Read about START-PROF pipe stress analysis software
Steel pipe nominal wall thickness is calculated as:
S ≥ SR + С,
SR - design wall thickness. Minimum wall thickness necessary to withstand internal pressure, determined by standards
С - total allowance for design wall thickness
Total allowance C is calculated as:
С = С1 + С2,
С1 - manufacturing allowance (mill tolerance), taken as the sum of the negative wall thickness deviation C11 and mill tolerance C12. For ASME code it is entered in percents (%). For Russian codes it is entered in millimeters.
С11 - allowance to compensate for negative tolerance. Determined by negative wall thickness deviation, set by standards or specifications. Allowance does not include design thickness rounding to standard sheet thickness.
С12 - mill tolerance to compensate element wall thinning during technical operations - stretch, press, bend, etc. Determined by the fitting manufacturing process and set according to technical manufacturing conditions.
С2 - mill tolerance to compensate for corrosion and wear (erosion). Determined by design standards or industry standards taking into account projected service life. When contact with corrosion (erosion) zone is two-sided, С2 should be increased.
Note that nominal wall thickness S should not be lower than values in standards, and that S is rounded up to the closest wall thickness according to standards or technical conditions.
Prior to analysis (at the input data logic check stage), START-PROF checks wall thickness for design pressure using:
S - C ≥ SR
Nominal wall thickness S and total allowance C values are input in element properties. SR - minimum design wall thickness for design pressure according to standards selected in Project Settings.
For ASME code wall thickness is checked only for straight pipes and pipe bends. For all other fittings wall thickness is not checked.
If test pressure value at manufacturing facility is lower than required, there must be a guarantee of the ability to run hydraulic tests during construction up to the pressure causing equivalent stress equal to:
for SNIP 2.05.06-85 - 95% of standard yield strength σ = 0.95
for RD 10-249-98 (table 2,8) - σ = /1.1 ~ 0.9
for PB 03-585-03  - 90% of material yield strength at test pressure for . σ = 0.9
Based on this, START-PROF checks wall thickness for test pressure using the formula (for GOST 32388-2013 only for sustained mode):
S ≥ ST
ST - minimum wall thickness for test pressure calculated according to standards selected in Project Settings. Test temperature Тtest is input in Project Settings, test pressure Ptest is input in element properties, material yield strength is in the materials database.
Note that, according to GOST 8731 and GOST 8733, carbon and low allow steel seamless pipes must withstand hydraulic pressure at allowable stress equal to 40% of tensile strength for the given steel brand σ = 0.4, which does not always correspond to requirements of standards.
Pipes will meet GOST 8731 and GOST 8733 criteria only if they meet the following condition:
Let's look at an example - steel 20. According to , tensile strength value at 20°C = 460 MPa, yield strength = 250 MPa; the condition is then:
0.4 ∙ 460 < 0.9 ∙ 250 MPa
184 < 225 MPa,
i.e., the condition is not met. This indicates that standard requirements regarding allowable test pressure are not met and the manufacturing plant must be given more strict conditions for tests at 225MPa pressure (GOST documents have a note regarding this).
Reinforced wall thickness is calculated as:
- nominal wall thickness
- internal (non-reinforced) protective layer thickness
- external (non-reinforced) protective layer thickness
1. PB 03-585-03, rules of design of process piping, Moscow, 2003
2. GOST 34233.1-2017 Boiler and pressure vessels code