Allowable stress of material which is used is an important parameter in the stress analysis of piping system. Working stress in the piping system should not exceed allowable stress of that material based on code and standard. In the piping system, stress ratio is generally used. Stress ratio is comparison between actual working pressure in the piping system and allowable stress based on code and standard.
Allowable stress has different value in each code. For code ASME 31.3, the pipe material can be expressed in safe condition when the stresses which are experienced by the material meet the following criteria:
1. Sustain Load
The resultant of longitudinal stress (SL) in each component of piping system due to sustain load such as internal load and weight of pipe load, should not exceed the value of multiplication between Sh and W. Sh is allowable stress of pipe material at maximum temperature of operating condition. W is reduction factor of welding joint in the manufacturing process of pipe. W has value equal with 1.0 if longitudinal connection type is used in the manufacturing process of pipe.
Displacement stress range (SE) in piping system should not exceed allowable displacement stress range (SA) which can be calculated as follow:
SA = f(1.25 Sc + 0.25 Sh)
Where:
F = stress range factor
Sc = allowable stress of pipe on minimum working temperature.
Sh = allowable stress of pipe on maximum working temperature
2. Occasional Load
The resultant of longitudinal stress due to sustained load and all of loads which are caused by occasional load such as wind load and seismic load, should not exceed the value of 1.33 is multiplied with allowable stress of pipe on maximum working temperature (Sh). For casting material, the Sh value must be multiplied by factor quality casting (Ec). Loading due to wind and seismic are not generally occur simultaneously, so in the pipe stress analysis, the calculation is performed only to one of that loads in accordance with environmental condition.
3. Thermal Expansion Load
The stress that occurs due to thermal expansion load is stress range from the resultant of bending stress and torsional stress due to thermal expansion. Range expansion stress should not exceed allowable stress SA according to the following equation:
SE = √(Sb2 + St2) ≤ f(1.25 Sc + 0.25Sh)
The value of Sb and St are bending stress and torsional stress respectively.