Code ASME B31.1 for Piping Stress Analysis

Piping stress analysis can use code ASME B31.1 as reference calculation and design. Code ASME B31.1 for power piping analysis is belonging to the American society of mechanical engineers. In code ASME B31.1 there are empirical formulas that apply to the sustain load, expansion load, and the combination of sustain and expansion load (during operations), and also occasional load are described as the following below:

1. Sustain load

The stresses (S) that occur due to sustain the load such as pressure, weight and other mechanical loads can be expressed by the equation as follows:

(P D_o / 4 t_n) + 1000(0.75 i M_a / Z) ≤ 1.0 S_h

2. Range Thermal Expansion Load

The stresses that occur due to thermal expansion can be expressed as the equation below:

S_E = 1000(i Mc / Z) ≤ S_A + f(S_h – S_L)

3. Combination Load of Sustain load and Thermal Expansion Load

The stresses due to combination of sustained load and thermal expansion load (S_ls + S_E), can be calculated with the equation:

S_ls + S_E = (P D_o / 4 t_n) + 1000(0.75 i M_a / Z) + 1000(i Mc / Z) ≤ (S_h + S_a)

4. Occasional Load

The stresses that occur due to pressure, weight, and other sustain load can be expressed as the equation below:

(P D_o / 4 t_n) + 1000(0.75 i M_a / Z) + 1000(i Mc / Z) ≤ K S_h

Where:

P          = internal design pressure (psi)

Do       = outside diameter (in)

Ma       = Moment due to sustain load (in-lbs)

Mb       = Moment due to occasional load (in-lbs)

Mc       = Range of the moment due to thermal expansion (in-lbs)

Z          = section modulus of the pipe (in³)

tn         = nominal wall thickness of pipe (in)

i           = stress intensification factor

K equal to 1.15 for the occasional load which work less than 1% of the operating period and is equal to 1.20 for the occasional load which work less than 10% of the operating period.

Allowable Stress Based on ASME B31.3

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 (S_L) 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 (S_E) in piping system should not exceed allowable displacement stress range (S_A) which can be calculated as follow:

S_A = 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 S_A according to the following equation:

S_E = √(Sb² + St²) ≤ f(1.25 Sc + 0.25Sh)

The value of Sb and St are bending stress and torsional stress respectively.

Flexibility Analysis Based on ASME B31.3

Based on code ASME B31.3, flexibility analysis in the piping system needs formal analysis or does not need formal analysis. Piping system can be does not need formal analysis it meet the following requirement:

1. Piping system is a duplicate of an existing piping system, which is in operation showed a satisfactory performance.
2. Piping system can be judged easily that has sufficient flexibility when compared with the flexibility of existing piping system which had been analyzed before.
3. Piping system with uniform size, which is supported by only two support without any restraint point between them and meet the empirical equation as following below:

(D_y / (L-U)²) ≤ K₁

Where:

K₁        = 208000 SA/Ea (mm/m)²

U         = anchor distance, straight line between anchor, m

L          = developed length of pipe between anchor, m

Y         = resultant of total displacement strain to be absorbed by piping system, mm

D         = outside diameter of pipe, mm

Ea        = reference modulus of elasticity at 201 C (700 F), Mpa

SA       = allowable displacement stress range, Mpa

While a piping system can be said require for analysis of flexibility when meeting the following requirements:

1. Piping system that does not meet of the three requirements above must be analyzed by one of methods analysis of the following: simple analysis method, approximate analysis method or comprehensive analysis method.
2. Comprehensive analyses that are acceptable include analysis method and method that use charts which can calculate force, moment and stress which is caused by the displacement strain.
3. The factors of stress intensity in piping component except straight pipe must be taken into calculation in comprehensive analysis method.
4. In the flexibility analysis, all of piping components between two anchor points must be analyzed as a complete system.

Code Standard for Design Piping System

Piping system must pay attention to the good of plan qualification both in technical term and economical terms. Qualification plan of piping system in term of mechanical can be found by doing some analysis, such as determination of pipe wall thickness, hydraulic analysis, pipe stress analysis, and analysis flexible pipe. While in terms of economical, plan of qualification piping system depends on the financial policy of the company or industry but keeping in terms of eligibility based on the mechanics that have been set in the Code Standards. This condition is to ensure the security and use current piping system plan which is operated safely for the environment.

Planning piping system can use rules code and standard. Code and standard that are applicable in the planning piping system, namely:

a. ASME B31.9 Building Services Piping
b. ASME B31.8 Gas Transmission & Distribution Piping
c. ASME B31.7 Nuclear Power Piping
d. ASME B31.5 Refrigeration Piping
e.ASME B31.4 Liquid-petroleum transportation piping system
f. ASME B31.3 Process Piping on Petroleum Refineries, Chemical, Pharmaceutical, Textile, Papper, Semiconductor, and Crycogenic Plant.
g. ASME B31.2 Fuel Gas Piping
h. ASME B31.1 Power Piping
i. API 5L Spesification of Line Pipe Material
j. API 576 Pipeline Coating
k. DnV 1981 Rules For Submarine Pipe Systems
l. DnV RP F105 Free Spanning Pipelines
m. DnV RP E305 On Bottom Stability Design Of Submarine Pipeline
n. ANSI B16.5 Pipe Flange and Flange Fitting