Power Piping

KJ’s Services has been fabricating pipe per B31.1.1, B31.3, and B31.8 for over 40 years with a specialty in power piping (ASME B31.1) (ASME Code S and PP stamps) for over 20 years. Power Piping (sometimes referred to as Boiler Pressure Vessel Piping) is constructed to a much higher constraint than your standard fabricated piping systems.

What is Power Piping?

Power piping is found in electric power generating stations, industrial plants and refineries, geothermal heating systems, and central and district heating and cooling systems. The standard for these systems is ASME B31.1 which prescribes requirements for the design, materials, fabrication, erection, examination, testing, inspection, operation, and maintenance of piping systems typically found in the aforementioned systems. It also covers boiler-external piping for power boilers and high-temperature, high pressure water boilers in which steam or vapor is generated at a pressure of more than 15 psig [100 kPa (gage)]; and high temperature water is generated at pressures exceeding 160 psig [1,103 kPa (gage)] and/or temperatures exceeding 250 degrees F (120 degrees C). ASME B31.1 or Power Piping Code provides rules important for Power piping Professionals as this code governs the design rules for power generation throughout the world.

ASME B31.3 and ASME B31.1 codes (i.e Process Piping Code and Power Piping code) are different. Both are related to piping design aspects but vary widely in design considerations. The following table lists the major differences between ASME B31.3 and ASME B31.1.

Sr. No

Parameter

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 S_c and S_h are limited to 138 Mpa or 20 ksi.

For the Power piping code (ASME B31.1), the maximum value of S_c and S_h 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 S_h

As per ASME B31.1, the allowable value of occasional stress is 1.15 to 1.20 times S_h

Equation for Pipe Wall Thickness Calculation

The equation for pipe wall thickness calculation in B31.3 is valid for t<D/6.

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

Design Life

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

B31.3 code does not provide specific equations for PSV reaction force calculation.

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 S_T/S in the case of process piping. Here, S_T=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)