

For reinforced thermoplastic piping we recommend to use fiberglass piping code.

For flexible plastic piping we recommend to use flexible plastic piping function in START-Elements.

The main features of thermoplastic piping unlike steel piping is:

PASS/START-PROF uses GOST 32388-2013 method for thermoplastic piping analysis.

Axial Expansion

Axial expansion of the thermoplastic pipe is:

Thermal expansion e1 is

k - Temperature range factor, taking into account the nonlinear temperature distribution across the wall thickness, from pipe additional properties, a - thermal expansion factor from material database

Pressure elongation e2 is

t - wall thickness, D - pipe outer diameter, m - Poisson's ratio at operating temperature, p - operating pressure, Ecm - average creep modulus

Swelling elongation e3 is

Elastic Modulus (Creep Modulus)

Creep modulus depends on the service life from Project Settings, temperature and stress value.

There are 3 types of creep modulus:

EminT - creep modulus at ambient temperature, service life, and current element stress taken from material database

EmaxT - creep modulus at operating temperature, service life, and current element stress taken from material database

Ambient temperature and service life are taken from Project Settings

E1.6minT - creep modulus at ambient temperature, service life=100 minutes=1.667 hour, and current element stress taken from material database

E1.6maxT - creep modulus at operating temperature, service life=100 minutes=1.667 hour, and current element stress taken from material database

E0.1minT - creep modulus at ambient temperature, service life=0.1 hour, and current element stress taken from material database

E0.1maxT - creep modulus at operating temperature, service life=0.1 hour, and current element stress taken from material database

The creep modulus in material database depends on service life, stress, and temperature:

Allowable Stress Calculation

Stresses from primary (sustained) loads (weight, pressure)

Ky - Safety factor from pipe properties,

Kc - Joint strength factor from pipe properties,

Kx - Chemical resistance factor from pipe properties,

Kp - Laying condition factor from pipe properties,

Kt=1

Kcyc=1

- nominal long-term allowable stress

where

A1, B1, G1, J1 - characteristic factors for left curve from material database

A2, B2, G2, J2 - characteristic factors for right curve. If only one curve used then set A2=0, B2=0, G2=0, J2=0

- service life from project settings,

- operation temperature from pipe properties,

- safety factor for operation temperature from material database. Ki=K20 for ti<=20, Ki=Kope for ti>20

Stresses from primary + secondary loads (weight, pressure, thermal expansions, support movements, cold spring, etc.) in operating state

- service life from project settings,

Kt - secondary allowable stress factor from material database. Recommended values PVC, PVC-C: 1.75, PE, PE-RT: 2.5, PVDF: 3.5, PP: 2.5

Kcyc - fatigue factor

N - number of cycles per year, taken from Temperature Cycles multiplied by service life from Project Settings

Kcyc can't be greater than 1.0 or less than 0.4

All other factors calculated using the same rules, as for weight (primary) loads

Stresses from primary + secondary loads (weight, pressure, thermal expansions, support movements, cold spring, etc.) in cold State

- service life from project settings,

- Ambient temperature from Project Settings,

All other factors calculated using the same rules, as for operating (primary+secondary) loads

Stresses in the test state

Ky=1, Kx=1, Kp=1, Kt=1, Kcyc=1

=24 hours,

- test temperature from pipe properties,

- safety factor for occasional loads Kacc from material database.

Stresses from occasional loads

Ky=1, Kx=1, Kp=1, Kt=1, Kcyc=1

=24 hours,

- operation temperature from project settings,

- safety factor for occasional loads Kacc from material database.

Stress Calculation Equations

Hoop Stress

Axial Stress

Torsion Stress

Equivalent Stress

p - pressure

D - pipe outer diameter

t - pipe wall thickness

A - pipe cross section area

Z - pipe section modulus

Mt - torsion moment

Mi - in-plane moment

Mo - out-plane moment

- ring bending stresses, calculated using built-in nonlinear FEM model

Stress Intensification Factors

Bends:

For straight tees:

For reducing tees:

Reducer:

Thermoplastic Pipe Damage Example

The crack in PVC-C pipe caused by too high torsion moment caused by thermal expansion. The pipe stress analysis using START-PROF software at design stage can help to avoid this situation.

See Also

See video:

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