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:

The allowable stress of plastic piping is dependent on service life and temperature. To determine the allowable stress used the that depends on temperature and service life. The A,B,G,J factors are stored in START-PROF material database. For details about how the stress and allowable stresses are calculated, see Stresses in Piping

Linear thermal expansion factor for plastic piping is much higher than for steel piping, so thermal expansion e1 is much greater than in steel piping

Linear expansion from pressure load e2, caused by Bourdon effect, for plastic piping is much greater than for steel piping and must be taken into account

In some cases the swelling elongation due to chemical reaction e3 with product should be considered. Swelling strain should be specified in pipe additional properties

Unlike steel piping, the stresses are calculated using Young's modulus (creep modulus) Ecm that depends on service life and stress level. For higher service life - the lower creep modulus is used. The support loads, displacements etc. calculated at 100 minutes creep modulus Esm. The analysis for occasional loads is performed using 0.1 hour creep modulus Est

In operating condition the average creep modulus is used (average between installation and operating temperature)

Poisson's ratio is highly dependent on temperature. At high temperatures the 0.5 value often used

Allowable stress for plastic piping depends on chemical resistance factor, laying condition factor, safety factor and joint strength factor

The wall thickness check is performed only for straight pipes and not performed for fittings

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

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

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

There are 3 types of creep modulus:

- Average creep modulus Ecm - used for sustained (primary), operating (primary+secondary) and cold (primary+secondary) stress calculation

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

- Average creep modulus Esm - used for support and equipment load calculation at operating temperature

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

- Average creep modulus Est - used for occasional stress calculation (from wind, seismic, water hammer, slug flow, etc.)

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:

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

- 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

- service life from project settings,

- Ambient temperature from Project Settings,

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

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.

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.

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

Bends:

For straight tees:

For reducing tees:

Reducer:

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 video:

- HDPE Piping Stress Analysis With PASS/START-PROF Tutorial. Part 1
- HDPE Piping Stress Analysis Tutorial With PASS/START-PROF Training. Part 2