ASME B31.3-Process Piping
ASME B31.1-Power Piping
Scope (B31.3 vs B31.1)
ASME B31.3 provides rules for Process or Chemical Plant piping.
ASME B 31.1 provides rules for Power Plant piping.
Basic Allowable Material Stress
As per ASME B31.3, the basic allowable material stress value is higher (For example the allowable stress value for A 106 B material at 250 Deg C is 132117.328 Kpa as per ASME B 31.3) than the same as per B31.1.
The basic allowable material stress value as per ASME B31.1 is lower (for example, the allowable stress value for A 106 B material at 250 Deg C is 117900.344 Kpa as per ASME B 31.1) than that of ASME B31.3.
Allowable Sagging (Sustained)
ASME B31.3 code does not specifically say about any limit of allowable sagging. An allowable sagging of up to 15 mm is acceptable in general. B31.3 does not provide a suggested support span.
ASME B31.1 clearly specifies the allowable sagging value as 2.5 mm. Table 121.5-1 of ASME B 31.1 provides a suggested support span.
SIF on Reducers
Process Piping Code ASME B31.3 does not use SIF (SIF=1.0) for reducer stress calculation
Power Piping code ASME B31.1 uses a maximum SIF of 2.0 for reducers while pipe stress calculation.
Factor of Safety
ASME B31.3 uses a factor of safety of 3; relatively lower than ASME B31.1.
ASME B31.1 uses a safety factor of 4 to have higher reliability as compared to Process plants
SIF for Butt Welded Joints
B31.3 uses a SIF of 1.0 for butt-welded joints
B31.1 uses a SIF of up to 1.9 max in stress calculation.
Approach towards SIF
ASME B31.3 uses a complex in-plane, out-of-plane SIF approach.
ASME B31.1 uses a simplified single SIF Approach.
Maximum values of Sc and Sh
As per the Process Piping code ASME B31.3, the maximum value of Sc and Sh are limited to 138 Mpa or 20 ksi.
For the Power piping code (ASME B31.1), the maximum value of Sc and Sh are 138 Mpa only if the minimum tensile strength of the material is 70 ksi (480 Mpa) otherwise it is dependent on the values provided in the mandatory Appendix A as per temperature.
Allowable Stress for Occasional Stresses
The allowable value of occasional stress as per ASME B31.3 is 1.33 times Sh
As per ASME B31.1, the allowable value of occasional stress is 1.15 to 1.20 times Sh
Equation for Pipe Wall Thickness Calculation
There is no such limitation in the Power Piping (ASME B31.1) wall thickness calculation. However, they add a limitation on maximum design pressure.
Section Modulus, Z for Sustained and Occasional Stresses
While Sustained and Occasional stress calculation, the Process Piping code ASME B31.3 reduces the thickness by corrosion and other allowances.
ASME B31.1 calculates the section modulus using nominal thickness. Thickness is not reduced by corrosion and other allowances.
Rules for material usage below -29 Deg. C
B31.3 provides extensive rules for use of materials below-29 Deg C
The power piping code, B31.1 does not provide any such rules for pipe materials below -29 deg C.
Maximum Value of Cyclic Stress Range Factor
The maximum value of cyclic stress range factor, f as per B31.3 is 1.2.
As per ASME B31.1, the maximum value of f is 1.0
Allowance for Pressure Temperature Variation
As per clause 302.2.4 of ASME B31.3, occasional pressure-temperature variation can exceed the allowable by (a) 33% for no more than 10 hours at any one time and no more than 100 hours/year or (b) 20% for no more than 50 hours at any one time and no more than 500 hours/year.
As per clause 102.2.4 of ASME B31.1, occasional pressure-temperature variation can exceed the allowable by (a) 15% if the event duration occurs for no more than 8 hours at any one time and not more than 800 hours/year, or (b) 20% if the event duration occurs for not more than 1 hour at any one time and not more than 80 hours/year
Process Piping following ASME B31.3 is normally designed for 20 to 30 years of service life.
Power Piping using ASME B31.1 is generally designed for 40 years or more of service life.
PSV reaction force
ASME B31.1 provides specific equations for PSV reaction force calculation.
Hydrostatic Test Pressure
As per ASME B31.3, the hydrostatic test
for the piping system need to be performed at 1.5 times the design pressure corrected for temperature means the design pressure must be multiplied by ST
/S in the case of process piping. Here, ST
=pipe material allowable stress at test temperature, and S=pipe material allowable stress at component design temperature. (Clause 345.4.2)
The hydrostatic test pressure following ASME B31.1 is 1.5 times the piping design pressure. (Clause 137.4.5)
Pneumatic Test Pressure
The pneumatic test pressure as per ASME B31.3 is (1.1 to 1.33) times the design pressure of the piping system. (Clause 345.5.4)
B31.1 instructs to use a pneumatic test pressure between (1.2 to 1.5) times the design pressure for the piping system. (Clause 137.5.5)